Computer Networks ISE: A Systems Approach, Fourth Edition

Computer Networks ISE: A Systems Approach, Fourth Edition
EDITION
COMPUTER
NETWORKS
A
S Y S T E M S
A P P R O A C H
4
The Morgan Kaufmann Series in Networking
Series Editor, David Clark, M.I.T.
Computer Networks: A Systems Approach, 4e
Larry L. Peterson and Bruce S. Davie
Network Routing: Algorithms, Protocols, and
Architectures
Deepankar Medhi and Karthikeyan Ramaswami
Deploying IP and MPLS QoS for Multiservice Networks:
Theory and Practice
John Evans and Clarence Filsfils
Traffic Engineering and QoS Optimization of Integrated
Voice & Data Networks
Gerald R. Ash
IPv6 Core Protocols Implementation
Qing Li, Tatuya Jinmei, and Keiichi Shima
Smart Phone and Next-Generation Mobile Computing
Pei Zheng and Lionel Ni
GMPLS: Architecture and Applications
Adrian Farrel and Igor Bryskin
Network Security: A Practical Approach
Jan L. Harrington
Content Networking: Architecture, Protocols, and Practice
Markus Hofmann and Leland R. Beaumont
Network Algorithmics: An Interdisciplinary Approach to
Designing Fast Networked Devices
George Varghese
Network Recovery: Protection and Restoration of Optical,
SONET-SDH, IP, and MPLS
Jean Philippe Vasseur, Mario Pickavet, and Piet
Demeester
Routing, Flow, and Capacity Design in Communication
and Computer Networks
Michał Pióro and Deepankar Medhi
Wireless Sensor Networks: An Information Processing
Approach
Feng Zhao and Leonidas Guibas
Communication Networking: An Analytical Approach
Anurag Kumar, D. Manjunath, and Joy Kuri
The Internet and Its Protocols: A Comparative Approach
Adrian Farrel
Modern Cable Television Technology: Video, Voice, and
Data Communications, 2e
Walter Ciciora, James Farmer, David Large, and
Michael Adams
Bluetooth Application Programming with the Java APIs
C Bala Kumar, Paul J. Kline, and Timothy J.
Thompson
Policy-Based Network Management: Solutions for the
Next Generation
John Strassner
Network Architecture, Analysis, and Design, 2e
James D. McCabe
MPLS Network Management: MIBs, Tools, and
Techniques
Thomas D. Nadeau
Developing IP-Based Services: Solutions for Service
Providers and Vendors
Monique Morrow and Kateel Vijayananda
Telecommunications Law in the Internet Age
Sharon K. Black
Optical Networks: A Practical Perspective, 2e
Rajiv Ramaswami and Kumar N. Sivarajan
Internet QoS: Architectures and Mechanisms
Zheng Wang
TCP/IP Sockets in Java: Practical Guide for Programmers
Michael J. Donahoo and Kenneth L. Calvert
TCP/IP Sockets in C: Practical Guide for Programmers
Kenneth L. Calvert and Michael J. Donahoo
Multicast Communication: Protocols, Programming, and
Applications
Ralph Wittmann and Martina Zitterbart
MPLS: Technology and Applications
Bruce Davie and Yakov Rekhter
High-Performance Communication Networks, 2e
Jean Walrand and Pravin Varaiya
Internetworking Multimedia
Jon Crowcroft, Mark Handley, and Ian Wakeman
Understanding Networked Applications: A First Course
David G. Messerschmitt
Integrated Management of Networked Systems: Concepts,
Architectures, and Their Operational
Application
Heinz-Gerd Hegering, Sebastian Abeck, and
Bernhard Neumair
Virtual Private Networks: Making the Right Connection
Dennis Fowler
Networked Applications: A Guide to the New Computing
Infrastructure
David G. Messerschmitt
Wide Area Network Design: Concepts and Tools for
Optimization
Robert S. Cahn
For further information on these books and for a list of forthcoming titles, please visit our Web site at
http://www.mkp.com.
EDITION
COMPUTER
NETWORKS
A
S Y S T E M S
A P P R O A C H
Larry L. Peterson & Bruce S. Davie
AMSTERDAM • BOSTON • HEIDELBERG • LONDON
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ISBN 13: 978-0-12-370548-8 (Case bound)
ISBN 10: 0-12-370548-7 (Case bound)
ISBN 13: 978-0-12-374013-7 (Paperback)
ISBN 10: 0-12-374013-4 (Paperback)
Library of Congress Cataloging-in-Publication Data
Peterson, Larry L.
Computer networks : a systems approach / Larry L. Peterson & Bruce S.
Davi – 4th ed.
p. cm.
Includes bibliographical references and index.
ISBN-13: 978-0-12-370548-8 (hardcover : alk. paper)
ISBN-10: 0-12-370548-7 (hardcover : alk. paper)
ISBN-13: 978-0-12-374013-7 (pbk. : alk. paper)
ISBN-10: 0-12-374013-4 (pbk. : alk. paper) 1. Computer networks. I.
Davie, Bruce S. II. Title.
TK5105.5.P479 2007
004.6’5–dc22
2006102454
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Printed in the United States of America.
06 07 08 09 10
5 4 3 2 1
To Lee Peterson and Robert Davie
F O R E W O R D
David D. Clark
Massachusetts Institute of Technology
I
t is now ten years since this classic book first appeared. Looking back, it is amazing
what has happened in that time. We have seen the transformation of the Web from
a small experiment to a World Wide phenomenon. We have seen the emergence
of voice over IP and peer-to-peer content sharing. We have seen technology speed up
a hundred-fold, the emergence of broadband to the home, and the rise of botnets and
other horrid security problems. Many things have changed, technology has come and
gone, and (perhaps equally amazing) much of the basics of the Internet are still there.
This book, too, has changed much in ten years, with four editions to keep up. But
the basic value of the book remains the same as the first edition. This book gives you the
facts you need, and puts those facts into the larger context so that the knowledge you
gain will be of value even as the details change. Reading this book informs you about
today and prepares you for tomorrow. One new feature is a set of sidebars that illustrate
the context of ideas being presented in the text—the why of the ideas. Why did an idea
fail? Why did it succeed?
What has changed in the book? Some technologies have faded from sight, and get
less attention in this edition. We bid a fond farewell to FDDI and ATM LANs. Some
technologies have mutated and emerged in new forms. Remote Procedure Call is no
longer a LAN-based low-level invocation mechanism, but the foundation of Internetwide Web Services. We welcome gigabit Ethernet, an updated and expanded section on
wireless, and more on router implementation. The material on TCP is up to date, with
discussion of new acknowledgment schemes and extensions for high speed.
With the increasing concern with security, there is a completely revised chapter
with a new emphasis on a systems approach to security, and a discussion of threats and
how to counter them. And at the end, there is a chapter that helps you “put it all together,” using case studies at the application layer (VOIP, multimedia, and peer to peer)
to show how all the concepts from the previous chapters combine to provide the system
that supports these applications.
viii
Foreword
The evolution of networks is not going to slow down. Soon we will be talking
about the impact of television over IP, the collision of the Internet and sensor networks,
and lots of other very new and exciting ideas. But relax—if you read this book today you
will have the insights you need for tomorrow.
F O R E W O R D T O T H E
F I R S T
E D I T I O N
David Clark
Massachusetts Institute of Technology
T
he term spaghetti code is universally understood as an insult. All good computer
scientists worship the god of modularity, since modularity brings many benefits,
including the all-powerful benefit of not having to understand all parts of a
problem at the same time in order to solve it. Modularity thus plays a role in presenting
ideas in a book, as well as in writing code. If a book’s material is organized effectively—
modularly—the reader can start at the beginning and actually make it to the end.
The field of network protocols is perhaps unique in that the “proper” modularity
has been handed down to us in the form of an international standard: the seven-layer
reference model of network protocols from the ISO. This model, which reflects a layered
approach to modularity, is almost universally used as a starting point for discussions of
protocol organization, whether the design in question conforms to the model or deviates
from it.
It seems obvious to organize a networking book around this layered model. However, there is a peril to doing so, because the OSI model is not really successful at organizing the core concepts of networking. Such basic requirements as reliability, flow control,
or security can be addressed at most, if not all, of the OSI layers. This fact has led to
great confusion in trying to understand the reference model. At times it even requires a
suspension of disbelief. Indeed, a book organized strictly according to a layered model
has some of the attributes of spaghetti code.
Which brings us to this book. Peterson and Davie follow the traditional layered
model, but they do not pretend that this model actually helps in the understanding
of the big issues in networking. Instead, the authors organize discussion of fundamental concepts in a way that is independent of layering. Thus, after reading the book,
readers will understand flow control, congestion control, reliability enhancement, data
representation, and synchronization, and will separately understand the implications of
addressing these issues in one or another of the traditional layers.
This is a timely book. It looks at the important protocols in use today—especially
the Internet protocols. Peterson and Davie have a long involvement in and much ex-
x
Foreword to the First Edition
perience with the Internet. Thus their book reflects not just the theoretical issues in
protocol design, but the real factors that matter in practice. The book looks at some of
the protocols that are just emerging now, so the reader can be assured of an up-to-date
perspective. But most importantly, the discussion of basic issues is presented in a way
that derives from the fundamental nature of the problem, not the constraints of the layered reference model or the details of today’s protocols. In this regard, what this book
presents is both timely and timeless. The combination of real-world relevance, current
examples, and careful explanation of fundamentals makes this book unique.
P R E FA C E
W
hen the first edition of this book was published in 1996, it was a novelty to
be able to order merchandise on the Internet, and a company that advertised
its domain name was considered cutting edge. Today, Internet commerce
is a fact of life, and “.com” stocks have gone through an entire boom and bust cycle.
A host of new technologies ranging from optical switches to wireless networks are now
becoming mainstream. It seems the only predictable thing about the Internet is constant
change.
Despite these changes the question we asked in the first edition is just as valid
today: What are the underlying concepts and technologies that make the Internet work?
The answer is that much of the TCP/IP architecture continues to function just as was
envisioned by its creators more than 30 years ago. This isn’t to say that the Internet
architecture is uninteresting; quite the contrary. Understanding the design principles that
underly an architecture that has not only survived but fostered the kind of growth and
change that the Internet has seen over the past three decades is precisely the right place
to start. Like the previous editions, the third edition makes the “why” of the Internet
architecture its cornerstone.
Audience
Our intent is that the book should serve as the text for a comprehensive networking
class, at either the graduate or upper-division undergraduate level. We also believe that
the book’s focus on core concepts should be appealing to industry professionals who are
retraining for network-related assignments, as well as current network practitioners who
want to understand the “whys” behind the protocols they work with every day and to see
the big picture of networking.
It is our experience that both students and professionals learning about networks
for the first time often have the impression that network protocols are some sort of edict
handed down from on high, and that their job is to learn as many TLAs (three-letter
acronyms) as possible. In fact, protocols are the building blocks of a complex system
developed through the application of engineering design principles. Moreover, they are
constantly being refined, extended, and replaced based on real-world experience. With
xii
Preface
this in mind, our goal with this book is to do more than survey the protocols in use
today. Instead, we explain the underlying principles of sound network design. We feel
that this grasp of underlying principles is the best tool for handling the rate of change in
the networking field.
Changes in the Fourth Edition
Even though our focus is on the underlying principles of networking, we illustrate these
principles using examples from today’s working Internet. Therefore, we added a significant amount of new material to track many of the important recent advances in networking. We also deleted, reorganized, and changed the focus of existing material to reflect
changes that have taken place over the past decade.
Perhaps the most significant change we have noticed since writing the first edition
is that almost every reader now has some familiarity with networked applications such as
the World Wide Web and email. For this reason, we have increased the focus on applications, starting in the first chapter. We use applications as the motivation for the study of
networking, and to derive a set of requirements that a useful network must meet if it is
to support both current and future applications on a global scale. However, we retain the
problem-solving approach of previous editions that starts with the problem of interconnecting hosts and works its way up the layers to conclude with a detailed examination
of application layer issues. We believe it is important to make the topics covered in the
book relevant by starting with applications and their needs. At the same time, we feel
that higher-layer issues, such as application layer and transport layer protocols, are best
understood after the basic problems of connecting hosts and switching packets have been
explained.
As we did in the second and third editions, we have added or increased coverage of
important new topics, and brought other topics up to date. Major new or substantially
updated topics in this edition are:
■
Comprehensively revised and updated coverage of security, with a focus on
building secure systems, not just on cryptographic algorithms;
■
Expanded and updated coverage of XML (extensible markup language);
■
An updated section on overlay networks, including “peer-to-peer” networking
and “content distribution networks”;
■
A new section on web services, including the SOAP and REST (Representational State Transfer) architectures;
Preface
xiii
■
Updated material on wireless technology, including the 802.11 (WiFi) and
802.16 (WiMAX) standards as well as cellular wireless technologies including
the 3G (third generation) standards;
■
Expanded coverage of interdomain routing;
■
Expanded coverage on protocols and quality of service for multimedia applications such as voiceover IP (VOIP) and video streaming;
■
Updated coverage of congestion control mechanisms, particularly for high
bandwidth-delay product networks.
In addition, we have added a new feature to this edition: “Where are they now?”
sidebars. These short discussions focus on the success and failure of protocols in the real
world. Sometimes they describe a protocol that most people have written off but which
is actually enjoying unheralded success; other times they trace the fate of a protocol
that failed to thrive over the long run. The goal of these sidebars is to make the material
relevant by showing how technologies have fared in the competitive world of networking.
Approach
For an area that’s as dynamic and changing as computer networks, the most important
thing a textbook can offer is perspective—to distinguish between what’s important and
what’s not, and between what’s lasting and what’s superficial. Based on our experience
over the past 20-plus years doing research that has led to new networking technology,
teaching undergraduate and graduate students about the latest trends in networking, and
delivering advanced networking products to market, we have developed a perspective—
which we call the systems approach—that forms the soul of this book. The systems approach has several implications:
■
Rather than accept existing artifacts as gospel, we start first with principles and
walk you through the thought process that led to today’s networks. This allows
us to explain why networks look like they do. It is our experience that once you
understand the underlying concepts, any new protocol that you are confronted
with will be relatively easy to digest.
■
Although the material is loosely organized around the traditional network layers, starting at the bottom and moving up the protocol stack, we do not adopt
a rigidly layerist approach. Many topics—congestion control and security are
good examples—have implications up and down the hierarchy, and so we discuss them outside the traditional layered model. In short, we believe layering
makes a good servant but a poor master; it’s more often useful to take an endto-end perspective.
xiv
Preface
■
Rather than explain how protocols work in the abstract, we use the most important protocols in use today—many of them from the TCP/IP Internet—to
illustrate how networks work in practice. This allows us to include real-world
experiences in the discussion.
■
Although at the lowest levels networks are constructed from commodity hardware that can be bought from computer vendors and communication services
that can be leased from the phone company, it is the software that allows networks to provide new services and adapt quickly to changing circumstances. It is
for this reason that we emphasize how network software is implemented, rather
than stopping with a description of the abstract algorithms involved. We also
include code segments taken from a working protocol stack to illustrate how
you might implement certain protocols and algorithms.
■
Networks are constructed from many building-block pieces, and while it is necessary to be able to abstract away uninteresting elements when solving a particular problem, it is essential to understand how all the pieces fit together to form a
functioning network. We therefore spend considerable time explaining the overall end-to-end behavior of networks, not just the individual components, so that
it is possible to understand how a complete network operates, all the way from
the application to the hardware.
■
The systems approach implies doing experimental performance studies, and
then using the data you gather both to quantitatively analyze various design
options and to guide you in optimizing the implementation. This emphasis on
empirical analysis pervades the book.
■
Networks are like other computer systems—for example, operating systems,
processor architectures, distributed and parallel systems, and so on. They are all
large and complex. To help manage this complexity, system builders often draw
on a collection of design principles. We highlight these design principles as they
are introduced throughout the book, illustrated, of course, with examples from
computer networks.
Pedagogy and Features
The fourth edition retains several features from prior editions, and adds one more, that
we encourage you to take advantage of:
■
Problem statements. At the start of each chapter, we describe a problem that
identifies the next set of issues that must be addressed in the design of a network.
This statement introduces and motivates the issues to be explored in the chapter.
Preface
xv
■
Shaded sidebars. Throughout the text, shaded sidebars elaborate on the topic being discussed or introduce a related advanced topic. In many cases, these sidebars
relate real-world anecdotes about networking.
■
“Where are they now?” sidebars. These new elements trace the success and failure
of protocols in real-world deployment.
■
Highlighted paragraphs. These paragraphs summarize an important nugget of
information that we want you to take away from the discussion, such as a widely
applicable system design principle.
■
Real protocols. Even though the book’s focus is on core concepts rather than existing protocol specifications, real protocols are used to illustrate most of the
important ideas. As a result, the book can be used as a source of reference for
many protocols. To help you find the descriptions of the protocols, each applicable section heading parenthetically identifies the protocols described in that
section. For example, Section 5.2, which describes the principles of reliable endto-end protocols, provides a detailed description of TCP, the canonical example
of such a protocol.
■
Open issues. We conclude the main body of each chapter with an important
issue that is currently being debated in the research community, the commercial
world, or society as a whole. We have found that discussing these issues helps to
make the subject of networking more relevant and exciting.
■
Recommended reading. These highly selective lists appear at the end of each chapter. Each list generally contains the seminal papers on the topics just discussed.
We strongly recommend that advanced readers (e.g., graduate students) study
the papers in this reading list to supplement the material covered in the chapter.
Road Map and Course Use
The book is organized as follows:
■
Chapter 1 introduces the set of core ideas that are used throughout the rest of the
text. Motivated by widespread applications, it discusses what goes into a network
architecture, provides an introduction to protocol implementation issues, and
defines the quantitative performance metrics that often drive network design.
■
Chapter 2 surveys a wide range of low-level network technologies, ranging from
Ethernet to token ring to wireless. It also describes many of the issues that
all data link protocols must address, including encoding, framing, and error
detection.
xvi
Preface
■
Chapter 3 introduces the basic models of switched networks (datagrams versus
virtual circuits) and describes two prevalent switching technologies—switched
Ethernet and ATM—in some detail. It also discusses the design of hardwarebased switches.
■
Chapter 4 introduces internetworking and describes the key elements of the
Internet Protocol (IP). A central question addressed in this chapter is how networks that scale to the size of the Internet are able to route packets. Unicast,
multicast, and interdomain routing are covered.
■
Chapter 5 moves up to the transport level, describing both the Internet’s Transmission Control Protocol (TCP) and Remote Procedure Call (RPC) used to
build client-server applications in detail. The Real-time Transport Protocol
(RTP), which supports multimedia applications, is also described.
■
Chapter 6 discusses congestion control and resource allocation. The issues in
this chapter cut across both the network level (Chapters 3 and 4) and the transport level (Chapter 5). Of particular note, this chapter describes how congestion
control works in TCP, and it introduces the mechanisms used to provide quality
of service in IP.
■
Chapter 7 considers the data sent through a network. This includes both the
problems of presentation formatting and data compression. XML is covered
here, and the compression section includes explanations of how MPEG video
compression and MP3 audio compression work.
■
Chapter 8 discusses network security, beginning with an overview of cryptographic tools, the problems of key distribution, and a discussion of several
authentication techniques using both public and private keys. The main focus of this chapter is the building of secure systems, using examples including
Pretty Good Privacy (PGP), Secure Shell (SSH), and the IP Security architecture
(IPSEC). Firewalls are also covered here.
■
Chapter 9 describes a representative sample of network applications and the
protocols they use, including traditional applications like email and the Web,
multimedia applications such as IP telephony and video streaming, and overlay
networks like peer-to-peer file sharing and content distribution networks. The
Web Services architectures for developing new application protocols are also
presented here.
For an undergraduate course, extra class time will most likely be needed to help
students digest the introductory material in the first chapter, probably at the expense
Preface
xvii
of the more advanced topics covered in Chapters 6 through 8. Chapter 9 then returns
to the popular topic of network applications. In contrast, the instructor for a graduate
course should be able to cover the first chapter in only a lecture or two—with students
studying the material more carefully on their own—thereby freeing up additional class
time to cover the last four chapters in depth. Both graduate and undergraduate classes
will want to cover the core material contained in the middle four chapters (Chapters
2–5), although an undergraduate class might choose to skim the more advanced sections
(e.g., Sections 2.2, 4.4, and 4.5).
For those of you using the book in self-study, we believe that the topics we have
selected cover the core of computer networking, and so we recommend that the book
be read sequentially, from front to back. In addition, we have included a liberal supply
of references to help you locate supplementary material that is relevant to your specific
areas of interest, and we have included solutions to select exercises.
The book takes a unique approach to the topic of congestion control by pulling all
topics related to congestion control and resource allocation together in a single place—
Chapter 6. We do this because the problem of congestion control cannot be solved at
any one level, and we want you to consider the various design options at the same time.
(This is consistent with our view that strict layering often obscures important design
trade-offs.) A more traditional treatment of congestion control is possible, however, by
studying Section 6.2 in the context of Chapter 3 and Section 6.4 in the context of
Chapter 5.
Exercises
Significant effort has gone into improving the exercises with each new edition. In the
second edition we greatly increased the number of problems and, based on class testing,
dramatically improved their quality. In the third edition we made two other important
changes, which we retained here:
■
For those exercises that we felt are particularly challenging or require special
knowledge not provided in the book (e.g., probability expertise), we have added
an icon ★ to indicate the extra level of difficulty.
■
In each chapter we added some extra representative exercises for which worked
solutions are provided at the back of the book. These exercises, marked ✓, are
intended to provide some help in tackling the other exercises in the book.
In this edition we have added new exercises to reflect the updated content. The
current set of exercises are of several different styles:
xviii
Preface
■
Analytical exercises that ask the student to do simple algebraic calculations that
demonstrate their understanding of fundamental relationships.
■
Design questions that ask the student to propose and evaluate protocols for
various circumstances.
■
Hands-on questions that ask the student to write a few lines of code to test an
idea or to experiment with an existing network utility.
■
Library research questions that ask the student to learn more about a particular
topic.
Also, as described in more detail below, socket-based programming assignments, as
well as simulation labs, are available online.
Supplemental Materials and Online Resources
To assist instructors, we have prepared an instructor’s manual that contains solutions to
selected exercises. The manual is available from the publisher.
Additional support materials, including lecture slides, figures from the text, socketbased programming assignments, and sample exams and programming assignments are
available through the Morgan Kaufmann website at http://www.mkp.com/pd4e.
We suggest that you visit the page for this book every few weeks, as we will be adding
support materials and establishing links to networking-related sites on a regular basis.
And finally, as with the third edition, a set of laboratory experiments supplement
the book. These labs, developed by Professor Emad Aboelela from the University of
Massachusetts Dartmouth, use simulation to explore the behavior, scalability, and performance of protocols covered in the book. Sections that discuss material covered by the
laboratory exercises are marked with the icon shown in the margin. The simulations use
the OPNET simulation toolset, which is available for free to any one using Computer
Networks in their course.
Acknowledgments
This book would not have been possible without the help of many people. We would
like to thank them for their efforts in improving the end result. Before we do so, however,
we should mention that we have done our best to correct the mistakes that the reviewers
have pointed out and to accurately describe the protocols and mechanisms that our
colleagues have explained to us. We alone are responsible for any remaining errors. If
you should find any of these, please send email to our publisher, Morgan Kaufmann,
at [email protected], and we will endeavor to correct them in future
printings of this book.
Preface
xix
First, we would like to thank the many people who reviewed drafts of all or parts
of the manuscript. In addition to those who reviewed prior editions, we wish to thank
David Maltz, Bobby Bhattacharjee, and Sarvesh Kaulkarni for their thorough reviews.
Thanks also to Ric Pruss and Mike Takefman for their reviews of various sections. We
also wish to thank all those who provided feedback and input to help us decide what to
do in this edition: Tim Batten, Julio Pontes, and Kevin Mills.
Several members of the Network Systems Group at Princeton contributed ideas,
examples, corrections, data, and code to earlier editions of this book. In particular, we
would like to thank Andy Bavier, Tammo Spalink, Mike Wawrzoniak, Zuki Gottlieb,
George Tzanetakis, and Chad Mynhier. KyoungSoo Park provided valuable help on the
exercise solutions, instructor’s manual, and lecture slides. As before, we want to thank
the Defense Advanced Research Projects Agency, the National Science Foundation, Intel
Corporation, and Cisco Systems, Inc. for supporting our networking research over the
past several years. Thanks also to Cisco for providing the time for one of us to work on
the book.
This edition could not have been produced without the substantial contributions
of Mark Abbott, who crafted a great deal of new material for this book in return for not
much more than these few lines of thanks.
Finally, we would like to thank our series editor, David Clark, as well as all the
people at Morgan Kaufmann who helped shepherd us through the book-writing process.
A special thanks is due to our original sponsoring editor, Jennifer Young; our editor for
this edition, Rick Adams; our developmental editor, Rachel Roumeliotis; and to Karyn
Johnson, assistant editor on prior editions. The whole crew at MKP has been a delight
to work with over the lifetime of this book.
C O N T E N T S
Foreword
vii
Foreword to the First Edition
ix
Preface
xi
1 Foundation
Problem: Building a Network
2
1.1 Applications
4
1.2 Requirements
6
1.2.1 Connectivity
7
1.2.2 Cost-Effective Resource Sharing
11
1.2.3 Support for Common Services
14
1.3 Network Architecture
19
1.3.1 Layering and Protocols
20
1.3.2 OSI Architecture
26
1.3.3 Internet Architecture
28
1.4 Implementing Network Software
30
1.4.1 Application Programming Interface (Sockets)
31
1.4.2 Example Application
33
1.4.3 Protocol Implementation Issues
37
1.5 Performance
1.5.1 Bandwidth and Latency
40
40
1.5.2 Delay × Bandwidth Product
44
1.5.3 High-Speed Networks
46
1.5.4 Application Performance Needs
48
1.6 Summary
50
Open Issue: Ubiquitous Networking
51
Further Reading
52
Exercises
55
xxii
Contents
2 Direct Link Networks
Problem: Physically Connecting Hosts
64
2.1 Hardware Building Blocks
66
2.1.1 Nodes
66
2.1.2 Links
71
2.2 Encoding (NRZ, NRZI, Manchester, 4B/5B)
79
2.3 Framing
84
2.3.1 Byte-Oriented Protocols (PPP)
84
2.3.2 Bit-Oriented Protocols (HDLC)
87
2.3.3 Clock-Based Framing (SONET)
89
2.4 Error Detection
92
2.4.1 Two-Dimensional Parity
93
2.4.2 Internet Checksum Algorithm
94
2.4.3 Cyclic Redundancy Check
96
2.5 Reliable Transmission
101
2.5.1 Stop-and-Wait
102
2.5.2 Sliding Window
105
2.5.3 Concurrent Logical Channels
115
2.6 Ethernet (802.3)
116
2.6.1 Physical Properties
116
2.6.2 Access Protocol
119
2.6.3 Experience with Ethernet
123
2.7 Rings (802.5, FDDI, RPR)
124
2.7.1 Token Ring Media Access Control
127
2.7.2 Token Ring Maintenance
129
2.7.3 FDDI
130
2.7.4 Resilient Packet Ring (802.17)
131
2.8 Wireless
133
2.8.1 Bluetooth (802.15.1)
136
2.8.2 Wi-Fi (802.11)
137
2.8.3 WiMAX (802.16)
143
2.8.4 Cell Phone Technologies
2.9 Summary
145
147
Open Issue: Sensor Networks
148
Further Reading
149
Exercises
151
3 Packet Switching
Problem: Not All Networks Are Directly Connected
3.1 Switching and Forwarding
166
168
Contents
xxiii
3.1.1 Datagrams
170
3.1.2 Virtual Circuit Switching
172
3.1.3 Source Routing
179
3.2 Bridges and LAN Switches
183
3.2.1 Learning Bridges
184
3.2.2 Spanning Tree Algorithm
187
3.2.3 Broadcast and Multicast
192
3.2.4 Limitations of Bridges
193
3.3 Cell Switching (ATM)
195
3.3.1 Cells
195
3.3.2 Segmentation and Reassembly
200
3.3.3 Virtual Paths
205
3.3.4 Physical Layers for ATM
3.4 Implementation and Performance
206
208
3.4.1 Ports
210
3.4.2 Fabrics
214
3.5 Summary
Open Issue: The Future of Switching
218
219
Further Reading
219
Exercises
221
4 Internetworking
Problem: There Is More Than One Network
4.1 Simple Internetworking (IP)
4.1.1 What Is an Internetwork?
232
234
234
4.1.2 Service Model
236
4.1.3 Global Addresses
248
4.1.4 Datagram Forwarding in IP
250
4.1.5 Address Translation (ARP)
254
4.1.6 Host Configuration (DHCP)
259
4.1.7 Error Reporting (ICMP)
262
4.1.8 Virtual Networks and Tunnels
262
4.2 Routing
266
4.2.1 Network as a Graph
268
4.2.2 Distance Vector (RIP)
269
4.2.3 Link State (OSPF)
277
4.2.4 Metrics
286
4.2.5 Routing for Mobile Hosts
289
4.2.6 Router Implementation
294
4.3 Global Internet
297
xxiv
Contents
4.3.1 Subnetting
299
4.3.2 Classless Routing (CIDR)
303
4.3.3 Interdomain Routing (BGP)
306
4.3.4 Routing Areas
316
4.3.5 IP Version 6 (IPv6)
318
4.4 Multicast
329
4.4.1 Multicast Addresses
331
4.4.2 Multicast Routing (DVMRP, PIM, MSDP)
332
4.5 Multiprotocol Label Switching
343
4.5.1 Destination-Based Forwarding
344
4.5.2 Explicit Routing
350
4.5.3 Virtual Private Networks and Tunnels
352
4.6 Summary
Open Issue: Deployment of IPv6
356
358
Further Reading
359
Exercises
360
5 End-to-End Protocols
Problem: Getting Processes to Communicate
380
5.1 Simple Demultiplexer (UDP)
382
5.2 Reliable Byte Stream (TCP)
384
5.2.1 End-to-End Issues
385
5.2.2 Segment Format
387
5.2.3 Connection Establishment and Termination
390
5.2.4 Sliding Window Revisited
394
5.2.5 Triggering Transmission
400
5.2.6 Adaptive Retransmission
403
5.2.7 Record Boundaries
407
5.2.8 TCP Extensions
408
5.2.9 Alternative Design Choices
410
5.3 Remote Procedure Call
5.3.1 RPC Fundamentals
5.3.2 RPC Implementations (SunRPC, DCE)
5.4 Transport for Real-Time Applications (RTP)
411
412
419
426
5.4.1 Requirements
428
5.4.2 RTP Details
429
5.4.3 Control Protocol
433
5.5 Performance
437
5.6 Summary
440
Open Issue: Application-Specific Protocols
441
Contents
xxv
Further Reading
442
Exercises
443
6 Congestion Control and Resource Allocation
Problem: Allocating Resources
6.1 Issues in Resource Allocation
6.1.1 Network Model
456
458
458
6.1.2 Taxonomy
462
6.1.3 Evaluation Criteria
464
6.2 Queuing Disciplines
467
6.2.1 FIFO
468
6.2.2 Fair Queuing
469
6.3 TCP Congestion Control
474
6.3.1 Additive Increase/Multiplicative Decrease
474
6.3.2 Slow Start
477
6.3.3 Fast Retransmit and Fast Recovery
483
6.4 Congestion-Avoidance Mechanisms
486
6.4.1 DECbit
486
6.4.2 Random Early Detection (RED)
487
6.4.3 Source-Based Congestion Avoidance
493
6.5 Quality of Service
499
6.5.1 Application Requirements
500
6.5.2 Integrated Services (RSVP)
506
6.5.3 Differentiated Services (EF, AF)
516
6.5.4 Equation-Based Congestion Control
522
6.6 Summary
Open Issue: Inside versus Outside the Network
524
525
Further Reading
526
Exercises
527
7 End-to-End Data
Problem: What Do We Do with the Data?
7.1
7.2
542
Presentation Formatting
544
7.1.1
Taxonomy
545
7.1.2
Examples (XDR, ASN.1, NDR)
549
7.1.3
Markup Languages (XML)
553
Data Compression
557
7.2.1 Lossless Compression Algorithms
559
7.2.2 Image Compression (JPEG)
561
7.2.3 Video Compression (MPEG)
566
7.2.4 Transmitting MPEG over a Network
571
xxvi
7.3
Contents
7.2.5 Audio Compression (MP3)
575
Summary
576
Open Issue: Computer Networks Meet Consumer Electronics
577
Further Reading
578
Exercises
579
8 Network Security
Problem: Security Attacks
8.1 Cryptographic Tools
586
589
8.1.1 Principles of Ciphers
589
8.1.2 Symmetric-Key Ciphers
591
8.1.3 Public-Key Ciphers
593
8.1.4 Authenticators
595
8.2 Key Predistribution
599
8.2.1 Predistribution of Public Keys
599
8.2.2 Predistribution of Symmetric Keys
604
8.3 Authentication Protocols
8.3.1 Originality and Timeliness Techniques
604
605
8.3.2 Public-Key Authentication Protocols
606
8.3.3 Symmetric-Key Authentication Protocols
607
8.3.4 Diffie-Hellman Key Agreement
8.4 Secure Systems
8.4.1 Pretty Good Privacy (PGP)
611
613
613
8.4.2 Secure Shell (SSH)
615
8.4.3 Transport Layer Security (TLS, SSL, HTTPS)
618
8.4.4 IP Security (IPsec)
622
8.4.5 Wireless Security (802.11i)
625
8.5 Firewalls
8.5.1 Strengths and Weaknesses of Firewalls
8.6 Summary
Open Issue: Denial-of-Service Attacks
626
629
631
632
Further Reading
633
Exercises
634
9 Applications
Problem: Applications Need Their Own Protocols
9.1 Traditional Applications
640
642
9.1.1 Electronic Mail (SMTP, MIME, IMAP)
643
9.1.2 World Wide Web (HTTP)
650
9.1.3 Name Service (DNS)
657
9.1.4 Network Management (SNMP)
666
Contents
9.2 Web Services
9.2.1 Custom Application Protocols (WSDL, SOAP)
9.2.2 A Generic Application Protocol (REST)
9.3 Multimedia Applications
xxvii
668
670
676
678
9.3.1 Session Control and Call Control (SDP, SIP, H.323)
679
9.3.2 Resource Allocation for Multimedia Applications
688
9.4 Overlay Networks
693
9.4.1 Routing Overlays
695
9.4.2 Peer-to-Peer Networks (Gnutella, BitTorrent)
702
9.4.3 Content Distribution Networks
714
9.5 Summary
Open Issue: New Network Architecture
719
720
Further Reading
721
Exercises
722
Solutions to Select Exercises
729
Glossary
743
Bibliography
769
Index
785
EDITION
COMPUTER
NETWORKS
A
S Y S T E M S
A P P R O A C H
4
Foundation
I must Create a System, or be enslav’d by another Man’s; I will not
Reason and Compare: my business is to Create.
—William Blake
S
uppose you want to build a computer network, one that has the potential to
grow to global proportions and to support applications as diverse as teleconferencing, video-on-demand, electronic commerce, distributed computing, and
digital libraries. What available technologies would serve as the underlying building
blocks, and what kind of software architecture would you design to integrate these
building blocks into an effective communication service? Answering this
P R O B L E M
question is the overriding goal of
Building a Network
this book—to describe the available
building materials and then to show
how they can be used to construct
a network from the ground up.
Before we can understand how to design a computer network, we should
first agree on exactly what a computer network is. At one time, the term network
meant the set of serial lines used to attach dumb terminals to mainframe computers. To some, the term implies the voice telephone network. To others, the
only interesting network is the cable network used to disseminate video signals.
The main thing these networks have in common is that they are specialized to
handle one particular kind of data (keystrokes, voice, or video) and they typically
connect to special-purpose devices (terminals, hand receivers, and television sets).
What distinguishes a computer network from these other types of networks? Probably the most important characteristic of a computer network is its generality. Computer networks are built primarily from general-purpose programmable hardware, and
they are not optimized for a particular application like making phone calls or delivering television signals. Instead, they are able to carry many different types of data,
and they support a wide, and ever-growing, range of applications. This chapter looks
2
at some typical applications of computer networks and discusses
the requirements that a network designer who wishes to support
such applications must be aware of.
Once we understand the requirements, how do we proceed? Fortunately, we will not be building the first network.
Others, most notably the community of researchers responsible
for the Internet, have gone before us. We will use the wealth
of experience generated from the Internet to guide our design.
This experience is embodied in a network architecture that identifies the available hardware and software components and shows
how they can be arranged to form a complete network system.
To start us on the road toward understanding how to build
a network, this chapter does four things. First, it explores the requirements that different applications and different communities
of people (such as network users and network operators) place
on the network. Second, it introduces the idea of a network architecture, which lays the foundation for the rest of the book.
Third, it introduces some of the key elements in the implementation of computer networks. Finally, it identifies the key metrics
that are used to evaluate the performance of computer networks.
1
4
1.1
1
Foundation
Applications
Most people know the Internet through its applications: the World Wide Web, email,
streaming audio and video, chat rooms, and music (file) sharing. The Web, for example,
presents an intuitively simple interface. Users view pages full of textual and graphical
objects, click on objects that they want to learn more about, and a corresponding new
page appears. Most people are also aware that just under the covers, each selectable object
on a page is bound to an identifier for the next page to be viewed. This identifier, called a
Uniform Resource Locator (URL), is used to provide a way of identifying all the possible
pages that can be viewed from your web browser. For example,
http://www.cs.princeton.edu/~llp/index.html
is the URL for a page providing information about one of this book’s authors: the string
http indicates that the HyperText Transfer Protocol (HTTP) should be used to download the page, www.cs.princeton.edu is the name of the machine that serves the
page, and
/~llp/index.html
uniquely identifies Larry’s home page at this site.
What most Web users are not aware of, however, is that by clicking on just one such
URL, as many as 17 messages may be exchanged over the Internet, and this assumes
the page itself is small enough to fit in a single message. This number includes up to
six messages to translate the server name (www.cs.princeton.edu) into its Internet
address (128.112.136.35), three messages to set up a Transmission Control Protocol
(TCP) connection between your browser and this server, four messages for your browser
to send the HTTP “get” request and the server to respond with the requested page (and
for each side to acknowledge receipt of that message), and four messages to tear down the
TCP connection. Of course, this does not include the millions of messages exchanged
by Internet nodes throughout the day, just to let each other know that they exist and
are ready to serve web pages, translate names to addresses, and forward messages toward
their ultimate destination.
Another widespread application of the Internet is the delivery of “streaming” audio
and video. While an entire video file could first be fetched from a remote machine and
then played on the local machine, similar to the process of downloading and displaying
a web page, this would entail waiting for the last second of the video file to be delivered
before starting to look at it. Streaming video implies that the sender and the receiver
are, respectively, the source and the sink for the video stream. That is, the source generates a video stream (perhaps using a video capture card), sends it across the Internet in
messages, and the sink displays the stream as it arrives.
1.1 Applications
5
There are a variety of different classes of video applications. One class of video application is video-on-demand, which reads a preexisting movie from disk and transmits
it over the network. Another kind of application is videoconferencing, which is in some
ways the more challenging (and, for networking people, interesting) case because it has
very tight timing constraints. Just as when using the telephone, the interactions among
the participants must be timely. When a person at one end gestures, then that action
must be displayed at the other end as quickly as possible. Too much delay makes the
system unusable. Contrast this with video-on-demand where, if it takes several seconds
from the time the user starts the video until the first image is displayed, the service is still
deemed satisfactory. Also, interactive video usually implies that video is flowing in both
directions, while a video-on-demand application is most likely sending video in only one
direction.
One pioneering example of a videoconferencing tool, developed in the early and
mid-1990s, is vic. Figure 1.1 shows the control panel for a vic session. vic is actually
Figure 1.1
The vic video application. This shot is from a 1995 release of the tool.
6
1
Foundation
one of a suite of conferencing tools designed at Lawrence Berkeley Laboratory and UC
Berkeley. The others include a whiteboard application (wb) that allows users to send
sketches and slides to each other, a visual audio tool called vat, and a session directory
(sdr) that is used to create and advertise videoconferences. All these tools run on Unix—
hence their lowercase names—and are freely available on the Internet. Many similar tools
are available for other operating systems. It is interesting to note that while video over the
Internet is still considered to be in its relative infancy at the time of this writing (2006),
that the tools to support video over IP have existed for well over a decade.
Although they are just two examples, downloading pages from the Web and participating in a videoconference demonstrate the diversity of applications that can be built
on top of the Internet, and hint at the complexity of the Internet’s design. Starting from
the beginning, and addressing one problem at time, the rest of this book explains how
to build a network that supports such a wide range of applications. Chapter 9 concludes
the book by revisiting these two specific applications, as well as several others that have
become popular on today’s Internet.
1.2
Requirements
We have just established an ambitious goal for ourselves: to understand how to build a
computer network from the ground up. Our approach to accomplishing this goal will
be to start from first principles, and then ask the kinds of questions we would naturally
ask if building an actual network. At each step, we will use today’s protocols to illustrate
various design choices available to us, but we will not accept these existing artifacts as
gospel. Instead, we will be asking (and answering) the question of why networks are
designed the way they are. While it is tempting to settle for just understanding the way
it’s done today, it is important to recognize the underlying concepts because networks are
constantly changing as the technology evolves and new applications are invented. It is
our experience that once you understand the fundamental ideas, any new protocol that
you are confronted with will be relatively easy to digest.
The first step is to identify the set of constraints and requirements that influence
network design. Before getting started, however, it is important to understand that the
expectations you have of a network depend on your perspective:
■
An application programmer would list the services that his application needs, for
example, a guarantee that each message the application sends will be delivered
without error within a certain amount of time.
■
A network designer would list the properties of a cost-effective design, for example, that network resources are efficiently utilized and fairly allocated to different
users.
1.2 Requirements
■
7
A network provider would list the characteristics of a system that is easy to administer and manage, for example, in which faults can be easily isolated and
where it is easy to account for usage.
This section attempts to distill these different perspectives into a high-level introduction to the major considerations that drive network design, and in doing so, identifies
the challenges addressed throughout the rest of this book.
1.2.1 Connectivity
Starting with the obvious, a network must provide connectivity among a set of computers. Sometimes it is enough to build a limited network that connects only a few select
machines. In fact, for reasons of privacy and security, many private (corporate) networks
have the explicit goal of limiting the set of machines that are connected. In contrast,
other networks (of which the Internet is the prime example) are designed to grow in a
way that allows them the potential to connect all the computers in the world. A system
that is designed to support growth to an arbitrarily large size is said to scale. Using the
Internet as a model, this book addresses the challenge of scalability.
Links, Nodes, and Clouds
Network connectivity occurs at many different levels. At the lowest level, a network can
consist of two or more computers directly connected by some physical medium, such as
a coaxial cable or an optical fiber. We call such a physical medium a link, and we often
refer to the computers it connects as nodes. (Sometimes a node is a more specialized piece
of hardware rather than a computer, but we overlook that distinction for the purposes
of this discussion.) As illustrated in Figure 1.2, physical links are sometimes limited to a
pair of nodes (such a link is said to be point-to-point), while in other cases, more than two
nodes may share a single physical link (such a link is said to be multiple-access). Whether
Figure 1.2
Direct links: (a) point-to-point; (b) multiple-access.
8
1
Foundation
a given link supports point-to-point or multiple-access connectivity depends on how the
node is attached to the link. It is also the case that multiple-access links are often limited
in size, in terms of both the geographical distance they can cover and the number of
nodes they can connect.
If computer networks were limited to situations in which all nodes are directly
connected to each other over a common physical medium, then networks would either
be very limited in the number of computers they could connect, or the number of wires
coming out of the back of each node would quickly become both unmanageable and
very expensive. Fortunately, connectivity between two nodes does not necessarily imply a
direct physical connection between them—indirect connectivity may be achieved among
a set of cooperating nodes. Consider the following two examples of how a collection of
computers can be indirectly connected.
Figure 1.3 shows a set of nodes, each of which is attached to one or more pointto-point links. Those nodes that are attached to at least two links run software that
forwards data received on one link out on another. If organized in a systematic way,
these forwarding nodes form a switched network. There are numerous types of switched
networks, of which the two most common are circuit-switched and packet-switched. The
former is most notably employed by the telephone system, while the latter is used for the
overwhelming majority of computer networks and will be the focus of this book. The
important feature of packet-switched networks is that the nodes in such a network send
Figure 1.3
Switched network.
1.2 Requirements
9
discrete blocks of data to each other. Think of these blocks of data as corresponding to
some piece of application data such as a file, a piece of email, or an image. We call each
block of data either a packet or a message, and for now we use these terms interchangeably;
we discuss the reason they are not always the same in Section 1.2.2.
Packet-switched networks typically use a strategy called store-and-forward. As the
name suggests, each node in a store-and-forward network first receives a complete packet
over some link, stores the packet in its internal memory, and then forwards the complete packet to the next node. In contrast, a circuit-switched network first establishes a
dedicated circuit across a sequence of links and then allows the source node to send a
stream of bits across this circuit to a destination node. The major reason for using packet
switching rather than circuit switching in a computer network is efficiency, discussed in
the next subsection.
The cloud in Figure 1.3 distinguishes between the nodes on the inside that implement the network (they are commonly called switches, and their primary function is to
store and forward packets) and the nodes on the outside of the cloud that use the network
(they are commonly called hosts, and they support users and run application programs).
Also note that the cloud in Figure 1.3 is one of the most important icons of computer
networking. In general, we use a cloud to denote any type of network, whether it is a
single point-to-point link, a multiple-access link, or a switched network. Thus, whenever you see a cloud used in a figure, you can think of it as a placeholder for any of the
networking technologies covered in this book.
A second way in which a set of computers can be indirectly connected is shown in
Figure 1.4. In this situation, a set of independent networks (clouds) are interconnected
to form an internetwork, or internet for short. We adopt the Internet’s convention of
referring to a generic internetwork of networks as a lowercase i internet, and the currently
operational TCP/IP Internet as the capital I Internet. A node that is connected to two or
more networks is commonly called a router or gateway, and it plays much the same role
as a switch—it forwards messages from one network to another. Note that an internet
can itself be viewed as another kind of network, which means that an internet can be
built from an interconnection of internets. Thus, we can recursively build arbitrarily
large networks by interconnecting clouds to form larger clouds.
Just because a set of hosts are directly or indirectly connected to each other does not
mean that we have succeeded in providing host-to-host connectivity. The final requirement is that each node must be able to state which of the other nodes on the network
it wants to communicate with. This is done by assigning an address to each node. An
address is a byte string that identifies a node; that is, the network can use a node’s address to distinguish it from the other nodes connected to the network. When a source
node wants the network to deliver a message to a certain destination node, it specifies
the address of the destination node. If the sending and receiving nodes are not directly
10
1
Figure 1.4
Foundation
Interconnection of networks.
▲
connected, then the switches and routers of the network use this address to decide how
to forward the message toward the destination. The process of determining systematically how to forward messages toward the destination node based on its address is called
routing.
This brief introduction to addressing and routing has presumed that the source
node wants to send a message to a single destination node (unicast). While this is the
most common scenario, it is also possible that the source node might want to broadcast a
message to all the nodes on the network. Or a source node might want to send a message
to some subset of the other nodes, but not all of them, a situation called multicast.
Thus, in addition to node-specific addresses, another requirement of a network is that it
supports multicast and broadcast addresses.
The main idea to take away from this discussion is that we can define a network
recursively as consisting of two or more nodes connected by a physical link, or as two
or more networks connected by a node. In other words, a network can be constructed
from a nesting of networks, where at the bottom level, the network is implemented by
some physical medium. One of the key challenges in providing network connectivity is
to define an address for each node that is reachable on the network (including support
for broadcast and multicast connectivity), and to be able to use this address to route
messages toward the appropriate destination node(s).
1.2 Requirements
11
1.2.2 Cost-Effective Resource Sharing
As stated above, this book focuses on packet-switched networks. This section explains the
key requirement of computer networks—efficiency—that leads us to packet switching as
the strategy of choice.
Given a collection of nodes indirectly connected by a nesting of networks, it is
possible for any pair of hosts to send messages to each other across a sequence of links
and nodes. Of course, we want to do more than support just one pair of communicating
hosts—we want to provide all pairs of hosts with the ability to exchange messages. The
question, then, is how do all the hosts that want to communicate share the network,
especially if they want to use it at the same time? And, as if that problem isn’t hard
enough, how do several hosts share the same link when they all want to use it at the same
time?
To understand how hosts share a network, we need to introduce a fundamental
concept, multiplexing, which means that a system resource is shared among multiple
users. At an intuitive level, multiplexing can be explained by analogy to a timesharing
computer system, where a single physical CPU is shared (multiplexed) among multiple
jobs, each of which believes it has its own private processor. Similarly, data being sent by
multiple users can be multiplexed over the physical links that make up a network.
To see how this might work, consider the simple network illustrated in Figure 1.5,
where the three hosts on the left side of the network (senders S1–S3) are sending data to
the three hosts on the right (receivers R1–R3) by sharing a switched network that contains only one physical link. (For simplicity, assume that host S1 is sending data to host
R1, and so on.) In this situation, three flows of data—corresponding to the three pairs
of hosts—are multiplexed onto a single physical link by switch 1 and then demultiplexed
back into separate flows by switch 2. Note that we are being intentionally vague about
Figure 1.5
Multiplexing multiple logical flows over a single physical link.
12
1
Foundation
exactly what a “flow of data” corresponds to. For the purposes of this discussion, assume
that each host on the left has a large supply of data that it wants to send to its counterpart
on the right.
There are several different methods for multiplexing multiple flows onto one physical link. One common method is synchronous time-division multiplexing (STDM). The
idea of STDM is to divide time into equal-sized quanta and, in a round-robin fashion,
give each flow a chance to send its data over the physical link. In other words, during
time quantum 1, data from S1 to R1 is transmitted; during time quantum 2, data from
S2 to R2 is transmitted; in quantum 3, S3 sends data to R3. At this point, the first flow
(S1 to R1) gets to go again, and the process repeats. Another method is frequency-division
multiplexing (FDM). The idea of FDM is to transmit each flow over the physical link at
a different frequency, much the same way that the signals for different TV stations are
transmitted at a different frequency on a physical cable TV link.
Although simple to understand, both STDM and FDM are limited in two ways.
First, if one of the flows (host pairs) does not have any data to send, its share of the physical link—that is, its time quantum or its frequency—remains idle, even if one of the
other flows has data to transmit. For example, S3 had to wait its turn behind S1 and S2
in the previous paragraph, even if S1 and S2 had nothing to send. For computer communication, the amount of time that a link is idle can be very large—for example, consider
the amount of time you spend reading a web page (leaving the link idle) compared to
the time you spend fetching the page. Second, both STDM and FDM are limited to
situations in which the maximum number of flows is fixed and known ahead of time. It
is not practical to resize the quantum or to add additional quanta in the case of STDM
or to add new frequencies in the case of FDM.
The form of multiplexing that we make most use of in this book is called statistical
multiplexing. Although the name is not all that helpful for understanding the concept,
statistical multiplexing is really quite simple, with two key ideas. First, it is like STDM
in that the physical link is shared over time—first data from one flow is transmitted
over the physical link, then data from another flow is transmitted, and so on. Unlike
STDM, however, data is transmitted from each flow on demand rather than during a
predetermined time slot. Thus, if only one flow has data to send, it gets to transmit that
data without waiting for its quantum to come around and thus without having to watch
the quanta assigned to the other flows go by unused. It is this avoidance of idle time that
gives packet switching its efficiency.
As defined so far, however, statistical multiplexing has no mechanism to ensure that
all the flows eventually get their turn to transmit over the physical link. That is, once a
flow begins sending data, we need some way to limit the transmission, so that the other
flows can have a turn. To account for this need, statistical multiplexing defines an upper
bound on the size of the block of data that each flow is permitted to transmit at a given
1.2 Requirements
13
time. This limited-size block of data is typically referred to as a packet, to distinguish it
from the arbitrarily large message that an application program might want to transmit.
Because a packet-switched network limits the maximum size of packets, a host may not
be able to send a complete message in one packet. The source may need to fragment
the message into several packets, with the receiver reassembling the packets back into the
original message.
In other words, each flow sends a sequence of packets over the physical link, with
a decision made on a packet-by-packet basis as to which flow’s packet to send next.
Notice that if only one flow has data to send, then it can send a sequence of packets
back-to-back. However, should more than one of the flows have data to send, then their
packets are interleaved on the link. Figure 1.6 depicts a switch multiplexing packets from
multiple sources onto a single shared link.
The decision as to which packet to send next on a shared link can be made in a
number of different ways. For example, in a network consisting of switches interconnected by links such as the one in Figure 1.5, the decision would be made by the switch
that transmits packets onto the shared link. (As we will see later, not all packet-switched
networks actually involve switches, and they may use other mechanisms to determine
whose packet goes onto the link next.) Each switch in a packet-switched network makes
this decision independently, on a packet-by-packet basis. One of the issues that faces a
network designer is how to make this decision in a fair manner. For example, a switch
could be designed to service packets on a first-in-first-out (FIFO) basis. Another approach would be to transmit the packets from each of the different flows that are currently sending data through the switch in a round-robin manner. This might be done to
Figure 1.6
A switch multiplexing packets from multiple sources onto one shared link.
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ensure that certain flows receive a particular share of the link’s bandwidth, or that they
never have their packets delayed in the switch for more than a certain length of time.
A network that attempts to allocate bandwidth to particular flows is sometimes said to
support quality of service (QoS), a topic that we return to in Chapter 6.
Also, notice in Figure 1.6 that since the switch has to multiplex three incoming
packet streams onto one outgoing link, it is possible that the switch will receive packets
faster than the shared link can accommodate. In this case, the switch is forced to buffer
these packets in its memory. Should a switch receive packets faster than it can send them
for an extended period of time, then the switch will eventually run out of buffer space,
and some packets will have to be dropped. When a switch is operating in this state, it is
said to be congested.
The bottom line is that statistical multiplexing defines a cost-effective way for multiple users (e.g., host-to-host flows of data) to share network resources (links and nodes)
in a fine-grained manner. It defines the packet as the granularity with which the links of
the network are allocated to different flows, with each switch able to schedule the use of
the physical links it is connected to on a per-packet basis. Fairly allocating link capacity
to different flows and dealing with congestion when it occurs are the key challenges of
statistical multiplexing.
1.2.3
Support for Common Services
While the previous section outlined the
challenges involved in providing costeffective connectivity among a group of
hosts, it is overly simplistic to view a computer network as simply delivering packets among a collection of computers. It
is more accurate to think of a network
as providing the means for a set of application processes that are distributed over
those computers to communicate. In other
words, the next requirement of a computer
network is that the application programs
running on the hosts connected to the network must be able to communicate in a
meaningful way.
When two application programs
need to communicate with each other,
SANs, LANs, MANs, and WANs
One way to characterize networks
is according to their size. Two wellknown examples are local area networks (LANs) and wide area networks
(WANs); the former typically extend
less than 1 km, while the latter can be
worldwide. Other networks are classified as metropolitan area networks
(MANs), which usually span tens of
kilometers. The reason such classifications are interesting is that the size
of a network often has implications
for the underlying technology that can
be used, with a key factor being the
amount of time it takes for data to
1.2 Requirements
propagate from one end of the network to the other; we discuss this issue more in later chapters.
An interesting historical note
is that the term “wide area network”
was not applied to the first WANs
because there was no other sort of
network to differentiate them from.
When computers were incredibly rare
and expensive, there was no point in
thinking about how to connect all
the computers in the local area—there
was only one computer in that area.
Only as computers began to proliferate did LANs become necessary, and
the term “WAN” was then introduced
to describe the larger networks that
interconnected geographically distant
computers.
Another kind of network that
we need to be aware of is a storage
area network (SAN). SANs are usually
confined to a single room and connect the various components of a large
computing system, such as disk arrays
and servers. For example, High Performance Parallel Interface (HiPPI)
and Fiber Channel are two common
SAN technologies used to connect
massively parallel processors to scalable storage servers and data vaults.
Although this book does not describe
such networks in detail, they are
worth knowing about because they
are often at the leading edge in terms
of performance, and because it is increasingly common to connect such
networks into LANs and WANs.
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there are a lot of complicated things that
need to happen beyond simply sending a
message from one host to another. One
option would be for application designers to build all that complicated functionality into each application program.
However, since many applications need
common services, it is much more logical
to implement those common services once
and then to let the application designer
build the application using those services.
The challenge for a network designer is to
identify the right set of common services.
The goal is to hide the complexity of
the network from the application without overly constraining the application
designer.
Intuitively, we view the network
as providing logical channels over which
application-level processes can communicate with each other; each channel provides the set of services required by that
application. In other words, just as we use
a cloud to abstractly represent connectivity
among a set of computers, we now think
of a channel as connecting one process
to another. Figure 1.7 shows a pair of
application-level processes communicating
over a logical channel that is, in turn, implemented on top of a cloud that connects
a set of hosts. We can think of the channel
as being like a pipe connecting two applications, so that a sending application can
put data in one end and expect that data
to be delivered by the network to the application at the other end of the pipe.
The challenge is to recognize what
functionality the channels should provide to application programs. For example,
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Processes communicating over an abstract channel.
does the application require a guarantee that messages sent over the channel are delivered,
or is it acceptable if some messages fail to arrive? Is it necessary that messages arrive at the
recipient process in the same order in which they are sent, or does the recipient not care
about the order in which messages arrive? Does the network need to ensure that no third
parties are able to eavesdrop on the channel, or is privacy not a concern? In general, a
network provides a variety of different types of channels, with each application selecting
the type that best meets its needs. The rest of this section illustrates the thinking involved
in defining useful channels.
Identifying Common Communication Patterns
Designing abstract channels involves first understanding the communication needs of a
representative collection of applications, then extracting their common communication
requirements, and finally incorporating the functionality that meets these requirements
in the network.
One of the earliest applications supported on any network is a file access program
like FTP (File Transfer Protocol) or NFS (Network File System). Although many details
vary—for example, whether whole files are transferred across the network or only single
blocks of the file are read/written at a given time—the communication component of
remote file access is characterized by a pair of processes, one that requests that a file be
1.2 Requirements
17
read or written and a second process that honors this request. The process that requests
access to the file is called the client, and the process that supports access to the file is
called the server.
Reading a file involves the client sending a small request message to a server and the
server responding with a large message that contains the data in the file. Writing works
in the opposite way—the client sends a large message containing the data to be written
to the server, and the server responds with a small message confirming that the write to
disk has taken place. A digital library, as exemplified by the World Wide Web, is another
application that behaves in a similar way: a client process makes a request, and a server
process responds by returning the requested data.
Using file access, a digital library, and the two video applications described in the
Preface (videoconferencing and video-on-demand) as a representative sample, we might
decide to provide the following two types of channels: request/reply channels and message
stream channels. The request/reply channel would be used by the file transfer and digital
library applications. It would guarantee that every message sent by one side is received
by the other side and that only one copy of each message is delivered. The request/reply
channel might also protect the privacy and integrity of the data that flows over it, so that
unauthorized parties cannot read or modify the data being exchanged between the client
and server processes.
The message stream channel could be used by both the video-on-demand and
videoconferencing applications, provided it is parameterized to support both one-way
and two-way traffic and to support different delay properties. The message stream channel might not need to guarantee that all messages are delivered, since a video application
can operate adequately even if some video frames are not received. It would, however,
need to ensure that those messages that are delivered arrive in the same order in which
they were sent, to avoid displaying frames out of sequence. Like the request/reply channel, the message stream channel might want to ensure the privacy and integrity of the
video data. Finally, the message stream channel might need to support multicast, so that
multiple parties can participate in the teleconference or view the video.
While it is common for a network designer to strive for the smallest number of
abstract channel types that can serve the largest number of applications, there is a danger
in trying to get away with too few channel abstractions. Simply stated, if you have a
hammer, then everything looks like a nail. For example, if all you have are message stream
and request/reply channels, then it is tempting to use them for the next application
that comes along, even if neither type provides exactly the semantics needed by the
application. Thus, network designers will probably be inventing new types of channels—
and adding options to existing channels—for as long as application programmers are
inventing new applications.
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Also note that independent of exactly what functionality a given channel provides,
there is the question of where that functionality is implemented. In many cases, it is easiest to view the host-to-host connectivity of the underlying network as simply providing
a bit pipe, with any high-level communication semantics provided at the end hosts. The
advantage of this approach is it keeps the switches in the middle of the network as simple
as possible—they simply forward packets—but it requires the end hosts to take on much
of the burden of supporting semantically rich process-to-process channels. The alternative is to push additional functionality onto the switches, thereby allowing the end hosts
to be “dumb” devices (e.g., telephone handsets). We will see this question of how various
network services are partitioned between the packet switches and the end hosts (devices)
as a recurring issue in network design.
Reliability
As suggested by the examples just considered, reliable message delivery is one of the
most important functions that a network can provide. It is difficult to determine how
to provide this reliability, however, without first understanding how networks can fail.
The first thing to recognize is that computer networks do not exist in a perfect world.
Machines crash and later are rebooted, fibers are cut, electrical interference corrupts bits
in the data being transmitted, switches run out of buffer space, and if these sorts of
physical problems aren’t enough to worry about, the software that manages the hardware
sometimes forwards packets into oblivion. Thus, a major requirement of a network is
to recover from certain kinds of failures, so that application programs don’t have to deal
with them, or even be aware of them.
There are three general classes of failure that network designers have to worry
about. First, as a packet is transmitted over a physical link, bit errors may be introduced
into the data; that is, a 1 is turned into a 0 or vice versa. Sometimes single bits are
corrupted, but more often than not, a burst error occurs—several consecutive bits are
corrupted. Bit errors typically occur because outside forces, such as lightning strikes,
power surges, and microwave ovens, interfere with the transmission of data. The good
news is that such bit errors are fairly rare, affecting on average only one out of every 106
to 107 bits on a typical copper-based cable and one out of every 1012 to 1014 bits on a
typical optical fiber. As we will see, there are techniques that detect these bit errors with
high probability. Once detected, it is sometimes possible to correct for such errors—if
we know which bit or bits are corrupted, we can simply flip them—while in other cases
the damage is so bad that it is necessary to discard the entire packet. In such a case, the
sender may be expected to retransmit the packet.
The second class of failure is at the packet, rather than the bit, level; that is, a
complete packet is lost by the network. One reason this can happen is that the packet
contains an uncorrectable bit error and therefore has to be discarded. A more likely
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reason, however, is that one of the nodes that has to handle the packet—for example,
a switch that is forwarding it from one link to another—is so overloaded that it has
no place to store the packet, and therefore is forced to drop it. This is the problem of
congestion mentioned in Section 1.2.2. Less commonly, the software running on one
of the nodes that handles the packet makes a mistake. For example, it might incorrectly
forward a packet out on the wrong link, so that the packet never finds its way to the
ultimate destination. As we will see, one of the main difficulties in dealing with lost
packets is distinguishing between a packet that is indeed lost and one that is merely late
in arriving at the destination.
The third class of failure is at the node and link level; that is, a physical link is cut,
or the computer it is connected to crashes. This can be caused by software that crashes,
a power failure, or a reckless backhoe operator. Failures due to misconfiguration of a
network device are also common. While any of these failures can eventually be corrected,
they can have a dramatic effect on the network for an extended period of time. However,
they need not totally disable the network. In a packet-switched network, for example,
it is sometimes possible to route around a failed node or link. One of the difficulties in
dealing with this third class of failure is distinguishing between a failed computer and
one that is merely slow, or in the case of a link, between one that has been cut and one
that is very flaky and therefore introducing a high number of bit errors.
The key idea to take away from this discussion is that defining useful channels
involves both understanding the applications’ requirements and recognizing the limitations of the underlying technology. The challenge is to fill in the gap between what the
application expects and what the underlying technology can provide. This is sometimes
called the semantic gap.
1.3
Network Architecture
In case you hadn’t noticed, the previous section established a pretty substantial set of
requirements for network design—a computer network must provide general, costeffective, fair, and robust connectivity among a large number of computers. As if this
weren’t enough, networks do not remain fixed at any single point in time, but must
evolve to accommodate changes in both the underlying technologies upon which they
are based as well as changes in the demands placed on them by application programs.
Designing a network to meet these requirements is no small task.
To help deal with this complexity, network designers have developed general
blueprints—usually called network architectures—that guide the design and implementation of networks. This section defines more carefully what we mean by a network architecture by introducing the central ideas that are common to all network architectures.
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It also introduces two of the most widely referenced architectures—the OSI architecture
and the Internet architecture.
1.3.1 Layering and Protocols
When a system gets complex, the system designer introduces another level of abstraction.
The idea of an abstraction is to define a unifying model that can capture some important
aspect of the system, encapsulate this model in an object that provides an interface that
can be manipulated by other components of the system, and hide the details of how
the object is implemented from the users of the object. The challenge is to identify
abstractions that simultaneously provide a service that proves useful in a large number
of situations and that can be efficiently implemented in the underlying system. This is
exactly what we were doing when we introduced the idea of a channel in the previous
section: We were providing an abstraction for applications that hides the complexity of
the network from application writers.
Abstractions naturally lead to layering, especially in network systems. The general
idea is that you start with the services offered by the underlying hardware, and then
add a sequence of layers, each providing a higher (more abstract) level of service. The
services provided at the high layers are implemented in terms of the services provided by
the low layers. Drawing on the discussion of requirements given in the previous section,
for example, we might imagine a simple network as having two layers of abstraction
sandwiched between the application program and the underlying hardware, as illustrated
in Figure 1.8. The layer immediately above the hardware in this case might provide hostto-host connectivity, abstracting away the fact that there may be an arbitrarily complex
network topology between any two hosts. The next layer up builds on the available hostto-host communication service and provides support for process-to-process channels,
abstracting away the fact that the network occasionally loses messages, for example.
Layering provides two nice features. First, it decomposes the problem of building
a network into more manageable components. Rather than implementing a monolithic
piece of software that does everything you will ever want, you can implement several
Figure 1.8
Example of a layered network system.
1.3 Network Architecture
Figure 1.9
21
Layered system with alternative abstractions available at a given layer.
layers, each of which solves one part of the problem. Second, it provides a more modular
design. If you decide that you want to add some new service, you may only need to
modify the functionality at one layer, reusing the functions provided at all the other
layers.
Thinking of a system as a linear sequence of layers is an oversimplification, however.
Many times there are multiple abstractions provided at any given level of the system,
each providing a different service to the higher layers but building on the same low-level
abstractions. To see this, consider the two types of channels discussed in Section 1.2.3:
One provides a request/reply service and one supports a message stream service. These
two channels might be alternative offerings at some level of a multilevel networking
system, as illustrated in Figure 1.9.
Using this discussion of layering as a foundation, we are now ready to discuss the
architecture of a network more precisely. For starters, the abstract objects that make up
the layers of a network system are called protocols. That is, a protocol provides a communication service that higher-level objects (such as application processes, or perhaps
higher-level protocols) use to exchange messages. For example, we could imagine a network that supports a request/reply protocol and a message stream protocol, corresponding to the request/reply and message stream channels discussed above.
Each protocol defines two different interfaces. First, it defines a service interface to
the other objects on the same computer that want to use its communication services. This
service interface defines the operations that local objects can perform on the protocol.
For example, a request/reply protocol would support operations by which an application
can send and receive messages. An implementation of the HTTP protocol could support
an operation to fetch a page of hypertext from a remote server. An application such as
a web browser would invoke such an operation whenever the browser needs to obtain a
new page, for example, when the user clicks on a link in the currently displayed page.
Second, a protocol defines a peer interface to its counterpart (peer) on another
machine. This second interface defines the form and meaning of messages exchanged
between protocol peers to implement the communication service. This would determine
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Figure 1.10
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Service and peer interfaces.
the way in which a request/reply protocol on one machine communicates with its peer on
another machine. In the case of HTTP, for example, the protocol specification defines
in detail how a “GET” command is formatted, what arguments can be used with the
command, and how a web server should respond when it receives such a command. (We
will look more closely at this particular protocol in Section 9.1.2.)
To summarize, a protocol defines a communication service that it exports locally
(the service interface), along with a set of rules governing the messages that the protocol
exchanges with its peer(s) to implement this service (the peer interface). This situation is
illustrated in Figure 1.10.
Except at the hardware level where peers directly communicate with each other
over a link, peer-to-peer communication is indirect—each protocol communicates with
its peer by passing messages to some lower-level protocol, which in turn delivers the
message to its peer. In addition, there are potentially multiple protocols at any given
level, each providing a different communication service. We therefore represent the suite
of protocols that make up a network system with a protocol graph. The nodes of the graph
correspond to protocols, and the edges represent a depends on relation. For example,
Figure 1.11 illustrates a protocol graph for the hypothetical layered system we have been
discussing—the protocols Request/Reply Protocol (RRP) and Message Stream Protocol
(MSP) implement two different types of process-to-process channels, and both depend
on Host-to-Host Protocol (HHP), which provides a host-to-host connectivity service.
In this example, suppose that the file access program on host 1 wants to send a
message to its peer on host 2 using the communication service offered by protocol RRP.
In this case, the file application asks RRP to send the message on its behalf. To communicate with its peer, RRP then invokes the services of HHP, which in turn transmits the
message to its peer on the other machine. Once the message has arrived at protocol HHP
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Figure 1.11 Example of a protocol graph.
on host 2, HHP passes the message up to RRP, which in turn delivers the message to the
file application. In this particular case, the application is said to employ the services of
the protocol stack RRP/HHP.
Note that the term protocol is used in two different ways. Sometimes it refers to
the abstract interfaces—that is, the operations defined by the service interface and the
form and meaning of messages exchanged between peers—and sometimes it refers to
the module that actually implements these two interfaces. To distinguish between the
interfaces and the module that implements these interfaces, we generally refer to the former as a protocol specification. Specifications are generally expressed using a combination
of prose, pseudocode, state transition diagrams, pictures of packet formats, and other
abstract notations. It should be the case that a given protocol can be implemented in
different ways by different programmers, as long as each adheres to the specification.
The challenge is ensuring that two different implementations of the same specification
can successfully exchange messages. Two or more protocol modules that do accurately
implement a protocol specification are said to interoperate with each other.
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We can imagine many different protocols and protocol graphs that satisfy the communication requirements of a collection of applications. Fortunately, there exist standardization bodies, such as the International Standards Organization (ISO) and the Internet Engineering Task Force (IETF), that establish policies for a particular protocol
graph. We call the set of rules governing the form and content of a protocol graph a
network architecture. Although beyond the scope of this book, standardization bodies
such as the ISO and the IETF have established well-defined procedures for introducing,
validating, and finally approving protocols in their respective architectures. We briefly
describe the architectures defined by the ISO and the IETF shortly, but first there are
two additional things we need to explain about the mechanics of a protocol graph.
Encapsulation
Consider what happens in Figure 1.11 when one of the application programs sends a
message to its peer by passing the message to protocol RRP. From RRP’s perspective, the
message it is given by the application is an uninterpreted string of bytes. RRP does not
care that these bytes represent an array of integers, an email message, a digital image, or
whatever; it is simply charged with sending them to its peer. However, RRP must communicate control information to its peer, instructing it how to handle the message when
it is received. RRP does this by attaching a header to the message. Generally speaking,
a header is a small data structure—from a few bytes to a few dozen bytes—that is used
among peers to communicate with each other. As the name suggests, headers are usually attached to the front of a message. In some cases, however, this peer-to-peer control
information is sent at the end of the message, in which case it is called a trailer. The
exact format for the header attached by RRP is defined by its protocol specification. The
rest of the message—that is, the data being transmitted on behalf of the application—is
called the message’s body or payload. We say that the application’s data is encapsulated in
the new message created by protocol RRP.
This process of encapsulation is then repeated at each level of the protocol graph;
for example, HHP encapsulates RRP’s message by attaching a header of its own. If we
now assume that HHP sends the message to its peer over some network, then when
the message arrives at the destination host, it is processed in the opposite order: HHP
first interprets the HHP header at the front of the message (i.e., takes whatever action
is appropriate given the contents of the header), and passes the body of the message
(but not the HHP header) up to RRP, which takes whatever action is indicated by the
RRP header that its peer attached, and passes the body of the message (but not the
RRP header) up to the application program. The message passed up from RRP to the
application on host 2 is exactly the same message as the application passed down to RRP
on host 1; the application does not see any of the headers that have been attached to it to
implement the lower-level communication services. This whole process is illustrated in
1.3 Network Architecture
Figure 1.12
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High-level messages are encapsulated inside of low-level messages.
Figure 1.12. Note that in this example, nodes in the network (e.g., switches and routers)
may inspect the HHP header at the front of the message.
Note that when we say a low-level protocol does not interpret the message it is
given by some high-level protocol, we mean that it does not know how to extract any
meaning from the data contained in the message. It is sometimes the case, however, that
the low-level protocol applies some simple transformation to the data it is given, such as
to compress or encrypt it. In this case, the protocol is transforming the entire body of
the message, including both the original application’s data and all the headers attached
to that data by higher-level protocols.
Multiplexing and Demultiplexing
Recall from Section 1.2.2 that a fundamental idea of packet switching is to multiplex
multiple flows of data over a single physical link. This same idea applies up and down
the protocol graph, not just to switching nodes. In Figure 1.11, for example, we can
think of RRP as implementing a logical communication channel, with messages from
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two different applications multiplexed over this channel at the source host and then
demultiplexed back to the appropriate application at the destination host.
Practically speaking, all this means is that the header that RRP attaches to its messages contains an identifier that records the application to which the message belongs.
We call this identifier RRP’s demultiplexing key, or demux key for short. At the source
host, RRP includes the appropriate demux key in its header. When the message is delivered to RRP on the destination host, it strips its header, examines the demux key, and
demultiplexes the message to the correct application.
RRP is not unique in its support for multiplexing; nearly every protocol implements this mechanism. For example, HHP has its own demux key to determine which
messages to pass up to RRP and which to pass up to MSP. However, there is no uniform
agreement among protocols—even those within a single network architecture—on exactly what constitutes a demux key. Some protocols use an 8-bit field (meaning they can
support only 256 high-level protocols), and others use 16- or 32-bit fields. Also, some
protocols have a single demultiplexing field in their header, while others have a pair of
demultiplexing fields. In the former case, the same demux key is used on both sides of
the communication, while in the latter case, each side uses a different key to identify the
high-level protocol (or application program) to which the message is to be delivered.
1.3.2 OSI Architecture
The ISO was one of the first organizations to formally define a common way to connect
computers. Their architecture, called the Open Systems Interconnection (OSI) architecture
and illustrated in Figure 1.13, defines a partitioning of network functionality into seven
layers, where one or more protocols implement the functionality assigned to a given
layer. In this sense, the schematic given in Figure 1.13 is not a protocol graph, per se,
but rather a reference model for a protocol graph. The ISO, usually in conjunction with
a second standards organization known as the International Telecommunications Union
(ITU),1 publishes a series of protocol specifications based on the OSI architecture. This
series is sometimes called the “X dot” series since the protocols are given names like X.25,
X.400, X.500, and so on.
Starting at the bottom and working up, the physical layer handles the transmission
of raw bits over a communications link. The data link layer then collects a stream of bits
into a larger aggregate called a frame. Network adaptors, along with device drivers running in the node’s OS, typically implement the data link level. This means that frames,
not raw bits, are actually delivered to hosts. The network layer handles routing among
nodes within a packet-switched network. At this layer, the unit of data exchanged among
nodes is typically called a packet rather than a frame, although they are fundamentally
1 A subcommittee of the ITU on telecommunications (ITU-T) replaces an earlier subcommittee of the ITU, which was
known by its French name, Comité Consultatif International de Télégraphique et Téléphonique (CCITT).
1.3 Network Architecture
Figure 1.13
27
OSI network architecture.
the same thing. The lower three layers are implemented on all network nodes, including
switches within the network and hosts connected along the exterior of the network. The
transport layer then implements what we have up to this point been calling a process-toprocess channel. Here, the unit of data exchanged is commonly called a message rather
than a packet or a frame. The transport layer and higher layers typically run only on the
end hosts and not on the intermediate switches or routers.
There is less agreement about the definition of the top three layers. Skipping ahead
to the top (seventh) layer, we find the application layer. Application layer protocols include things like the File Transfer Protocol (FTP), which defines a protocol by which
file transfer applications can interoperate. Below that, the presentation layer is concerned
with the format of data exchanged between peers, for example, whether an integer is 16,
32, or 64 bits long and whether the most significant byte is transmitted first or last, or
how a video stream is formatted. Finally, the session layer provides a name space that is
used to tie together the potentially different transport streams that are part of a single
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application. For example, it might manage an audio stream and a video stream that are
being combined in a teleconferencing application.
1.3.3 Internet Architecture
The Internet architecture, which is also sometimes called the TCP/IP architecture after its two main protocols, is depicted in Figure 1.14. An alternative representation is
given in Figure 1.15. The Internet architecture evolved out of experiences with an earlier
packet-switched network called the ARPANET. Both the Internet and the ARPANET
were funded by the Advanced Research Projects Agency (ARPA), one of the R&D funding agencies of the U.S. Department of Defense. The Internet and ARPANET were
around before the OSI architecture, and the experience gained from building them was
a major influence on the OSI reference model.
While the seven-layer OSI model can, with some imagination, be applied to the
Internet, a four-layer model is often used instead. At the lowest level are a wide variety
of network protocols, denoted NET1 , NET2 , and so on. In practice, these protocols are
implemented by a combination of hardware (e.g., a network adaptor) and software (e.g.,
a network device driver). For example, you might find Ethernet or Fiber Distributed
Figure 1.14
Internet protocol graph.
Figure 1.15 Alternative view of the Internet architecture. The “Network” layer shown
here is sometimes referred to as the “subnetwork” or “link” layer.
1.3 Network Architecture
29
Data Interface (FDDI) protocols at this layer. (These protocols in turn may actually involve several sublayers, but the Internet architecture does not presume anything about
them.) The second layer consists of a single protocol—the Internet Protocol (IP). This
is the protocol that supports the interconnection of multiple networking technologies
into a single, logical internetwork. The third layer contains two main protocols—the
Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP). TCP and
UDP provide alternative logical channels to application programs: TCP provides a reliable byte-stream channel, and UDP provides an unreliable datagram delivery channel
(datagram may be thought of as a synonym for message). In the language of the Internet,
TCP and UDP are sometimes called end-to-end protocols, although it is equally correct
to refer to them as transport protocols.
Running above the transport layer are a range of application protocols, such as
FTP, TFTP (Trivial File Transport Protocol), Telnet (remote login), and SMTP (Simple Mail Transfer Protocol, or electronic mail), that enable the interoperation of popular
applications. To understand the difference between an application layer protocol and
an application, think of all the different World Wide Web browsers that are available
(Firefox, Safari, Internet Explorer, Lynx, etc.). There is a similarly large number of different implementations of web servers. The reason that you can use any one of these
application programs to access a particular site on the Web is because they all conform
to the same application layer protocol: HTTP (HyperText Transport Protocol). Confusingly, the same word sometimes applies to both an application and the application layer
protocol that it uses (e.g., FTP).
The Internet architecture has three features that are worth highlighting. First, as
best illustrated by Figure 1.15, the Internet architecture does not imply strict layering.
The application is free to bypass the defined transport layers and to directly use IP or
one of the underlying networks. In fact, programmers are free to define new channel
abstractions or applications that run on top of any of the existing protocols.
Second, if you look closely at the protocol graph in Figure 1.14, you will notice an
hourglass shape—wide at the top, narrow in the middle, and wide at the bottom. This
shape actually reflects the central philosophy of the architecture. That is, IP serves as
the focal point for the architecture—it defines a common method for exchanging packets among a wide collection of networks. Above IP can be arbitrarily many transport
protocols, each offering a different channel abstraction to application programs. Thus,
the issue of delivering messages from host to host is completely separated from the issue
of providing a useful process-to-process communication service. Below IP, the architecture allows for arbitrarily many different network technologies, ranging from Ethernet
to wireless to single point-to-point links.
A final attribute of the Internet architecture (or more accurately, of the IETF culture) is that in order for a new protocol to be officially included in the architecture, there
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needs to be both a protocol specification and at least one (and preferably two) representative implementations of the specification. The existence of working implementations
is required for standards to be adopted by the IETF. This cultural assumption of the
design community helps to ensure that the architecture’s protocols can be efficiently implemented. Perhaps the value the Internet culture places on working software is best
exemplified by a quote on T-shirts commonly worn at IETF meetings:
We reject kings, presidents, and voting. We believe in rough consensus and running code.
(Dave Clark)
▲
Of these three attributes of the Internet architecture, the hourglass design philosophy is important enough to bear repeating. The hourglass’s narrow waist represents
a minimal and carefully chosen set of global capabilities that allows both higher-level
applications and lower-level communication technologies to coexist, share capabilities,
and evolve rapidly. The narrow-waisted model is critical to the Internet’s ability to adapt
rapidly to new user demands and changing technologies.
1.4
Implementing Network Software
Network architectures and protocol specifications are essential things, but a good blueprint is not enough to explain the phenomenal success of the Internet: The number of
computers connected to the Internet has roughly doubled every 12 to 18 months since
1981, and is now estimated at 350 million; the number of people that use the Internet
is estimated at 1 billion; and it is believed that the number of bits transmitted over the
Internet, which has also grown exponentially, surpassed the corresponding figure for the
voice phone system sometime in 2001.
What explains the success of the Internet? There are certainly many contributing
factors (including a good architecture), but one thing that has made the Internet such
a runaway success is the fact that so much of its functionality is provided by software
running in general-purpose computers. The significance of this is that new functionality
can be added readily with “just a small matter of programming.” As a result, new applications and services—electronic commerce, videoconferencing, and packet telephony, to
name a few—have been showing up at a phenomenal pace.
A related factor is the massive increase in computing power available in commodity
machines. Although computer networks have always been capable in principle of transporting any kind of information, such as digital voice samples, digitized images, and so
on, this potential was not particularly interesting if the computers sending and receiving
that data were too slow to do anything useful with the information. Virtually all of today’s computers are capable of playing back digitized voice at full speed and can display
video at a speed and resolution that is useful for some (but by no means all) applications.
1.4
Implementing Network Software
31
Thus, today’s networks have begun to support multimedia, and their support for it will
only improve as computing hardware becomes faster.
The point to take away from this is that knowing how to implement network software is an essential part of understanding computer networks. With this in mind, this
section first introduces some of the issues involved in implementing an application program on top of a network, and then goes on to identify the issues involved in implementing the protocols running within the network. In many respects, network applications
and network protocols are very similar—the way an application engages the services of
the network is pretty much the same as the way a high-level protocol invokes the services
of a low-level protocol. As we will see later in the section, however, there are a couple of
important differences.
1.4.1 Application Programming Interface (Sockets)
The place to start when implementing a network application is the interface exported by
the network. Since most network protocols are implemented in software (especially those
high in the protocol stack), and nearly all computer systems implement their network
protocols as part of the operating system, when we refer to the interface “exported by
the network,” we are generally referring to the interface that the OS provides to its
networking subsystem. This interface is often called the network application programming
interface (API).
Although each operating system is free to define its own network API (and most
have), over time certain of these APIs have become widely supported; that is, they have
been ported to operating systems other than their native system. This is what has happened with the socket interface originally provided by the Berkeley distribution of Unix,
which is now supported in virtually all popular operating systems. The advantage of
industry-wide support for a single API is that applications can be easily ported from one
OS to another, and that developers can easily write applications for multiple OSs. It is
important to keep in mind, however, that application programs typically interact with
many parts of the OS other than the network; for example, they read and write files,
fork concurrent processes, and output to the graphical display. Just because two systems
support the same network API does not mean that their file system, process, or graphic
interfaces are the same. Still, understanding a widely adopted API like Unix sockets gives
us a good place to start.
Before describing the socket interface, it is important to keep two concerns separate
in your mind. Each protocol provides a certain set of services, and the API provides a
syntax by which those services can be invoked in this particular OS. The implementation
is then responsible for mapping the tangible set of operations and objects defined by the
API onto the abstract set of services defined by the protocol. If you have done a good
job of defining the interface, then it will be possible to use the syntax of the interface to
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invoke the services of many different protocols. Such generality was certainly a goal of
the socket interface, although it’s far from perfect.
The main abstraction of the socket interface, not surprisingly, is the socket. A good
way to think of a socket is as the point where a local application process attaches to the
network. The interface defines operations for creating a socket, attaching the socket to
the network, sending/receiving messages through the socket, and closing the socket. To
simplify the discussion, we will limit ourselves to showing how sockets are used with
TCP.
The first step is to create a socket, which is done with the following operation:
int socket(int domain, int type, int protocol)
The reason that this operation takes three arguments is that the socket interface was
designed to be general enough to support any underlying protocol suite. Specifically, the domain argument specifies the protocol family that is going to be used:
PF_INET denotes the Internet family; PF_UNIX denotes the Unix pipe facility; and
PF_PACKET denotes direct access to the network interface (i.e., it bypasses the TCP/IP
protocol stack). The type argument indicates the semantics of the communication.
SOCK_STREAM is used to denote a byte stream. SOCK_DGRAM is an alternative
that denotes a message-oriented service, such as that provided by UDP. The protocol
argument identifies the specific protocol that is going to be used. In our case, this argument is UNSPEC because the combination of PF_INET and SOCK_STREAM
implies TCP. Finally, the return value from socket is a handle for the newly created
socket, that is, an identifier by which we can refer to the socket in the future. It is given
as an argument to subsequent operations on this socket.
The next step depends on whether you are a client or a server. On a server machine,
the application process performs a passive open—the server says that it is prepared to
accept connections, but it does not actually establish a connection. The server does this
by invoking the following three operations:
int bind(int socket, struct sockaddr *address, int addr_len)
int listen(int socket, int backlog)
int accept(int socket, struct sockaddr *address, int *addr_len)
The bind operation, as its name suggests, binds the newly created socket to the
specified address. This is the network address of the local participant—the server. Note
that, when used with the Internet protocols, address is a data structure that includes
both the IP address of the server and a TCP port number. (As we will see in Chapter 5,
ports are used to indirectly identify processes. They are a form of demux keys as defined
in Section 1.3.1.) The port number is usually some well-known number specific to the
service being offered; for example, web servers commonly accept connections on port 80.
1.4
Implementing Network Software
33
The listen operation then defines how many connections can be pending on the
specified socket. Finally, the accept operation carries out the passive open. It is a
blocking operation that does not return until a remote participant has established a connection, and when it does complete, it returns a new socket that corresponds to this justestablished connection, and the address argument contains the remote participant’s
address. Note that when accept returns, the original socket that was given as an argument still exists and still corresponds to the passive open; it is used in future invocations
of accept.
On the client machine, the application process performs an active open; that is, it
says who it wants to communicate with by invoking the following single operation:
int connect(int socket, struct sockaddr *address, intaddr_len)
This operation does not return until TCP has successfully established a connection, at
which time the application is free to begin sending data. In this case, address contains
the remote participant’s address. In practice, the client usually specifies only the remote
participant’s address and lets the system fill in the local information. Whereas a server
usually listens for messages on a well-known port, a client typically does not care which
port it uses for itself; the OS simply selects an unused one.
Once a connection is established, the application processes invoke the following
two operations to send and receive data:
int send(int socket, char *message, int msg_len, int flags)
int recv(int socket, char *buffer, int buf_len, int flags)
The first operation sends the given message over the specified socket, while the second operation receives a message from the specified socket into the given buffer. Both
operations take a set of flags that control certain details of the operation.
1.4.2 Example Application
We now show the implementation of a simple client/server program that uses the socket
interface to send messages over a TCP connection. The program also uses other Unix
networking utilities, which we introduce as we go. Our application allows a user on one
machine to type in and send text to a user on another machine. It is a simplified version
of the Unix talk program, which is similar to the program at the core of a web chat
room.
Client
We start with the client side, which takes the name of the remote machine as an argument. It calls the Unix utility gethostbyname to translate this name into the remote
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host’s IP address. The next step is to construct the address data structure (sin) expected
by the socket interface. Notice that this data structure specifies that we’ll be using the
socket to connect to the Internet (AF_INET). In our example, we use TCP port 5432 as
the well-known server port; this happens to be a port that has not been assigned to any
other Internet service. The final step in setting up the connection is to call socket and
connect. Once the connect operation returns, the connection is established and the
client program enters its main loop, which reads text from standard input and sends it
over the socket.
#include
#include
#include
#include
#include
<stdio.h>
<sys/types.h>
<sys/socket.h>
<netinet/in.h>
<netdb.h>
#define SERVER_PORT 5432
#define MAX_LINE 256
int
main(int argc, char * argv[])
{
FILE *fp;
struct hostent *hp;
struct sockaddr_in sin;
char *host;
char buf[MAX_LINE];
int s;
int len;
if (argc==2) {
host = argv[1];
}
else {
fprintf(stderr, "usage: simplex-talk host\n");
exit(1);
}
/* translate host name into peer’s IP address */
hp = gethostbyname(host);
if (!hp) {
fprintf(stderr, "simplex-talk: unknown host: %s\n", host);
exit(1);
1.4
Implementing Network Software
35
}
/* build address data structure */
bzero((char *)&sin, sizeof(sin));
sin.sin_family = AF_INET;
bcopy(hp->h_addr, (char *)&sin.sin_addr, hp->h_length);
sin.sin_port = htons(SERVER_PORT);
/* active open */
if ((s = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
perror("simplex-talk: socket");
exit(1);
}
if (connect(s, (struct sockaddr *)&sin, sizeof(sin)) < 0) {
perror("simplex-talk: connect");
close(s);
exit(1);
}
/* main loop: get and send lines of text */
while (fgets(buf, sizeof(buf), stdin)) {
buf[MAX_LINE-1] = ’\0’;
len = strlen(buf) + 1;
send(s, buf, len, 0);
}
}
Server
The server is equally simple. It first constructs the address data structure by filling in
its own port number (SERVER_PORT). By not specifying an IP address, the application program is willing to accept connections on any of the local host’s IP addresses.
Next, the server performs the preliminary steps involved in a passive open: creates the
socket, binds it to the local address, and sets the maximum number of pending connections to be allowed. Finally, the main loop waits for a remote host to try to connect,
and when one does, receives and prints out the characters that arrive on the connection.
#include
#include
#include
#include
#include
<stdio.h>
<sys/types.h>
<sys/socket.h>
<netinet/in.h>
<netdb.h>
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#define SERVER_PORT
#define MAX_PENDING
#define MAX_LINE
5432
5
256
int
main()
{
struct sockaddr_in sin;
char buf[MAX_LINE];
int len;
int s, new_s;
/* build address data structure */
bzero((char *)&sin, sizeof(sin));
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = INADDR_ANY;
sin.sin_port = htons(SERVER_PORT);
/* setup passive open */
if ((s = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
perror("simplex-talk: socket");
exit(1);
}
if ((bind(s, (struct sockaddr *)&sin, sizeof(sin))) < 0) {
perror("simplex-talk: bind");
exit(1);
}
listen(s, MAX_PENDING);
/* wait for connection, then receive and print text */
while(1) {
if ((new_s = accept(s, (struct sockaddr *)&sin, &len)) < 0) {
perror("simplex-talk: accept");
exit(1);
}
while (len = recv(new_s, buf, sizeof(buf), 0))
fputs(buf, stdout);
close(new_s);
}
}
1.4
Implementing Network Software
37
1.4.3 Protocol Implementation Issues
As mentioned at the beginning of this section, the way application programs interact
with the underlying network is similar to the way a high-level protocol interacts with a
low-level protocol. For example, TCP needs an interface to send outgoing messages to IP,
and IP needs to be able to deliver incoming messages to TCP. This is exactly the service
interface introduced in Section 1.3.1.
Since we already have a network API (e.g., sockets), we might be tempted to use
this same interface between every pair of protocols in the protocol stack. Although certainly an option, in practice the socket interface is not used in this way. The reason is
that there are inefficiencies built into the socket interface that protocol implementers
are not willing to tolerate. Application programmers tolerate them because they simplify
their programming task, and because the inefficiency only has to be tolerated once, but
protocol implementers are often obsessed with performance and must worry about getting a message through several layers of protocols. The rest of this section discusses the
two primary differences between the network API and the protocol-to-protocol interface
found lower in the protocol graph.
Process Model
Most operating systems provide an abstraction called a process, or alternatively, a thread.
Each process runs largely independently of other processes, and the OS is responsible
for making sure that resources, such as address space and CPU cycles, are allocated to all
the current processes. The process abstraction makes it fairly straightforward to have a
lot of things executing concurrently on one machine; for example, each user application
might execute in its own process, and various things inside the OS might execute as
other processes. When the OS stops one process from executing on the CPU and starts
up another one, we call the change a context switch.
When designing the network subsystem, one of the first questions to answer is,
“Where are the processes?” There are essentially two choices, as illustrated in Figure 1.16.
In the first, which we call the process-per-protocol model, each protocol is implemented
by a separate process. This means that as a message moves up or down the protocol
stack, it is passed from one process/protocol to another—the process that implements
protocol i processes the message, then passes it to protocol i − 1, and so on. How
one process/protocol passes a message to the next process/protocol depends on the support the host OS provides for interprocess communication. Typically, there is a simple
mechanism for enqueuing a message with a process. The important point, however, is
that a context switch is required at each level of the protocol graph—typically a timeconsuming operation.
The alternative, which we call the process-per-message model, treats each protocol
as a static piece of code and associates the processes with the messages. That is, when a
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Figure 1.16 Alternative process models: (a) process-per-protocol;
(b) process-per-message.
message arrives from the network, the OS dispatches a process that it makes responsible
for the message as it moves up the protocol graph. At each level, the procedure that
implements that protocol is invoked, which eventually results in the procedure for the
next protocol being invoked, and so on. For outbound messages, the application’s process
invokes the necessary procedure calls until the message is delivered. In both directions,
the protocol graph is traversed in a sequence of procedure calls.
Although the process-per-protocol model is sometimes easier to think about—
I implement my protocol in my process, and you implement your protocol in your
process—the process-per-message model is generally more efficient for a simple reason:
A procedure call is an order of magnitude more efficient than a context switch on most
1.4
Implementing Network Software
39
computers. The former model requires the expense of a context switch at each level,
while the latter model costs only a procedure call per level.
Message Buffers
A second inefficiency of the socket interface is that the application process provides the
buffer that contains the outbound message when calling send, and similarly it provides
the buffer into which an incoming message is copied when invoking the receive operation. This forces the topmost protocol to copy the message from the application’s buffer
into a network buffer, and vice versa, as shown in Figure 1.17. It turns out that copying
data from one buffer to another is one of the most expensive things a protocol implementation can do. This is because while processors are becoming faster at an incredible
pace, memory is not getting faster as quickly as processors are. Relative to processors,
memory is getting slower.
Instead of copying message data from one buffer to another at each layer in the
protocol stack, most network subsystems define an abstract data type for messages that
is shared by all protocols in the protocol graph. Not only does this abstraction permit
messages to be passed up and down the protocol graph without copying, but it usually provides copy-free ways of manipulating messages in other ways, such as adding
and stripping headers, fragmenting large messages into a set of small messages, and reassembling a collection of small messages into a single large message. The exact form of
this message abstraction differs from OS to OS, but it generally involves a linked-list of
pointers to message buffers, similar to the one shown in Figure 1.18. We leave it as an
exercise for the reader to define a general copy-free message abstraction.
Figure 1.17 Copying incoming/outgoing messages between application buffer and
network buffer.
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Figure 1.18
1.5
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Example message data structure.
Performance
Up to this point, we have focused primarily on the functional aspects of a network. Like
any computer system, however, computer networks are also expected to perform well.
This is because the effectiveness of computations distributed over the network often
depends directly on the efficiency with which the network delivers the computation’s
data. While the old programming adage “first get it right and then make it fast” is valid
in many settings, in networking it is usually necessary to “design for performance.” It is,
therefore, important to understand the various factors that impact network performance.
1.5.1 Bandwidth and Latency
Network performance is measured in two fundamental ways: bandwidth (also called
throughput) and latency (also called delay). The bandwidth of a network is given by the
number of bits that can be transmitted
Bandwidth and Throughput
over the network in a certain period of
time. For example, a network might have
Bandwidth and throughput are two
a bandwidth of 10 million bits/second
of the most confusing terms used in
(Mbps), meaning that it is able to denetworking. While we could try to
liver 10 million bits every second. It is
give you a precise definition of each
sometimes useful to think of bandwidth
term, it is important that you know
in terms of how long it takes to transmit
how other people might use them and
each bit of data. On a 10-Mbps network,
for you to be aware that they are offor example, it takes 0.1 microsecond (µs)
ten used interchangeably. First of all,
to transmit each bit.
bandwidth is literally a measure of the
While you can talk about the bandwidth of a frequency band. For examwidth of the network as a whole, someple, a voice-grade telephone line suptimes you want to be more precise,
ports a frequency band ranging from
focusing, for example, on the bandwidth
1.5
Performance
41
of a single physical link or a logical processto-process channel. At the physical level,
bandwidth is constantly improving, with
no end in sight. Intuitively, if you think
of a second of time as a distance you
could measure with a ruler, and bandwidth as how many bits fit in that distance, then you can think of each bit
as a pulse of some width. For example, each bit on a 1-Mbps link is 1 µs
wide, while each bit on a 2-Mbps link
is 0.5 µs wide, as illustrated in Figure 1.19. The more sophisticated the
transmitting and receiving technology, the
narrower each bit can become, and thus,
the higher the bandwidth. For logical
process-to-process channels, bandwidth
is also influenced by other factors, including how many times the software
that implements the channel has to handle, and possibly transform, each bit of
data.
The second performance metric, latency, corresponds to how long it takes a message to travel from one end of a network to the other. (As with bandwidth, we could be
focused on the latency of a single link or an end-to-end channel.) Latency is measured
strictly in terms of time. For example, a transcontinental network might have a latency of
300 to 3,300 Hz; it is said to have a
bandwidth of 3,300 Hz − 300 Hz =
3,000 Hz. If you see the word “bandwidth” used in a situation in which
it is being measured in hertz, then it
probably refers to the range of signals
that can be accommodated.
When we talk about the bandwidth of a communication link, we
normally refer to the number of bits
per second that can be transmitted
on the link. We might say that the
bandwidth of an Ethernet is 10 Mbps.
A useful distinction might be made,
however, between the bandwidth that
is available on the link and the number of bits per second that we can actually transmit over the link in practice. We tend to use the word “throughput” to refer to the measured performance of a system. Thus, because of
Figure 1.19 Bits transmitted at a particular bandwidth can be regarded as having
some width: (a) bits transmitted at 1 Mbps (each bit 1 µs wide); (b) bits transmitted at
2 Mbps (each bit 0.5 µs wide).
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24 milliseconds (ms); that is, it takes a
message 24 ms to travel from one end
various inefficiencies of implementaof North America to the other. There are
tion, a pair of nodes connected by a
many situations in which it is more imporlink with a bandwidth of 10 Mbps
tant to know how long it takes to send a
might achieve a throughput of only
message from one end of a network to the
2 Mbps. This would mean that an apother and back, rather than the one-way
plication on one host could send data
latency. We call this the round-trip time
to the other host at 2 Mbps.
(RTT) of the network.
Finally, we often talk about the
We often think of latency as having
bandwidth requirements of an applithree components. First, there is the speedcation. This is the number of bits
of-light propagation delay. This delay ocper second that it needs to transmit
curs because nothing, including a bit on
over the network to perform accepta wire, can travel faster than the speed of
ably. For some applications, this might
light. If you know the distance between
be “whatever I can get”; for others, it
two points, you can calculate the speedmight be some fixed number (preferof-light latency, although you have to be
ably no more than the available link
careful because light travels across different
bandwidth); and for others, it might
mediums at different speeds: It travels at
be a number that varies with time. We
3.0 × 108 m/s in a vacuum, 2.3 × 108 m/s
will provide more on this topic later in
8
in a cable, and 2.0 × 10 m/s in a fiber.
this section.
Second, there is the amount of time it
takes to transmit a unit of data. This is a function of the network bandwidth and the
size of the packet in which the data is carried. Third, there may be queuing delays inside
the network, since packet switches generally need to store packets for some time before
forwarding them on an outbound link, as discussed in Section 1.2.2. So, we could define
the total latency as
Latency = Propagation + Transmit + Queue
Propagation = Distance/SpeedOfLight
Transmit = Size/Bandwidth
where Distance is the length of the wire over which the data will travel, SpeedOfLight is the effective speed of light over that wire, Size is the size of the packet,
and Bandwidth is the bandwidth at which the packet is transmitted. Note that if the
message contains only one bit and we are talking about a single link (as opposed to a
whole network), then the Transmit and Queue terms are not relevant, and latency
corresponds to the propagation delay only.
Bandwidth and latency combine to define the performance characteristics of a
given link or channel. Their relative importance, however, depends on the application.
1.5
Performance
43
For some applications, latency dominates bandwidth. For example, a client that sends
a 1-byte message to a server and receives a 1-byte message in return is latency bound.
Assuming that no serious computation is involved in preparing the response, the application will perform much differently on a transcontinental channel with a 100-ms RTT
than it will on an across-the-room channel with a 1-ms RTT. Whether the channel is
1 Mbps or 100 Mbps is relatively insignificant, however, since the former implies that the
time to transmit a byte (Transmit) is 8 µs and the latter implies Transmit = 0.08 µs.
In contrast, consider a digital library program that is being asked to fetch a 25megabyte (MB) image—the more bandwidth that is available, the faster it will be able to
return the image to the user. Here, the bandwidth of the channel dominates performance.
To see this, suppose that the channel has a bandwidth of 10 Mbps. It will take 20 seconds
to transmit the image, making it relatively unimportant if the image is on the other side
of a 1-ms channel or a 100-ms channel; the difference between a 20.001-second response
time and a 20.1-second response time is negligible.
Figure 1.20 gives you a sense of how latency or bandwidth can dominate performance in different circumstances. The graph shows how long it takes to move objects of
Figure 1.20 Perceived latency (response time) versus round-trip time for various
object sizes and link speeds.
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various sizes (1 byte, 2 KB, 1 MB) across networks with RTTs ranging from 1 to 100 ms
and link speeds of either 1.5 or 10 Mbps. We use logarithmic scales to show relative
performance. For a 1-byte object (say, a keystroke), latency remains almost exactly equal
to the RTT, so that you cannot distinguish between a 1.5-Mbps network and a 10-Mbps
network. For a 2-KB object (say, an email message), the link speed makes quite a difference on a 1-ms-RTT network but a negligible difference on a 100-ms-RTT network.
And for a 1-MB object (say, a digital image), the RTT makes no difference—it is the
link speed that dominates performance across the full range of RTT.
Note that throughout this book we use the terms latency and delay in a generic way,
that is, to denote how long it takes to perform a particular function such as delivering
a message or moving an object. When we are referring to the specific amount of time it
takes a signal to propagate from one end of a link to another, we use the term propagation
delay. Also, we make it clear in the context of the discussion whether we are referring to
the one-way latency or the round-trip time.
As an aside, computers are becoming so fast that when we connect them to networks, it is sometimes useful to think, at least figuratively, in terms of instructions per
mile. Consider what happens when a computer that is able to execute 1 billion instructions per second sends a message out on a channel with a 100-ms RTT. (To make the
math easier, assume that the message covers a distance of 5,000 miles.) If that computer
sits idle the full 100 ms waiting for a reply message, then it has forfeited the ability to
execute 100 million instructions, or 20,000 instructions per mile. It had better have been
worth going over the network to justify this waste.
1.5.2 Delay × Bandwidth Product
It is also useful to talk about the product of these two metrics, often called the delay ×
bandwidth product. Intuitively, if we think of a channel between a pair of processes as a
hollow pipe (see Figure 1.21), where the latency corresponds to the length of the pipe
and the bandwidth gives the diameter of the pipe, then the delay × bandwidth product
gives the volume of the pipe—the maximum number of bits that could be in transit
through the pipe at any given instant. Said another way, if latency (measured in time)
corresponds to the length of the pipe, then given the width of each bit (also measured in
time), you can calculate how many bits fit in the pipe. For example, a transcontinental
channel with a one-way latency of 50 ms and a bandwidth of 45 Mbps is able to hold
50 × 10−3 sec × 45 × 106 bits/sec
= 2.25 × 106 bits
or approximately 280 KB of data. In other words, this example channel (pipe) holds as
many bytes as the memory of a personal computer from the early 1980s could hold.
1.5
Figure 1.21
Performance
45
Network as a pipe.
How Big Is a Mega?
There are several pitfalls you need to
be aware of when working with the
common units of networking—MB,
Mbps, KB, and Kbps. The first is to
distinguish carefully between bits and
bytes. Throughout this book, we always use a lowercase b for bits and a
capital B for bytes. The second is to
be sure you are using the appropriate
definition of mega (M) and kilo (K).
Mega, for example, can mean either
220 or 106 . Similarly, kilo can be either
210 or 103 . What is worse, in networking we typically use both definitions. Here’s why.
Network bandwidth, which is
often specified in terms of Mbps, is
typically governed by the speed of the
clock that paces the transmission of
the bits. A clock that is running at
10 MHz is used to transmit bits at
10 Mbps. Because the mega in MHz
means 106 hertz, Mbps is usually also
defined as 106 bits per second. (Similarly, Kbps is 103 bits per second.)
On the other hand, when we talk
about a message that we want to transmit, we often give its size in kilobytes.
The delay × bandwidth product is
important to know when constructing
high-performance networks because it corresponds to how many bits the sender
must transmit before the first bit arrives
at the receiver. If the sender is expecting
the receiver to somehow signal that bits
are starting to arrive, and it takes another
channel latency for this signal to propagate back to the sender (i.e., we are interested in the channel’s RTT rather than
just its one-way latency), then the sender
can send up to two delay × bandwidths
worth of data before hearing from the receiver that all is well. The bits in the pipe
are said to be “in flight,” which means
that if the receiver tells the sender to stop
transmitting, it might receive up to a delay × bandwidth’s worth of data before
the sender manages to respond. In our
example above, that amount corresponds
to 5.5 × 106 bits (671 KB) of data. On
the other hand, if the sender does not fill
the pipe—send a whole delay × bandwidth
product’s worth of data before it stops to
wait for a signal—the sender will not fully
utilize the network.
Note that most of the time we are
interested in the RTT scenario, which we
simply refer to as the delay × bandwidth
product, without explicitly saying that this
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Bandwidth
Distance
Link Type
(Typical)
(Typical)
Round-trip Delay
Delay × BW
Dial-up
56 Kbps
10 km
87 µs
5 bits
Wireless LAN
54 Mbps
50 m
0.33 µs
18 bits
Satellite
45 Mbps
35,000 km
230 ms
10 Mb
Cross-country fiber
10 Gbps
4,000 km
40 ms
400 Mb
Table 1.1 Sample delay × bandwidth products.
product is multiplied by two. Again,
whether the “delay” in “delay × bandwidth” means one-way latency or RTT is
made clear by the context. Table 1.1 shows
some examples of delay × bandwidth
products for some typical network links.
1.5.3 High-Speed Networks
The bandwidths available on today’s networks are increasing at a dramatic rate,
and there is eternal optimism that network
bandwidth will continue to improve. This
causes network designers to start thinking
about what happens in the limit, or stated
another way, what is the impact on network design of having infinite bandwidth
available.
Although high-speed networks bring
a dramatic change in the bandwidth available to applications, in many respects their
impact on how we think about networking comes in what does not change as
bandwidth increases: the speed of light.
To quote Scotty from Star Trek, “You cannae change the laws of physics.” In other
words, “high speed” does not mean that
latency improves at the same rate as band-
Because messages are stored in the
computer’s memory, and memory is
typically measured in powers of two,
the K in KB is usually taken to mean
210 . (Similarly, MB usually means
220 .) When you put the two together,
it is not uncommon to talk about
sending a 32-KB message over a 10Mbps channel, which should be interpreted to mean 32 × 210 × 8 bits are
being transmitted at a rate of 10×106
bits per second. This is the interpretation we use throughout the book, unless explicitly stated otherwise.
The good news is that many
times we are satisfied with a backof-the-envelope calculation, in which
case it is perfectly reasonable to pretend that a byte has 10 bits in it (making it easy to convert between bits
and bytes) and that 106 is really equal
to 220 (making it easy to convert between the two definitions of mega).
Notice that the first approximation
introduces a 20% error, while the latter introduces only a 5% error.
1.5
Performance
47
▲
width; the transcontinental RTT of a
1-Gbps link is the same 100 ms as it is
To help you in your quick-andfor a 1-Mbps link.
dirty calculations, 100 ms is a reasonTo appreciate the significance of
able number to use for a cross-country
ever-increasing
bandwidth in the face of
round-trip time—at least when the
fixed
latency,
consider
what is required to
country in question is the United
transmit a 1-MB file over a 1-Mbps netStates—and 1 ms is a good approxiwork versus over a 1-Gbps network, both
mation of an RTT across a local area
of which have an RTT of 100 ms. In the
network. In the case of the former, we
case of the 1-Mbps network, it takes 80
increase the 48-ms round-trip time
round-trip times to transmit the file; durimplied by the speed of light over
ing each RTT, 1.25% of the file is sent. In
a fiber to 100 ms because there are,
contrast, the same 1-MB file doesn’t even
as we have said, other sources of decome close to filling 1 RTT’s worth of the
lay, such as the queueing time in the
1-Gbps link, which has a delay × bandswitches inside the network. You can
width product of 12.5 MB.
also be sure that the path taken by the
Figure 1.22 illustrates the difference
fiber between two points will not be a
between the two networks. In effect, the
straight line.
1-MB file looks like a stream of data that
needs to be transmitted across a 1-Mbps network, while it looks like a single packet on a
1-Gbps network. To help drive this point home, consider that a 1-MB file is to a 1-Gbps
network what a 1-KB packet is to a 1-Mbps network.
Another way to think about the situation is that more data can be transmitted
during each RTT on a high-speed network, so much so that a single RTT becomes a
significant amount of time. Thus, while you wouldn’t think twice about the difference
between a file transfer taking 101 RTTs rather than 100 RTTs (a relative difference of
only 1%), suddenly the difference between 1 RTT and 2 RTTs is significant—a 100%
increase. In other words, latency, rather than throughput, starts to dominate our thinking
about network design.
Perhaps the best way to understand the relationship between throughput and latency is to return to basics. The effective end-to-end throughput that can be achieved
over a network is given by the simple relationship
Throughput = TransferSize/TransferTime
where TransferTime includes not only the elements of one-way Latency identified
earlier in this section, but also any additional time spent requesting or setting up the
transfer. Generally, we represent this relationship as
TransferTime = RTT + 1/Bandwidth x TransferSize
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Figure 1.22 Relationship between bandwidth and latency. A 1-MB file would fill the
1-Mbps link 80 times, but only fill the 1-Gbps link 1/12 of one time.
We use RTT in this calculation to account for a request message being sent across the
network and the data being sent back. For example, consider a situation where a user
wants to fetch a 1-MB file across a 1-Gbps network with a round-trip time of 100 ms.
The TransferTime includes both the transmit time for 1 MB (1/1 Gbps × 1 MB =
8 ms), and the 100-ms RTT, for a total transfer time of 108 ms. This means that the
effective throughput will be
1 MB/108 ms = 74.1 Mbps
not 1 Gbps. Clearly, transferring a larger amount of data will help improve the effective
throughput, where in the limit, an infinitely large transfer size will cause the effective
throughput to approach the network bandwidth. On the other hand, having to endure
more than 1 RTT—for example, to retransmit missing packets—will hurt the effective
throughput for any transfer of finite size and will be most noticeable for small transfers.
1.5.4 Application Performance Needs
The discussion in this section has taken a network-centric view of performance; that
is, we have talked in terms of what a given link or channel will support. The unstated
assumption has been that application programs have simple needs—they want as much
bandwidth as the network can provide. This is certainly true of the aforementioned
digital library program that is retrieving a 25-MB image; the more bandwidth that is
available, the faster the program will be able to return the image to the user.
1.5
Performance
49
However, some applications are able to state an upper limit on how much bandwidth they need. Video applications are a prime example. Suppose one wants to stream
a video image; that is one-quarter the size of a standard TV image; that is, it has a resolution of 352 by 240 pixels. If each pixel is represented by 24 bits of information, as
would be the case for 24-bit color, then the size of each frame would be
(352 × 240 × 24)/8 = 247.5 KB
If the application needs to support a frame rate of 30 frames per second, then it might
request a throughput rate of 75 Mbps. The ability of the network to provide more bandwidth is of no interest to such an application because it has only so much data to transmit
in a given period of time.
Unfortunately, the situation is not as simple as this example suggests. Because the
difference between any two adjacent frames in a video stream is often small, it is possible
to compress the video by transmitting only the differences between adjacent frames. This
compressed video does not flow at a constant rate, but varies with time according to factors such as the amount of action and detail in the picture and the compression algorithm
being used. Therefore, it is possible to say what the average bandwidth requirement will
be, but the instantaneous rate may be more or less.
The key issue is the time interval over which the average is computed. Suppose
that this example video application can be compressed down to the point that it needs
only 2 Mbps, on average. If it transmits 1 Mb in a 1-second interval and 3 Mb in
the following 1-second interval, then over the 2-second interval it is transmitting at an
average rate of 2 Mbps; however, this will be of little consolation to a channel that was
engineered to support no more than 2 Mb in any one second. Clearly, just knowing the
average bandwidth needs of an application will not always suffice.
Generally, however, it is possible to put an upper bound on how large a burst an
application like this is likely to transmit. A burst might be described by some peak rate
that is maintained for some period of time. Alternatively, it could be described as the
number of bytes that can be sent at the peak rate before reverting to the average rate or
some lower rate. If this peak rate is higher than the available channel capacity, then the
excess data will have to be buffered somewhere, to be transmitted later. Knowing how big
of a burst might be sent allows the network designer to allocate sufficient buffer capacity
to hold the burst. We will return to the subject of describing bursty traffic accurately in
Chapter 6.
Analogous to the way an application’s bandwidth needs can be something other
than “all it can get,” an application’s delay requirements may be more complex than
simply “as little delay as possible.” In the case of delay, it sometimes doesn’t matter so
much whether the one-way latency of the network is 100 ms or 500 ms as how much
the latency varies from packet to packet. The variation in latency is called jitter.
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Foundation
Network-induced jitter.
Consider the situation in which the source sends a packet once every 33 ms, as
would be the case for a video application transmitting frames 30 times a second. If the
packets arrive at the destination spaced out exactly 33 ms apart, then we can deduce that
the delay experienced by each packet in the network was exactly the same. If the spacing
between when packets arrive at the destination—sometimes called the interpacket gap—
is variable, however, then the delay experienced by the sequence of packets must have also
been variable, and the network is said to have introduced jitter into the packet stream,
as shown in Figure 1.23. Such variation is generally not introduced in a single physical
link, but it can happen when packets experience different queuing delays in a multihop
packet-switched network. This queuing delay corresponds to the Queue component of
latency defined earlier in this section, which varies with time.
To understand the relevance of jitter, suppose that the packets being transmitted
over the network contain video frames, and in order to display these frames on the screen
the receiver needs to receive a new one every 33 ms. If a frame arrives early, then it can
simply be saved by the receiver until it is time to display it. Unfortunately, if a frame
arrives late, then the receiver will not have the frame it needs in time to update the screen,
and the video quality will suffer; it will not be smooth. Note that it is not necessary to
eliminate jitter, only to know how bad it is. The reason for this is that if the receiver
knows the upper and lower bounds on the latency that a packet can experience, it can
delay the time at which it starts playing back the video (i.e., displays the first frame) long
enough to ensure that in the future it will always have a frame to display when it needs
it. The receiver delays the frame, effectively smoothing out the jitter, by storing it in a
buffer. We return to the topic of jitter in Chapter 6.
1.6
Summary
Computer networks like the Internet have experienced enormous growth over the past
decade and are now positioned to provide a wide range of services—remote file access, digital libraries, videoconferencing—to hundreds of millions of users. Much of this
growth can be attributed to the general-purpose nature of computer networks, and in
1.6 Summary
51
particular to the ability to add new functionality to the network by writing software that
runs on affordable, high-performance computers. With this in mind, the overriding goal
of this book is to describe computer networks in such a way that when you finish reading
it, you should feel that if you had an army of programmers at your disposal, you could
actually build a fully-functional computer network from the ground up. This chapter
lays the foundation for realizing this goal.
The first step we have taken toward this goal is to carefully identify exactly what we
expect from a network. For example, a network must first provide cost-effective connectivity among a set of computers. This is accomplished through a nested interconnection
of nodes and links, and by sharing this hardware base through the use of statistical multiplexing. This results in a packet-switched network, on top of which we then define a
collection of process-to-process communication services.
The second step is to define a layered architecture that will serve as a blueprint
for our design. The central objects of this architecture are network protocols. Protocols
both provide a communication service to higher-level protocols and define the form and
meaning of messages exchanged with their peers running on other machines. We have
briefly surveyed two of the most widely used architectures: the OSI architecture and the
Internet architecture. This book most closely follows the Internet architecture, both in
its organization and as a source of examples.
The third step is to implement the network’s protocols and application programs,
usually in software. Both protocols and applications need an interface by which they invoke the services of other protocols in the network subsystem. The socket interface is the
most widely used interface between application programs and the network subsystem,
but a slightly different interface is typically used within the network subsystem.
Finally, the network as a whole must offer high performance, where the two performance metrics we are most interested in are latency and throughput. As we will see in
later chapters, it is the product of these two metrics—the so-called delay × bandwidth
product—that often plays a critical role in protocol design.
There is little doubt that computer networks are becoming an inO P E N
I S S U E
tegral part of the everyday lives of
Ubiquitous Networking
vast numbers of people. What began
over 35 years ago as experimental systems like the ARPANET—connecting
mainframe computers over longdistance telephone lines—has turned into big business. And where there is big business,
there are lots of players. In this case, there is the computing industry, which has become
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increasingly involved in supporting packet-switched networking products; the telephone
carriers, which recognize the market for carrying all sorts of data, not just voice; and the
cable TV industry, which in parts of the world involved in both the delivery of “content”
(e.g. video-on-demand) and the provision of high-speed residential connections to the
Internet. And this list does not even include the many players involved in delivery of
services over the Internet such as voiceover IP (VoIP) and electronic commerce.
Assuming that the goal is ubiquitous networking—to bring the network into every
household—the first problem that must be addressed is how to establish the necessary
physical links. The most widely discussed options in most parts of the world make use of
either the existing cable TV facilities or the copper pairs used to deliver telephone service.
Fiber to the home, or to the apartment building, which not long ago looked like a pipe
dream, is gathering momentum in some areas. There have also been developments in the
technology to deliver network connectivity over power lines, and, as we will see in the
next chapter, there is now an abundance of wireless networking technologies. Increasingly this is leading to an expectation that access to the Internet is available everywhere,
not just in the workplace or at home.
How the struggle between the computer companies, the telephone companies, the
cable industry, and other stakeholders in the networking business will play out in the
marketplace is anyone’s guess. (If we knew the answer, we’d be charging a lot more for
this book.) All we know is that there are many technical obstacles—issues of connectivity,
levels of service, performance, reliability, security, and fairness—that stand between the
current state-of-the-art and the sort of global, ubiquitous, heterogeneous network that
we believe is possible and desirable. It is these challenges that are the focus of this book.
F U R T H E R
R E A D I N G
Computer networks are not the first communication-oriented technology to have found
their way into the everyday fabric of our society. For example, the early part of this
century saw the introduction of the telephone, and then during the 1950s television
became widespread. When considering the future of networking—how widely it will
spread and how we will use it—it is instructive to study this history. Our first reference
is a good starting point for doing this (the entire issue is devoted to the first 100 years of
telecommunications).
The second and third papers are the seminal papers on the OSI and Internet architectures, respectively. The Zimmerman paper introduces the OSI architecture, and
the Clark paper is a retrospective. The final two papers are not specific to networking,
but present viewpoints that capture the “systems approach” of this book. The Saltzer
et al. paper motivates and describes one of the most widely applied rules of network
architecture—the end-to-end argument. The paper by Mashey describes the thinking be-
Further Reading
53
hind RISC architectures; as we will soon discover, making good judgments about where
to place functionality in a complex system is what system design is all about.
■
Pierce, J. “Telephony—A Personal View.” IEEE Communications 22(5):116–
120, May 1984.
■
Zimmerman, H. “OSI Reference Model—The ISO Model of Architecture for
Open Systems Interconnection.” IEEE Transactions on Communications COM28(4):425–432, April 1980.
■
Clark, D. “The Design Philosophy of the DARPA Internet Protocols.” Proceedings of the SIGCOMM ’88 Symposium, pp. 106–114, August 1988.
■
Saltzer, J., D. Reed, and D. Clark. “End-to-End Arguments in System Design.”
ACM Transactions on Computer Systems 2(4):277–288, November 1984.
■
Mashey, J. “RISC, MIPS, and the Motion of Complexity.” UniForum 1986
Conference Proceedings, pp. 116–124, 1986.
Several texts offer an introduction to computer networking: Stallings gives an encyclopedic treatment of the subject, with an emphasis on the lower levels of the OSI hierarchy [Sta07]; Tanenbaum uses the OSI architecture as an organizational model [Tan03];
Comer gives an overview of the Internet architecture [Com00]; and Bertsekas and Gallager discuss networking from a performance modeling perspective [BG92].
To put computer networking into a larger context, two books—one dealing with
the past and the other looking toward the future—are must reading. The first is Holzmann and Pehrson’s The Early History of Data Networks [HP95]. Surprisingly, many of
the ideas covered in the book you are now reading were invented during the 1700s. The
second is Realizing the Information Future: The Internet and Beyond, a book prepared by
the Computer Science and Telecommunications Board of the National Research Council
[NRC94].
To follow the history of the Internet from its beginning, the reader is encouraged to
peruse the Internet’s Request for Comments (RFC) series of documents. These documents,
which include everything from the TCP specification to April Fools’ jokes, are retrievable
at http://www.ietf.org/rfc.html. For example, the protocol specifications for TCP,
UDP, and IP are available in RFC 793, 768, and 791, respectively.
To gain a better appreciation for the Internet philosophy and culture, two references are recommended; both are also quite entertaining. Padlipsky gives a good description of the early days, including a pointed comparison of the Internet and OSI
architectures [Pad85]. For an account of what really happens behind the scenes at the
Internet Engineering Task Force, we recommend Boorsook’s article [Boo95].
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There are a wealth of articles discussing various aspects of protocol implementations. A good starting point is to understand two complete protocol implementation
environments: the Stream mechanism from System V Unix [Rit84] and the x-kernel
[HP91]. In addition, [LMKQ89] and [SW95] describe the widely used Berkeley Unix
implementation of TCP/IP.
More generally, there is a large body of work addressing the issue of structuring and
optimizing protocol implementations. Clark was one of the first to discuss the relationship between modular design and protocol performance [Cla82]. Later papers then introduce the use of upcalls in structuring protocol code [Cla85] and study the processing
overheads in TCP [CJRS89]. Finally, [WM87] describes how to gain efficiency through
appropriate design and implementation choices.
Several papers have introduced specific techniques and mechanisms that can be
used to improve protocol performance. For example, [HMPT89] describes some of the
mechanisms used in the x-kernel, [MD93] discusses various implementations of demultiplexing tables, [VL87] introduces the timing-wheel mechanism used to manage protocol events, and [DP93] describes an efficient buffer management strategy. Also, the
performance of protocols running on parallel processors—locking is a key issue in such
environments—is discussed in [BG93] and [NYKT94].
Because many aspects of protocol implementation depend on an understanding of
the basics of operating systems, we recommend Finkel [Fin88], Bic and Shaw [BS88],
and Tanenbaum [Tan01] for an introduction to OS concepts.
Finally, we conclude the Further Reading section of each chapter with a set of live
references; that is, URLs for locations on the World Wide Web where you can learn more
about the topics discussed in that chapter. Since these references are live, it is possible that
they will not remain active for an indefinite period of time. For this reason, we limit the
set of live references at the end of each chapter to sites that either export software, provide
a service, or report on the activities of an ongoing working group or standardization
body. In other words, we only give URLs for the kinds of material that cannot easily be
referenced using standard citations. For this chapter, we include four live references:
■
http://www.mkp.com/pd4e: Information about this book, including supplements, addenda, and so on.
■
http://www.acm.org/sigcomm/sos.html: Status of various networking
standards, including those of the IETF, ISO, and IEEE.
■
http://www.ietf.org/: Information about the IETF and its working groups.
■
http://edas.info/S.cgi?search=1: Searchable bibliography of networkrelated research papers.
Exercises
55
E X E R C I S E S
1
Use anonymous FTP to connect to ftp.isi.edu (directory in-notes), and
retrieve the RFC index. Also retrieve the protocol specifications for TCP, IP,
and UDP.
2
Look up the website
http://www.cs.princeton.edu/nsg
Here you can read about current network research underway at Princeton University and see a picture of author Larry Peterson. Follow links to find a biography of author Bruce Davie.
3
Use a Web search tool to locate useful, general, and noncommercial information about the following topics: MBone, ATM, MPEG, IPv6, and Ethernet.
4
The Unix utility whois can be used to find the domain name corresponding to an organization, or vice versa. Read the man page documentation for
whois and experiment with it. Try whois princeton.edu and whois
princeton, for starters. As an alternative, explore the whois interface at
http://www.internic.net/whois.html.
5
Calculate the total time required to transfer a 1,000-KB file in the following
cases, assuming an RTT of 100 ms, a packet size of 1-KB data, and an initial
2 × RTT of “handshaking” before data is sent.
(a) The bandwidth is 1.5 Mbps, and data packets can be sent continuously.
(b) The bandwidth is 1.5 Mbps, but after we finish sending each data packet
we must wait one RTT before sending the next.
(c) The bandwidth is “infinite,” meaning that we take transmit time to be
zero, and up to 20 packets can be sent per RTT.
(d) The bandwidth is infinite, and during the first RTT we can send one
packet (21−1 ), during the second RTT we can send two packets (22−1 ),
during the third we can send four (23−1 ), and so on. (A justification for
such an exponential increase will be given in Chapter 6.)
✓
6
Calculate the total time required to transfer a 1.5 MB file in the following cases,
assuming a RTT of 80 ms, a packet size of 1 KB data, and an initial 2×RTT of
“handshaking” before data is sent.
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(a) The bandwidth is 10 Mbps, and data packets can be sent continuously.
(b) The bandwidth is 10 Mbps, but after we finish sending each data packet
we must wait one RTT before sending the next.
(c) The link allows infinitely fast transmit, but limits bandwidth such that
only 20 packets can be sent per RTT.
(d) Zero transmit time as in (c), but during the first RTT we can send one
packet, during the second RTT we can send two packets, during the third
we can send four = 23−1 , and so on. (A justification for such an exponential increase will be given in Chapter 6.)
✓
7
Consider a point-to-point link 2 km in length. At what bandwidth would propagation delay (at a speed of 2 × 108 m/sec) equal transmit delay for 100-byte
packets? What about 512-byte packets?
8
Consider a point-to-point link 50 km in length. At what bandwidth would
propagation delay (at a speed of 2 × 108 m/sec) equal transmit delay for 100byte packets? What about 512-byte packets?
9
What properties of postal addresses would be likely to be shared by a network
addressing scheme? What differences might you expect to find? What properties of telephone numbering might be shared by a network addressing scheme?
10
One property of addresses is that they are unique; if two nodes had the same
address it would be impossible to distinguish between them. What other properties might be useful for network addresses to have? Can you think of any
situations in which network (or postal or telephone) addresses might not be
unique?
11
Give an example of a situation in which multicast addresses might be beneficial.
12
What differences in traffic patterns account for the fact that STDM is a costeffective form of multiplexing for a voice telephone network and FDM is a
cost-effective form of multiplexing for television and radio networks, yet we
reject both as not being cost-effective for a general-purpose computer network?
13
How “wide” is a bit on a 1-Gbps link? How long is a bit in copper wire, where
the speed of propagation is 2.3 × 108 m/s?
14
How long does it take to transmit x KB over a y-Mbps link? Give your answer
as a ratio of x and y.
Exercises
15
57
Suppose a 100-Mbps point-to-point link is being set up between Earth and
a new lunar colony. The distance from the moon to Earth is approximately
385,000 km, and data travels over the link at the speed of light—3 × 108 m/s.
(a) Calculate the minimum RTT for the link.
(b) Using the RTT as the delay, calculate the delay × bandwidth product for
the link.
(c) What is the significance of the delay × bandwidth product computed
in (b)?
(d) A camera on the lunar base takes pictures of Earth and saves them in digital
format to disk. Suppose Mission Control on Earth wishes to download the
most current image, which is 25 MB. What is the minimum amount of
time that will elapse between when the request for the data goes out and
the transfer is finished?
✓
16
Suppose a 128-Kbps point-to-point link is set up between Earth and a rover
on Mars. The distance from Earth to Mars (when they are closest together) is
approximately 55 Gm, and data travels over the link at the speed of light—
3 × 108 m/sec.
(a) Calculate the minimum RTT for the link.
(b) Calculate the delay × bandwidth product for the link.
(c) A camera on the rover takes pictures of its surroundings and sends these to
Earth. How quickly after a picture is taken can it reach Mission Control
on Earth? Assume that each image is 5 MB in size.
17
For each of the following operations on a remote file server, discuss whether
they are more likely to be delay sensitive or bandwidth sensitive.
(a) Open a file.
(b) Read the contents of a file.
(c) List the contents of a directory.
(d) Display the attributes of a file.
58
1
18
Foundation
Calculate the latency (from first bit sent to last bit received) for the following:
(a) A 10-Mbps Ethernet with a single store-and-forward switch in the path,
and a packet size of 5,000 bits. Assume that each link introduces a propagation delay of 10 µs, and that the switch begins retransmitting immediately
after it has finished receiving the packet.
(b) Same as (a) but with three switches.
(c) Same as (a) but assume the switch implements “cut-through” switching: it
is able to begin retransmitting the packet after the first 200 bits have been
received.
✓
19
Calculate the latency (from first bit sent to last bit received) for:
(a) A 1-Gbps Ethernet with a single store-and-forward switch in the path, and
a packet size of 5,000 bits. Assume that each link introduces a propagation
delay of 10 µs and that the switch begins retransmitting immediately after
it has finished receiving the packet.
(b) Same as (a) but with three switches.
(c) Same as (b) but assume the switch implements cut-through switching: it
is able to begin retransmitting the packet after the first 128 bits have been
received.
20
Calculate the effective bandwidth for the following cases. For (a) and (b) assume there is a steady supply of data to send; for (c) simply calculate the average
over 12 hours.
(a) A 10-Mbps Ethernet through three store-and-forward switches as in Exercise 18(b). Switches can send on one link while receiving on the other.
(b) Same as (a) but with the sender having to wait for a 50-byte acknowledgment packet after sending each 5,000-bit data packet.
(c) Overnight (12-hour) shipment of 100 compact discs (650 MB each).
21
Calculate the bandwidth × delay product for the following links. Use one-way
delay, measured from first bit sent to first bit received.
(a) A 10-Mbps Ethernet with a delay of 10 µs.
Exercises
Figure 1.24
59
Diagram for Exercise 22.
(b) A 10-Mbps Ethernet with a single store-and-forward switch like that of
Exercise 18(a), packet size 5,000 bits, and 10 µs per link propagation delay.
(c) A 1.5-Mbps T1 link, with a transcontinental one-way delay of 50 ms.
(d) A 1.5-Mbps T1 link through a satellite in geosynchronous orbit, 35,900km high. The only delay is speed-of-light propagation delay.
22
Hosts A and B are each connected to a switch S via 10-Mbps links as in Figure 1.24. The propagation delay on each link is 20 µs. S is a store-and-forward
device; it begins retransmitting a received packet 35 µs after it has finished
receiving it. Calculate the total time required to transmit 10,000 bits from
A to B.
(a) As a single packet.
(b) As two 5,000-bit packets sent one right after the other.
23
Suppose a host has a 1-MB file that is to be sent to another host. The file takes
1 second of CPU time to compress 50%, or 2 seconds to compress 60%.
(a) Calculate the bandwidth at which each compression option takes the same
total compression + transmission time.
(b) Explain why latency does not affect your answer.
24
Suppose that a certain communications protocol involves a per-packet overhead of 100 bytes for headers and framing. We send 1 million bytes of data
using this protocol; however, one data byte is corrupted and the entire packet
containing it is thus lost. Give the total number of overhead + loss bytes for
packet data sizes of 1,000, 5,000, 10,000, and 20,000 bytes. Which size is
optimal?
25
Assume you wish to transfer an n B file along a path composed of the source,
destination, seven point-to-point links, and five switches. Suppose each link
has a propagation delay of 2 ms, bandwidth of 4 Mbps, and that the switches
support both circuit and packet switching. Thus, you can either break the file
60
1
Foundation
up into 1-KB packets, or set up a circuit through the switches and send the file
as one contiguous bitstream. Suppose that packets have 24 B of packet header
information and 1,000 B of payload, that store-and-forward packet processing
at each switch incurs a 1-ms delay after the packet had been completely received, that packets may be sent continuously without waiting for acknowledgments, and that circuit setup requires a 1-KB message to make one round-trip
on the path incurring a 1-ms delay at each switch after the message has been
completely received. Assume switches introduce no delay to data traversing a
circuit. You may also assume that file size is a multiple of 1,000 B.
(a) For what file size n B is the total number of bytes sent across the network
less for circuits than for packets?
(b) For what file size n B is the total latency incurred before the entire file
arrives at the destination less for circuits than for packets?
(c) How sensitive are these results to the number of switches along the path?
To the bandwidth of the links? To the ratio of packet size to packet header
size?
(d) How accurate do you think this model of the relative merits of circuits and
packets is? Does it ignore important considerations that discredit one or
the other approach? If so, what are they?
26
Consider a network with a ring topology, link bandwidths of 100 Mbps, and
propagation speed 2 × 108 m/s. What would the circumference of the loop
be to exactly contain one 250-byte packet, assuming nodes do not introduce
delay? What would the circumference be if there was a node every 100 m, and
each node introduced 10 bits of delay?
27
Compare the channel requirements for voice traffic with the requirements for
the real-time transmission of music, in terms of bandwidth, delay, and jitter.
What would have to improve? By approximately how much? Could any channel requirements be relaxed?
28
For the following, assume that no data compression is done; this would in
practice almost never be the case. For (a)–(c), calculate the bandwidth necessary
for transmitting in real time:
(a) Video at a resolution of 640 × 480, 3 bytes/pixel, 30 frames/second.
(b) 160 × 120 video, 1 byte/pixel, 5 frames/second.
Exercises
61
(c) CD-ROM music, assuming one CD holds 75 minutes’ worth and takes
650 MB.
(d) Assume a fax transmits an 8 × 10-inch black-and-white image at a resolution of 72 pixels per inch. How long would this take over a 14.4-Kbps
modem?
✓
29
For the following, as in the previous problem, assume that no data compression
is done. Calculate the bandwidth necessary for transmitting in real time:
(a) HDTV high-definition video at a resolution of 1,920 × 1,080, 24
bits/pixel, 30 frames/sec.
(b) Plain old telephone service (POTS) voice audio of 8-bit samples at 8 KHz.
(c) GSM mobile voice audio of 260-bit samples at 50 Hz.
(d) HDCD high-definition audio of 24-bit samples at 88.2 kHz.
30
Discuss the relative performance needs of the following applications, in terms
of average bandwidth, peak bandwidth, latency, jitter, and loss tolerance:
(a) File server.
(b) Print server.
(c) Digital library.
(d) Routine monitoring of remote weather instruments.
(e) Voice.
(f ) Video monitoring of a waiting room.
(g) Television broadcasting.
★
31
Suppose a shared medium M offers to hosts A1 , A2 , . . . , AN in round-robin
fashion an opportunity to transmit one packet; hosts that have nothing to send
immediately relinquish M. How does this differ from STDM? How does network utilization of this scheme compare with STDM?
32
Consider a simple protocol for transferring files over a link. After some initial
negotiation, A sends data packets of size 1 KB to B; B then replies with an
acknowledgment. A always waits for each ACK before sending the next data
62
1
Foundation
packet; this is known as stop-and-wait. Packets that are overdue are presumed
lost and are retransmitted.
(a) In the absence of any packet losses or duplications, explain why it is not
necessary to include any “sequence number” data in the packet headers.
(b) Suppose that the link can lose occasional packets, but that packets that do
always arrive in the order sent. Is a 2-bit sequence number (that is, N mod
4) enough for A and B to detect and resend any lost packets? Is a 1-bit
sequence number enough?
(c) Now suppose that the link can deliver out of order, and that sometimes
a packet can be delivered as much as 1 minute after subsequent packets.
How does this change the sequence number requirements?
★
33
Suppose hosts A and B are connected by a link. Host A continuously transmits
the current time from a high-precision clock, at a regular rate, fast enough to
consume all the available bandwidth. Host B reads these time values and writes
them each paired with its own time from a local clock synchronized with A’s.
Give qualitative examples of B’s output assuming the link has
(a) High bandwidth, high latency, low jitter.
(b) Low bandwidth, high latency, high jitter.
(c) High bandwidth, low latency, low jitter, occasional lost data.
For example, a link with zero jitter, a bandwidth high enough to write on
every other clock tick, and a latency of 1 tick might yield something like
(0000, 0001), (0002, 0003), (0004, 0005).
34
Obtain and build the simplex-talk sample socket program shown in the text.
Start one server and one client in separate windows. While the first client is running, start 10 other clients that connect to the same server; these other clients
should most likely be started in the background with their input redirected
from a file. What happens to these 10 clients? Do their connect()s fail, or
time out, or succeed? Do any other calls block? Now let the first client exit.
What happens? Try this with the server value MAX_PENDING set to 1 as
well.
35
Modify the simplex-talk socket program so that each time the client sends a
line to the server, the server sends the line back to the client. The client (and
server) will now have to make alternating calls to recv() and send().
Exercises
63
36
Modify the simplex-talk socket program so that it uses UDP as the transport protocol, rather than TCP. You will have to change SOCK_STREAM to
SOCK_DGRAM in both client and server. Then, in the server, remove the
calls to listen() and accept(), and replace the two nested loops at the end
with a single loop that calls recv() with socket s. Finally, see what happens
when two such UDP clients simultaneously connect to the same UDP server,
and compare this to the TCP behavior.
37
Investigate the different options and parameters one can set for a TCP connection. (Do “man tcp” on Unix.) Experiment with various parameter settings
to see how they effect TCP performance.
38
The Unix utility ping can be used to find the RTT to various Internet hosts. Read the man page for ping, and use it to find the RTT to
www.cs.princeton.edu in New Jersey and www.cisco.com in California. Measure the RTT values at different times of day, and compare the results.
What do you think accounts for the differences?
39
The Unix utility traceroute, or its Windows equivalent tracert, can be used
to find the sequence of routers through which a message is routed. Use this to
find the path from your site to some others. How well does the number of hops
correlate with the RTT times from ping? How well does the number of hops
correlate with geographical distance?
40
Use traceroute, above, to map out some of the routers within your organization (or to verify none are used).
Direct Link Networks
It is a mistake to look too far ahead. Only one link in the chain of
destiny can be handled at a time.
—Winston Churchill
T
he simplest network possible is one in which two hosts are directly connected
by some physical medium. The medium be a length of wire, a piece of optical
fiber, or a medium (such as air or even free space) through which electromagnetic
radiation (e.g., radio waves) can be transmitted. It may cover a small area (e.g., an office
building) or a wide area (e.g., transcontinental). Connecting two or more nodes with a
suitable medium is only the first step, however. There are five additional problems that
must be addressed before the nodes can successfully exchange packets.
The first is encoding bits onto
the
transmission
medium so that they
P R O B L E M
can be understood by a receiving host.
Physically Connecting
Second is the matter of delineating the
Hosts
sequence of bits transmitted over the
link into complete messages that can
be delivered to the end node. This is
called the framing problem, and the messages delivered to the end hosts are often called
frames. Third, because frames are sometimes corrupted during transmission, it is necessary to detect these errors and take the appropriate action; this is the error detection
problem. The fourth issue is making a link appear reliable in spite of the fact that it
corrupts frames from time to time. Finally, in those cases where the link is shared by
multiple hosts—as opposed to a simple point-to-point link—it is necessary to mediate
access to this link. This is the media access control problem.
Although these five issues—encoding, framing, error detection, reliable delivery,
and access mediation—can be discussed in the abstract, they are very real problems that
are addressed in different ways by different networking technologies. This chapter considers these issues in the context of four specific network technologies: point-to-point
links, carrier sense multiple access (CSMA) networks (of which Ethernet is the most
64
famous example), token rings (of which IEEE Standard 802.5 and
FDDI are the most famous examples), and wireless networks (for
which 802.11 is the most widespread standard1 ). The goal of this
chapter is simultaneously to survey the available network technology and to explore these five fundamental issues.
Before tackling the specific issues of connecting hosts, this
chapter begins by examining the building blocks that will be used:
nodes and links. We then explore the first three issues—encoding,
framing, and error detection—in the context of a simple point-topoint link. The techniques introduced in these three sections are
general and therefore apply equally well to multiple-access networks. The problem of reliable delivery is considered next. Since
link-level reliability is usually not implemented in shared-access
networks, this discussion focuses on point-to-point links only.
Finally, we address the media access problem in the context of
CSMA, token rings, and wireless.
1 Strictly speaking, 802.11 is a set of standards.
2
66
2.1
2
Direct Link Networks
Hardware Building Blocks
As we saw in Chapter 1, networks are constructed from two classes of hardware building
blocks: nodes and links. This statement is just as true for the simplest possible network—
one in which a single point-to-point link connects a pair of nodes—as it is for a worldwide internet. This section gives a brief overview of what we mean by nodes and links
and, in so doing, defines the underlying technology that we will assume throughout the
rest of this book.
2.1.1 Nodes
Nodes are often general-purpose computers, like a desktop workstation, a multiprocessor,
or a PC. For our purposes, let’s assume it’s a workstation-class machine. This workstation
can serve as a host that users run application programs on, it might be used inside the
network as a switch that forwards messages from one link to another, or it might be
configured as a router that forwards internet packets from one network to another. In
some cases, a network node—most commonly a switch or router inside the network,
rather than a host—is implemented by special-purpose hardware. This is usually done
for reasons of performance and cost: It is generally possible to build custom hardware that
performs a particular function faster and cheaper than a general-purpose processor can
perform it. When this happens, we will first describe the basic function being performed
by the node as though this function is being implemented in software on a generalpurpose workstation, and then explain why and how this functionality might instead be
implemented by special hardware.
Although we could leave it at that, it is useful to know a little bit about what a
workstation looks like on the inside. This information becomes particularly important
when we become concerned about how well the network performs. Figure 2.1 gives a
simple block diagram of the workstation-class machine we assume throughout this book.
Two aspects of the memory component are important to note. First, the memory
on any given machine is finite. It may be 64 MB or it may be 1 GB, but it is not infinite.
As pointed out in Section 1.2.2, this is important because memory turns out to be one
of the two scarce resources in the network (the other is link bandwidth) that must be
carefully managed if we are to provide a fair amount of network capacity to each user.
Memory is a scarce resource because, on any node that forwards packets, those packets
must be buffered in memory while waiting their turn to be transmitted over an outgoing
link.
Second, while CPUs are becoming faster at an unbelievable pace, the same is
not true of memory. Recent performance trends show processor speeds doubling every
18 months, but memory latency improving at a rate of only 7% each year. The relevance
of this difference is that as a network node, a workstation runs at memory speeds, not
processor speeds, to a first approximation. This means that the network software needs
2.1
Figure 2.1
Hardware Building Blocks
67
Example workstation architecture.
to be careful about how it uses memory and, in particular, about how many times it
accesses memory as it processes each message. We do not have the luxury of being sloppy
just because processors are becoming infinitely fast.
The workstation’s network adaptor component connects the rest of the workstation
to the link. More than just a physical connection, it is an active intermediary between
node and link, with its own internal processor. Its role is to transmit data from the
workstation onto the link, and receive data from the link, storing it for the workstation.
The adaptor implements nearly all the networking functionality, to be discussed in the
course of this chapter, that makes it possible to convey data over a dumb wire (or radio
airwaves) between adaptors. For example, adaptors break data into frames that the link
can transport, detect errors introduced as a frame travels over the link, and follow fairness
rules that allow a link to be shared by multiple workstations.
A network adaptor can be thought of as having two main components: a bus interface that understands how to communicate with the host and a link interface that understands how to use the link. There must also be a communication path between these two
components, over which incoming and outgoing data is passed. A simple block diagram
of a network adaptor is depicted in Figure 2.2.
Different kinds of links require network adaptors with very different link interfaces.
In this chapter we will see the tasks performed by the link interface for a variety of link
technologies. Different I/O buses likewise require different adaptors. From the perspective of the host, however, bus interfaces tend to be similar to each other. Typically, the
adaptor exports a control status register (CSR) that is readable and writable from the CPU.
The CSR is typically located at some address in the memory, thereby making it possible
68
2
Figure 2.2
Direct Link Networks
Block diagram of a typical network adaptor.
for the CPU to read and write just like any other memory location. Software on the
host—a device driver—writes to the CSR to instruct it to transmit and/or receive data
and reads from the CSR to learn the current state of the adaptor. To notify the host of
an asynchronous event such as the reception of a frame, the adaptor interrupts the host.
One of the most important issues in network adaptor design is how bytes of data are
transferred between the adaptor and the host memory. There are two basic mechanisms:
direct memory access (DMA) and programmed I/O (PIO). With DMA, the adaptor directly
reads and writes the host’s memory without any CPU involvement; the host simply gives
the adaptor a memory address and the adaptor reads from (or writes to) it. With PIO, the
CPU is directly responsible for moving data between the adaptor and the host memory:
To send a frame, the CPU executes a tight loop that first reads a word from host memory
and then writes it to the adaptor; to reFrames, Buffers, and Messages
ceive a frame, the CPU reads words from
the adaptor and writes them to memory.
As this section has suggested, the network adaptor is the place where the
As noted earlier, host memory pernetwork comes in physical contact
formance is often the limiting factor in
with the host. It also happens to be
network performance. Nowhere is this
the place where three different worlds
possibility more critical than at the host/
intersect: the network, the host archiadaptor interface. To help drive this point
tecture, and the host operating syshome, consider Figure 2.3. This diagram
tem. It turns out that each of these
shows the bandwidth available between
has a different terminology for talking
various components of a modern PC.
about the same thing. It is important
While the I/O bus is fast enough to transto recognize when this is happening.
fer frames between the network adaptor
From the network’s perspective,
and host memory at gigabit rates, there are
the
adaptor
transmits frames from the
two potential problems.
2.1
Figure 2.3
Hardware Building Blocks
69
Memory bandwidth on a modern PC-class machine.
The first is that the advertised I/O bus speed corresponds to its peak bandwidth;
it is the product of the bus’s width and clock speed (e.g., a 64-bit-wide bus running at
133 MHz has a peak transfer rate of 8,512 Mbps). The real limitation is the size of the
data block that is being transferred across the I/O bus, since there is a certain amount
of overhead involved in each bus transfer. On some architectures, for example, it takes
8 clock cycles to acquire the bus for the purpose of transferring data from the adaptor
to host memory. This overhead is independent of the number of data bytes transferred.
Thus, if you want to transfer a 64-byte payload across the I/O bus—this happens to
be the size of a minimum Ethernet packet—then the whole transfer takes 16 cycles:
8 cycles to acquire the bus and 8 cycles to transfer the data. (The bus is 64 bits wide,
which means that it can transfer 8 bytes
during each clock cycle; 64 bytes divided
host and receives frames into the host.
by 8 bytes per cycle equals 8 cycles.) This
From the perspective of the host armeans that the maximum sustained bandchitecture, each frame is received into
width you can achieve for such packets is
or transmitted from a buffer, which
only half the peak (i.e., 4,256 Mbps).
is simply a region of main memThe second problem is that the
ory of some length and starting at
memory/CPU bandwidth, which in this
some address. Finally, from the opexample is 3,200 MBps (25.6 Gbps), is
erating system’s perspective, a mesthe same order of magnitude as the bandsage is an abstract object that holds
width of the I/O bus. Fortunately, this is
network frames. Messages are implea measured number rather than an advermented by a data structure that intised peak rate. The ramification is that
cludes pointers to different memory
while it is possible to deliver frames across
locations (buffers). We saw an exthe I/O bus and into memory and then to
ample of a message data structure in
load the data from memory into the CPU’s
Chapter 1.
registers at network bandwidths, it is impractical for the device driver, operating
70
2
Direct Link Networks
▲
system, and application to go to memory multiple times for each word of data in a network packet, possibly because it needs to copy the data from one buffer to another. In
particular, if the memory/CPU path is crossed n times, then it might be the case that the
bandwidth your application sees is 3,200/n MBps. (The performance might be better
if the data is cached, but often caches don’t help with data arriving from the network.)
For example, if the various software layers need to copy the message from one buffer
to another four times—not an uncommon situation—then the application might see a
throughput of 800 MBps (6,400 Mbps), less than the 8,512 Mbps that the I/O bus can
support.
As an aside, it is important to recognize that there are many parallels between
moving a message to and from memory and moving a message across a network. In
particular, the effective throughput of the memory system is defined by the same two
formulas given in Section 1.5.
Throughput = TransferSize/TransferTime
TransferTime = RTT + 1/Bandwidth × TransferSize
In the case of the memory system, however, the transfer size corresponds to how big
a unit of data we can move across the bus in one transfer (i.e., cache line versus small
cells versus large message), and the RTT corresponds to the memory latency, that is,
whether the memory is on-chip cache, off-chip cache, or main memory. Just as in the
case of the network, the larger the transfer size and the smaller the latency, the better
the effective throughput. Also similar to a network, the effective memory throughput
does not necessarily equal the peak memory bandwidth (i.e., the bandwidth that can be
achieved with an infinitely large transfer).
The main point of this discussion is that we must be aware of the limits memory
bandwidth places on network performance. If carefully designed, the system can work
around these limits. For example, it is possible to integrate the buffers used by the device
driver, the operating system, and the application in a way that minimizes data copies.
The system also needs to be aware of when data is brought into cache, so it can perform
all necessary operations on the data before it gets bumped from the cache. The details of
how this is accomplished are beyond the scope of this book, but can be found in papers
referenced at the end of the chapter.
Finally, there is a second important lesson lurking in this discussion: when the
network isn’t performing as well as you think it should, it’s not always the network’s fault.
In many cases, the actual bottleneck in the system is one of the machines connected to
the network. For example, when it takes a long time for a web page to appear on your
browser, it might be network congestion, but it’s just as likely the case that the server at
2.1
Hardware Building Blocks
71
the other end of the network—which may be trying to serve many users at the same time
as you—can’t keep up with the workload.
2.1.2 Links
Network links are implemented on a variety of different physical media, including
twisted pair (the wire that your phone connects to), coaxial cable (the wire that your
TV connects to), optical fiber (the medium most commonly used for high-bandwidth,
long-distance links), and space (the stuff that radio waves, microwaves, and infrared
beams propagate through). Whatever the physical medium, it is used to propagate signals. These signals are actually electromagnetic waves traveling at the speed of light. (The
speed of light is, however, medium dependent—electromagnetic waves traveling through
copper and fiber do so at about two-thirds the speed of light in a vacuum.)
One important property of an electromagnetic wave is the frequency, measured in
hertz, with which the wave oscillates. The distance between a pair of adjacent maxima
or minima of a wave, typically measured in meters, is called the wave’s wavelength. Since
all electromagnetic waves travel at the speed of light, that speed divided by the wave’s
frequency is equal to its wavelength. We have already seen the example of a voice-grade
telephone line, which carries continuous electromagnetic signals ranging between 300
and 3,300 Hz; a 300-Hz wave traveling through copper would have a wavelength of
SpeedOfLightInCopper ÷ Frequency
= 2/3 × 3 × 108 ÷ 300
= 667 × 103 meters
Generally, electromagnetic waves span a much wider range of frequencies, ranging from
radio waves, to infrared light, to visible light, to X-rays and gamma rays. Figure 2.4
depicts the electromagnetic spectrum and shows which media are commonly used to
carry which frequency bands.
So far we understand a link to be a physical medium carrying signals in the form
of electromagnetic waves. Such links provide the foundation for transmitting all sorts of
information, including the kind of data we are interested in transmitting—binary data
(1s and 0s). We say that the binary data is encoded in the signal. The problem of encoding
binary data onto electromagnetic signals is a complex topic. To help make the topic
more manageable, we can think of it as being divided into two layers. The lower layer
is concerned with modulation—varying the frequency, amplitude, or phase of the signal
to effect the transmission of information. A simple example of modulation is to vary the
power (amplitude) of a single wavelength. Intuitively, this is equivalent to turning a light
on and off. Because the issue of modulation is secondary to our discussion of links as a
72
2
Figure 2.4
Direct Link Networks
Electromagnetic spectrum.
building block for computer networks, we simply assume that it is possible to transmit a
pair of distinguishable signals—think of them as a “high” signal and a “low” signal—and
we consider only the upper layer, which is concerned with the much simpler problem of
encoding binary data onto these two signals. Section 2.2 discusses such encodings.
Another attribute of a link is how many bitstreams can be encoded on it at a
given time. If the answer is only one, then the nodes connected to the link must share
access to the link. This is the case for the multiple-access links described in Sections 2.6
and 2.7. For point-to-point links, however, it is often the case that two bitstreams can be
simultaneously transmitted over the link at the same time, one going in each direction.
Such a link is said to be full-duplex. A point-to-point link that supports data flowing in
only one direction at a time—such a link is called half-duplex—requires that the two
nodes connected to the link alternate using it. For the purposes of this book, we assume
that all point-to-point links are full-duplex.
The only other property of a link that we are interested in at this stage is a very
pragmatic one—how do you go about getting one? The answer depends on how far the
link needs to reach, how much money you have to spend, and whether or not you know
how to operate earth-moving equipment. The following is a survey of different link types
you might use to build a computer network.
Cables
If the nodes you want to connect are in the same room, in the same building, or even
on the same site (e.g., a campus), then you can buy a piece of cable and physically string
it between the nodes. Exactly what type of cable you choose to install depends on the
technology you plan to use to transmit data over the link; we’ll see several examples later
in this chapter. For now, a list of the common cable (fiber) types is given in Table 2.1.
2.1
Hardware Building Blocks
Cable
Typical Bandwidths
Distances
Category 5 twisted pair
10–100 Mbps
100m
Thin-net coax
10–100 Mbps
200m
Thick-net coax
10–100 Mbps
500m
Multimode fiber
100 Mbps
2 km
Single-mode fiber
0.1–10 Gbps
40 km
Table 2.1
Common types of cables and fibers available for local links.
Service
Bandwidth
DS1
1.544 Mbps
DS3
44.736 Mbps
STS-1
51.840 Mbps
STS-3
155.250 Mbps
STS-12
622.080 Mbps
STS-24
1.244160 Gbps
STS-48
2.488320 Gbps
Table 2.2
73
Common bandwidths available from the carriers.
Of these, Category 5 (Cat-5) twisted pair—it uses a thicker gauge than the twisted
pair you find in your home—is quickly becoming the within-building norm. Because
of the difficulty and cost in pulling new cable through a building, every effort is made
to make new technologies use existing cable; Gigabit Ethernet, for example, has been
designed to run over Cat-5 wiring. Fiber is typically used to connect buildings at a site.
Leased Lines
If the two nodes you want to connect are on opposite sides of the country, or even across
town, then it is not practical to install the link yourself. Until recently, your only option
was to lease a dedicated link from the telephone company. Table 2.2 gives the common
“leased line” services that can be obtained from the average phone company. Again, more
details are given throughout this chapter.
While these bandwidths appear somewhat arbitrary, there is actually some method
to the madness. DS1 and DS3 (they are also sometimes called T1 and T3, respectively)
are relatively old technologies that were originally defined for copper-based transmission
74
2
Direct Link Networks
media. DS1 is equal to the aggregation of 24 digital voice circuits of 64 Kbps each, and
DS3 is equal to 28 DS1 links. All the STS-N links are for optical fiber (STS stands
for Synchronous Transport Signal). STS-1 is the base link speed, and each STS-N has
N times the bandwidth of STS-1. An STS-N link is also sometimes called an OC-N
link (OC stands for optical carrier). The difference between STS and OC is subtle: The
former refers to the electrical transmission on the devices connected to the link, and the
latter refers to the actual optical signal that is propagated over the fiber.
More recently, many providers, both traditional telephone companies and some
upstart competitors, have started to offer a range of alternatives to leased lines based on
Ethernet, a technology we will discuss in detail in Section 2.6. One consequence of this
development is a proliferation of different access speeds beyond those in Table 2.2.
Keep in mind that the phone company does not implement the “link” we just
ordered as a single, unbroken piece of cable or fiber. Instead, it implements the link on
its own network. Although the telephone network has historically looked much different
from the kind of network described in this book—it was built primarily to provide a
voice service and used circuit-switching technology—the current trend is toward the
style of packet-switched networking described in this book. This is not surprising—the
potential market for carrying data, voice, and video on one packet-switched network is
huge.
In any case, whether the link is physical or a logical connection through the telephone network, the problem of building a computer network on top of a collection of
such links remains the same. So, we will proceed as though each link is implemented by
a single cable/fiber, and only when we are done will we worry about whether we have just
built a computer network on top of the underlying telephone network, or the computer
network we have just built could itself serve as the backbone for the telephone network.
Last-Mile Links
If you can’t afford a dedicated leased line—they range in price from several hundred dollars a month for a DS1 link across the United States to “if you have to ask, you can’t
afford it”—then there are less expensive options available. We call these “last-mile” links
because they often span the last mile from the home to a network service provider. These
services, which are summarized in Table 2.3, typically connect a home to an existing network. This means they are probably not suitable for use in building a complete network
from scratch, but if you’ve already succeeded in building a network—and “you” happen
to be either the telephone company or the cable company—then you can use these links
to reach millions of customers.
The first option is a conventional modem over POTS. Today it is possible to buy
a modem that transmits data at 56 Kbps over a standard voice-grade line for less than
a hundred dollars. The technology is already at its bandwidth limit, however, which
2.1
Service
Bandwidth
POTS
28.8–56 Kbps
ISDN
64–128 Kbps
xDSL
128 Kbps–100 Mbps
CATV
1–40 Mbps
Table 2.3
Hardware Building Blocks
75
Common services available to connect your home.
led to the development of the second option: Integrated Services Digital Network
(ISDN). An ISDN connection includes two 64-Kbps channels, one that can be used
to transmit data and another that can be used for digitized voice. (A device that encodes analog voice into a digital ISDN link is called a CODEC, for coder/decoder.)
When the voice channel is not in use, it can be combined with the data channel to
support up to 128 Kbps of data bandShannon’s Theorem Meets
width.
Your Modem
For many years ISDN was viewed
as
the
future for modest bandwidth into
There has been an enormous body of
the home. ISDN, however, has now been
work done in the related areas of siglargely overtaken by two newer technal processing and information thenologies: digital subscriber line (xDSL)
ory, studying everything from how
and cable modems. The former is acsignals degrade over distance to how
tually a collection of technologies that
much data a given signal can effecare able to transmit data at high speeds
tively carry. The most notable piece of
over the standard twisted pair lines that
work in this area is a formula known
currently come into most homes in the
as Shannon’s theorem. Simply stated,
Shannon’s theorem gives an upper
United States (and many other places).
bound to the capacity of a link, in
The one in most widespread use toterms of bits per second (bps), as a
day is asymmetric digital subscriber line
function of the signal-to-noise ratio of
(ADSL). As its name implies, ADSL prothe link, measured in decibels (dB).
vides a different bandwidth from the
Shannon’s theorem can be used
subscriber to the telephone company’s
to determine the data rate at which a
central office (upstream) than it does
modem can be expected to transmit
from the central office to the subscriber
binary data over a voice-grade phone
(downstream). The exact bandwidth deline without suffering from too high
pends on the length of the line running
an error rate. For example, we assume
from the subscriber to the central office.
that a voice-grade phone connection
This line is called the local loop, as illustrated in Figure 2.5, and runs over
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2
Direct Link Networks
existing copper. Downstream bandwidths
range from 1.544 Mbps (18,000 feet) to
8.448 Mbps (9,000 feet), while upstream
bandwidths range from 16 to 640 Kbps.
An alternative technology that has
yet to be widely deployed—very high
data rate digital subscriber line (VDSL)—
is symmetric, with data rates ranging
from 12.96 to 55.2 Mbps. VDSL runs
over much shorter distances—1,000 to
4,500 feet—which means that it will not
typically reach from the home to the central office. Instead, the telephone company
would have to put VDSL transmission
hardware in neighborhoods, with some
other technology (e.g., STS-N running
over fiber) connecting the neighborhood
to the central office, as illustrated in Figure 2.6. This is sometimes called “fiber to
the neighborhood” (contrasting with more
ambitious schemes such as “fiber to the
home” and “fiber to the curb”).
Cable modems are an alternative to
the various types of DSL. As the name suggests, this technology uses the cable TV
(CATV) infrastructure, which currently
reaches 95% of the households in the
United States. (Only 65% of U.S. homes
actually subscribe.) In this approach, some
subset of the available CATV channels
are made available for transmitting digital data, where a single CATV channel
has a bandwidth of 6 MHz. CATV, like
ADSL, is used in an asymmetric way, with
downstream rates much greater than upstream rates. The technology is currently
able to achieve 40 Mbps downstream on a
single CATV channel, with 100 Mbps as
the theoretical capacity. The upstream rate
supports a frequency range of 300 to
3,300 Hz.
Shannon’s theorem is typically
given by the following formula:
C = B log2 (1 + S/N )
where C is the achievable channel capacity measured in hertz, B is the
bandwidth of the line (3,300 Hz −
300 Hz = 3,000 Hz), S is the average signal power, and N is the average
noise power. The signal-to-noise ratio
(S/N ) is usually expressed in decibels,
related as follows:
dB = 10 × log10 (S/N )
Assuming a typical decibel ratio of
30 dB, this means that S/N = 1,000.
Thus, we have
C = 3,000 × log2 (1001)
which equals approximately 30 Kbps,
roughly the limit of a 28.8-Kbps
modem.
Given this fundamental limit,
why is it possible to buy 56-Kbps
modems at any electronics store? One
reason is that such rates depend on improved line quality, that is, a higher
signal-to-noise ratio than 30 dB. Another reason is that changes within the
phone system have largely eliminated
analog lines that are bandwidth limited to 3,300 Hz.
2.1
Figure 2.5
Hardware Building Blocks
77
ADSL connects the subscriber to the central office via the local loop.
Figure 2.6 VDSL connects the subscriber to the optical network that reaches the
neighborhood.
is roughly half the downstream rate (i.e., 20 Mbps) due to a 1,000-fold decrease in the
signal-to-noise ratio. It is also the case that fewer CATV channels are dedicated to upstream traffic than to downstream traffic. Unlike DSL, the bandwidth is shared among
all subscribers in a neighborhood (a fact that led to some amusing advertising from DSL
providers). This means that some method for arbitrating access to the shared medium—
similar to the 802 standards described later in this chapter—needs to be used. Finally,
like DSL, it is unlikely that cable modems will be used to connect arbitrary node A at
one site to arbitrary node B at some other site. Instead, cable modems are seen as a means
to connect node A in your home to the cable company, with the cable company then
defining what the rest of the network looks like.
Wireless Links
Up to this point we have discussed links that channel signals through a physical medium
like a wire or optical fiber. Wireless links transmit electromagnetic signals—radio, microwave, infrared, or even visible light—through space, even through vacuum. Wireless
communication is used not only by computer networks, of course. The ability to exchange signals without physical connectivity is what makes mobile communication devices possible. The further ability to broadcast that signal, and the fact that the hardware
and power burden is primarily on the transmitting end, makes wireless communication
well-suited to television and radio broadcasting.
Because wireless links all share the same wire, so to speak, the challenge is to share
it efficiently, without unduly interfering with each other. Most of this sharing is accomplished by dividing the “wire” along the dimensions of frequency and space. Exclusive use
of a particular frequency in a particular geographic area may be allocated to an individual
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2
Direct Link Networks
entity such as a corporation. It is feasible to limit the area covered by an electromagnetic
signal because such signals weaken, or attenuate, with the distance from their origin. To
reduce the area covered by your signal, reduce the power of your transmitter.
These allocations are determined by government agencies, such as the Federal
Communications Commission (FCC) in the United States. Specific bands (frequency
ranges) are allocated to certain uses. Some bands are reserved for government use. Other
bands are reserved for uses such as AM radio, FM radio, television, satellite communication, and cell phones. Specific frequencies within these bands are then licensed to
individual organizations for use within certain geographical areas. Finally, there are several frequency bands set aside for “license-exempt” usage—bands in which a license is
not needed.
Devices that use license-exempt frequencies are still subject to certain restrictions
to make that otherwise unconstrained sharing work. The first is a limit on transmission
power. This limits the range of a signal, making it less likely to interfere with another
signal. For example, a cordless phone might have a range of about 100 feet.
The second restriction requires the use of spread spectrum techniques. The idea
behind spread spectrum is to spread the signal over a wider frequency band than normal in such a way as to minimize the impact of interference from other devices. (Spread
spectrum was originally designed for military use, so these “other devices” were often
attempting to jam the signal.) For example, frequency hopping is a spread spectrum technique that involves transmitting the signal over a random sequence of frequencies, that
is, first transmitting at one frequency, then a second, then a third, and so on. The sequence of frequencies is not truly random, but is instead computed algorithmically by a
pseudorandom number generator. The receiver uses the same algorithm as the sender—
and initializes it with the same seed—and hence is able to hop frequencies in sync with
the transmitter to correctly receive the frame. This scheme reduces interference by making it unlikely that two signals would be using the same frequency for more than the
infrequent isolated bit.
A second spread spectrum technique, called direct sequence, adds redundancy for
greater tolerance of interference. Each bit of data is represented by multiple bits in the
transmitted signal so that, if some of the transmitted bits are damaged by interference,
there is usually enough redundancy to recover the original bit. For each bit the sender
wants to transmit, it actually sends the exclusive-OR of that bit and n random bits. As
with frequency hopping, the sequence of random bits is generated by a pseudorandom
number generator known to both the sender and the receiver. The transmitted values,
known as an n-bit chipping code, spread the signal across a frequency band that is n times
wider than the frame would have otherwise required. Figure 2.7 gives an example of a
4-bit chipping sequence.
2.2
Figure 2.7
Encoding (NRZ, NRZI, Manchester, 4B/5B)
79
Example 4-bit chipping sequence.
It is these license-exempt frequencies, with their spread spectrum techniques and
limited range, that are used by 802.11 (Wi-Fi), 802.15.1 (Bluetooth), and some flavors
of 802.16 (WiMAX). These technologies are discussed further in Section 2.8.
Different parts of the electromagnetic spectrum have different properties, making
some better suited to communication, and some less so. For example, some can penetrate
buildings and some cannot. Governments regulate only the prime communication portion: the radio and microwave ranges. As demand for prime spectrum increases, there is
great interest in the spectrum that will become available when analog television is phased
out in favor of digital. There is also an effort to devise a wireless network technology that
could squeeze into the currently unused frequencies that separate television channels
without interfering with them.
Among the unregulated spectra are the infrared and visual light ranges, which cannot penetrate walls. Infrared is widely used in remote controls for television and the like.
Increasingly, it is also used in a variety of short-range data exchange applications, based
on standards established by the Infrared Data Association (IrDA). For example, IrDA has
defined a standard for “Point & Pay,” using infrared to conduct a financial transaction
between a handheld device, such as a mobile phone or PDA, and a stationary financial
terminal such as a cash register. Visual light or infrared can also be focused by a laser to
provide a high-bandwidth link between two stationary points—even though no mobility
is involved—in situations where a wired link is less practical for some reason. For example, two buildings belonging to the same organization but separated by a busy highway
could communicate using lasers.
2.2
Encoding (NRZ, NRZI, Manchester, 4B/5B)
The first step in turning nodes and links into usable building blocks is to understand how
to connect them in such a way that bits can be transmitted from one node to another.
As mentioned in the preceding section, signals propagate over physical links. The task,
80
2
Direct Link Networks
therefore, is to encode the binary data that the source node wants to send into the signals
that the links are able to carry, and then to decode the signal back into the corresponding
binary data at the receiving node. We ignore the details of modulation and assume we
are working with two discrete signals: high and low. In practice, these signals might
correspond to two different voltages on a copper-based link, or two different power
levels on an optical link.
As we have said, most of the functions discussed in this chapter are performed by
a network adaptor—a piece of hardware that connects a node to a link. The network
adaptor contains a signalling component that actually encodes bits into signals at the
sending node and decodes signals into bits at the receiving node. Thus, as illustrated in
Figure 2.8, signals travel over a link between two signalling components, and bits flow
between network adaptors.
Let’s return to the problem of encoding bits onto signals. The obvious thing to do is
to map the data value 1 onto the high signal and the data value 0 onto the low signal. This
is exactly the mapping used by an encoding scheme called, cryptically enough, nonreturn
to zero (NRZ). For example, Figure 2.9 schematically depicts the NRZ-encoded signal
(bottom) that corresponds to the transmission of a particular sequence of bits (top).
The problem with NRZ is that a sequence of several consecutive 1s means that the
signal stays high on the link for an extended period of time, and similarly, several consecutive 0s means that the signal stays low for a long time. There are two fundamental
problems caused by long strings of 1s or 0s. The first is that it leads to a situation known
as baseline wander. Specifically, the receiver keeps an average of the signal it has seen so
far, and then uses this average to distinguish between low and high signals. Whenever the
signal is significantly lower than this averBit Rates and Baud Rates
age, the receiver concludes that it has just
seen a 0, and likewise, a signal that is sigMany people use the terms bit rate
nificantly higher than the average is interand baud rate interchangeably, even
preted to be a 1. The problem, of course,
though as we see with the Manchester
is that too many consecutive 1s or 0s cause
encoding, they are not the same thing.
this average to change, making it more difWhile the Manchester encoding is an
ficult to detect a significant change in the
example of a case in which a link’s
signal.
baud rate is greater than its bit rate, it
The second problem is that frequent
is also possible to have a bit rate that is
transitions from high to low and vice versa
greater than the baud rate. This would
are necessary to enable clock recovery. Inimply that more than one bit is entuitively, the clock recovery problem is
coded on each pulse sent over the link.
that both the encoding and the decodTo see how this might happen,
ing processes are driven by a clock—every
suppose you could transmit four
clock cycle the sender transmits a bit and
2.2
Encoding (NRZ, NRZI, Manchester, 4B/5B)
81
Figure 2.8
Signals travel between signalling components; bits flow between adaptors.
Figure 2.9
NRZ encoding of a bitstream.
the receiver recovers a bit. The sender’s and the receiver’s clocks have to be precisely
synchronized in order for the receiver to recover the same bits the sender transmits.
If the receiver’s clock is even slightly faster or slower than the sender’s clock, then it
does not correctly decode the signal. You could imagine sending the clock to the receiver over a separate wire, but this is typically avoided because it makes the cost of
cabling twice as high. So instead, the receiver derives the clock from the received
distinguished signals over a link rather
signal—the clock recovery process. Whenthan just two. On an analog link, for
ever the signal changes, such as on a tranexample, these four signals might corsition from 1 to 0 or from 0 to 1, then the
respond to four different frequencies.
receiver knows it is at a clock cycle boundGiven four different signals, it is possiary, and it can resynchronize itself. Howble to encode two bits of information
ever, a long period of time without such a
on each signal. That is, the first signal
transition leads to clock drift. Thus, clock
means 00, the second signal means
recovery depends on having lots of transi01, and so on. Now, a sender (retions in the signal, no matter what data is
ceiver) that is able to transmit (detect)
being sent.
1,000 pulses per second would be able
One approach that addresses this
to send (receive) 2,000 bits of inforproblem, called nonreturn to zero inverted
mation per second. That is, it would
(NRZI), has the sender make a transition
be a 1,000-baud/2,000-bps link.
from the current signal to encode a 1 and
stay at the current signal to encode a 0.
82
2
Figure 2.10
Direct Link Networks
Different encoding strategies.
This solves the problem of consecutive 1s, but obviously does nothing for consecutive
0s. NRZI is illustrated in Figure 2.10. An alternative, called Manchester encoding, does
a more explicit job of merging the clock with the signal by transmitting the exclusiveOR of the NRZ-encoded data and the clock. (Think of the local clock as an internal
signal that alternates from low to high; a low/high pair is considered one clock cycle.)
The Manchester encoding is also illustrated in Figure 2.10. Observe that the Manchester
encoding results in 0 being encoded as a low-to-high transition and 1 being encoded
as a high-to-low transition. Because both 0s and 1s result in a transition to the signal, the clock can be effectively recovered at the receiver. (There is also a variant of
the Manchester encoding, called differential Manchester, in which a 1 is encoded with
the first half of the signal equal to the last half of the previous bit’s signal and a 0 is
encoded with the first half of the signal opposite to the last half of the previous bit’s
signal.)
The problem with the Manchester encoding scheme is that it doubles the rate at
which signal transitions are made on the link, which means that the receiver has half the
time to detect each pulse of the signal. The rate at which the signal changes is called the
link’s baud rate. In the case of the Manchester encoding, the bit rate is half the baud rate,
so the encoding is considered only 50% efficient. Keep in mind that if the receiver had
been able to keep up with the faster baud rate required by the Manchester encoding in
Figure 2.10, then both NRZ and NRZI could have been able to transmit twice as many
bits in the same time period.
A final encoding that we consider, called 4B/5B, attempts to address the inefficiency
of the Manchester encoding without suffering from the problem of having extended durations of high or low signals. The idea of 4B/5B is to insert extra bits into the bitstream
so as to break up long sequences of 0s or 1s. Specifically, every 4 bits of actual data are
encoded in a 5-bit code that is then transmitted to the receiver; hence the name 4B/5B.
The 5-bit codes are selected in such a way that each one has no more than one leading 0
2.2
4-Bit Data Symbol
5-Bit Code
0000
11110
0001
01001
0010
10100
0011
10101
0100
01010
0101
01011
0110
01110
0111
01111
1000
10010
1001
10011
1010
10110
1011
10111
1100
11010
1101
11011
1110
11100
1111
11101
Table 2.4
Encoding (NRZ, NRZI, Manchester, 4B/5B)
83
4B/5B encoding.
and no more than two trailing 0s. Thus, when sent back-to-back, no pair of 5-bit codes
results in more than three consecutive 0s being transmitted. The resulting 5-bit codes are
then transmitted using the NRZI encoding, which explains why the code is only concerned about consecutive 0s—NRZI already solves the problem of consecutive 1s. Note
that the 4B/5B encoding results in 80% efficiency.
Table 2.4 gives the 5-bit codes that correspond to each of the 16 possible 4-bit data
symbols. Notice that since 5 bits are enough to encode 32 different codes, and we are
using only 16 of these for data, there are 16 codes left over that we can use for other
purposes. Of these, code 11111 is used when the line is idle, code 00000 corresponds to
when the line is dead, and 00100 is interpreted to mean halt. Of the remaining 13 codes,
7 of them are not valid because they violate the “one leading 0, two trailing 0s,” rule, and
the other 6 represent various control symbols. As we will see later in this chapter, some
framing protocols (e.g., FDDI) make use of these control symbols.
84
2
Figure 2.11
2.3
Direct Link Networks
Bits flow between adaptors, frames between hosts.
Framing
Now that we have seen how to transmit a sequence of bits over a point-to-point link—
from adaptor to adaptor—let’s consider the scenario illustrated in Figure 2.11. Recall
from Chapter 1 that we are focusing on packet-switched networks, which means that
blocks of data (called frames at this level), not bitstreams, are exchanged between nodes.
It is the network adaptor that enables the nodes to exchange frames. When node A
wishes to transmit a frame to node B, it tells its adaptor to transmit a frame from the
node’s memory. This results in a sequence of bits being sent over the link. The adaptor
on node B then collects together the sequence of bits arriving on the link and deposits
the corresponding frame in B’s memory. Recognizing exactly what set of bits constitute
a frame—that is, determining where the frame begins and ends—is the central challenge
faced by the adaptor.
There are several ways to address the framing problem. This section uses several
different protocols to illustrate the various points in the design space. Note that while we
discuss framing in the context of point-to-point links, the problem is a fundamental one
that must also be addressed in multiple-access networks like Ethernet and token rings.
2.3.1 Byte-Oriented Protocols (PPP)
One of the oldest approaches to framing—it has its roots in connecting terminals to
mainframes—is to view each frame as a collection of bytes (characters) rather than a
collection of bits. Such a byte-oriented approach is exemplified by older protocols such as
the Binary Synchronous Communication (BISYNC) protocol developed by IBM in the
late 1960s, and the Digital Data Communication Message Protocol (DDCMP) used in
Digital Equipment Corporation’s DECNET. The more recent and widely used Point-toPoint Protocol (PPP) provides another example of this approach.
Sentinel-based Approaches
Figure 2.12 illustrates the BISYNC protocol’s frame format. This figure is the first of
many that you will see in this book that are used to illustrate frame or packet formats,
2.3
Figure 2.12
BISYNC frame format.
Figure 2.13
PPP frame format.
Framing
85
so a few words of explanation are in order. We show a packet as a sequence of labeled
fields. Above each field is a number indicating the length of that field in bits. Note that
the packets are transmitted beginning with the leftmost field.
BISYNC uses special characters known as sentinel characters to indicate where
frames start and end. The beginning of a frame is denoted by sending a special SYN
(synchronization) character. The data portion of the frame is then contained between
two more special characters: STX (start of text) and ETX (end of text). The SOH (start
of header) field serves much the same purpose as the STX field. The problem with the
sentinel approach, of course, is that the ETX character might appear in the data portion of the frame. BISYNC overcomes this problem by “escaping” the ETX character by
preceding it with a data-link-escape (DLE) character whenever it appears in the body of
a frame; the DLE character is also escaped (by preceding it with an extra DLE) in the
frame body. (C programmers may notice that this is analogous to the way a quotation
mark is escaped by the backslash when it occurs inside a string.) This approach is often
called character stuffing because extra characters are inserted in the data portion of the
frame.
The frame format also includes a field labeled cyclic redundancy check (CRC) that
is used to detect transmission errors; various algorithms for error detection are presented
in Section 2.4. Finally, the frame contains additional header fields that are used for,
among other things, the link-level reliable delivery algorithm. Examples of these algorithms are given in Section 2.5.
The more recent PPP, which is commonly used to carry Internet Protocol packets over various sorts of point-to-point links, is similar to BISYNC in that it also uses
sentinels and character stuffing. The format for a PPP frame is given in Figure 2.13.
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2
Direct Link Networks
The special start-of-text character,
denoted as the Flag field in Figure 2.13,
is 01111110. The Address and Control fields usually contain default values,
and so are uninteresting. The Protocol
field is used for demultiplexing: it identifies the high-level protocol such as IP
or IPX (an IP-like protocol developed by
Novell). The frame payload size can be
negotiated, but it is 1,500 bytes by default. The Checksum field is either 2
(by default) or 4 bytes long.
The PPP frame format is unusual
in that several of the field sizes are negotiated rather than fixed. This negotiation is
conducted by a protocol called Link Control Protocol (LCP). PPP and LCP work
in tandem: LCP sends control messages
encapsulated in PPP frames—such messages are denoted by an LCP identifier in
the PPP Protocol field—and then turns
around and changes PPP’s frame format
based on the information contained in
those control messages. LCP is also involved in establishing a link between two
peers when both sides detect that communication over the link is possible (e.g.,
when each optical receiver detects an incoming signal from the fiber to which it
connects).
Byte-Counting Approach
As every Computer Sciences 101 student
knows, the alternative to detecting the
end of a file with a sentinel value is to
include the number of items in the file
at the beginning of the file. The same
is true in framing—the number of bytes
contained in a frame can be included
What’s in a Layer?
One of the important contributions of
the OSI reference model presented in
Chapter 1 was to provide some vocabulary for talking about protocols and,
in particular, protocol layers. This vocabulary has provided fuel for plenty
of arguments along the lines of “Your
protocol does function X at layer Y,
and the OSI reference model says it
should be done at layer Z—that’s a
layer violation.” In fact, figuring out
the right layer at which to perform
a given function can be very difficult, and the reasoning is usually a lot
more subtle than “What does the OSI
model say?” It is partly for this reason that this book avoids a rigidly layerist approach. Instead, it shows you
a lot of functions that need to be performed by protocols and looks at some
ways that they have been successfully
implemented.
In spite of our nonlayerist approach, sometimes we need convenient ways to talk about classes of
protocols, and the name of the layer
at which they operate is often the
best choice. Thus, for example, this
chapter focuses primarily on link-layer
protocols. (Bit encoding, described in
Section 2.2, is the exception, being
considered a physical-layer function.)
Link-layer protocols can be identified
by the fact that they run over single links—the type of network discussed in this chapter. Network-layer
2.3
Figure 2.14
Framing
87
DDCMP frame format.
protocols, by contrast, run over
switched networks that contain lots
of links interconnected by switches
or routers. Topics related to networklayer protocols are discussed in Chapters 3 and 4.
Note that protocol layers are
supposed to be helpful—they provide
helpful ways to talk about classes of
protocols, and they help us divide the
problem of building networks into
manageable subtasks. However, they
are not meant to be overly restrictive—
the mere fact that something is a layer
violation does not end the argument
about whether it is a worthwhile thing
to do. In other words, layering makes
a good slave, but a poor master. A particularly interesting argument about
the best layer in which to place a certain function comes up when we look
at congestion control in Chapter 6.
as a field in the frame header. The DECNET’s DDCMP protocol uses this approach, as illustrated in Figure 2.14. In this
example, the COUNT field specifies how
many bytes are contained in the frame’s
body.
One danger with this approach is
that a transmission error could corrupt
the count field, in which case the end of
the frame would not be correctly detected.
(A similar problem exists with the sentinelbased approach if the ETX field becomes
corrupted.) Should this happen, the receiver will accumulate as many bytes as the
bad COUNT field indicates and then use
the error detection field to determine that
the frame is bad. This is sometimes called
a framing error. The receiver will then wait
until it sees the next SYN character to start
collecting the bytes that make up the next
frame. It is therefore possible that a framing error will cause back-to-back frames to
be incorrectly received.
2.3.2 Bit-Oriented Protocols (HDLC)
Unlike byte-oriented protocols, a bit-oriented protocol is not concerned with byte
boundaries—it simply views the frame as a collection of bits. These bits might come
from some character set, such as ASCII, they might be pixel values in an image, or they
could be instructions and operands from an executable file. The Synchronous Data Link
Control (SDLC) protocol developed by IBM is an example of a bit-oriented protocol;
SDLC was later standardized by the ISO as the High-Level Data Link Control (HDLC)
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2
Figure 2.15
Direct Link Networks
HDLC frame format.
protocol. In the following discussion, we use HDLC as an example; its frame format is
given in Figure 2.15.
HDLC denotes both the beginning and the end of a frame with the distinguished
bit sequence 01111110. This sequence is also transmitted during any times that the link
is idle so that the sender and receiver can keep their clocks synchronized. In this way,
both protocols essentially use the sentinel approach. Because this sequence might appear anywhere in the body of the frame—in fact, the bits 01111110 might cross byte
boundaries—bit-oriented protocols use the analog of the DLE character, a technique
known as bit stuffing.
Bit stuffing in the HDLC protocol works as follows. On the sending side, any time
five consecutive 1s have been transmitted from the body of the message (i.e., excluding
when the sender is trying to transmit the distinguished 01111110 sequence), the sender
inserts a 0 before transmitting the next bit. On the receiving side, should five consecutive
1s arrive, the receiver makes its decision based on the next bit it sees (i.e., the bit following
the five 1s). If the next bit is a 0, it must have been stuffed, and so the receiver removes
it. If the next bit is a 1, then one of two things is true: Either this is the end-of-frame
marker or an error has been introduced into the bitstream. By looking at the next bit,
the receiver can distinguish between these two cases: if it sees a 0 (i.e., the last eight bits
it has looked at are 01111110), then it is the end-of-frame marker; if it sees a 1 (i.e., the
last eight bits it has looked at are 01111111), then there must have been an error and the
whole frame is discarded. In the latter case, the receiver has to wait for the next 01111110
before it can start receiving again, and as a consequence, there is the potential that the
receiver will fail to receive two consecutive frames. Obviously, there are still ways that
framing errors can go undetected, such as when an entire spurious end-of-frame pattern
is generated by errors, but these failures are relatively unlikely. Robust ways of detecting
errors are discussed in Section 2.4.
An interesting characteristic of bit stuffing, as well as character stuffing, is that the
size of a frame is dependent on the data that is being sent in the payload of the frame.
It is in fact not possible to make all frames exactly the same size, given that the data that
might be carried in any frame is arbitrary. (To convince yourself of this, consider what
happens if the last byte of a frame’s body is the ETX character.) A form of framing that
ensures that all frames are the same size is described in the next subsection.
2.3
Framing
89
2.3.3 Clock-Based Framing (SONET)
A third approach to framing is exemplified by the Synchronous Optical Network
(SONET) standard. For lack of a widely accepted generic term, we refer to this approach simply as clock-based framing. SONET was first proposed by Bell Communications Research (Bellcore), and then developed under the American National Standards
Institute (ANSI) for digital transmission over optical fiber; it has since been adopted by
the ITU-T. Who standardized what and when is not the interesting issue, though. The
thing to remember about SONET is that it is the dominant standard for long-distance
transmission of data over optical networks.
An important point to make about SONET before we go any further is that the
full specification is substantially larger than this book. Thus, the following discussion
will necessarily cover only the high points of the standard. Also, SONET addresses both
the framing problem and the encoding problem. It also addresses a problem that is very
important for phone companies—the multiplexing of several low-speed links onto one
high-speed link. We begin with framing and discuss the other issues following.
As with the previously discussed framing schemes, a SONET frame has some special information that tells the receiver where the frame starts and ends. However, that is
about as far as the similarities go. Notably, no bit stuffing is used, so that a frame’s length
does not depend on the data being sent. So the question to ask is, “How does the receiver
know where each frame starts and ends?” We consider this question for the lowest-speed
SONET link, which is known as STS-1 and runs at 51.84 Mbps. An STS-1 frame is
shown in Figure 2.16. It is arranged as nine rows of 90 bytes each, and the first 3 bytes
of each row are overhead, with the rest being available for data that is being transmitted
over the link. The first 2 bytes of the frame contain a special bit pattern, and it is these
bytes that enable the receiver to determine where the frame starts. However, since bit
stuffing is not used, there is no reason why this pattern will not occasionally turn up in
the payload portion of the frame. To guard against this, the receiver looks for the special
bit pattern consistently, hoping to see it appearing once every 810 bytes, since each frame
is 9 × 90 = 810 bytes long. When the special pattern turns up in the right place enough
times, the receiver concludes that it is in sync and can then interpret the frame correctly.
One of the things we are not describing due to the complexity of SONET is the
detailed use of all the other overhead bytes. Part of this complexity can be attributed to
the fact that SONET runs across the carrier’s optical network, not just over a single link.
(Recall that we are glossing over the fact that the carriers implement a network, and we
are instead focusing on the fact that we can lease a SONET link from them and then use
this link to build our own packet-switched network.) Additional complexity comes from
the fact that SONET provides a considerably richer set of services than just data transfer.
For example, 64 Kbps of a SONET link’s capacity is set aside for a voice channel that is
used for maintenance.
90
2
Figure 2.16
Direct Link Networks
A SONET STS-1 frame.
The overhead bytes of a SONET frame are encoded using NRZ, the simple encoding described in the previous section where 1s are high and 0s are low. However, to
ensure that there are plenty of transitions to allow the receiver to recover the sender’s
clock, the payload bytes are scrambled. This is done by calculating the exclusive-OR
(XOR) of the data to be transmitted and by the use of a well-known bit pattern. The bit
pattern, which is 127 bits long, has plenty of transitions from 1 to 0, so that XORing
it with the transmitted data is likely to yield a signal with enough transitions to enable
clock recovery.
SONET supports the multiplexing of multiple low-speed links in the following
way. A given SONET link runs at one of a finite set of possible rates, ranging from
51.84 Mbps (STS-1) to 2,488.32 Mbps (STS-48), and beyond. (See Table 2.2 in Section 2.1 for the full set of SONET data rates.) Note that all of these rates are integer
multiples of STS-1. The significance for framing is that a single SONET frame can contain subframes for multiple lower-rate channels. A second related feature is that each
frame is 125 µs long. This means that at STS-1 rates, a SONET frame is 810 bytes long,
while at STS-3 rates, each SONET frame is 2,430 bytes long. Notice the synergy between these two features: 3 × 810 = 2,430, meaning that three STS-1 frames fit exactly
in a single STS-3 frame.
Intuitively, the STS-N frame can be thought of as consisting of N STS-1 frames,
where the bytes from these frames are interleaved, that is, a byte from the first frame is
transmitted, then a byte from the second frame is transmitted, and so on. The reason for
interleaving the bytes from each STS-N frame is to ensure that the bytes in each STS-1
frame are evenly paced, that is, bytes show up at the receiver at a smooth 51 Mbps, rather
than all bunched up during one particular 1/N th of the 125-µs interval.
Although it is accurate to view an STS-N signal as being used to multiplex N
STS-1 frames, the payload from these STS-1 frames can be linked together to form a
larger STS-N payload; such a link is denoted STS-N c (for concatenated ). One of the
2.3
Figure 2.17
Three STS-1 frames multiplexed onto one STS-3c frame.
Figure 2.18
SONET frames out of phase.
Framing
91
fields in the overhead is used for this purpose. Figure 2.17 schematically depicts concatenation in the case of three STS-1 frames being concatenated into a single STS-3c
frame. The significance of a SONET link being designated as STS-3c rather than STS-3
is that, in the former case, the user of the link can view it as a single 155.25-Mbps pipe,
whereas an STS-3 should really be viewed as three 51.84-Mbps links that happen to
share a fiber.
Finally, the preceding description of SONET is overly simplistic in that it assumes
that the payload for each frame is completely contained within the frame. (Why wouldn’t
it be?) In fact, we should view the STS-1 frame just described as simply a placeholder for
the frame, where the actual payload may float across frame boundaries. This situation
is illustrated in Figure 2.18. Here we see both the STS-1 payload floating across two
STS-1 frames, and the payload shifted some number of bytes to the right and, therefore,
wrapped around. One of the fields in the frame overhead points to the beginning of the
payload. The value of this capability is that it simplifies the task of synchronizing the
clocks used throughout the carriers’ networks, which is something that carriers spend a
lot of their time worrying about.
92
2.4
2
Direct Link Networks
Error Detection
As discussed in Chapter 1, bit errors are sometimes introduced into frames. This happens, for example, because of electrical interference or thermal noise. Although errors are
rare, especially on optical links, some mechanism is needed to detect these errors so that
corrective action can be taken. Otherwise, the end user is left wondering why the C program that successfully compiled just a moment ago now suddenly has a syntax error in it,
when all that happened in the interim is that it was copied across a network file system.
There is a long history of techniques for dealing with bit errors in computer systems, dating back to at least the 1940s. Hamming and Reed/Solomon codes are two
notable examples that were developed for use in punch card readers and when storing
data on magnetic disks and in early core memories. This section describes some of the
error detection techniques most commonly used in networking.
Detecting errors is only one part of the problem. The other part is correcting errors
once detected. There are two basic approaches that can be taken when the recipient of
a message detects an error. One is to notify the sender that the message was corrupted
so that the sender can retransmit a copy of the message. If bit errors are rare, then in all
probability the retransmitted copy will be error free. Alternatively, there are some types
of error detection algorithms that allow the recipient to reconstruct the correct message
even after it has been corrupted; such algorithms rely on error correcting codes, discussed
below.
One of the most common techniques for detecting transmission errors is a technique known as the cyclic redundancy check (CRC). It is used in nearly all the link-level
protocols discussed in the previous section—for example, HDLC, DDCMP—as well as
in the CSMA and token ring protocols described later in this chapter. Section 2.4.3 outlines the basic CRC algorithm. Before discussing that approach, we consider two simpler
schemes that are also widely used: two-dimensional parity and checksums. The former is
used by the BISYNC protocol when it is transmitting ASCII characters (CRC is used as
the error code when BISYNC is used to transmit EBCDIC), and the latter is used by
several Internet protocols.
The basic idea behind any error detection scheme is to add redundant information
to a frame that can be used to determine if errors have been introduced. In the extreme,
we could imagine transmitting two complete copies of the data. If the two copies are
identical at the receiver, then it is probably the case that both are correct. If they differ,
then an error was introduced into one (or both) of them, and they must be discarded.
This is a rather poor error detection scheme for two reasons. First, it sends n redundant bits for an n-bit message. Second, many errors will go undetected—any error that
happens to corrupt the same bit positions in the first and second copies of the message.
Fortunately, we can do a lot better than this simple scheme. In general, we can
provide quite strong error detection capability while sending only k redundant bits for
2.4
Error Detection
93
an n-bit message, where k n. On an Ethernet, for example, a frame carrying up to
12,000 bits (1,500 bytes) of data requires only a 32-bit CRC code, or as it is commonly
expressed, uses CRC-32. Such a code will catch the overwhelming majority of errors, as
we will see below.
We say that the extra bits we send are redundant because they add no new information to the message. Instead, they are derived directly from the original message using
some well-defined algorithm. Both the sender and the receiver know exactly what that
algorithm is. The sender applies the algorithm to the message to generate the redundant bits. It then transmits both the message and those few extra bits. When the receiver
applies the same algorithm to the received message, it should (in the absence of errors)
come up with the same result as the sender. It compares the result with the one sent to it
by the sender. If they match, it can conclude (with high likelihood) that no errors were
introduced in the message during transmission. If they do not match, it can be sure that
either the message or the redundant bits were corrupted, and it must take appropriate
action, that is, discarding the message, or correcting it if that is possible.
One note on the terminology for these extra bits. In general, they are referred to
as error-detecting codes. In specific cases, when the algorithm to create the code is based
on addition, they may be called a checksum. We will see that the Internet checksum is
appropriately named: It is an error check that uses a summing algorithm. Unfortunately,
the word “checksum” is often used imprecisely to mean any form of error detecting code,
including CRCs. This can be confusing, so we urge you to use the word “checksum” only
to apply to codes that actually do use addition and to use “error detecting code” to refer
to the general class of codes described in this section.
2.4.1 Two-Dimensional Parity
Two-dimensional parity is exactly what the name suggests. It is based on “simple” (onedimensional) parity, which usually involves adding one extra bit to a 7-bit code to balance
the number of 1s in the byte. For example, odd parity sets the eighth bit to 1 if needed
to give an odd number of 1s in the byte, and even parity sets the eighth bit to 1 if
needed to give an even number of 1s in the byte. Two-dimensional parity does a similar
calculation for each bit position across each of the bytes contained in the frame. This
results in an extra parity byte for the entire frame, in addition to a parity bit for each byte.
Figure 2.19 illustrates how two-dimensional even parity works for an example frame
containing 6 bytes of data. Notice that the third bit of the parity byte is 1 since there is
an odd number of 1s in the third bit across the 6 bytes in the frame. It can be shown
that two-dimensional parity catches all 1-, 2-, and 3-bit errors, and most 4-bit errors. In
this case, we have added 14 bits of redundant information to a 42-bit message, and yet
we have stronger protection against common errors than the “repetition code” described
above.
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2
Figure 2.19
Direct Link Networks
Two-dimensional parity.
2.4.2 Internet Checksum Algorithm
A second approach to error detection is exemplified by the Internet checksum. Although
it is not used at the link level, it nevertheless provides the same sort of functionality as
CRCs and parity, so we discuss it here. We will see examples of its use in Sections 4.1,
5.1, and 5.2.
The idea behind the Internet checksum is very simple—you add up all the words
that are transmitted and then transmit the result of that sum. The result is called the
checksum. The receiver performs the same calculation on the received data and compares
the result with the received checksum. If any transmitted data, including the checksum
itself, is corrupted, then the results will not match, so the receiver knows that an error
occurred.
You can imagine many different variations on the basic idea of a checksum. The
exact scheme used by the Internet protocols works as follows. Consider the data being
checksummed as a sequence of 16-bit integers. Add them together using 16-bit ones
complement arithmetic (explained below) and then take the ones complement of the
result. That 16-bit number is the checksum.
In ones complement arithmetic, a negative integer −x is represented as the complement of x, that is, each bit of x is inverted. When adding numbers in ones complement
arithmetic, a carryout from the most significant bit needs to be added to the result. Consider, for example, the addition of −5 and −3 in ones complement arithmetic on 4-bit
integers: +5 is 0101, so −5 is 1010; +3 is 0011, so −3 is 1100. If we add 1010 and
1100 ignoring the carry, we get 0110. In ones complement arithmetic, the fact that this
2.4
Error Detection
95
operation caused a carry from the most significant bit causes us to increment the result,
giving 0111, which is the ones complement representation of −8 (obtained by inverting
the bits in 1000), as we would expect.
The following routine gives a straightforward implementation of the Internet’s
checksum algorithm. The count argument gives the length of buf measured in
16-bit units. The routine assumes that buf has already been padded with 0s to a 16-bit
boundary.
u_short
cksum(u_short *buf, int count)
{
register u_long sum = 0;
while (count--)
{
sum += *buf++;
if (sum & 0xFFFF0000)
{
/* carry occurred,
so wrap around */
sum &= 0xFFFF;
sum++;
}
}
return ~(sum & 0xFFFF);
}
This code ensures that the calculation uses ones complement arithmetic, rather
than the twos complement that is used in most machines. Note the if statement inside
the while loop. If there is a carry into the top 16 bits of sum, then we increment sum
just as in the previous example.
Compared to our repetition code, this algorithm scores well for using a small number of redundant bits—only 16 for a message of any length—but it does not score extremely well for strength of error detection. For example, a pair of single-bit errors, one
of which increments a word and one of which decrements another word by the same
amount, will go undetected. The reason for using an algorithm like this in spite of its
relatively weak protection against errors (compared to a CRC, for example) is simple:
This algorithm is much easier to implement in software. Experience in the ARPANET
suggested that a checksum of this form was adequate. One reason it is adequate is that
this checksum is the last line of defense in an end-to-end protocol; the majority of errors
are picked up by stronger error detection algorithms, such as CRCs, at the link level.
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Direct Link Networks
2.4.3 Cyclic Redundancy Check
It should be clear by now that a major goal in designing error detection algorithms is to
maximize the probability of detecting errors using only a small number of redundant bits.
Cyclic redundancy checks use some fairly powerful mathematics to achieve this goal. For
example, a 32-bit CRC gives strong protection against common bit errors in messages
that are thousands of bytes long. The theoretical foundation of the cyclic redundancy
check is rooted in a branch of mathematics called finite fields. While this may sound
daunting, the basic ideas can be easily understood.
To start, think of an (n + 1)-bit message as being represented by an n degree polynomial, that is, a polynomial whose highest-order term is x n . The message is represented by a polynomial by using the value of each bit in the message as the coefficient
for each term in the polynomial, starting
Simple Probability Calculations
with the most significant bit to represent
When dealing with network errors
the highest-order term. For example,
and other unlikely (we hope) events,
an 8-bit message consisting of the bits
we often have use for simple back10011010 corresponds to the polynoof-the-envelope probability estimates.
mial
A useful approximation here is that
M (x) = 1 × x 7 + 0 × x 6 + 0 × x 5
if two independent events have small
probabilities p and q, then the proba+1 × x 4 + 1 × x 3 + 0 × x 2
bility of either event is p + q; the exact answer is 1 − (1 − p)(1 − q) =
+1 × x 1 + 0 × x 0
p
+ q − pq. For p = q = .01, this es= x7 + x4 + x3 + x1
timate is .02, while the exact value is
.0199.
We can thus think of a sender and a
For a simple application of this,
receiver as exchanging polynomials with
suppose that the per-bit error rate on
each other.
a link is 1 in 107 . Now suppose we are
For the purposes of calculating a
interested in estimating the probabilCRC, a sender and receiver have to agree
ity of at least one bit in a 10,000-bit
on a divisor polynomial, C(x). C(x) is
packet being errored. Using the above
a polynomial of degree k. For example,
approximation repeatedly over all the
suppose C(x) = x 3 + x 2 + 1. In this
bits, we can say that we are interested
case, k = 3. The answer to the quesin the probability of either the first
tion “Where did C(x) come from?” is,
bit being errored, or the second bit,
in most practical cases, “You look it up
or the third, and so on. Assuming bit
in a book.” In fact, the choice of C(x)
errors are all independent (which they
has a significant impact on what types
aren’t), we can therefore estimate that
of errors can be reliably detected, as we
2.4
Error Detection
97
discuss below. There are a handful of divisor polynomials that are very good choices
for various environments, and the exact choice is normally made as part of the protocol design. For example, the Ethernet standard uses a well-known polynomial of degree 32.
When a sender wishes to transmit a message M (x) that is n + 1 bits long, what
is actually sent is the (n + 1)-bit message plus k bits. We call the complete transmitted message, including the redundant bits, P(x). What we are going to do is contrive to make the polynomial representing P(x) exactly divisible by C(x); we explain
how this is achieved below. If P(x) is transmitted over a link and there are no errors introduced during transmission, then the receiver should be able to divide P(x)
by C(x) exactly, leaving a remainder of zero. On the other hand, if some error is introduced into P(x) during transmission, then in all likelihood the received polynomial
will no longer be exactly divisible by C(x), and thus the receiver will obtain a nonzero
remainder implying that an error has octhe probability of at least one error in
curred.
a 10,000-bit (104 bit) packet is 104 ×
It will help to understand the follow10−7 = 10−3 . The exact answer, coming if you know a little about polynomial
puted as 1 − P(no errors), would be
arithmetic; it is just slightly different from
1 − (1 − 10−7 )10,000 = .00099950.
normal integer arithmetic. We are dealing
For a slightly more complex apwith a special class of polynomial arithplication, we compute the probability
metic here, where coefficients may be only
of exactly two errors in such a packet;
one or zero, and operations on the coeffithis is the probability of an error that
cients are performed using modulo 2 arithwould sneak past a 1-parity-bit checkmetic. This is referred to as polynomial
sum. If we consider two particular bits
arithmetic modulo 2. Since this is a netin the packet, say bit i and bit j, the
working book, not a mathematics text, let’s
probability of those exact bits being
focus on the key properties of this type of
errored is 10−7 × 10−7 . Now the toarithmetic for our purposes (which we ask
tal number of possible bit pairs in the
you to accept on faith):
packet is
104 2
= 104 × (104 − 1)/2 ≈ 5 × 107
So again using the approximation of
repeatedly adding the probabilities of
many rare events (in this case, of any
possible bit pair being errored), our
total probability of at least two errored
bits is 5 × 107 × 10−14 = 5 × 10−7 .
■
Any polynomial B(x) can be divided by a divisor polynomial
C(x) if B(x) is of higher degree
than C(x);
■
Any polynomial B(x) can be divided once by a divisor polynomial C(x) if B(x) is of the same
degree as C(x);
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Direct Link Networks
■
The remainder obtained when B(x) is divided by C(x) is obtained by subtracting C(x) from B(x);
■
To subtract C(x) from B(x), we simply perform the exclusive-OR (XOR) operation on each pair of matching coefficients.
For example, the polynomial x 3 + 1 can be divided by x 3 + x 2 + 1 (because they
are both of degree 3) and the remainder would be 0 × x 3 + 1 × x 2 + 0 × x 1 + 0 × x 0 = x 2
(obtained by XORing the coefficients of each term). In terms of messages, we could say
that 1001 can be divided by 1101 and leaves a remainder of 0100. You should be able to
see that the remainder is just the bitwise exclusive-OR of the two messages.
Now that we know the basic rules for dividing polynomials, we are able to do long
division, which is necessary to deal with longer messages. An example appears below.
Recall that we wanted to create a polynomial for transmission that is derived from
the original message M (x), is k bits longer than M (x), and is exactly divisible by C(x).
We can do this in the following way:
1
Multiply M (x) by x k , that is, add k zeros at the end of the message. Call this
zero-extended message T (x).
2
Divide T (x) by C(x) and find the remainder.
3
Subtract the remainder from T (x).
It should be obvious that what is left at this point is a message that is exactly
divisible by C(x). We may also note that the resulting message consists of M (x) followed
by the remainder obtained in step 2, because when we subtracted the remainder (which
can be no more than k bits long), we were just XORing it with the k zeros added in
step 1. This part will become clearer with an example.
Consider the message x 7 + x 4 + x 3 + x 1 , or 10011010. We begin by multiplying by
x 3 , since our divisor polynomial is of degree 3. This gives 10011010000. We divide this
by C(x), which corresponds to 1101 in this case. Figure 2.20 shows the polynomial longdivision operation. Given the rules of polynomial arithmetic described above, the long
division operation proceeds much as it would if we were dividing integers. Thus, in the
first step of our example, we see that the divisor 1101 divides once into the first four bits
of the message (1001), since they are of the same degree, and leaves a remainder of 100
(1101 XOR 1001). The next step is to bring down a digit from the message polynomial
until we get another polynomial with the same degree as C(x), in this case 1001. We
calculate the remainder again (100) and continue until the calculation is complete. Note
that the “result” of the long division, which appears at the top of the calculation, is not
really of much interest—it is the remainder at the end that matters.
2.4
Figure 2.20
Error Detection
99
CRC calculation using polynomial long division.
You can see from the very bottom of Figure 2.20 that the remainder of the example
calculation is 101. So we know that 10011010000 minus 101 would be exactly divisible
by C(x), and this is what we send. The minus operation in polynomial arithmetic is the
logical XOR operation, so we actually send 10011010101. As noted above, this turns
out to be just the original message with the remainder from the long-division calculation
appended to it. The recipient divides the received polynomial by C(x) and, if the result
is 0, concludes that there were no errors. If the result is nonzero, it may be necessary
to discard the corrupted message; with some codes, it may be possible to correct a small
error (e.g., if the error affected only one bit). A code that enables error correction is called
an error correcting code (ECC).
Now we will consider the question of where the polynomial C(x) comes from.
Intuitively, the idea is to select this polynomial so that it is very unlikely to divide evenly
into a message that has errors introduced into it. If the transmitted message is P(x), we
may think of the introduction of errors as the addition of another polynomial E(x), so
the recipient sees P(x)+E(x). The only way that an error could slip by undetected would
be if the received message could be evenly divided by C(x), and since we know that P(x)
can be evenly divided by C(x), this could only happen if E(x) can be divided evenly by
C(x). The trick is to pick C(x) so that this is very unlikely for common types of errors.
One common type of error is a single-bit error, which can be expressed as E(x) = x i
when it affects bit position i. If we select C(x) such that the first and the last term are
nonzero, then we already have a two-term polynomial that cannot divide evenly into the
one term E(x). Such a C(x) can, therefore, detect all single-bit errors. In general, it is
possible to prove that the following types of errors can be detected by a C(x) with the
stated properties:
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■
All single-bit errors, as long as the x k and x 0 terms have nonzero coefficients;
■
All double-bit errors, as long as C(x) has a factor with at least three terms;
■
Any odd number of errors, as long as C(x) contains the factor (x + 1);
■
Any “burst” error (i.e., sequence
of consecutive errored bits) for
which the length of the burst is
less than k bits. (Most burst errors of larger than k bits can also
be detected.)
Six versions of C(x) are widely used
in link-level protocols (shown in Table 2.5).
For example, the Ethernet and 802.5
networks described later in this chapter
use CRC-32, while HDLC uses CRCCCITT. ATM, as described in Chapter 3,
uses CRC-8, CRC-10, and CRC-32.
Finally, we note that the CRC algorithm, while seemingly complex, is easily
implemented in hardware using a k-bit
shift register and XOR gates. The number
of bits in the shift register equals the degree of the generator polynomial (k). Figure 2.21 shows the hardware that would
be used for the generator x 3 + x 2 + 1
from our previous example. The message is shifted in from the left, beginning with the most significant bit and
ending with the string of k zeros that
is attached to the message, just as in
the long-division example. When all the
bits have been shifted in and appropriately XORed, the register contains the
remainder, that is, the CRC (most significant bit on the right). The position
of the XOR gates is determined as follows: If the bits in the shift register are
labeled 0 through k − 1, left to right,
Error Detection or Error
Correction?
We have mentioned that it is possible to use codes that not only detect
the presence of errors but also enable
errors to be corrected. Since the details of such codes require yet more
complex mathematics than that required to understand CRCs, we will
not dwell on them here. However, it
is worth considering the merits of correction versus detection.
At first glance, it would seem
that correction is always better, since
with detection we are forced to throw
away the message and, in general, ask
for another copy to be transmitted.
This uses up bandwidth and may introduce latency while waiting for the
retransmission. However, there is a
downside to correction: It generally
requires a greater number of redundant bits to send an error correcting
code that is as strong (that is, able to
cope with the same range of errors) as
a code that only detects errors. Thus,
while error detection requires more
bits to be sent when errors occur, error correction requires more bits to be
sent all the time. As a result, error correction tends to be most useful when
(1) errors are quite probable, as they
2.5 Reliable Transmission
CRC
C (x)
CRC-8
x8 + x2 + x1 + 1
CRC-10
x 10 + x 9 + x 5 + x 4 + x 1 + 1
CRC-12
x 12 + x 11 + x 3 + x 2 + 1
CRC-16
x 16 + x 15 + x 2 + 1
CRC-CCITT
x 16 + x 12 + x 5 + 1
CRC-32
x 32 + x 26 + x 23 + x 22 + x 16 + x 12 + x 11
101
+ x 10 + x 8 + x 7 + x 5 + x 4 + x 2 + x + 1
Table 2.5
Common CRC polynomials.
Figure 2.21
CRC calculation using shift register.
may be, for example, in a wireless
environment, or (2) the cost of retransmission is too high, for example, because of the latency involved retransmitting a packet over a satellite
link.
The use of error correcting codes
in networking is sometimes referred
to as forward error correction (FEC)
because the correction of errors is handled “in advance” by sending extra
information, rather than waiting for
errors to happen and dealing with
them later by retransmission.
then put an XOR gate in front of bit n if
there is a term x n in the generator polynomial. Thus, we see an XOR gate in
front of positions 0 and 2 for the generator x 3 + x 2 + x 0 .
2.5
Reliable
Transmission
As we saw in the previous section, frames
are sometimes corrupted while in transit, with an error code like CRC used
to detect such errors. While some error
codes are strong enough also to correct errors, in practice the overhead is typically
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too large to handle the range of bit and burst errors that can be introduced on a network
link. Even when error correcting codes are used (e.g., on wireless links) some errors will
be too severe to be corrected. As a result, some corrupt frames must be discarded. A linklevel protocol that wants to deliver frames reliably must somehow recover from these
discarded (lost) frames.
This is usually accomplished using a combination of two fundamental mechanisms
—acknowledgments and timeouts. An acknowledgment (ACK for short) is a small control
frame that a protocol sends back to its peer saying that it has received an earlier frame.
By control frame we mean a header without any data, although a protocol can piggyback
an ACK on a data frame it just happens to be sending in the opposite direction. The
receipt of an acknowledgment indicates to the sender of the original frame that its frame
was successfully delivered. If the sender does not receive an acknowledgment after a
reasonable amount of time, then it retransmits the original frame. This action of waiting
a reasonable amount of time is called a timeout.
The general strategy of using acknowledgments and timeouts to implement reliable
delivery is sometimes called automatic repeat request (normally abbreviated ARQ). This
section describes three different ARQ algorithms using generic language, that is, we do
not give detailed information about a particular protocol’s header fields.
2.5.1 Stop-and-Wait
The simplest ARQ scheme is the stop-and-wait algorithm. The idea of stop-and-wait is
straightforward: After transmitting one frame, the sender waits for an acknowledgment
before transmitting the next frame. If the acknowledgment does not arrive after a certain
period of time, the sender times out and retransmits the original frame.
Figure 2.22 illustrates four different scenarios that result from this basic algorithm.
This figure is a timeline, a common way to depict a protocol’s behavior (see also the
sidebar on this sort of diagram). The sending side is represented on the left, the receiving
side is depicted on the right, and time flows from top to bottom. Figure 2.22(a) shows
the situation in which the ACK is received before the timer expires, (b) and (c) show the
situation in which the original frame and the ACK, respectively, are lost, and (d) shows
the situation in which the timeout fires too soon. Recall that by “lost” we mean that the
frame was corrupted while in transit, that this corruption was detected by an error code
on the receiver, and that the frame was subsequently discarded.
There is one important subtlety in the stop-and-wait algorithm. Suppose the sender
sends a frame and the receiver acknowledges it, but the acknowledgment is either lost or
delayed in arriving. This situation is illustrated in timelines (c) and (d) of Figure 2.22.
In both cases, the sender times out and retransmits the original frame, but the receiver
will think that it is the next frame, since it correctly received and acknowledged the first
2.5 Reliable Transmission
103
Figure 2.22 Timeline showing four different scenarios for the stop-and-wait algorithm.
(a) The ACK is received before the timer expires; (b) the original frame is lost; (c) the
ACK is lost; (d) the timeout fires too soon.
Timelines and Packet Exchange
Diagrams
Figures 2.22 and 2.23 are two examples of a frequently-used tool in
teaching, explaining, and designing
protocols: the timeline or packet exchange diagram. You are going to see
frame. This has the potential to cause duplicate copies of a frame to be delivered.
To address this problem, the header for a
stop-and-wait protocol usually includes a
1-bit sequence number—that is, the sequence number can take on the values 0
and 1—and the sequence numbers used
for each frame alternate, as illustrated in
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2
Figure 2.23
Direct Link Networks
Timeline for stop-and-wait with 1-bit sequence number.
Figure 2.23. Thus, when the sender retransmits frame 0, the receiver can determine that
it is seeing a second copy of frame 0 rather than the first copy of frame 1 and therefore
can ignore it (the receiver still acknowledges it, in case the first ACK was lost).
The main shortcoming of the stopmany more of them in this book—
and-wait algorithm is that it allows the
see Figures 9.12 and 9.16 for more
sender to have only one outstanding
complex examples. They are very useframe on the link at a time, and this
ful because they capture visually the
may be far below the link’s capacity. Conbehavior over time of a distributed
sider, for example, a 1.5-Mbps link with
system—something that can be quite
a 45-ms round-trip time. This link has a
hard to analyze. When designing a
delay × bandwidth product of 67.5 Kb,
protocol, you often have to be preor approximately 8 KB. Since the sender
pared for the unexpected—a system
can send only one frame per RTT, and ascrashes, a message gets lost, or somesuming a frame size of 1 KB, this implies
thing that you expected to happen
a maximum sending rate of
quickly turns out to take a long time.
These sorts of diagrams can often help
Bits Per Frame ÷ Time Per Frame
understand what might go wrong in
= 1024 × 8 ÷ 0.045
such cases and thus help a protocol
designer be prepared for every eventu= 182 Kbps
ality.
or about one-eighth of the link’s capacity.
2.5 Reliable Transmission
Figure 2.24
105
Timeline for the sliding window algorithm.
▲
To use the link fully, then, we’d like the sender to be able to transmit up to eight frames
before having to wait for an acknowledgment.
The significance of the bandwidth × delay product is that it represents the amount
of data that could be in transit. We would like to be able to send this much data without
waiting for the first acknowledgment. The principle at work here is often referred to
as keeping the pipe full. The algorithms presented in the following two subsections do
exactly this.
2.5.2 Sliding Window
Consider again the scenario in which the link has a delay × bandwidth product of 8 KB
and frames are of 1-KB size. We would like the sender to be ready to transmit the ninth
frame at pretty much the same moment that the ACK for the first frame arrives. The
algorithm that allows us to do this is called sliding window, and an illustrative timeline is
given in Figure 2.24.
The Sliding Window Algorithm
The sliding window algorithm works as follows. First, the sender assigns a sequence number, denoted SeqNum, to each frame. For now, let’s ignore the fact that SeqNum
is implemented by a finite-size header field and instead assume that it can grow infinitely large. The sender maintains three variables: The send window size, denoted SWS,
gives the upper bound on the number of outstanding (unacknowledged) frames that the
sender can transmit; LAR denotes the sequence number of the last acknowledgment received ; and LFS denotes the sequence number of the last frame sent. The sender also
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Direct Link Networks
Figure 2.25
Sliding window on sender.
Figure 2.26
Sliding window on receiver.
maintains the following invariant:
LFS − LAR ≤ SWS
This situation is illustrated in Figure 2.25.
When an acknowledgment arrives, the sender moves LAR to the right, thereby
allowing the sender to transmit another frame. Also, the sender associates a timer with
each frame it transmits, and it retransmits the frame should the timer expire before an
ACK is received. Notice that the sender has to be willing to buffer up to SWS frames
since it must be prepared to retransmit them until they are acknowledged.
The receiver maintains the following three variables: The receive window size, denoted RWS, gives the upper bound on the number of out-of-order frames that the
receiver is willing to accept; LAF denotes the sequence number of the largest acceptable
frame; and LFR denotes the sequence number of the last frame received. The receiver also
maintains the following invariant:
LAF − LFR ≤ RWS
This situation is illustrated in Figure 2.26.
When a frame with sequence number SeqNum arrives, the receiver takes the
following action. If SeqNum ≤ LFR or SeqNum > LAF, then the frame is outside the receiver’s window and it is discarded. If LFR < SeqNum ≤ LAF, then the
2.5 Reliable Transmission
107
frame is within the receiver’s window and it is accepted. Now the receiver needs to decide whether or not to send an ACK. Let SeqNumToAck denote the largest sequence
number not yet acknowledged, such that all frames with sequence numbers less than or
equal to SeqNumToAck have been received. The receiver acknowledges the receipt of
SeqNumToAck, even if higher-numbered packets have been received. This acknowledgment is said to be cumulative. It then sets LFR = SeqNumToAck and adjusts
LAF = LFR + RWS.
For example, suppose LFR = 5 (i.e., the last ACK the receiver sent was for sequence
number 5), and RWS = 4. This implies that LAF = 9. Should frames 7 and 8 arrive,
they will be buffered because they are within the receiver’s window. However, no ACK
needs to be sent since frame 6 is yet to arrive. Frames 7 and 8 are said to have arrived out
of order. (Technically, the receiver could resend an ACK for frame 5 when frames 7 and 8
arrive.) Should frame 6 then arrive—perhaps it is late because it was lost the first time and
had to be retransmitted, or perhaps it was simply delayed2 —the receiver acknowledges
frame 8, bumps LFR to 8, and sets LAF to 12. If frame 6 was in fact lost, then a timeout
will have occurred at the sender, causing it to retransmit frame 6.
We observe that when a timeout occurs, the amount of data in transit decreases,
since the sender is unable to advance its window until frame 6 is acknowledged. This
means that when packet losses occur, this scheme is no longer keeping the pipe full. The
longer it takes to notice that a packet loss has occurred, the more severe this problem
becomes.
Notice that in this example, the receiver could have sent a negative acknowledgment (NAK) for frame 6 as soon as frame 7 arrived. However, this is unnecessary since
the sender’s timeout mechanism is sufficient to catch this situation, and sending NAKs
adds additional complexity to the receiver. Also, as we mentioned, it would have been
legitimate to send additional acknowledgments of frame 5 when frames 7 and 8 arrived;
in some cases, a sender can use duplicate ACKs as a clue that a frame was lost. Both
approaches help to improve performance by allowing early detection of packet losses.
Yet another variation on this scheme would be to use selective acknowledgments.
That is, the receiver could acknowledge exactly those frames it has received, rather than
just the highest-numbered frame received in order. So, in the above example, the receiver
could acknowledge the receipt of frames 7 and 8. Giving more information to the sender
makes it potentially easier for the sender to keep the pipe full, but adds complexity to
the implementation.
The sending window size is selected according to how many frames we want to have
outstanding on the link at a given time; SWS is easy to compute for a given delay ×
2 It’s unlikely that a packet could be delayed in this way on a point-to-point link, but later on we will see this same
algorithm used on more complex networks where such delays are possible.
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bandwidth product.3 On the other hand, the receiver can set RWS to whatever it wants.
Two common settings are RWS = 1, which implies that the receiver will not buffer any
frames that arrive out of order, and RWS = SWS, which implies that the receiver can
buffer any of the frames the sender transmits. It makes no sense to set RWS > SWS
since it’s impossible for more than SWS frames to arrive out of order.
Finite Sequence Numbers and Sliding Window
We now return to the one simplification we introduced into the algorithm—our assumption that sequence numbers can grow infinitely large. In practice, of course, a frame’s
sequence number is specified in a header field of some finite size. For example, a 3-bit
field means that there are eight possible sequence numbers, 0 . . . 7. This makes it necessary to reuse sequence numbers or, stated another way, sequence numbers wrap around.
This introduces the problem of being able to distinguish between different incarnations
of the same sequence numbers, which implies that the number of possible sequence
numbers must be larger than the number of outstanding frames allowed. For example,
stop-and-wait allowed one outstanding frame at a time and had two distinct sequence
numbers.
Suppose we have one more number in our space of sequence numbers than we
have potentially outstanding frames, that is, SWS ≤ MaxSeqNum − 1, where MaxSeqNum is the number of available sequence numbers. Is this sufficient? The answer
depends on RWR. If RWS = 1, then MaxSeqNum ≥ SWS + 1 is sufficient. If
RWS is equal to SWS, then having a MaxSeqNum just one greater than the sending
window size is not good enough. To see this, consider the situation in which we have
the eight sequence numbers 0 through 7, and SWS = RWS = 7. Suppose the sender
transmits frames 0..6, they are successfully received, but the ACKs are lost. The receiver
is now expecting frames 7, 0..5, but the sender times out and sends frames 0..6. Unfortunately, the receiver is expecting the second incarnation of frames 0..5, but gets the first
incarnation of these frames. This is exactly the situation we wanted to avoid.
It turns out that the sending window size can be no more than half as big as the
number of available sequence numbers when RWS = SWS, or stated more precisely,
SWS < (MaxSeqNum + 1)/2
Intuitively, what this is saying is that the sliding window protocol alternates between
the two halves of the sequence number space, just as stop-and-wait alternates between
sequence numbers 0 and 1. The only difference is that it continually slides between the
two halves rather than discretely alternating between them.
3 Easy, that is, if we know the delay and the bandwidth. Sometimes we do not, and estimating them well is a challenge to
protocol designers. We discuss this further in Chapter 5.
2.5 Reliable Transmission
109
Note that this rule is specific to the situation where RWS = SWS. We leave it as
an exercise to determine the more general rule that works for arbitrary values of RWS
and SWS. Also note that the relationship between the window size and the sequence
number space depends on an assumption that is so obvious that it is easy to overlook,
namely, that frames are not reordered in transit. This cannot happen on a direct point-topoint link since there is no way for one frame to overtake another during transmission.
However, we will see the sliding window algorithm used in a different environment in
Chapter 5, and we will need to devise another rule.
Implementation of Sliding Window
The following routines illustrate how we might implement the sending and receiving
sides of the sliding window algorithm. The routines are taken from a working protocol
named, appropriately enough, Sliding Window Protocol (SWP). So as not to concern
ourselves with the adjacent protocols in the protocol graph, we denote the protocol sitting above SWP as high-level protocol (HLP) and the protocol sitting below SWP as a
link-level protocol (LINK).
We start by defining a pair of data structures. First, the frame header is very
simple: It contains a sequence number (SeqNum) and an acknowledgment number
(AckNum). It also contains a Flags field that indicates whether the frame is an ACK
or carries data.
typedef u_char SwpSeqno;
typedef struct {
SwpSeqno SeqNum; /* sequence number of this frame */
SwpSeqno AckNum; /* ack of received frame */
u_char
Flags;
/* up to 8 bits worth of flags */
} SwpHdr;
Next, the state of the sliding window algorithm has the following structure. For
the sending side of the protocol, this state includes variables LAR and LFS, as described
earlier in this section, as well as a queue that holds frames that have been transmitted but not yet acknowledged (sendQ). The sending state also includes a counting
semaphore called sendWindowNotFull. We will see how this is used below, but generally a semaphore is a synchronization primitive that supports semWait and semSignal operations. Every invocation of semSignal increments the semaphore by 1,
and every invocation of semWait decrements s by 1, with the calling process blocked
(suspended) should decrementing the semaphore cause its value to become less than 0. A
process that is blocked during its call to semWait will be allowed to resume as soon as
enough semSignal operations have been performed to raise the value of the semaphore
above 0.
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For the receiving side of the protocol, the state includes the variable NFE. This
is the next frame expected (i.e., the frame with a sequence number one more than the
last frame received (LFR), described earlier in this section). There is also a queue that
holds frames that have been received out of order (recvQ). Finally, although not shown,
the sender and receiver sliding window sizes are defined by constants SWS and RWS,
respectively.
typedef struct {
/* sender side state: */
SwpSeqno
LAR;
/* seqno of last ACK received */
SwpSeqno
LFS;
/* last frame sent */
Semaphore
sendWindowNotFull;
SwpHdr
hdr;
/* pre-initialized header */
struct sendQ_slot {
Event
timeout;
/* event associated with send-timeout */
Msg
msg;
} sendQ[SWS];
/* receiver side state: */
SwpSeqno
NFE;
/* seqno of next frame expected */
struct recvQ_slot {
int
received; /* is msg valid? */
Msg
msg;
} recvQ[RWS];
} SwpState;
The sending side of SWP is implemented by procedure sendSWP. This routine
is rather simple. First, semWait causes this process to block on a semaphore until it
is OK to send another frame. Once allowed to proceed, sendSWP sets the sequence
number in the frame’s header, saves a copy of the frame in the transmit queue (sendQ),
schedules a timeout event to handle the case in which the frame is not acknowledged,
and sends the frame to the next-lower-level protocol, which we denote as LINK.
One detail worth noting is the call to store_swp_hdr just before the call to
msgAddHdr. This routine translates the C structure that holds the SWP header
(state->hdr) into a byte string that can be safely attached to the front of the message (hbuf). This routine (not shown) must translate each integer field in the header
into network byte order and remove any padding that the compiler has added to the C
structure. The issue of byte order is discussed more fully in Section 7.1, but for now it is
enough to assume that this routine places the most significant bit of a multiword integer
in the byte with the highest address.
2.5 Reliable Transmission
111
Another piece of complexity in this routine is the use of semWait and the sendWindowNotFull semaphore. sendWindowNotFull is initialized to the size of the
sender’s sliding window, SWS (this initialization is not shown). Each time the sender
transmits a frame, the semWait operation decrements this count and blocks the sender
should the count go to 0. Each time an ACK is received, the semSignal operation invoked in deliverSWP (see below) increments this count, thus unblocking any waiting
sender.
static int
sendSWP(SwpState *state, Msg *frame)
{
struct sendQ_slot *slot;
hbuf[HLEN];
/* wait for send window to open */
semWait(&state->sendWindowNotFull);
state->hdr.SeqNum = ++state->LFS;
slot = &state->sendQ[state->hdr.SeqNum % SWS];
store_swp_hdr(state->hdr, hbuf);
msgAddHdr(frame, hbuf, HLEN);
msgSaveCopy(&slot->msg, frame);
slot->timeout = evSchedule(swpTimeout, slot,
SWP_SEND_TIMEOUT);
return send(LINK, frame);
}
Before continuing to the receive side of SWP, we need to reconcile a seeming inconsistency. On the one hand, we have been saying that a high-level protocol invokes the
services of a low-level protocol by calling the send operation, so we would expect that a
protocol that wants to send a message via SWP would call send(SWP, packet). On the
other hand, the procedure that implements SWP’s send operation is called sendSWP,
and its first argument is a state variable (SwpState). What gives? The answer is that
the operating system provides glue code that translates the generic call to send into a
protocol-specific call to sendSWP. This glue code maps the first argument to send
(the magic protocol variable SWP) into both a function pointer to sendSWP, and
a pointer to the protocol state that SWP needs to do its job. The reason we have the
high-level protocol indirectly invoke the protocol-specific function through the generic
function call is that we want to limit how much information the high-level protocol has
coded in it about the low-level protocol. This makes it easier to change the protocol
graph configuration at some time in the future.
Now to SWP’s protocol-specific implementation of the deliver operation, which
is given in procedure deliverSWP. This routine actually handles two different kinds
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of incoming messages: ACKs for frames sent earlier from this node and data frames
arriving at this node. In a sense, the ACK half of this routine is the counterpart to the
sender side of the algorithm given in sendSWP. A decision as to whether the incoming
message is an ACK or a data frame is made by checking the Flags field in the header.
Note that this particular implementation does not support piggybacking ACKs on data
frames.
When the incoming frame is an ACK, deliverSWP simply finds the slot in the
transmit queue (sendQ) that corresponds to the ACK, cancels the timeout event, and
frees the frame saved in that slot. This work is actually done in a loop since the ACK
may be cumulative. The only other thing to notice about this case is the call to subroutine swpInWindow. This subroutine, which is given below, ensures that the sequence
number for the frame being acknowledged is within the range of ACKs that the sender
currently expects to receive.
When the incoming frame contains data, deliverSWP first calls msgStripHdr
and load_swp_hdr to extract the header from the frame. Routine load_swp_hdr is
the counterpart to store_swp_hdr discussed earlier; it translates a byte string into the
C data structure that holds the SWP header. deliverSWP then calls swpInWindow
to make sure the sequence number of the frame is within the range of sequence numbers
that it expects. If it is, the routine loops over the set of consecutive frames it has received
and passes them up to the higher-level protocol by invoking the deliverHLP routine. It
also sends a cumulative ACK back to the sender, but does so by looping over the receive
queue (it does not use the SeqNumToAck variable used in the prose description given
earlier in this section).
static int
deliverSWP(SwpState state, Msg *frame)
{
SwpHdr
hdr;
char
*hbuf;
hbuf = msgStripHdr(frame, HLEN);
load_swp_hdr(&hdr, hbuf)
if (hdr->Flags & FLAG_ACK_VALID)
{
/* received an acknowledgment---do SENDER side */
if (swpInWindow(hdr.AckNum, state->LAR + 1,
state->LFS))
{
do
{
struct sendQ_slot *slot;
2.5 Reliable Transmission
113
slot = &state->sendQ[++state->LAR % SWS];
evCancel(slot->timeout);
msgDestroy(&slot->msg);
semSignal(&state->sendWindowNotFull);
} while (state->LAR != hdr.AckNum);
}
}
if (hdr.Flags & FLAG_HAS_DATA)
{
struct recvQ_slot *slot;
/* received data packet---do RECEIVER side */
slot = &state->recvQ[hdr.SeqNum % RWS];
if (!swpInWindow(hdr.SeqNum, state->NFE,
state->NFE + RWS - 1))
{
/* drop the message */
return SUCCESS;
}
msgSaveCopy(&slot->msg, frame);
slot->received = TRUE;
if (hdr.SeqNum == state->NFE)
{
Msg m;
while (slot->received)
{
deliver(HLP, &slot->msg);
msgDestroy(&slot->msg);
slot->received = FALSE;
slot = &state->recvQ[++state->NFE % RWS];
}
/* send ACK: */
prepare_ack(&m, state->NFE - 1);
send(LINK, &m);
msgDestroy(&m);
}
}
return SUCCESS;
}
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Finally, swpInWindow is a simple subroutine that checks to see if a given sequence number falls between some minimum and maximum sequence number.
static bool
swpInWindow(SwpSeqno seqno, SwpSeqno min, SwpSeqno max)
{
SwpSeqno pos, maxpos;
pos
= seqno - min;
/* pos *should* be in range [0..MAX) */
maxpos = max - min + 1;
/* maxpos is in range [0..MAX] */
return pos < maxpos;
}
Frame Order and Flow Control
The sliding window protocol is perhaps the best-known algorithm in computer networking. What is easily confusing about the algorithm, however, is that it can be used to
serve three different roles. The first role is the one we have been concentrating on in this
section—to reliably deliver frames across an unreliable link. (In general, the algorithm
can be used to reliably deliver messages across an unreliable network.) This is the core
function of the algorithm.
The second role that the sliding window algorithm can serve is to preserve the order
in which frames are transmitted. This is easy to do at the receiver—since each frame has
a sequence number, the receiver just makes sure that it does not pass a frame up to the
next-higher-level protocol until it has already passed up all frames with a smaller sequence
number. That is, the receiver buffers (i.e., does not pass along) out-of-order frames. The
version of the sliding window algorithm described in this section does preserve frame
order, although we could imagine a variation in which the receiver passes frames to
the next protocol without waiting for all earlier frames to be delivered. A question we
should ask ourselves is whether we really need the sliding window protocol to keep the
frames in order, or whether, instead, this is unnecessary functionality at the link level.
Unfortunately, we have not yet seen enough of the network architecture to answer this
question; we first need to understand how a sequence of point-to-point links is connected
by switches to form an end-to-end path.
The third role that the sliding window algorithm sometimes plays is to support
flow control—a feedback mechanism by which the receiver is able to throttle the sender.
Such a mechanism is used to keep the sender from overrunning the receiver, that is, from
transmitting more data than the receiver is able to process. This is usually accomplished
by augmenting the sliding window protocol so that the receiver not only acknowledges
2.5 Reliable Transmission
115
▲
frames it has received, but also informs the sender of how many frames it has room to
receive. The number of frames that the receiver is capable of receiving corresponds to
how much free buffer space it has. As in the case of ordered delivery, we need to make
sure that flow control is necessary at the link level before incorporating it into the sliding
window protocol.
One important concept to take away from this discussion is the system design principle we call separation of concerns. That is, you must be careful to distinguish between
different functions that are sometimes rolled together in one mechanism, and you must
make sure that each function is necessary and being supported in the most effective way.
In this particular case, reliable delivery, ordered delivery, and flow control are sometimes
combined in a single sliding window protocol, and we should ask ourselves if this is the
right thing to do at the link level. With this question in mind, we revisit the sliding
window algorithm in Chapter 3 (we show how X.25 networks use it to implement hopby-hop flow control) and in Chapter 5 (we describe how TCP uses it to implement a
reliable byte-stream channel).
2.5.3 Concurrent Logical Channels
The data link protocol used in the ARPANET provides an interesting alternative to the
sliding window protocol, in that it is able to keep the pipe full while still using the
simple stop-and-wait algorithm. One important consequence of this approach is that
the frames sent over a given link are not kept in any particular order. The protocol also
implies nothing about flow control.
The idea underlying the ARPANET protocol, which we refer to as concurrent logical channels, is to multiplex several logical channels onto a single point-to-point link and
to run the stop-and-wait algorithm on each of these logical channels. There is no relationship maintained among the frames sent on any of the logical channels, yet because
a different frame can be outstanding on each of the several logical channels, the sender
can keep the link full.
More precisely, the sender keeps 3 bits of state for each channel: a boolean, saying
whether the channel is currently busy; the 1-bit sequence number to use the next time a
frame is sent on this logical channel; and the next sequence number to expect on a frame
that arrives on this channel. When the node has a frame to send, it uses the lowest idle
channel, and otherwise it behaves just like stop-and-wait.
In practice, the ARPANET supported 8 logical channels over each ground link and
16 over each satellite link. In the ground-link case, the header for each frame included
a 3-bit channel number and a 1-bit sequence number, for a total of 4 bits. This is exactly the number of bits the sliding window protocol requires to support up to eight
outstanding frames on the link when RWS = SWS.
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2.6
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Ethernet (802.3)
The Ethernet is easily the most successful local area networking technology of the last 20
years. Developed in the mid-1970s by researchers at the Xerox Palo Alto Research Center
(PARC), the Ethernet is a working example of the more general carrier sense, multiple
access with collision detect (CSMA/CD) local area network technology.
As indicated by the CSMA name, the Ethernet is a multiple-access network, meaning that a set of nodes send and receive frames over a shared link. You can, therefore,
think of an Ethernet as being like a bus that has multiple stations plugged into it. The
“carrier sense” in CSMA/CD means that all the nodes can distinguish between an idle
and a busy link, and “collision detect” means that a node listens as it transmits and can
therefore detect when a frame it is transmitting has interfered (collided) with a frame
transmitted by another node.
The Ethernet has its roots in an early packet radio network, called Aloha, developed
at the University of Hawaii to support computer communication across the Hawaiian
Islands. Like the Aloha network, the fundamental problem faced by the Ethernet is how
to mediate access to a shared medium fairly and efficiently (in Aloha the medium was
the atmosphere, while in Ethernet the medium is a coax cable). That is, the core idea in
both Aloha and the Ethernet is an algorithm that controls when each node can transmit.
Digital Equipment Corporation and Intel Corporation joined Xerox to define a
10-Mbps Ethernet standard in 1978. This standard then formed the basis for IEEE
standard 802.3. With one exception that we will see in Section 2.6.2, it is fair to view
the 1978 Ethernet standard as a proper subset of the 802.3 standard; 802.3 additionally
defines a much wider collection of physical media over which Ethernet can operate, and
more recently, it has been extended to include a 100-Mbps version called Fast Ethernet,
and a 1,000-Mbps version called Gigabit Ethernet. The rest of this section focuses on the
10-Mbps Ethernet since it is typically used in multiple-access mode, and we are interested
in how multiple hosts share a single link. Both 100- and 1,000-Mbps Ethernets are
designed to be used in full-duplex, point-to-point configurations, which means that they
are typically used in switched networks, as described in the next chapter.
2.6.1 Physical Properties
An Ethernet segment is implemented on a coaxial cable of up to 500 m. This cable is similar to the type used for cable TV, except that it typically has an impedance of 50 ohms
instead of cable TV’s 75 ohms. Hosts connect to an Ethernet segment by tapping into
it; taps must be at least 2.5 m apart. A transceiver—a small device directly attached to
the tap—detects when the line is idle and drives the signal when the host is transmitting.
It also receives incoming signals. The transceiver is, in turn, connected to an Ethernet
adaptor, which is plugged into the host. All the logic that makes up the Ethernet pro-
2.6 Ethernet (802.3)
Figure 2.27
117
Ethernet transceiver and adaptor.
tocol, as described in this section, is implemented in the adaptor (not the transceiver).
This configuration is shown in Figure 2.27.
Multiple Ethernet segments can be joined together by repeaters. A repeater is a device that forwards digital signals, much like an amplifier forwards analog signals. However, no more than four repeaters may be positioned between any pair of hosts, meaning
that an Ethernet has a total reach of only 2,500 m. For example, using just two repeaters
between any pair of hosts supports a configuration similar to the one illustrated in Figure 2.28, that is, a segment running down the spine of a building with a segment on
each floor. All told, an Ethernet is limited to supporting a maximum of 1,024 hosts.
Any signal placed on the Ethernet by a host is broadcast over the entire network,
that is, the signal is propagated in both directions, and repeaters forward the signal on all
outgoing segments. Terminators attached to the end of each segment absorb the signal
and keep it from bouncing back and interfering with trailing signals. The Ethernet uses
the Manchester encoding scheme described in Section 2.2.
In addition to the system of segments and repeaters just described, alternative technologies have been introduced over the years. For example, rather than using a 50-ohm
coax cable, an Ethernet can be constructed from a thinner cable known as 10Base2; the
original cable is called 10Base5 (the two cables are commonly called thin-net and thicknet, respectively). The “10” in 10Base2 means that the network operates at 10 Mbps,
“Base” refers to the fact that the cable is used in a baseband system, and the “2” means
that a given segment can be no longer than 200 m (a segment of the original 10Base5
cable can be up to 500 m long). Today, a third cable technology is predominantly used,
called 10BaseT, where the “T” stands for twisted pair. Typically, Category 5 twisted pair
wiring is used. A 10BaseT segment is usually limited to under 100 m in length. (Both
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Figure 2.28
Direct Link Networks
Ethernet repeater.
100- and 1,000-Mbps Ethernets also run up over Category 5 twisted pair, up to distances
of 100 m.)
Because the cable is so thin, you do not tap into a 10Base2 or 10BaseT cable
in the same way as you would with 10Base5 cable. With 10Base2, a T-joint is spliced
into the cable. In effect, 10Base2 is used to daisy-chain a set of hosts together. With
10BaseT, the common configuration is to have several point-to-point segments coming
out of a multiway repeater, sometimes called a hub, as illustrated in Figure 2.29. Multiple
Figure 2.29
Ethernet hub.
2.6 Ethernet (802.3)
119
100-Mbps Ethernet segments can also be connected by a hub, but the same is not true
of 1,000-Mbps segments.
It is important to understand that whether a given Ethernet spans a single segment,
a linear sequence of segments connected by repeaters, or multiple segments connected
in a star configuration by a hub, data transmitted by any one host on that Ethernet
reaches all the other hosts. This is the good news. The bad news is that all these hosts are
competing for access to the same link, and as a consequence, they are said to be in the
same collision domain.
2.6.2 Access Protocol
We now turn our attention to the algorithm that controls access to the shared Ethernet
link. This algorithm is commonly called the Ethernet’s media access control (MAC). It
is typically implemented in hardware on the network adaptor. We will not describe the
hardware per se, but instead focus on the algorithm it implements. First, however, we
describe the Ethernet’s frame format and addresses.
Frame Format
Each Ethernet frame is defined by the format given in Figure 2.30. The 64-bit preamble allows the receiver to synchronize with the signal; it is a sequence of alternating 0s
and 1s. Both the source and destination hosts are identified with a 48-bit address. The
packet type field serves as the demultiplexing key, that is, it identifies to which of possibly many higher-level protocols this frame should be delivered. Each frame contains
up to 1,500 bytes of data. Minimally, a frame must contain at least 46 bytes of data,
even if this means the host has to pad the frame before transmitting it. The reason for
this minimum frame size is that the frame must be long enough to detect a collision; we
discuss this more below. Finally, each frame includes a 32-bit CRC. Like the HDLC protocol described in Section 2.3.2, the Ethernet is a bit-oriented framing protocol. Note
that from the host’s perspective, an Ethernet frame has a 14-byte header: two 6-byte addresses and a 2-byte type field. The sending adaptor attaches the preamble, CRC, and
postamble before transmitting, and the receiving adaptor removes them.
The frame format just described is taken from the Digital-Intel-Xerox Ethernet
standard. The 802.3 frame format is exactly the same, except it substitutes a 16-bit length
Figure 2.30
Ethernet frame format.
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field for the 16-bit type field. 802.3 is usually paired with an encapsulation standard that
defines a type field used to demultiplex incoming frames. This type field is the first thing
in the data portion of the 802.3 frames, that is, it immediately follows the 802.3 header.
Fortunately, since the Ethernet standard has avoided using any type values less than 1,500
(the maximum length found in an 802.3 header), and the type and length fields are in
the same location in the header, it is possible for a single device to accept both formats,
and for the device driver running on the host to interpret the last 16 bits of the header
as either a type or a length. In practice, most hosts follow the Digital-Intel-Xerox format
and interpret this field as the frame’s type.
Addresses
Each host on an Ethernet—in fact, every Ethernet host in the world—has a unique Ethernet address. Technically, the address belongs to the adaptor, not the host; it is usually
burned into ROM. Ethernet addresses are typically printed in a form humans can read
as a sequence of six numbers separated by colons. Each number corresponds to 1 byte
of the 6-byte address and is given by a pair of hexadecimal digits, one for each of the
4-bit nibbles in the byte; leading 0s are dropped. For example, 8:0:2b:e4:b1:2 is the
human-readable representation of Ethernet address
00001000 00000000 00101011 11100100 10110001 00000010
To ensure that every adaptor gets a unique address, each manufacturer of Ethernet
devices is allocated a different prefix that must be prepended to the address on every
adaptor they build. For example, Advanced Micro Devices has been assigned the 24-bit
prefix x080020 (or 8:0:20). A given manufacturer then makes sure the address suffixes
it produces are unique.
Each frame transmitted on an Ethernet is received by every adaptor connected to
that Ethernet. Each adaptor recognizes those frames addressed to its address and passes
only those frames on to the host. (An adaptor can also be programmed to run in promiscuous mode, in which case it delivers all received frames to the host, but this is not the
normal mode.) In addition to these unicast addresses, an Ethernet address consisting of
all 1s is treated as a broadcast address; all adaptors pass frames addressed to the broadcast
address up to the host. Similarly, an address that has the first bit set to 1 but is not the
broadcast address is called a multicast address. A given host can program its adaptor to
accept some set of multicast addresses. Multicast addresses are used to send messages to
some subset of the hosts on an Ethernet (e.g., all file servers). To summarize, an Ethernet
adaptor receives all frames and accepts
■
Frames addressed to its own address;
■
Frames addressed to the broadcast address;
2.6 Ethernet (802.3)
121
■
Frames addressed to a multicast address, if it has been instructed to listen to that
address;
■
All frames, if it has been placed in promiscuous mode.
It passes to the host only the frames that it accepts.
Transmitter Algorithm
As we have just seen, the receiver side of the Ethernet protocol is simple; the real smarts
are implemented at the sender’s side. The transmitter algorithm is defined as follows.
When the adaptor has a frame to send and the line is idle, it transmits the frame
immediately; there is no negotiation with the other adaptors. The upper bound of
1,500 bytes in the message means that the adaptor can occupy the line for only a fixed
length of time.
When an adaptor has a frame to send and the line is busy, it waits for the line to go
idle and then transmits immediately.4 The Ethernet is said to be a 1-persistent protocol
because an adaptor with a frame to send transmits with probability 1 whenever a busy
line goes idle. In general, a p-persistent algorithm transmits with probability 0 ≤ p ≤ 1
after a line becomes idle, and defers with probability q = 1 − p. The reasoning behind
choosing a p < 1 is that there might be multiple adaptors waiting for the busy line to
become idle, and we don’t want all of them to begin transmitting at the same time.
If each adaptor transmits immediately with a probability of, say, 33%, then up to three
adaptors can be waiting to transmit and the odds are that only one will begin transmitting
when the line becomes idle. Despite this reasoning, an Ethernet adaptor always transmits
immediately after noticing that the network has become idle and has been very effective
in doing so.
To complete the story about p-persistent protocols for the case when p < 1, you
might wonder how long a sender that loses the coin flip (i.e., decides to defer) has to wait
before it can transmit. The answer for the Aloha network, which originally developed this
style of protocol, was to divide time into discrete slots, with each slot corresponding to
the length of time it takes to transmit a full frame. Whenever a node has a frame to
send and it senses an empty (idle) slot, it transmits with probability p and defers until
the next slot with probability q = 1 − p. If that next slot is also empty, the node again
decides to transmit or defer, with probabilities p and q, respectively. If that next slot is
not empty—that is, some other station has decided to transmit—then the node simply
waits for the next idle slot and the algorithm repeats.
Returning to our discussion of the Ethernet, because there is no centralized control
it is possible for two (or more) adaptors to begin transmitting at the same time, either
4 To be more precise, all adaptors wait 9.6 µs after the end of one frame before beginning to transmit the next frame. This
is true for both the sender of the first frame, as well as those nodes listening for the line to become idle.
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because both found the line to be idle or because both had been waiting for a busy
line to become idle. When this happens, the two (or more) frames are said to collide on
the network. Each sender, because the Ethernet supports collision detection, is able to
determine that a collision is in progress. At the moment an adaptor detects that its frame
is colliding with another, it first makes sure to transmit a 32-bit jamming sequence and
then stops the transmission. Thus, a transmitter will minimally send 96 bits in the case
of a collision: 64-bit preamble plus 32-bit jamming sequence.
One way that an adaptor will send only 96 bits—which is sometimes called a runt
frame—is if the two hosts are close to each other. Had the two hosts been farther apart,
they would have had to transmit longer, and thus send more bits, before detecting the
collision. In fact, the worst-case scenario happens when the two hosts are at opposite
ends of the Ethernet. To know for sure that the frame it just sent did not collide with
another frame, the transmitter may need to send as many as 512 bits. Not coincidentally,
every Ethernet frame must be at least 512 bits (64 bytes) long: 14 bytes of header plus
46 bytes of data plus 4 bytes of CRC.
Why 512 bits? The answer is related to another question you might ask about an
Ethernet: Why is its length limited to only 2,500 m? Why not 10 or 1,000 km? The
answer to both questions has to do with the fact that the farther apart two nodes are, the
longer it takes for a frame sent by one to reach the other, and the network is vulnerable
to a collision during this time.
Figure 2.31 illustrates the worst-case scenario, where hosts A and B are at opposite
ends of the network. Suppose host A begins transmitting a frame at time t, as shown
in (a). It takes it one link latency (let’s denote the latency as d ) for the frame to reach
host B. Thus, the first bit of A’s frame arrives at B at time t + d , as shown in (b). Suppose
an instant before host A’s frame arrives (i.e., B still sees an idle line), host B begins
to transmit its own frame. B’s frame will immediately collide with A’s frame, and this
collision will be detected by host B (c). Host B will send the 32-bit jamming sequence,
as described above. (B’s frame will be a runt.) Unfortunately, host A will not know that
the collision occurred until B’s frame reaches it, which will happen one link latency
later, at time t + 2 × d , as shown in (d). Host A must continue to transmit until this
time in order to detect the collision. In other words, host A must transmit for 2 × d
to be sure that it detects all possible collisions. Considering that a maximally configured
Ethernet is 2,500 m long, and that there may be up to four repeaters between any two
hosts, the round-trip delay has been determined to be 51.2 µs, which on a 10-Mbps
Ethernet corresponds to 512 bits. The other way to look at this situation is that we need
to limit the Ethernet’s maximum latency to a fairly small value (e.g., 51.2 µs) for the
access algorithm to work; hence, an Ethernet’s maximum length must be something on
the order of 2,500 m.
2.6 Ethernet (802.3)
123
Figure 2.31 Worst-case scenario: (a) A sends a frame at time t ; (b) A’s frame arrives at
B at time t + d ; (c) B begins transmitting at time t + d and collides with A’s frame;
(d) B’s runt (32-bit) frame arrives at A at time t + 2d.
Once an adaptor has detected a collision and stopped its transmission, it waits a
certain amount of time and tries again. Each time it tries to transmit but fails, the adaptor doubles the amount of time it waits before trying again. This strategy of doubling
the delay interval between each retransmission attempt is a general technique known as
exponential backoff. More precisely, the adaptor first delays either 0 or 51.2 µs, selected
at random. If this effort fails, it then waits 0, 51.2, 102.4, or 153.6 µs (selected randomly) before trying again; this is k × 51.2 for k = 0..3. After the third collision, it
waits k × 51.2 for k = 0..23 − 1, again selected at random. In general, the algorithm
randomly selects a k between 0 and 2n − 1 and waits k × 51.2 µs, where n is the number
of collisions experienced so far. The adaptor gives up after a given number of tries and
reports a transmit error to the host. Adaptors typically retry up to 16 times, although the
backoff algorithm caps n in the above formula at 10.
2.6.3 Experience with Ethernet
Because Ethernets have been around for so many years and are so popular, we have a
great deal of experience in using them. One of the most important observations people
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have made about Ethernets is that they work best under lightly loaded conditions. This
is because under heavy loads—typically, a utilization of over 30% is considered heavy on
an Ethernet—too much of the network’s capacity is wasted by collisions.
Fortunately, most Ethernets are used in a far more conservative way than the standard allows. For example, most Ethernets have fewer than 200 hosts connected to them,
which is far fewer than the maximum of 1,024. (See if you can discover a reason for this
upper limit of around 200 hosts in Chapter 4.) Similarly, most Ethernets are far shorter
than 2,500 m, with a round-trip delay of closer to 5 µs than 51.2 µs. Another factor that
makes Ethernets practical is that, even though Ethernet adaptors do not implement linklevel flow control, the hosts typically provide an end-to-end flow-control mechanism.
As a result, it is rare to find situations in which any one host is continuously pumping
frames onto the network.
Finally, it is worth saying a few words about why Ethernets have been so successful,
so that we can understand the properties we should emulate with any LAN technology
that tries to replace it. First, an Ethernet is extremely easy to administer and maintain:
There are no switches that can fail, no routing or configuration tables that have to be kept
up-to-date, and it is easy to add a new host to the network. It is hard to imagine a simpler
network to administer. Second, it is inexpensive: Cable is cheap, and the only other
cost is the network adaptor on each host. Any switch-based approach will involve an
investment in some relatively expensive infrastructure (the switches), in addition to the
incremental cost of each adaptor. As we will see in the next chapter, the most successful
LAN switching technology in use today is itself based on Ethernet.
2.7
Rings (802.5, FDDI, RPR)
Ring networks, like Ethernets, are shared-media networks. This section will focus on
the type that was for years the most prevalent, known as the IBM Token Ring. Like
the Xerox Ethernet, IBM’s Token Ring has a nearly identical IEEE standard, known as
802.5. 802.5 and the later Fiber Distributed Data Interface (FDDI) token ring are no
longer in widespread use. Resilient Packet Ring (RPR) is a relatively recent technology,
and its corresponding IEEE standard is known as 802.17; it remains to be seen how
popular RPR will be.
As the name suggests, a ring network consists of a set of nodes connected in a
ring (see Figure 2.32). Data always flows in a particular direction around the ring, with
each node receiving frames from its upstream neighbor and then forwarding them to
its downstream neighbor. This ring-based topology is in contrast to the Ethernet’s bus
topology. Like the Ethernet, however, the ring is viewed as a single shared medium; it
does not behave as a collection of independent point-to-point links that just happen to
be configured in a loop. Thus, a ring network shares two key features with an Ethernet:
2.7
Figure 2.32
Rings (802.5, FDDI, RPR)
125
A ring network.
First, it involves a distributed algorithm that controls when each node is allowed to
transmit, and second, all nodes typically5 see all frames, with the node identified in the
frame header as the destination saving a copy of the frame as it flows past.
The most common early forms of ring network were all token rings. The word
“token” comes from the way access to the shared ring is managed. The idea is that a
token, which is really just a special sequence of bits, circulates around the ring; each node
receives and then forwards the token. When a node that has a frame to transmit sees the
token, it takes the token off the ring (i.e., it does not forward the special bit pattern)
and instead inserts its frame into the ring. Each node along the way simply forwards the
frame, with the destination node saving a copy and forwarding the message onto the next
node on the ring. When the frame makes its way back around to the sender, this node
strips its frame off the ring (rather than continuing to forward it) and reinserts the token.
In this way, some node downstream will have the opportunity to transmit a frame. The
media access algorithm is fair in the sense that as the token circulates around the ring,
each node gets a chance to transmit. Nodes are serviced in a round-robin fashion.
One of the first things you might worry about with a ring topology is that any link
or node failure would render the whole network useless. The problem of node failure
may be addressed by connecting each station into the ring using an electromechanical
relay. As long as the station is healthy, the relay is open and the station is included in
the ring. If the station stops providing power, the relay closes and the ring automatically
bypasses the station. This is illustrated in Figure 2.33. Note that this approach is only
effective when the transmission medium is electrical cable, not optical fiber.
5 We will see an exception to this in Section 2.7.4.
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Figure 2.33 Relay used on a token ring: (a) relay open—host active; (b) relay
closed—host bypassed.
Figure 2.34
Multistation access unit.
Several of these relays are usually packed into a single box, known as a multistation
access unit (MSAU). This has the interesting effect of making a token ring actually look
more like a star topology, as shown in Figure 2.34. Any failure of a link outside the
MSAU is then equivalent to a host failure, hence solved by the same relay mechanism.
It also makes it very easy to add stations to and remove stations from the network, since
they can just be plugged into or unplugged from the nearest MSAU, while the overall
wiring of the network can be left unchanged.
2.7
Rings (802.5, FDDI, RPR)
127
2.7.1 Token Ring Media Access Control
It is now time to look a little more closely at how the MAC protocol operates on a token
ring. The network adaptor for a token ring contains a receiver and a transmitter. Most
of the time, when a node is neither the source nor the destination of the data on the
ring, its adaptor is simply retransmitting the data that its receiver receives. When none
of the stations connected to the ring has anything to send, the token circulates around
the ring. As it does so, any station that has data to send may “seize” the token, that is,
not retransmit it and begin sending data. Once a station has the token, it is allowed to
send one or more packets—exactly how many more depends on some factors described
below.
Each transmitted packet contains the destination address of the intended receiver;
it may also contain a multicast (or broadcast) address if it is intended to reach more than
one (or all) receivers. As the packet flows past each node on the ring, each node looks
inside the packet to see if it is the intended recipient. If so, it copies the packet into a
buffer as it flows through the network adaptor, but it does not remove the packet from
the ring. The sending station has the responsibility of removing the packet from the
ring.
One issue we must address is how much data a given node is allowed to transmit
each time it possesses the token or, equivalently, how long a given node is allowed to hold
the token: the token holding time (THT). If we assume that most nodes on the network
do not have data to send at any given time—a reasonable assumption, and certainly one
that the Ethernet takes advantage of—then we could make a case for letting a node that
possesses the token transmit as much data as it has before passing the token on to the
next node, in effect setting the THT to infinity. The danger is that a single station could
monopolize the ring for an arbitrarily long time, but we could certainly set the THT to
significantly more than the time to send one packet.
It is easy to see that the more bytes a node can send each time it has the token, the
better the utilization of the ring you can achieve in the situation in which only a single
node has data to send. The downside, of course, is that this strategy does not work well
when multiple nodes have data to send—it favors nodes that have a lot of data to send
over nodes that have only a small message to send, even when it is important to get this
small message delivered as soon as possible.
That issue is addressed by the 802.5 protocol’s support for different levels of priority. The token contains a 3-bit priority field, so we can think of the token having a
certain priority n at any time. Each device that wants to send a packet assigns a priority
to that packet, and the device can only seize the token to transmit a packet if the packet’s
priority is at least as great as the token’s. The priority of the token changes over time due
to the use of three reservation bits in the frame header. For example, a station X waiting
to send a priority n packet may set these bits to n if it sees a data frame going past and
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Figure 2.35
Direct Link Networks
Token release: (a) early versus (b) delayed.
the bits have not already been set to a higher value. This causes the station that currently
holds the token to elevate its priority to n when it releases it. Station X is responsible for
lowering the token priority to its old value when it is done.
Note that this is a strict priority scheme, in the sense that no lower-priority packets
get sent when higher-priority packets are waiting. This may cause lower-priority packets
to be locked out of the ring for extended periods if there is a sufficient supply of highpriority packets.
The 802.5 protocol provides a form of reliable delivery using 2 bits in the packet
trailer, the A and C bits. These are both 0 initially. When a station sees a frame for
which it is the intended recipient, it sets the A bit in the frame. When it copies the
frame into its adaptor, it sets the C bit. If the sending station sees the frame come back
over the ring with the A bit still 0, it knows that the intended recipient is not functioning or absent. If the A bit is set but not the C bit, this implies that for some reason (e.g., lack of buffer space), the destination could not accept the frame. Thus, the
frame might reasonably be retransmitted later in the hope that buffer space had become
available.
One final issue will complete our discussion of the MAC protocol, which is the
matter of exactly when the sending node releases the token. As illustrated in Figure 2.35,
the sender can insert the token back onto the ring immediately following its frame (this is
called early release) or after the frame it transmits has gone all the way around the ring and
been removed (this is called delayed release). Clearly early release allows better bandwidth
utilization, especially on large rings. 802.5 originally used delayed token release, but
support for early release was subsequently added.
2.7
Rings (802.5, FDDI, RPR)
129
2.7.2 Token Ring Maintenance
Each 802.5 token ring has one station designated as a monitor. The monitor’s job is to
ensure the health of the ring by, for example, making sure that the token is not lost. Any
station on the ring can become the monitor, and there are defined procedures by which
the monitor is elected when the ring is first connected or on the failure of the current
monitor. A healthy monitor periodically announces its presence with a special control
message; if a station fails to see such a message for some period of time, it will assume
that the monitor has failed and will try to become the monitor. The procedures for
electing a monitor are the same whether the ring has just come up or the active monitor
has just failed.
When a station decides that a new monitor is needed, it transmits a “claim token”
frame, announcing its intent to become the new monitor. If that token circulates back
to the sender, it can assume that it is okay for it to become the monitor. If some other
station is also trying to become the monitor at the same instant, the sender might see
a claim token message from that other station first. In this case, it will be necessary to
break the tie using some well-defined rule like “highest address wins.”
One responsibility of the monitor is to make sure that there is always a token
somewhere in the ring, either circulating or currently held by a station. It should be clear
that a token may vanish for several reasons, such as a bit error, or a crash on the part of a
station that was holding it. To detect a missing token, the monitor watches for a passing
token and maintains a timer equal to the maximum possible token rotation time. This
interval equals
NumStations × THT + RingLatency
where NumStations is the number of stations on the ring, and RingLatency is the
total propagation delay of the ring. If the timer expires without the monitor seeing a
token, it creates a new one.
The monitor also checks for corrupted or orphaned frames. The former have checksum errors or invalid formats, and without monitor intervention, they could circulate
forever on the ring. The monitor drains them off the ring before reinserting the token.
An orphaned frame is one that was transmitted correctly onto the ring but whose “parent” died, that is, the sending station went down before it could remove the frame from
the ring. These are detected using another header bit, the “‘monitor” bit. This is 0 on
transmission and set to 1 the first time the packet passes the monitor. If the monitor
sees a packet with this bit set, it knows the packet is going by for the second time and it
drains the packet off the ring.
One additional ring maintenance function is the detection of dead stations. The
relays in the MSAU can automatically bypass a station that has been disconnected or
powered down, but may not detect more subtle failures. If any station suspects a failure
130
2
Figure 2.36
Direct Link Networks
802.5/token ring frame format.
on the ring, it can send a beacon frame to the suspect destination. Based on how far this
frame gets, the status of the ring can be established, and malfunctioning stations can be
bypassed by the relays in the MSAU.
Frame Format
We are now ready to define the 802.5 frame format, which is depicted in Figure 2.36.
802.5 uses differential Manchester encoding. This fact is used by the frame format, which
uses “illegal” Manchester codes in the start and end delimiters. After the start delimiter
comes the access control byte, which includes the frame priority and the reservation
priority mentioned above. The frame control byte is a demux key that identifies the
higher-layer protocol.
Similar to the Ethernet, 802.5 addresses are 48 bits long. The frame also includes a
32-bit CRC. This is followed by the frame status byte, which includes the A and C bits
for reliable delivery.
2.7.3 FDDI
Although FDDI is similar to 802.5 in many respects, there are significant differences.
For one, FDDI runs on fiber, not copper (although a later standard, CDDI, was defined
to allow copper links to be used). A more interesting difference is that an FDDI network
consists of a dual ring—two independent rings that transmit data in opposite directions,
as illustrated in Figure 2.37(a). The second ring is not used during normal operation
but instead comes into play only if the primary ring fails, as depicted in Figure 2.37(b).
That is, the ring loops back on the secondary fiber to form a complete ring, and as a
consequence, an FDDI network is able to tolerate a single break in the cable or the
failure of one station.
Another interesting difference is that instead of designating one node as a monitor,
all the nodes participate equally in maintaining the FDDI ring. Each node maintains an
estimate of the token rotation time (TRT)—the expected maximum time for the token
to make one complete trip around the ring. A node then measures the time between
successive arrivals of the token. If too much time elapses, suggesting that the token has
been lost, the node transmits a “claim” frame, in which it includes its current TRT
estimate. This claim serves two functions. First, the claim frame is a vote for a particular
value of the TRT. Any node that wants to vote for a shorter TRT will replace that claim
2.7
Figure 2.37
Rings (802.5, FDDI, RPR)
131
Dual-fiber ring: (a) normal operation; (b) failure of the primary ring.
frame with its own claim frame, otherwise it will accept the new time and forward the
claim frame. Second, the claim frame is a request for authorization to regenerate the
token. If a claim frame makes it all the way back around to the original sender, that node
knows not only that the TRT it voted for was the shortest and has been accepted by all
the other nodes, but also that it has been authorized to regenerate the token.
When the token arrives at a node with time to spare (i.e., in less than one TRT
since the last time this node saw the token), the node can transmit data so long as it
does not make the token fall behind schedule; otherwise, the node cannot transmit data.
A shortcoming of this basic scheme is that it cannot guarantee any particular node the
opportunity to transmit regularly, even if that node has data that is sensitive to jitter,
because an upstream node could consume all the available time. To account for this
possibility, FDDI defines two classes of traffic: synchronous and asynchronous. When a
node receives a token, it is always allowed to send synchronous data, without regard for
whether the token is early or late. In contrast, a node can send asynchronous traffic only
when the token is early.
2.7.4 Resilient Packet Ring (802.17)
Resilient Packet Ring (RPR) is a relatively recent technology that has been standardized
by the IEEE as 802.17. While it bears some similarity to the ring technologies described
above, it was designed with slightly different goals in mind, which led to some key differences in the protocol. Resiliency—the ability to recover quickly from a link or node
failure—was a key design goal, to make the technology suitable for service provider networks. Historically this had been provided at lower layers (e.g., by SONET’s protection
mechanisms). Other design goals included bandwidth efficiency and quality of service
(QoS) support, which had quite an impact on the protocol.
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Like FDDI, RPR consists of two counterrotating optical fiber rings; unlike FDDI,
it takes advantage of the bandwidth of both rings during normal operation. And unlike
the previously described rings, an RPR frame is removed from the ring by the receiving
node instead of leaving it to be removed by the sender, thereby freeing up some of the
bandwidth on the ring in what is called spatial reuse.
Most strikingly, RPR does not use tokens. Instead, RPR uses a technique called
buffer insertion. In a buffer insertion ring, a node can transmit its own frames whenever it has no other frames to forward. If a frame arrives while the node is transmitting
its own frame, then the node temporarily
buffers that frame. One of the major chalWhere Are They Now
lenges for buffer insertion rings is how to
avoid starvation and enforce QoS guarThe Future of Rings
antees, since in its simplest form a buffer
The history of rings has seen them
insertion ring could allow a station to hog
compete against Ethernet and ultithe link indefinitely. RPR addresses this
mately lose on several occasions. 802.5
issue with fairly sophisticated QoS and
eventually lost out to 10-Mbit Ethfairness mechanisms.
ernet for a variety of reasons, not
RPR supports three QoS classes:
least of which being the developclass A provides low latency and low jitment of switched Ethernet, a topic
ter (e.g., for phone calls), class B prowe will discuss in the next chapter.
vides predictable latency and jitter (e.g.,
FDDI was proposed as the faster alfor prerecorded multimedia), and class C
ternative to Ethernet, but then Ethprovides a best-effort transport.
ernet got faster too, and without the
To meet the resiliency goals, RPR
need for costly fiber optics, and FDDI
uses two mechanisms to recover from the
never really caught on. The one ring
failure of a link or node. The first, wraptechnology that is still seeing some
ping, is similar to the approach described
significant deployment is RPR, priabove for FDDI. The second, steering,
marily in metropolitan area networks
is more sophisticated: nodes adjacent to
(MANs), although it seems likely that
the failure notify the other nodes, which
“metro Ethernet” will eventually come
are then able to direct packets in the
to dominate here just as Ethernet has
correct (unbroken) direction around the
done in LANs. There is, however, at
ring toward any given destination, even if
least one reason RPR has had some
that is the “long” way around the ring—
success in MANs, which is the fact
assuming the destination is not the node
that rings are something of a natthat just failed, of course.
ural fit for this kind of network, in
A final interesting aspect of RPR is
a way that they are not in the LAN.
that it was designed to run over previWhereas it is cheap enough in a LAN
ously defined physical layers, including
????
2.8
Wireless
133
SONET and the physical layer specified for Ethernet. This saved the designers the time
and effort of developing their own physical layer specs and hardware—a good example
of the value of layered architectures.
2.8
Wireless
Wireless technologies differ in a variety of dimensions, most notably in how much bandwidth they provide and how far apart communicating nodes can be. Other important differences include which part of the electromagnetic spectrum they use (including whether
it requires a license) and how much power they consume (important for mobile nodes).
In this section we discuss four promiWhere Are They Now
nent wireless technologies: Bluetooth,
Wi-Fi (more formally known as 802.11),
to string cables in a hub-and-spoke
WiMAX (802.16), and third-generation
manner from a central switch to each
or 3G cellular wireless. In the following
workstation, a ring actually provides
sections we present them in order from
a very cost-effective way to interconshortest range to longest. Table 2.6 gives
nect nodes in a MAN, where the cost
an overview of these technologies and
of obtaining rights-of-way and layhow they relate to each other.
ing fiber can be significant. The reThe most widely used wireless links
siliency of a ring is also attractive in
today are usually asymmetric, that is, the
this environment—the fact that you
two endpoints are usually different kinds
have both a “clockwise” and an “counof nodes. One endpoint, sometimes deterclockwise” path between any two
scribed as the base station, usually has no
points ensures that a single fiber cut
mobility, but has a wired (or at least high
won’t cut off a customer. RPR was also
bandwidth) connection to the Internet or
developed with some fairness mechother networks as in Figure 2.38. The
anisms that ensure that a node’s lonode at the other end of the link—shown
cation on the ring doesn’t put it at
here as a “client node”—is often mobile,
an unfair advantage or disadvantage
and relies on its link to the base stato another node in another location
tion for all its communication with other
when it comes to getting access to the
nodes.
bandwidth—this is harder to achieve
Observe that in Figure 2.38 we
with Ethernet. Thus, while there is
have used a wavy pair of lines to reprecertainly plenty of momentum behind
sent the wireless “link” abstraction proEthernet in the MAN, it is probably
vided between two devices (e.g., between
too soon to predict the demise of RPR
a base station and one of its client nodes).
in this environment.
One of the interesting aspects of wireless
????
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Bluetooth
802.15.1
Wi-Fi 802.11
WiMAX
802.16
3G Cellular
Typical link
length
10 m
100 m
10 km
Tens of km
Typical
bandwidth
2.1 Mbps
(shared)
54 Mbps
(shared)
70 Mbps
(shared)
384+ Kbps (per
connection)
Typical use
Link a
peripheral to a
notebook
computer
Link a
notebook
computer to a
wired base
Link a building
to a wired tower
Link a cell
phone to a
wired tower
Wired technology analogy
USB
Ethernet
Coaxial cable
DSL
Table 2.6
Overview of leading wireless technologies.
Figure 2.38
A wireless network using a base station.
2.8
Figure 2.39
Wireless
135
A wireless ad hoc or mesh network.
communication is that it naturally supports point-to-multipoint communication, because radio waves sent by one device can be simultaneously received by many devices.
However, it is often useful to create a point-to-point link abstraction for higher-layer
protocols, and we will see examples of how this works later in this section.
Note that in Figure 2.38, communication between nonbase (client) nodes is routed
via the base station. This is in spite of the fact that radio waves emitted by one client node
may well be received by other client nodes—the common base station model does not
permit direct communication between the client nodes.
This topology implies three qualitatively different levels of mobility. The first level
is no mobility, such as when a receiver must be in a fixed location to receive a directional
transmission from the base station, as is the case with the initial version of WiMAX. The
second level is mobility within the range of a base, as is the case with Bluetooth. The
third level is mobility between bases, as is the case with cell phones and Wi-Fi.
An alternative topology that is seeing increasing interest is the mesh or ad hoc network. In a wireless mesh, nodes are peers (i.e., there is no special base station node).
Messages may be forwarded via a chain of peer nodes as long as each node is within
range of the preceding node. This is illustrated in Figure 2.39. This allows the wireless
portion of a network to extend beyond the limited range of a single radio. From the point
of view of competition between technologies, this allows a shorter-range technology to
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extend its range and potentially compete with a longer-range technology. Meshes also
offer fault tolerance by providing multiple routes for a message to get from point A to
point B. A mesh network can be extended incrementally, with incremental costs. On
the other hand, a mesh requires nonbase nodes to have a certain level of sophistication
in their hardware and software, potentially increasing per-unit costs—and power consumption, a critical consideration for battery-powered devices. Wireless mesh networks
are of considerable research interest, but they are still in their relative infancy compared
to networks with base stations, and thus we do not cover them further here.
We now turn our attention to the details of the four wireless technologies mentioned above, beginning with the most short-range technology, Bluetooth.
2.8.1 Bluetooth (802.15.1)
Bluetooth fills the niche of very short-range communication between mobile phones,
PDAs, notebook computers, and other personal or peripheral devices. For example, Bluetooth can be used to connect a mobile phone to a headset, or a notebook computer to a
printer. Roughly speaking, Bluetooth is a more convenient alternative to connecting two
devices with a wire. In such applications, it is not necessary to provide much range or
bandwidth. This is fortunate for some of the target battery-powered devices, since it is
important that they not consume much power.
Bluetooth operates in the license-exempt band at 2.45 GHz. It has a range of only
about 10 m. For this reason, and because the communicating devices typically belong to
one individual or group, Bluetooth is sometimes categorized as a personal area network
(PAN). Version 2.0 provides speeds up to 2.1 Mbps. Power consumption is low.
Bluetooth is specified by an industry consortium called the Bluetooth Special Interest Group. It specifies an entire suite of protocols, going beyond the link layer to define
application protocols, which it calls profiles, for a range of applications. For example,
there is a profile for synchronizing a PDA with a personal computer. Another profile
gives a mobile computer access to a wired LAN in the manner of 802.11, although this
was not Bluetooth’s original goal. The IEEE 802.15.1 standard is based on Bluetooth
but excludes the application protocols.
The basic Bluetooth network configuration, called a piconet, consists of a master device and up to seven slave devices, as in Figure 2.40. Any communication is between the
master and a slave; the slaves do not communicate directly with each other. Because slaves
have a simpler role, their Bluetooth hardware and software can be simpler and cheaper.
Since Bluetooth operates in an license-exempt band, it is required to use a spread
spectrum technique (as discussed in Section 2.1.2) to deal with possible interference
in the band. It uses frequency hopping with 79 channels (frequencies), using each for
625 µm at a time. This provides a natural time slot for Bluetooth to use for synchronous
time division multiplexing. A frame takes up 1, 3, or 5 consecutive time slots. Only
2.8
Figure 2.40
Wireless
137
A Bluetooth piconet.
the master can start to transmit in odd-numbered slots. A slave can start to transmit in
an even-numbered slot, but only in response to a request from the master during the
previous slot, thereby preventing any contention between the slave devices.
A slave device can be parked : set to an inactive, low-power state. A parked device
cannot communicate on the piconet; it can only be reactivated by the master. A piconet
can have up to 255 parked devices in addition to its active slave devices.
ZigBee is a newer technology that competes with Bluetooth to some extent. Devised by the ZigBee alliance and standardized as IEEE 802.15.4, it is designed for situations where the bandwidth requirements are low and power consumption must be
very low to give very long battery life. It is also intended to be simpler and cheaper than
Bluetooth, making it financially feasible to incorporate in cheaper devices such as a wall
switch that wirelessly communicates with a ceiling-mounted fan.
2.8.2 Wi-Fi (802.11)
This section takes a closer look at a specific technology centered around the emerging
IEEE 802.11 standard, also known as Wi-Fi.6 Wi-Fi is technically a trademark, owned by
6 There is some debate over whether Wi-Fi stands for “wireless fidelity,” by analogy to Hi-Fi, or whether it is just a catchy
name that doesn’t stand for anything other than 802.11.
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a trade group called the Wi-Fi alliance, that certifies product compliance with 802.11.
Like its Ethernet and token ring siblings, 802.11 is designed for use in a limited geographical area (homes, office buildings, campuses), and its primary challenge is to mediate access to a shared communication medium—in this case, signals propagating through
space. 802.11 supports additional features (e.g., time-bounded services, power management, and security mechanisms), but we focus our discussion on its base functionality.
Physical Properties
802.11 runs over six different physical layer protocols (so far). Five are based on spread
spectrum radio, and one on diffused infrared (and is of historical interest only at this
point). The fastest runs at a maximum of 54 Mbps.
The original 802.11 standard defined two radio-based physical layers standards,
one using frequency hopping (over 79 1-MHz-wide frequency bandwidths) and the
other using direct sequence (with an 11-bit chipping sequence). Both provide up to
2 Mbps. Then physical layer standard 802.11b was added. Using a variant of direct
sequence, 802.11b provides up to 11 Mbps. These three standards run in the licenseexempt 2.4 GHz frequency band of the electromagnetic spectrum. Then came 802.11a,
which delivers up to 54 Mbps using a variant of FDM called orthogonal frequency division
multiplexing (OFDM). 802.11a runs in the license-exempt 5-GHz band. On one hand,
this band is less used, so there is less interference. On the other hand, there is more absorption of the signal and it is limited to almost line of sight. The most recent standard
is 802.11g, which is backward compatible with 802.11b (and returns to the 2.4-GHz
band). 802.11g uses OFDM and delivers up to 54 Mbps. It is common for commercial
products to support all three of 802.11a, 802.11b, and 802.11g, which not only ensures
compatibility with any device that supports any one of the standards, but also makes it
possible for two such products to choose the highest bandwidth option for a particular
environment.
Collision Avoidance
At first glance, it might seem that a wireless protocol would follow the same algorithm as
the Ethernet—wait until the link becomes idle before transmitting and back off should
a collision occur—and to a first approximation, this is what 802.11 does. The additional
complication for wireless is that, while a node on an Ethernet receives every other node’s
transmissions, a node on an 802.11 network may be too far from certain other nodes to
receive their transmissions (and vice versa).
Consider the situation depicted in Figure 2.41, where A and C are both within
range of B but not each other. Suppose both A and C want to communicate with B and
so they each send it a frame. A and C are unaware of each other since their signals do
not carry that far. These two frames collide with each other at B, but unlike an Ethernet,
2.8
Wireless
139
Figure 2.41 The hidden node problem. Although A and C are hidden from each other,
their signals can collide at B. (B’s reach is not shown.)
Figure 2.42 The exposed node problem. Although B and C are exposed to each other’s
signals, there is no interference if B transmits to A while C transmits to D. (A’s and D’s
reaches are not shown.)
neither A nor C is aware of this collision. A and C are said to be hidden nodes with respect
to each other.
A related problem, called the exposed node problem, occurs under the circumstances
illustrated in Figure 2.42, where each of the four nodes is able to send and receive signals
that reach just the nodes to its immediate left and right. For example, B can exchange
frames with A and C but it cannot reach D, while C can reach B and D but not A.
Suppose B is sending to A. Node C is aware of this communication because it hears B’s
transmission. It would be a mistake, however, for C to conclude that it cannot transmit
to anyone just because it can hear B’s transmission. For example, suppose C wants to
transmit to node D. This is not a problem since C’s transmission to D will not interfere
with A’s ability to receive from B. (It would interfere with A sending to B, but B is
transmitting in our example.)
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802.11 addresses these two problems with an algorithm called multiple access with
collision avoidance (MACA). The idea is for the sender and receiver to exchange control
frames with each other before the sender actually transmits any data. This exchange
informs all nearby nodes that a transmission is about to begin. Specifically, the sender
transmits a Request to Send (RTS) frame to the receiver; the RTS frame includes a field
that indicates how long the sender wants to hold the medium (i.e., it specifies the length
of the data frame to be transmitted). The receiver then replies with a Clear to Send (CTS)
frame; this frame echoes this length field back to the sender. Any node that sees the CTS
frame knows that it is close to the receiver, and therefore cannot transmit for the period
of time it takes to send a frame of the specified length. Any node that sees the RTS frame
but not the CTS frame is not close enough to the receiver to interfere with it, and so is
free to transmit.
There are two more details to complete the picture. First, the receiver sends an
ACK to the sender after successfully receiving a frame. All nodes must wait for this ACK
before trying to transmit.7 Second, should two or more nodes detect an idle link and
try to transmit an RTS frame at the same time, their RTS frames will collide with each
other. 802.11 does not support collision detection, but instead the senders realize the
collision has happened when they do not receive the CTS frame after a period of time,
in which case they each wait a random amount of time before trying again. The amount
of time a given node delays is defined by the same exponential backoff algorithm used
on the Ethernet (see Section 2.6.2).
Distribution System
As described so far, 802.11 would be suitable for a network with a mesh (ad hoc) topology, and development of an 802.11s standard for mesh networks is nearing completion.
At the current time, however, nearly all 802.11 networks use a base-station-oriented
topology.
Instead of all nodes being created equal, some nodes are allowed to roam (e.g.,
your laptop) and some are connected to a wired network infrastructure. 802.11 calls
these base stations access points (AP), and they are connected to each other by a so-called
distribution system. Figure 2.43 illustrates a distribution system that connects three access
points, each of which services the nodes in some region. The details of the distribution
system are not important to this discussion—it could be an Ethernet or a token ring, for
example. The only important point is that the distribution network runs at layer 2 of
the ISO architecture (the link layer), that is, it operates at the same protocol layer as the
wireless links. In other words, it does not depend on any higher-level protocols (such as
the network layer).
7 This ACK was not part of the original MACA algorithm, but was instead proposed in an extended version called
MACAW: MACA for Wireless LANs.
2.8
Figure 2.43
Wireless
141
Access points connected to a distribution network.
Although two nodes can communicate directly with each other if they are within
reach of each other, the idea behind this configuration is that each node associates itself
with one access point. For node A to communicate with node E, for example, A first
sends a frame to its access point (AP-1), which forwards the frame across the distribution
system to AP-3, which finally transmits the frame to E. How AP-1 knew to forward the
message to AP-3 is beyond the scope of 802.11; it may have used the bridging protocol
described in the next chapter (Section 3.2). What 802.11 does specify is how nodes select
their access points and, more interestingly, how this algorithm works in light of nodes
moving from one cell to another.
The technique for selecting an AP is called scanning and involves the following four
steps:
1
The node sends a Probe frame;
2
All APs within reach reply with a Probe Response frame;
3
The node selects one of the access points, and sends that AP an Association
Request frame;
4
The AP replies with an Association Response frame.
A node engages this protocol whenever it joins the network, as well as when it becomes
unhappy with its current AP. This might happen, for example, because the signal from
its current AP has weakened due to the node moving away from it. Whenever a node
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Figure 2.44
Node mobility.
Figure 2.45
802.11 frame format.
acquires a new AP, the new AP notifies the old AP of the change (this happens in step 4)
via the distribution system.
Consider the situation shown in Figure 2.44, where node C moves from the cell
serviced by AP-1 to the cell serviced by AP-2. As it moves, it sends Probe frames, which
eventually result in Probe Response frames from AP-2. At some point, C prefers
AP-2 over AP-1, and so it associates itself with that access point.
The mechanism just described is called active scanning since the node is actively
searching for an access point. APs also periodically send a Beacon frame that advertises
the capabilities of the access point; these include the transmission rates supported by
the AP. This is called passive scanning, and a node can change to this AP based on the
Beacon frame simply by sending an Association Request frame back to the access
point.
Frame Format
Most of the 802.11 frame format, which is depicted in Figure 2.45, is exactly what we
would expect. The frame contains the source and destination node addresses, each of
which are 48 bits long, up to 2,312 bytes of data, and a 32-bit CRC. The Control
2.8
Wireless
143
field contains three subfields of interest (not shown): a 6-bit Type field that indicates
whether the frame carries data, is an RTS or CTS frame, or is being used by the scanning
algorithm; and a pair of 1-bit fields—called ToDS and FromDS—that are described
below.
The peculiar thing about the 802.11 frame format is that it contains four, rather
than two, addresses. How these addresses are interpreted depends on the settings of the
ToDS and FromDS bits in the frame’s Control field. This is to account for the possibility that the frame had to be forwarded across the distribution system, which would
mean that the original sender is not necessarily the same as the most recent transmitting
node. Similar reasoning applies to the destination address. In the simplest case, when
one node is sending directly to another, both the DS bits are 0, Addr1 identifies the
target node, and Addr2 identifies the source node. In the most complex case, both DS
bits are set to 1, indicating that the message went from a wireless node onto the distribution system, and then from the distribution system to another wireless node. With
both bits set, Addr1 identifies the ultimate destination, Addr2 identifies the immediate
sender (the one that forwarded the frame from the distribution system to the ultimate
destination), Addr3 identifies the intermediate destination (the one that accepted the
frame from a wireless node and forwarded it across the distribution system), and Addr4
identifies the original source. In terms of the example given in Figure 2.43, Addr1
corresponds to E, Addr2 identifies AP-3, Addr3 corresponds to AP-1, and Addr4
identifies A.
2.8.3 WiMAX (802.16)
WiMAX, which stands for Worldwide Interoperability for Microwave Access, was designed by the WiMAX Forum and standardized as IEEE 802.16. It was originally conceived as a last-mile technology (Section 2.1.2). In WiMAX’s case that “mile” is typically
1 to 6 miles, with a maximum of about 30 miles, leading to WiMAX being classified
as a metropolitan area network (MAN). In keeping with a last-mile role, WiMAX does
not incorporate mobility at the time of this writing, although efforts to add mobility are
nearing completion as IEEE 802.16e. Also in keeping with the last-mile niche, WiMAX’s
client systems, called subscriber stations, are assumed to be not end-user computing devices, but rather systems that multiplex all the communication of the computing devices
being used in a particular building. WiMAX provides up to 70 Mbps to a single subscriber station.
In order to adapt to different frequency bands and different conditions, WiMAX
defines several physical layer protocols. The original WiMAX physical layer protocol
is designed to use frequencies in the 10- to 66-GHz range. In this range waves travel
in straight lines, so communication is limited to line-of-sight (LOS). A WiMAX base
station uses multiple antennas pointed in different directions; the area covered by one
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antenna’s signal is a sector. To extend WiMAX to near-line-of-sight and nonline-of-sight
situations, several physical layer protocols were added that use the frequencies below
11 GHz (in the 10- to 11-GHz range, WiMAX can use either the original physical
layer or one of the newer ones). Since this range includes both licensed and licenseexempt frequencies, each of these physical layer protocols defines a variant better adapted
to the additional interference and the regulatory constraints of the license-exempt
frequencies.
The physical layer protocols provide two ways to divide the bandwidth between
upstream (i.e., from subscribers to base station) and downstream traffic: time division
duplexing (TDD) and frequency division duplexing (FDD). TDD is simply STDM
of the two streams; they take turns using the same frequency, and the proportion of
upstream to downstream time can be varied dynamically, adaptively, by the base station.
FDD is simply FDM of the two streams; one frequency is used for upstream and another
for downstream. In license-exempt bands, the protocols use only TDD.
Both channels, upstream and downstream, must be shared not just among the
many subscriber stations in a given sector, but also among the many WiMAX connections that each subscriber can have with the base station. WiMAX—unlike 802.11 and
Ethernet—is connection oriented. One reason for this is to be able to offer a variety
of QoS guarantees regarding properties such as latency and jitter, with the aim of supporting high-quality telephony and high-volume multimedia in addition to bursty data
traffic. This is conceptually similar to some of the wired last mile technologies (such as
DSL) with which WiMAX is intended to compete.
Sharing of the upstream and downstream channels is based on dividing them into
equal-sized time slots. A WiMAX frame generally takes up multiple slots, with different frames taking different numbers of slots. The downstream channel (from base to
subscribers) is relatively easy to subdivide into connections since only the base station
sends on that channel. The base station simply sends addressed frames, one after the
other. Each subscriber station in the sector receives all the frames, but ignores those not
addressed to one of its connections.
In the upstream direction, how a connection gets handled depends on its QoS
parameters. Some connections get slots at a fixed rate, some get polled to determine
how many slots they need currently, and some must request slots whenever they need
them. Connections in this last category must contend to place their requests in a limited
number of upstream slots set aside for contention. They use an exponential backoff
algorithm to minimize the chance of a collision, even on the first attempt.
A European alternative to WiMAX is HIPERMAN, which stands for highperformance radio metropolitan area network and uses the 2- to 11-GHz range. South
Korea’s WiBro (for wireless broadband) technology operates at 2.3 GHz, and is being
brought into line with the emerging IEEE 802.16e standard for mobile WiMAX.
2.8
Wireless
145
2.8.4 Cell Phone Technologies
Cell phone technology seems an obvious approach to mobile computer communication,
and indeed data services based on cellular standards are commercially available. One
drawback is the cost to users, due in part to cellular’s use of licensed spectrum (which
has historically been sold off to cellular phone operators for astronomical sums). The
frequency bands that are used for cellular telephones (and now for cellular data) vary
around the world. In Europe, for example, the main bands for cellular phones are at
900 and 1,800 MHz. In North America, 850- and 1,900-MHz bands are used. This
global variation in spectrum usage creates problems for users who want to travel from
one part of the world to another, and has created a market for phones that can operate
at multiple frequencies (e.g., a tri-band phone can operate at three of the four frequency
bands mentioned above). That problem, however, pales in comparison to the proliferation of incompatible standards that have plagued the cellular communication business.
Only recently have some signs of convergence on a small set of standards appeared.
And finally, there is the problem that most cellular technology was designed for voice
communication, and is only now starting to support moderately high-bandwidth data
communication.
Like 802.11 and WiMAX, cellular technology relies on the use of base stations
that are part of a wired network. The geographic area served by a base station’s antenna is
called a cell. A base station could serve a single cell, or use multiple directional antennas
to serve multiple cells. Cells don’t have crisp boundaries, and they overlap. Where they
overlap, a mobile phone could potentially communicate with multiple base stations. This
is somewhat similar to the 802.11 picture shown in Figure 2.43. At any time, however,
the phone is in communication with, and under the control of, just one base station. As
the phone begins to leave a cell, it moves into an area of overlap with one or more other
cells. The current base station senses the weakening signal from the phone, and gives
control of the phone to whichever base station is receiving the strongest signal from it.
If the phone is involved in a call at the time, the call must be transferred to the new base
station in what is called a handoff.
As we noted above, there is not one unique standard for cellular, but rather a collection of competing technologies that support data traffic in different ways and deliver
different speeds. These technologies are loosely categorized by “generation.” The first
generation (1G) was analog, and thus of limited interest from a data communications
perspective. Most of the cell phone technology currently deployed is considered second
generation (2G) or “2.5G” (not quite worthy of being called 3G, but more advanced
than 2G). The 2G and later technologies are digital. The most widely deployed 2G technology is referred to as GSM—the Global System for Mobile Communications, which
is used in more than 200 countries. North America, however, is a late adopter of GSM,
which helped prolong the proliferation of competing standards.
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Most 2G technologies use one of two approaches to sharing a limited amount of
spectrum between simultaneous calls. One way is a combination of FDM and TDM.
The spectrum available is divided into disjoint frequency bands, and each band is subdivided into time slots. A given call is allocated every nth slot in one of the bands. The
other approach is code division multiple access (CDMA). CDMA does not divide the
channel in either time or frequency, but rather uses different chipping codes to distinguish the transmissions of different cellphone users. (See Section 2.1.2 for a discussion
of chipping codes.)
The 2G and later cell phone technologies use compression algorithms tailored to
human speech to compress voice data to about 8 Kbps without losing quality. Since 2G
technologies focus on voice communication, they provide connections with just enough
bandwidth for that compressed speech—not enough for a decent data link. One of the
first cellular data standards to gain widespread adoption is the General Packet Radio
Service (GPRS), which is part of the GSM set of standards and is often referred to as a
2.5G technology.
GSM networks make use of a multiplexing technique called time division multiple
access (TDMA). (Confusingly, there is also a particular cellular standard that is sometimes called TDMA, but is known formally as IS-136.) You can think of TDMA as being
like TDM (time division multiplexing)—traditionally used for telephone services—with
the additional feature that the timeslots can be dynamically allocated to users or devices
that need them (and deallocated from devices that no longer need them). The number
of timeslots that are available for GPRS at a given frequency depends on how many cellular voice calls are currently in progress, since voice calls also consume timeslots. As a
result, GPRS data rates tend to be lower in busy cells. In practice, users often get between
30 and 70 Kbps—coincidentally, just about the same as a user of a dial-up modem on
a landline. Nevertheless, GPRS has proven quite useful and popular in some parts of
the world as a way to communicate wirelessly when faster connection methods (such
as 802.11) are not available. Other 2.5G data standards have also become available and
some manage to be quite a bit higher in bandwidth than GPRS.
The concept of a third generation (3G) was established before there was any
implementation of 3G technologies, with the aim of shaping a single international
standard that would provide much higher data bandwidth. Unfortunately, at the
time of writing, several mutually incompatible 3G standards are emerging. Thus,
the possibility that cellular standards will continue to diverge seems quite realistic.
Interestingly, all the 3G standards are based on variants of CDMA. For example,
the Universal Mobile Telecommunications System (UMTS) is based on wideband
CDMA (W-CDMA). UMTS appears poised to be the successor to GSM, and in
fact is sometimes referred to as 3GSM (i.e., the third generation version of GSM).
UMTS is intended to support data transfer rates of up to 1.92 Mbps, although
2.9
Summary
147
real network conditions will probably result in lower rates in practice. Nevertheless, it should represent a significant performance improvement over GPRS. And like
GSM, it should have quite widespread (if not actually universal) adoption around the
world.
There are a number of commercial UMTS networks in operation at the time of
writing with many more announced or planned. And to make it quite clear that work
in this field is far from complete, we note that 3.5G and 4G standards are also in the
works.
Finally, it should be noted that there is a class of mobile phones that are not cellular
phones but satellite phones, or satphones. Satphones use communication satellites as base
stations, communicating on frequency bands that have been reserved internationally for
satellite use. Consequently, service is available even where there are no cellular base stations. Satphones are rarely used where cellular is available, since service is typically much
more expensive. Satphones are also larger and heavier than modern cell phones because
of the need to transmit and receive over much longer distances, to reach satellites rather
than cellphone towers. Satellite communication is more extensively used in television
and radio broadcasting, taking advantage of the fact that the signal is broadcast, not
point-to-point. High-bandwidth data communication via satellite is commercially available, but its relatively high price (for both equipment and service) limits its use to regions
where no alternative is available.
2.9
Summary
This chapter introduced the hardware building blocks of a computer network—nodes
and links—and discussed the five key problems that must be solved so that two or more
nodes that are directly connected by a physical link can exchange messages with each
other. In practice, most of the algorithms that address these five problems are implemented on the adaptor that connects the host to the link. It turns out that the design of
this adaptor, and how the rest of the host interacts with it, is of critical importance in
how well the network performs overall.
The first problem is to encode the bits that make up a binary message into the signal
at the source node and then to recover the bits from the signal at the receiving node. This
is the encoding problem, and it is made challenging by the need to keep the sender’s and
receiver’s clocks synchronized. We discussed four different encoding techniques—NRZ,
NRZI, Manchester, and 4B/5B—which differ largely in how they encode clock information along with the data being transmitted. One of the key attributes of an encoding
scheme is its efficiency, that is, the ratio of signal pulses to encoded bits.
Once it is possible to transmit bits between nodes, the next step is to figure out
how to package these bits into frames. This is the framing problem, and it boils down
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to being able to recognize the beginning and end of each frame. Again, we looked at
several different techniques, including byte-oriented protocols, bit-oriented protocols,
and clock-based protocols.
Assuming that each node is able to recognize the collection of bits that make up
a frame, the third problem is to determine if those bits are in fact correct, or if they
have possibly been corrupted in transit. This is the error detection problem, and we
looked at three different approaches: cyclic redundancy check, two-dimensional parity,
and checksums. Of these, the CRC approach gives the strongest guarantees and is the
most widely used at the link level.
Given that some frames will arrive at the destination node containing errors and
thus will have to be discarded, the next problem is how to recover from such losses. The
goal is to make the link appear reliable. The general approach to this problem is called
ARQ and involves using a combination of acknowledgments and timeouts. We looked at
three specific ARQ algorithms: stop-and-wait, sliding window, and concurrent channels.
What makes these algorithms interesting is how effectively they use the link, with the
goal being to keep the pipe full.
The final problem is not relevant to point-to-point links, but it is the central issue
in multiple-access links: how to mediate access to a shared link so that all nodes eventually have a chance to transmit their data. In this case, we looked at a variety of media
access protocols—Ethernet, token ring, and several wireless protocols—which have been
put to practical use in building local area networks. The Ethernet and token ring media
access protocols are notable for their distributed nature—there is no central arbitrator
of access. Media access in wireless networks is made more complicated by the fact that
some nodes may be “hidden” from each other due to range limitations of radio transmission. Most of the common wireless protocols today designate some nodes as “wired” or
“base-station” nodes, while the other “mobile” nodes communicate with a base station.
Wireless standards and technologies are rapidly evolving, with mesh networks, in which
all nodes communicate as peers, now starting to emerge.
A sensor network is a wireless
network
of many nodes—up to tens of
O P E N
I S S U E
thousands—whose purpose is to monSensor Networks
itor some aspect of the geographic area
over which it is spread. The nodes are
equipped with one or more types of
sensor that allow them to detect, for
example, sound, motion, radiation, or chemicals. Some example applications of sensor
networks are monitoring a battleground to detect the locations of enemy forces, monitor-
Further Reading
149
ing a natural environment for pollutants or seismic activity, and monitoring temperature
throughout a building to optimize climate control.
The nodes in a sensor network must be low cost because of the quantity involved,
and must use very little power because they are generally battery powered. These minimal nodes are perhaps better described as devices rather than computers. There is an
open-source operating system called TinyOS designed specifically for the constraints
of these devices. Researchers are pursuing development of smart dust—sensor network
nodes called motes whose size is on the order of millimeters.
Although the sensor information is ultimately routed to a base station, most of the
nodes are not directly linked to the base station. Instead they communicate only with
their nearest neighbors, who forward the data to their neighbors until it reaches the base
station. This uses less power than transmitting over a longer distance, and allows the sensor network to extend beyond the range of a single link. One of the open questions about
sensor networks is how a node should determine which node to transmit or forward data
to. In one technique, the nodes form clusters. Each cluster designates one node as cluster
head, and all data is routed via cluster heads. In a technique that blurs the line between
network and application, nodes called aggregation points collect and process the data they
receive from neighboring nodes, then transmit the processed data. By processing the data
incrementally, instead of forwarding all the raw data to the base station, the amount of
traffic in the network (and the power consumed) is reduced. Further complicating the
issue of how to organize the network is the possibility of nodes failing, perhaps because of
battery exhaustion, and the possibility of nodes being dynamically added to an existing
network.
Another open issue for sensor networks is localization or location discovery—
determining the locations of nodes. Suppose the nodes are deployed by dropping them
from an aircraft, as might be the case for, say, military or environmental monitoring.
Then neither the node nor the base station would know where a node is. And yet that
geographical information is crucial; we need to know the location of that seismic activity
or enemy tank. GPS is considered too expensive and consumes too much power for the
majority of nodes. A typical solution requires a few nodes called beacons to determine
their own absolute locations based on GPS or manual configuration. The majority of
nodes can then derive their absolute location by combining an estimate of their position
relative to the beacons with the absolute location information provided by the beacons.
F U R T H E R
R E A D I N G
One of the most important contributions in computer networking over the last 20 years
is the original paper by Metcalf and Boggs (1976) introducing the Ethernet. Many years
later, Boggs, Mogul, and Kent (1988) reported their practical experiences with Ethernet,
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debunking many of the myths that had found their way into the literature over the years.
Both papers are must reading. The third and fourth papers discuss the issues involved in
integrating high-speed network adaptors with system software.
■
Metcalf, R., and D. Boggs. “Ethernet: Distributed Packet Switching for Local
Computer Networks.” Communications of the ACM 19(7):395–403, July 1976.
■
Boggs, D., J. Mogul, and C. Kent. “Measured Capacity of an Ethernet.” Proceedings of the SIGCOMM ’88 Symposium, pp. 222–234, August 1988.
■
Metcalf, R. “Computer/Network Interface Design Lessons from Arpanet and
Ethernet.” IEEE Journal of Selected Areas in Communication (JSAC) 11(2):173–
180, February 1993.
■
Druschel, P., M. Abbott, M. Pagels, and L. L. Peterson. “Network Subsystem
Design.” IEEE Network (Special Issue on End-System Support for High-Speed Networks) 7(4):8–17, July 1993.
There are countless textbooks with a heavy emphasis on the lower levels of the
network hierarchy, with a particular focus on telecommunications—networking from the
phone company’s perspective. Books by Spragins et al. [SHP91] and Minoli [Min93]
are two good examples. Several other books concentrate on various local area network
technologies. Of these, Stallings’s book is the most comprehensive [Sta00], while Jain
gives a thorough description of FDDI [Jai94]. Jain’s book also gives a good introduction
to the low-level details of optical communication. Also, a comprehensive overview of
FDDI can be found in Ross’s article [Ros86].
For an introduction to information theory, Blahut’s book is a good place to start
[Bla87], along with Shannon’s seminal paper on link capacity [Sha48].
For a general introduction to the mathematics behind error codes, Rao and Fujiwara [RF89] is recommended. For a detailed discussion of the mathematics of CRCs
in particular, along with some more information about the hardware used to calculate
them, see Peterson and Brown [PB61].
On the topic of network adaptor design, much work was done in the early 1990s
by researchers trying to connect hosts to networks running at higher and higher rates.
In addition to the two examples given in the reading list, see Traw and Smith [TS93],
Ramakrishnan [Ram93], Edwards et al. [EWL+ 94], Druschel et al. [DPD94], Kanakia
and Cheriton [KC88], Cohen et al. [CFFD93], and Steenkiste [Ste94a]. Recently, a new
generation of interface cards, ones that utilize network processors, are coming onto the
market. Spalink et al. demonstrate how these new processors can be programmed to
implement various network functionality [SKPG01].
Exercises
151
For general information on computer architecture, Hennessy and Patterson’s book
[HP06] is an excellent reference.
Finally, we recommend the following live reference:
■
http://standards.ieee.org/: Status of various IEEE network-related
standards.
E X E R C I S E S
1
Show the NRZ, Manchester, and NRZI encodings for the bit pattern shown
in Figure 2.46. Assume that the NRZI signal starts out low.
2
Show the 4B/5B encoding, and the resulting NRZI signal, for the following
bit sequence:
1110 0101 0000 0011
✓
3
Show the 4B/5B encoding, and the resulting NRZI signal, for the following
bit sequence:
1101 1110 1010 1101 1011 1110 1110 1111
4
In the 4B/5B encoding (see Table 2.4), only two of the 5-bit codes used end
in two 0s. How many possible 5-bit sequences are there (used by the existing
code or not) that meet the stronger restriction of having at most one leading
and at most one trailing 0? Could all 4-bit sequences be mapped to such 5-bit
sequences?
Figure 2.46
Diagram for Exercise 1.
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Direct Link Networks
Assuming a framing protocol that uses bit stuffing, show the bit sequence transmitted over the link when the frame contains the following bit sequence:
110101111101011111101011111110
Mark the stuffed bits.
6
Suppose the following sequence of bits arrives over a link:
1101011111010111110010111110110
Show the resulting frame after any stuffed bits have been removed. Indicate any
errors that might have been introduced into the frame.
✓
7
Suppose the following sequence of bits arrive over a link:
011010111110101001111111011001111110
Show the resulting frame after any stuffed bits have been removed. Indicate any
errors that might have been introduced into the frame.
8
Suppose you want to send some data using the BISYNC framing protocol, and
the last 2 bytes of your data are DLE and ETX. What sequence of bytes would
be transmitted immediately prior to the CRC?
9
For each of the following framing protocols, give an example of a byte/bit
sequence that should never appear in a transmission:
(a) BISYNC.
(b) HDLC.
★
10
Assume that a SONET receiver resynchronizes its clock whenever a 1 bit appears; otherwise, the receiver samples the signal in the middle of what it believes
is the bit’s timeslot.
(a) What relative accuracy of the sender’s and receiver’s clocks is required in
order to receive correctly 48 zero-bytes (one ATM AAL5 cell’s worth) in a
row?
(b) Consider a forwarding station A on a SONET STS-1 line, receiving frames
from the downstream end B and retransmitting them upstream. What relative accuracy of A’s and B’s clocks is required to keep A from accumulating
more than one extra frame per minute?
Exercises
153
11
Show that two-dimensional parity allows detection of all 3-bit errors.
12
Give an example of a 4-bit error that would not be detected by a twodimensional parity, as illustrated in Figure 2.19. What is the general set of
circumstances under which 4-bit errors will be undetected?
13
Show that two-dimensional parity provides the receiver enough information to
correct any 1-bit error (assuming the receiver knows only 1 bit is bad), but not
any 2-bit error.
14
Show that the Internet checksum will never be 0xFFFF (that is, the final value
of sum will not be 0x0000) unless every byte in the buffer is 0. (Internet specifications in fact require that a checksum of 0x0000 be transmitted as 0xFFFF;
the value 0x0000 is then reserved for an omitted checksum. Note that, in ones
complement arithmetic, 0x0000 and 0xFFFF are both representations of the
number 0.)
15
Prove the Internet checksum computation shown in the text is independent
of byte order (host order or network order) except that the bytes in the final
checksum should be swapped later to be in the correct order. Specifically, show
that the sum of 16-bit words can be computed in either byte order. For example, if the 1’s complement sum (denoted by +’) of 16-bit words is represented
as follows,
[A,B] +’ [C,D] +’ · · · +’ [Y,Z]
the following swapped sum is the same as the original sum above.
[B,A] +’ [D,C] +’ · · · +’ [Z,Y]
★
16
Suppose that one byte in a buffer covered by the Internet checksum algorithm
needs to be decremented (e.g., a header hop count field). Give an algorithm
to compute the revised checksum without rescanning the entire buffer. Your
algorithm should consider whether the byte in question is low order or high
order.
17
Show that the Internet checksum can be computed by first taking the 32-bit
ones complement sum of the buffer in 32-bit units, then taking the 16-bit ones
complement sum of the upper and lower halfwords, and finishing as before by
complementing the result. (To take a 32-bit ones complement sum on 32-bit
twos complement hardware, you need access to the “overflow” bit.)
18
Suppose we want to transmit the message 11001001 and protect it from errors
using the CRC polynomial x 3 + 1.
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(a) Use polynomial long division to determine the message that should be
transmitted.
(b) Suppose the leftmost bit of the message is inverted due to noise on the
transmission link. What is the result of the receiver’s CRC calculation?
How does the receiver know that an error has occurred?
✓
19
Suppose we want to transmit the message 1011 0010 0100 1011 and protect
it from errors using the CRC8 polynomial x 8 + x 2 + x 1 + 1.
(a) Use polynomial long division to determine the message that should be
transmitted.
(b) Suppose the leftmost bit of the message is inverted due to noise on the
transmission link. What is the result of the receiver’s CRC calculation?
How does the receiver know that an error has occurred?
20
The CRC algorithm as presented in this chapter requires lots of bit manipulations. It is, however, possible to do polynomial long division taking multiple
bits at a time, via a table-driven method, that enables efficient software implementations of CRC. We outline the strategy here for long division 3 bits at a
time (see Table 2.7); in practice we would divide 8 bits at a time, and the table
would have 256 entries.
Let the divisor polynomial C = C(x) be x 3 + x 2 + 1, or 1101. To build
the table for C , we take each 3-bit sequence, p, append three trailing 0s, and
p
q = p 000 ÷ C
C ×q
000
000
000 000
001
001
001 101
010
011
010 ____
011
0___
011 ____
100
111
100 011
101
110
101 110
110
100
110 ____
111
____
111 ____
Table 2.7
Table-driven CRC calculation.
Exercises
155
then find the quotient q = p 000 ÷ C , ignoring the remainder. The third
column is the product C × q, the first 3 bits of which should equal p.
(a) Verify, for p = 110, that the quotients p 000 ÷ C and p 111 ÷ C are the
same, that is, it doesn’t matter what the trailing bits are.
(b) Fill in the missing entries in the table.
(c) Use the table to divide 101 001 011 001 100 by C . Hint: The first 3 bits
of the dividend are p = 101, so from the table the corresponding first 3 bits
of the quotient are q = 110. Write the 110 above the second 3 bits of the
dividend, and subtract C × q = 101 110, again from the table, from the
first 6 bits of the dividend. Keep going in groups of 3 bits. There should
be no remainder.
★
21
With 1 parity bit we can detect all 1-bit errors. Show that at least one generalization fails, as follows:
(a) Show that if messages m are 8 bits long, then there is no error detection
code e = e(m) of size 2 bits that can detect all 2-bit errors. (Hint: Consider
the set M of all 8-bit messages with a single 1 bit; note that any message
from M can be transmuted into any other with a 2-bit error, and show that
some pair of messages m1 and m2 in M must have the same error code e.)
(b) Find an N (not necessarily minimal) such that no 32-bit error detection
code applied to N -bit blocks can detect all errors altering up to 8 bits.
22
Consider an ARQ protocol that uses only negative acknowledgments (NAKs),
but no positive acknowledgments (ACKs). Describe what timeouts would need
to be scheduled. Explain why an ACK-based protocol is usually preferred to a
NAK-based protocol.
23
Consider an ARQ algorithm running over a 20-km point-to-point fiber link.
(a) Compute the propagation delay for this link, assuming that the speed of
light is 2 × 108 m/s in the fiber.
(b) Suggest a suitable timeout value for the ARQ algorithm to use.
(c) Why might it still be possible for the ARQ algorithm to time out and
retransmit a frame, given this timeout value?
156
✓
2
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24
Suppose you are designing a sliding window protocol for a 1-Mbps point-topoint link to the moon, which has a one-way latency of 1.25 seconds. Assuming
that each frame carries 1 KB of data, what is the minimum number of bits you
need for the sequence number?
25
Suppose you are designing a sliding window protocol for a 1-Mbps pointto-point link to the stationary satellite evolving around Earth at 3 × 104 km
altitude. Assuming that each frame carries 1 KB of data, what is the minimum
number of bits you need for the sequence number in the following cases? Assume the speed of light is 3 × 108 meters per second.
(a) RWS=1.
(b) RWS=SWS.
26
The text suggests that the sliding window protocol can be used to implement
flow control. We can imagine doing this by having the receiver delay ACKs,
that is, not send the ACK until there is free buffer space to hold the next
frame. In doing so, each ACK would simultaneously acknowledge the receipt
of the last frame and tell the source that there is now free buffer space available
to hold the next frame. Explain why implementing flow control in this way is
not a good idea.
27
Implicit in the stop-and-wait scenarios of Figure 2.22 is the notion that the
receiver will retransmit its ACK immediately on receipt of the duplicate data
frame. Suppose instead that the receiver keeps its own timer, and retransmits
its ACK only after the next expected frame has not arrived within the timeout
interval. Draw timelines illustrating the scenarios in Figure 2.22(b)–(d); assume
the receiver’s timeout value is twice the sender’s. Also redraw (c) assuming the
receiver’s timeout value is half the sender’s.
28
In stop-and-wait transmission, suppose that both sender and receiver retransmit their last frame immediately on receipt of a duplicate ACK or data frame;
such a strategy is superficially reasonable because receipt of such a duplicate is
most likely to mean the other side has experienced a timeout.
(a) Draw a timeline showing what will happen if the first data frame is somehow duplicated, but no frame is lost. How long will the duplications continue? This situation is known as the Sorcerer’s Apprentice bug.
(b) Suppose that, like data, ACKs are retransmitted if there is no response
within the timeout period. Suppose also that both sides use the same time-
Exercises
157
out interval. Identify a reasonably likely scenario for triggering the Sorcerer’s Apprentice bug.
29
Give some details of how you might augment the sliding window protocol
with flow control by having ACKs carry additional information that reduces
the SWS as the receiver runs out of buffer space. Illustrate your protocol with
a timeline for a transmission; assume the initial SWS and RWS are 4, the
link speed is instantaneous, and the receiver can free buffers at the rate of one
per second (i.e., the receiver is the bottleneck). Show what happens at T = 0,
T = 1, . . . , T = 4 seconds.
30
Describe a protocol combining the sliding window algorithm with selective
ACKs. Your protocol should retransmit promptly, but not if a frame simply arrives one or two positions out of order. Your protocol should also make explicit
what happens if several consecutive frames are lost.
31
Draw a timeline diagram for the sliding window algorithm with SWS =
RWS = 3 frames, for the following two situations. Use a timeout interval of
about 2 × RTT.
(a) Frame 4 is lost.
(b) Frames 4–6 are lost.
✓
32
Draw a timeline diagram for the sliding window algorithm with SWS =
RWS = 4 frames in the following two situations. Assume the receiver sends
a duplicate acknowledgment if it does not receive the expected frame. For
example, it sends DUPACK[2] when it expects to see Frame[2] but receives
Frame[3] instead. Also, the receiver sends a cumulative acknowledgment after
it receives all the outstanding frames. For example, it sends ACK[5] when it
receives the lost frame Frame[2] after it already received Frame[3], Frame[4],
and Frame[5]. Use a timeout interval of about 2 × RTT.
(a) Frame[2] is lost. Retransmission takes place upon timeout (as usual).
(b) Frame[2] is lost. Retransmission takes place either upon receipt of the first
DUPACK or upon timeout. Does this scheme reduce the transaction time?
(Note that some end-to-end protocol (e.g., variant of TCP) uses a similar
scheme for fast retransmission.)
158
2
Direct Link Networks
33
Suppose that we attempt to run the sliding window algorithm with SWS =
RWS = 3 and with MaxSeqNum = 5. The N th packet DATA[N ] thus actually contains N mod 5 in its sequence number field. Give an example in
which the algorithm becomes confused, that is, a scenario in which the receiver
expects DATA[5] and accepts DATA[0]—which has the same transmitted sequence number—in its stead. No packets may arrive out of order. Note this
implies MaxSeqNum ≥ 6 is necessary as well as sufficient.
34
Consider the sliding window algorithm with SWS = RWS = 3, with no outof-order arrivals, and with infinite-precision sequence numbers.
(a) Show that if DATA[6] is in the receive window, then DATA[0] (or in
general any older data) cannot arrive at the receiver (and hence that
MaxSeqNum = 6 would have sufficed).
(b) Show that if ACK[6] may be sent (or, more literally, that DATA[5] is in
the sending window), then ACK[2] (or earlier) cannot be received.
These amount to a proof of the formula given in Section 2.5.2, particularized to
the case SWS = 3. Note that part (b) implies that the scenario of the previous
problem cannot be reversed to involve a failure to distinguish ACK[0] and
ACK[5].
35
Suppose that we run the sliding window algorithm with SWS = 5 and
RWS = 3, and no out-of-order arrivals.
(a) Find the smallest value for MaxSeqNum. You may assume that it suffices
to find the smallest MaxSeqNum such that if DATA[MaxSeqNum]
is in the receive window, then DATA[0] can no longer arrive.
(b) Give an example showing that MaxSeqNum − 1 is not sufficient.
(c) State a general rule for the minimum MaxSeqNum in terms of SWS
and RWS.
36
Suppose A is connected to B via an intermediate router R, as shown in Figure 2.47. The A–R and R–B links each accept and transmit only one packet
per second in each direction (so two packets take 2 seconds), and the two directions transmit independently. Assume A sends to B using the sliding window
protocol with SWS = 4.
Exercises
Figure 2.47
159
Diagram for Exercises 36–38.
(a) For Time = 0, 1, 2, 3, 4, 5, state what packets arrive at and leave each node,
or label them on a timeline.
(b) What happens if the links have a propagation delay of 1.0 seconds, but
accept immediately as many packets as are offered (i.e., latency = 1 second
but bandwidth is infinite)?
37
Suppose A is connected to B via an intermediate router R, as in the previous
problem. The A–R link is instantaneous, but the R–B link transmits only one
packet each second, one at a time (so two packets take 2 seconds). Assume A
sends to B using the sliding window protocol with SWS = 4. For Time =
0, 1, 2, 3, 4, state what packets arrive at and are sent from A and B. How large
does the queue at R grow?
38
Consider the situation in the previous exercise, except this time assume that
the router has a queue size of 1, that is, it can hold one packet in addition to
the one it is sending (in each direction). Let A’s timeout be 5 seconds, and let
SWS again be 4. Show what happens at each second from T = 0 until all four
packets from the first windowful are successfully delivered.
39
Why is it important for protocols configured on top of the Ethernet to have a
length field in their header indicating how long the message is?
40
What kind of problem can arise when two hosts on the same Ethernet share
the same hardware address? Describe what happens and why that behavior is a
problem.
41
The 1982 Ethernet specification allowed between any two stations up to
1,500 m of coaxial cable, 1,000 m of other point-to-point link cable, and two
repeaters. Each station or repeater connects to the coaxial cable via up to 50 m
of “drop cable.” Typical delays associated with each device are given in Table 2.8
(where c = speed of light in a vacuum = 3 × 108 m/s). What is the worst-case
round-trip propagation delay, measured in bits, due to the sources listed? (This
list is not complete; other sources of delay include sense time and signal rise
time.)
160
2
Direct Link Networks
Item
Delay
Coaxial cable
Propagation speed .77c
Link/drop cable
Propagation speed .65c
Repeaters
Approximately 0.6 µs each
Transceivers
Approximately 0.2 µs each
Table 2.8
★
Typical delays associated with various devices (Exercise 41).
42
Coaxial cable Ethernet was limited to a maximum of 500 m between repeaters,
which regenerate the signal to 100% of its original amplitude. Along one
500 m segment, the signal could decay to no less than 14% of its original
value (8.5 dB). Along 1,500 m, then, the decay might be (0.14)3 = 0.3%.
Such a signal, even along 2,500 m, is still strong enough to be read; why then
are repeaters required every 500 m?
43
Suppose the round-trip propagation delay for Ethernet is 46.4 µs. This yields
a minimum packet size of 512 bits (464 bits corresponding to propagation
delay + 48 bits of jam signal).
(a) What happens to the minimum packet size if the delay time is held constant, and the signaling rate rises to 100 Mbps?
(b) What are the drawbacks to so large a minimum packet size?
(c) If compatibility were not an issue, how might the specifications be written
so as to permit a smaller minimum packet size?
★
44
Let A and B be two stations attempting to transmit on an Ethernet. Each has a
steady queue of frames ready to send; A’s frames will be numbered A1 , A2 , and
so on, and B’s similarly. Let T = 51.2 µs be the exponential backoff base unit.
Suppose A and B simultaneously attempt to send frame 1, collide, and
happen to choose backoff times of 0 × T and 1 × T , respectively, meaning A
wins the race and transmits A1 while B waits. At the end of this transmission,
B will attempt to retransmit B1 while A will attempt to transmit A2 . These first
attempts will collide, but now A backs off for either 0 × T or 1 × T , while B
backs off for time equal to one of 0 × T , . . . , 3 × T .
(a) Give the probability that A wins this second backoff race immediately after
this first collision, that is, A’s first choice of backoff time k × 51.2 is less
than B’s.
Exercises
161
(b) Suppose A wins this second backoff race. A transmits A3 , and when it is
finished, A and B collide again as A tries to transmit A4 and B tries once
more to transmit B1 . Give the probability that A wins this third backoff
race immediately after the first collision.
(c) Give a reasonable lower bound for the probability that A wins all the remaining backoff races.
(d) What then happens to the frame B1 ?
This scenario is known as the Ethernet capture effect.
45
Suppose the Ethernet transmission algorithm is modified as follows: After each
successful transmission attempt, a host waits one or two slot times before attempting to transmit again, and otherwise backs off the usual way.
(a) Explain why the capture effect of the previous exercise is now much less
likely.
(b) Show how the strategy above can now lead to a pair of hosts capturing the
Ethernet, alternating transmissions, and locking out a third host.
(c) Propose an alternative approach, for example, by modifying the exponential backoff. What aspects of a station’s history might be used as parameters
to the modified backoff?
46
Ethernets use Manchester encoding. Assuming that hosts sharing the Ethernet
are not perfectly synchronized, why does this allow collisions to be detected
soon after they occur, without waiting for the CRC at the end of the packet?
47
Suppose A, B, and C all make their first carrier sense, as part of an attempt
to transmit, while a fourth station D is transmitting. Draw a timeline showing
one possible sequence of transmissions, attempts, collisions, and exponential
backoff choices. Your timeline should also meet the following criteria: (1) initial transmission attempts should be in the order A, B, C, but successful transmissions should be in the order C, B, A, and (2) there should be at least four
collisions.
48
Repeat the previous exercise, now with the assumption that Ethernet is p-persistent with p = 0.33 (that is, a waiting station transmits immediately with
probability p when the line goes idle, and otherwise defers one 51.2-µs slot
time and repeats the process). Your timeline should meet criterion (1) of the
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2
Direct Link Networks
previous problem, but in lieu of criterion (2), you should show at least one
collision and at least one run of four deferrals on an idle line. Again, note that
many solutions are possible.
★
49
Suppose Ethernet physical addresses are chosen at random (using true random
bits).
(a) What is the probability that on a 1,024-host network, two addresses will
be the same?
(b) What is the probability that the above event will occur on some one or
more of 220 networks?
(c) What is the probability that of the 230 hosts in all the networks of (b),
some pair has the same address?
Hint: The calculation for (a) and (c) is a variant of that used in solving the socalled Birthday Problem: Given N people, what is the probability that two of
their birthdays (addresses) will be the same? The second person has probability
1
1 − 365
of having a different birthday from the first, the third has probabil2
of having a different birthday from the first two, and so on. The
ity 1 − 365
probability all birthdays are different is thus
1 2 N − 1
1−
× 1−
× ··· × 1 −
365
365
365
which for smallish N is about
1−
50
1 + 2 + · · · + (N − 1)
365
Suppose five stations are waiting for another packet to finish on an Ethernet.
All transmit at once when the packet is finished and collide.
(a) Simulate this situation up until the point when one of the five waiting
stations succeeds. Use coin flips or some other genuine random source to
determine backoff times. Make the following simplifications: ignore interframe spacing, ignore variability in collision times (so that retransmission
is always after an exact integral multiple of the 51.2-µs slot time), and
assume that each collision uses up exactly one slot time.
(b) Discuss the effect of the listed simplifications in your simulation versus the
behavior you might encounter on a real Ethernet.
Exercises
163
51
Write a program to implement the simulation discussed above, this time with
N stations waiting to transmit. Again model time as an integer, T, in units of
slot times, and again treat collisions as taking one slot time (so a collision at
time T followed by a backoff of k = 0 would result in a retransmission attempt
at time T + 1). Find the average delay before one station transmits successfully,
for N = 20, N = 40, and N = 100. Does your data support the notion that
the delay is linear in N ? Hint: For each station, keep track of that station’s
NextTimeToSend and CollisionCount. You are done when you reach a
time T for which there is only one station with NextTimeToSend == T. If
there is no such station, increment T. If there are two or more, schedule the
retransmissions and try again.
52
Suppose that N Ethernet stations, all trying to send at the same time, require
N /2 slot times to sort out who transmits next. Assuming the average packet
size is 5 slot times, express the available bandwidth as a function of N .
53
Consider the following Ethernet model. Transmission attempts are at random
times with an average spacing of λ slot times; specifically, the interval between
consecutive attempts is an exponential random variable x = −λ log u, where u
is chosen randomly in the interval 0 ≤ u ≤ 1. An attempt at time t results in a
collision if there is another attempt in the range from t − 1 to t + 1, where t is
measured in units of the 51.2-µs slot time; otherwise the attempt succeeds.
(a) Write a program to simulate, for a given value of λ, the average number
of slot times needed before a successful transmission, called the contention
interval. Find the minimum value of the contention interval. Note that
you will have to find one attempt past the one that succeeds, in order to
determine if there was a collision. Ignore retransmissions, which probably
do not fit the random model above.
(b) The Ethernet alternates between contention intervals and successful transmissions. Suppose the average successful transmission lasts 8 slot times
(512 bytes). Using your minimum length of the contention interval from
above, what fraction of the theoretical 10-Mbps bandwidth is available for
transmissions?
54
What conditions would have to hold for a corrupted frame to circulate forever
on a token ring without a monitor? How does the monitor fix this problem?
164
2
Direct Link Networks
55
An IEEE 802.5 token ring has five stations and a total wire length of 230 m.
How many bits of delay must the monitor insert into the ring? Do this for both
4 and 16 Mbps; use a propagation rate of 2.3 × 108 m/s.
56
Consider a token ring network like FDDI in which a station is allowed to
hold the token for some period of time (the token holding time, or THT). Let
RingLatency denote the time it takes the token to make one complete rotation around the network when none of the stations have any data to send.
(a) In terms of THT and RingLatency, express the efficiency of this network
when only a single station is active.
(b) What setting of THT would be optimal for a network that had only one
station active (with data to send) at a time?
(c) In the case where N stations are active, give an upper bound on the token
rotation time, or TRT, for the network.
57
Consider a token ring with a ring latency of 200 µs. Assuming that the delayed token release strategy is used, what is the effective throughput rate that
can be achieved if the ring has a bandwidth of 4 Mbps? What is the effective
throughput rate that can be achieved if the ring has a bandwidth of 100 Mbps?
Answer for both a single active host and for “many” hosts; for the latter, assume
there are sufficiently many hosts transmitting that the time spent advancing the
token can be ignored. Assume a packet size of 1 KB.
58
For a 100-Mbps token ring network with a token rotation time of 200 µs and
that allows each station to transmit one 1-KB packet each time it possesses the
token, calculate the maximum effective throughput rate that any one host can
achieve. Do this assuming (a) immediate release and (b) delayed release.
59
Suppose a 100-Mbps delayed-release token ring has 10 stations, a ring latency
of 30 µs, and an agreed-on token rotation time (TRT) of 350 µs.
(a) How many synchronous frame bytes could each station send, assuming all
are allocated the same amount?
(b) Assume stations A, B, C are in increasing order on the ring. Due to uniform
synchronous traffic, the TRT without asynchronous data is 300 µs. B sends
a 200-µs (2.5-Kb) asynchronous frame. What TRT will A, B, and C then
see on their next measurement? Who may transmit such a frame next?
Exercises
165
60
How can an RPR network operate without using a token?
61
How can a wireless node interfere with the communications of another node
when the two nodes are separated by a distance greater than the transmission
range of either node?
62
Why might a mesh topology be superior to a base station topology for communications in a natural disaster?
63
Suppose a single computer is capable of generating output data at a rate higher
than Bluetooth’s bandwidth. If the computer were equipped with two or more
Bluetooth masters, each with its own slaves, would that work?
64
Why does it make sense that 802.16 (WiMAX) physical layer protocols use
only TDD (and not FDD) in license-exempt bands?
65
Which wireless protocol would you expect to provide a better foundation for a
packet-based telephony service: 802.11 (Wi-Fi) or 802.16 (WiMAX)? Why?
66
When a cell phone moves from an area served exclusively by a single base
station to an area where the cells of several base stations overlap, how is it
determined which base station will control the phone?
67
Why does a second generation (2G) cell phone connection provide much lower
bandwidth than a typical dial-up Internet connection?
68
Why is it important that nodes in sensor nets consume very little power?
69
Why isn’t it practical for each node in a sensor net to learn its location by using
GPS? Describe a practical alternative.
Packet Switching
Nature seems . . . to reach many of her ends by long circuitous routes.
—Rudolph Lotze
T
he directly connected networks described in the previous chapter suffer from
two limitations. First, there is a limit to how many hosts can be attached. For
example, only two hosts can be attached to a point-to-point link, and the Ethernet specification allows no more than 1,024 hosts. Second, there is a limit to how large
of a geographic area a single network can serve. For example, an Ethernet can span only
2,500 m, wireless networks are limited by the ranges of their radios, and even though
point-to-point links can be quite
long, they do not really serve the area
P R O B L E M
between the two ends. Since our goal
Not All Networks Are
is to build networks that can be global
Directly Connected
in scale, the next problem is therefore
to enable communication between
hosts that are not directly connected.
This problem is similar to one addressed in the telephone network: Your phone
is not directly connected to every person you might want to call, but instead is connected to an exchange that contains a switch. It is the switches that create the impression
that you have a connection to the person at the other end of the call. Similarly, computer networks use packet switches to enable packets to travel from one host to another,
even when no direct connection exists between those hosts. This chapter introduces the
major concepts of packet switching, which lies at the heart of computer networking.
A packet switch is a device with several inputs and outputs leading to and from the
hosts that the switch interconnects. The core job of a switch is to take packets that arrive
on an input and forward (or switch) them to the right output so that they will reach
their appropriate destination. There are a variety of ways that the switch can determine
the “right” output for a packet, which can be broadly categorized as connectionless and
connection-oriented approaches.
166
A key problem that a switch must deal with is the finite
bandwidth of its outputs. If packets destined for a certain output arrive at a switch and their arrival rate exceeds the capacity of that output, then we have a problem of contention. The
switch queues (buffers) packets until the contention subsides, but
if it lasts too long, the switch will run out of buffer space and
be forced to discard packets. When packets are discarded too
frequently, the switch is said to be congested. The ability of a
switch to handle contention is a key aspect of its performance.
This chapter introduces the issues of forwarding and contention in packet switches. We begin by considering the various approaches to switching, including the connectionless and
connection-oriented models. We then examine two particular
technologies in detail. The first is LAN switching, which has
evolved from Ethernet bridging to become one of the dominant technologies in today’s LAN environments. The second
noteworthy switching technology is asynchronous transfer mode
(ATM), which was initially developed to meet the needs of
telecommunications service providers in wide area networks. Finally, we consider some of the aspects of switch design that
must be taken into account when building large-scale networks.
3
168
3
3.1
Packet Switching
Switching and Forwarding
In the simplest terms, a switch is a mechanism that allows us to interconnect links to
form a larger network. A switch is a multi-input, multioutput device, which transfers
packets from an input to one or more outputs. Thus, a switch adds the star topology
(see Figure 3.1) to the point-to-point link, bus (Ethernet), and ring (802.5, 802.17, and
FDDI) topologies established in the last chapter. A star topology has several attractive
properties:
■
Even though a switch has a fixed number of inputs and outputs, which limits
the number of hosts that can be connected to a single switch, large networks can
be built by interconnecting a number of switches;
■
We can connect switches to each other and to hosts using point-to-point links,
which typically means that we can build networks of large geographic scope;
■
Adding a new host to the network by connecting it to a switch does not necessarily reduce the performance of the network for other hosts already connected.
This last claim cannot be made for the shared-media networks discussed in the last
chapter. For example, it is impossible for two hosts on the same 10-Mbps Ethernet to
transmit continuously at 10 Mbps because they share the same transmission medium.
Every host on a switched network has its own link to the switch, so it may be entirely
possible for many hosts to transmit at the full link speed (bandwidth), provided that the
switch is designed with enough aggregate capacity. Providing high aggregate throughput
Figure 3.1
A switch provides a star topology.
3.1 Switching and Forwarding
Figure 3.2
Example protocol graph running on a switch.
Figure 3.3
Example switch with three input and output ports.
169
is one of the design goals for a switch; we return to this topic below. In general, switched
networks are considered more scalable (i.e., more capable of growing to large numbers of
nodes) than shared-media networks because of this ability to support many hosts at full
speed.
A switch is connected to a set of links and, for each of these links, runs the appropriate data link protocol to communicate with the node at the other end of the link.
A switch’s primary job is to receive incoming packets on one of its links and to transmit
them on some other link. This function is sometimes referred to as either switching or
forwarding, and in terms of the OSI architecture, it is the main function of the network
layer. Figure 3.2 shows the protocol graph that would run on a switch that is connected
to two T3 links and one STS-1 SONET link. A representation of this same switch is
given in Figure 3.3. In this figure, we have split the input and output halves of each link,
and we refer to each input or output as a port. (In general, we assume that each link is
bidirectional, and hence supports both input and output.) In other words, this example
switch has three input ports and three output ports.
The question then is, how does the switch decide on which output port to place
each packet? The general answer is that it looks at the header of the packet for an identifier that it uses to make the decision. The details of how it uses this identifier vary, but
there are two common approaches. The first is the datagram or connectionless approach.
170
3
Packet Switching
The second is the virtual circuit or connection-oriented approach. A third approach, source
routing, is less common than these other two, but it is simple to explain and does have
some useful applications.
One thing that is common to all networks is that we need to have a way to identify
the end nodes. Such identifiers are usually called addresses. We have already seen examples of addresses in the previous chapter, such as the 48-bit address used for Ethernet.
The only requirement for Ethernet addresses is that no two nodes on a network have the
same address. This is accomplished by making sure that all Ethernet cards are assigned
a globally unique identifier. For the following discussions, we assume that each host has
a globally unique address. Later on, we consider other useful properties that an address
might have, but global uniqueness is adequate to get us started.
Another assumption that we need to make is that there is some way to identify the
input and output ports of each switch. There are at least two sensible ways to identify
ports: One is to number each port, and the other is to identify the port by the name of
the node (switch or host) to which it leads. For now, we use numbering of the ports.
3.1.1 Datagrams
The idea behind datagrams is incredibly simple: You just make sure that every packet
contains enough information to enable any switch to decide how to get it to its destination. That is, every packet contains the complete destination address. Consider the
example network illustrated in Figure 3.4, in which the hosts have addresses A, B, C, and
so on. To decide how to forward a packet, a switch consults a forwarding table (sometimes called a routing table), an example of which is depicted in Table 3.1. This particular
table shows the forwarding information that switch 2 needs to forward datagrams in the
Destination
Port
A
3
B
0
C
3
D
3
E
2
F
1
G
0
H
0
Table 3.1
Forwarding table for switch 2.
3.1 Switching and Forwarding
Figure 3.4
171
Datagram forwarding: an example network.
example network. It is pretty easy to figure out such a table when you have a complete
map of a simple network like that depicted here; we could imagine a network operator
configuring the tables statically. It is a lot harder to create the forwarding tables in large,
complex networks with dynamically changing topologies and multiple paths between
destinations. That harder problem is known as routing and is the topic of Section 4.2.
We can think of routing as a process that takes place in the background so that when a
data packet turns up, we will have the right information in the forwarding table to be
able to forward, or switch, the packet.
Connectionless (datagram) networks have the following characteristics:
■
A host can send a packet anywhere at any time, since any packet that turns
up at a switch can be immediately forwarded (assuming a correctly populated
forwarding table). As we will see, this contrasts with most connection-oriented
networks, in which some “connection state” needs to be established before the
first data packet is sent.
■
When a host sends a packet, it has no way of knowing if the network is capable
of delivering it or if the destination host is even up and running.
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3
Packet Switching
■
Each packet is forwarded independently of previous packets that might have
been sent to the same destination. Thus, two successive packets from host A to
host B may follow completely different paths (perhaps because of a change in
the forwarding table at some switch in the network).
■
A switch or link failure might not have any serious effect on communication
if it is possible to find an alternate route around the failure and to update the
forwarding table accordingly.
This last fact is particularly important to the history of datagram networks. One
of the important design goals of the Internet is robustness to failures, and history has
shown it to be quite effective at meeting this goal.1
3.1.2 Virtual Circuit Switching
A widely used technique for packet switching, which differs significantly from the datagram model, uses the concept of a virtual circuit (VC). This approach, which is also
called a connection-oriented model, requires that a virtual connection from the source
host to the destination host is set up before any data is sent. To understand how this
works, consider Figure 3.5, where host A again wants to send packets to host B. We can
think of this as a two-stage process. The first stage is connection setup. The second is data
transfer. We consider each in turn.
Figure 3.5
An example of a virtual circuit network.
1 The oft-repeated claim that the ARPANET was built to withstand nuclear attack does not appear to be substantiated by
those who actually worked on its design.
3.1 Switching and Forwarding
173
In the connection setup phase, it is necessary to establish “connection state” in
each of the switches between the source and destination hosts. The connection state for
a single connection consists of an entry in a VC table in each switch through which the
connection passes. One entry in the VC table on a single switch contains
■
A virtual circuit identifier (VCI) that uniquely identifies the connection at this
switch, and which will be carried inside the header of the packets that belong to
this connection;
■
An incoming interface on which packets for this VC arrive at the switch;
■
An outgoing interface in which packets for this VC leave the switch;
■
A potentially different VCI that will be used for outgoing packets.
The semantics of one such entry is as follows: If a packet arrives on the designated
incoming interface and that packet contains the designated VCI value in its header,
then that packet should be sent out the specified outgoing interface with the specified
outgoing VCI value having been first placed in its header.
Note that the combination of the VCI of packets as they are received at the switch
and the interface on which they are received uniquely identifies the virtual connection.
There may of course be many virtual connections established in the switch at one time.
Also, we observe that the incoming and outgoing VCI values are generally not the same.
Thus, the VCI is not a globally significant identifier for the connection; rather, it has
significance only on a given link (i.e., it has link-local scope).
Whenever a new connection is created, we need to assign a new VCI for that
connection on each link that the connection will traverse. We also need to ensure that
the chosen VCI on a given link is not currently in use on that link by some existing
connection.
There are two broad classes of approach to establishing connection state. One is to
have a network administrator configure the state, in which case the virtual circuit is “permanent.” Of course, it can also be deleted by the administrator, so a permanent virtual
circuit (PVC) might best be thought of as a long-lived, or administratively configured
VC. Alternatively, a host can send messages into the network to cause the state to be
established. This is referred to as signalling, and the resulting virtual circuits are said to
be switched. The salient characteristic of a switched virtual circuit (SVC) is that a host
may set up and delete such a VC dynamically without the involvement of a network administrator. Note that an SVC should more accurately be called a “signalled” VC, since
it is the use of signalling (not switching) that distinguishes an SVC from a PVC.
Let’s assume that a network administrator wants to manually create a new virtual connection from host A to host B. First, the administrator needs to identify a path
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3
Packet Switching
Incoming Interface
2
Table 3.2
Incoming VCI
Outgoing Interface
Outgoing VCI
5
1
11
Virtual circuit table entry for switch 1.
through the network from A to B. In the example network of Figure 3.5, there is only
one such path, but in general this may not be the case. The administrator then picks
a VCI value that is currently unused on each link for the connection. For the purposes
of our example, let’s suppose that the VCI value 5 is chosen for the link from host A
to switch 1, and that 11 is chosen for the link from switch 1 to switch 2. In that case,
switch 1 needs to have an entry in its VC table configured as shown in Table 3.2.
Similarly, suppose that the VCI of 7 is chosen to identify this connection on the
link from switch 2 to switch 3, and that a VCI of 4 is chosen for the link from switch 3
to host B. In that case, switches 2 and 3 need to be configured with VC table entries as
shown in Table 3.3. Note that the “outgoing” VCI value at one switch is the “incoming”
VCI value at the next switch.
Once the VC tables have been set up, the data transfer phase can proceed, as illustrated in Figure 3.6. For any packet that it wants to send to host B, A puts the VCI
value of 5 in the header of the packet and sends it to switch 1. Switch 1 receives any
such packet on interface 2, and it uses the combination of the interface and the VCI
in the packet header to find the appropriate VC table entry. As shown in Table 3.2, the
table entry in this case tells switch 1 to forward the packet out of interface 1 and to put
the VCI value 11 in the header when the packet is sent. Thus, the packet will arrive at
switch 2 on interface 3 bearing VCI 11. Switch 2 looks up interface 3 and VCI 11 in
its VC table (as shown in Table 3.3) and sends the packet on to switch 3 after updating
the VCI value in the packet header appropriately, as shown in Figure 3.7. This process
continues until it arrives at host B with the VCI value of 4 in the packet. To host B, this
identifies the packet as having come from host A.
Incoming Interface
Incoming VCI
Outgoing Interface
Outgoing VCI
3
VC table entry at switch 2
11
2
7
Incoming Interface
Incoming VCI
Outgoing Interface
Outgoing VCI
0
VC table entry at switch 3
7
1
4
Table 3.3
Virtual circuit table entries for switches 2 and 3.
3.1 Switching and Forwarding
Figure 3.6
A packet is sent into a virtual circuit network.
Figure 3.7
A packet makes its way through a virtual circuit network.
175
In real networks of reasonable size, the burden of configuring VC tables correctly in
a large number of switches would quickly become excessive using the above procedures.
Thus, some sort of signalling is almost always used, even when setting up “permanent”
VCs. In the case of PVCs, signalling is initiated by the network administrator, while
SVCs are usually set up using signalling by one of the hosts. We consider now how the
same VC just described could be set up by signalling from the host.
To start the signalling process, host A sends a setup message into the network, that
is, to switch 1. The setup message contains, among other things, the complete destination address of host B. The setup message needs to get all the way to B to create the
necessary connection state in every switch along the way. We can see that getting the
setup message to B is a lot like getting a datagram to B, in that the switches have to
know which output to send the setup message to so that it eventually reaches B. For
now, let’s just assume that the switches know enough about the network topology to
figure out how to do that, so that the setup message flows on to switches 2 and 3 before
finally reaching host B.
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When switch 1 receives the connection request, in addition to sending it on to
switch 2, it creates a new entry in its virtual circuit table for this new connection. This
entry is exactly the same as shown previously in Table 3.2. The main difference is that
now the task of assigning an unused VCI value on the interface is performed by the
switch for that port. In this example, the switch picks the value 5. The virtual circuit table
now has the following information: “When packets arrive on port 2 with identifier 5,
send them out on port 1.” Another issue is that, somehow, host A will need to learn that
it should put the VCI value of 5 in packets that it wants to send to B; we will see how
that happens below.
When switch 2 receives the setup message, it performs a similar process; in this
example it picks the value 11 as the incoming VCI value. Similarly switch 3 picks 7 as
the value for its incoming VCI. Each switch can pick any number it likes, as long as that
number is not currently in use for some other connection on that port of that switch. As
noted above, VCIs have link-local scope, that is, they have no global significance.
Finally the setup message arrives as host B. Assuming that B is healthy and willing
to accept a connection from host A, it too allocates an incoming VCI value, in this case 4.
This VCI value can be used by B to identify all packets coming from host A.
Now, to complete the connection, everyone needs to be told what their downstream
neighbor is using as the VCI for this connection. Host B sends an acknowledgment of
the connection setup to switch 3 and includes in that message the VCI that it chose (4).
Now switch 3 can complete the virtual circuit table entry for this connection, since it
knows the outgoing value must be 4. Switch 3 sends the acknowledgment on to switch 2,
specifying a VCI of 7. Switch 2 sends the message on to switch 1, specifying a VCI of 11.
Finally, switch 1 passes the acknowledgment on to host A, telling it to use the VCI of 5
for this connection.
At this point, everyone knows all that is necessary to allow traffic to flow from
host A to host B. Each switch has a complete virtual circuit table entry for the connection. Furthermore, host A has a firm acknowledgment that everything is in place all the
way to host B. At this point, the connection table entries are in place in all three switches
just as in the administratively configured example above, but the whole process happened
automatically in response to the signalling message sent from A. The data transfer phase
can now begin and is identical to that used in the PVC case.
When host A no longer wants to send data to host B, it tears down the connection
by sending a teardown message to switch 1. The switch removes the relevant entry from
its table and forwards the message on to the other switches in the path, which similarly
delete the appropriate table entries. At this point, if host A were to send a packet with a
VCI of 5 to switch 1, it would be dropped as if the connection had never existed.
3.1 Switching and Forwarding
177
There are several things to note about virtual circuit switching:
■
Since host A has to wait for the connection request to reach the far side of the
network and return before it can send its first data packet, there is at least one
RTT of delay before data is sent.2
■
While the connection request contains the full address for host B (which might
be quite large, being a global identifier on the network), each data packet contains only a small identifier, which is only unique on one link. Thus, the perpacket overhead caused by the header is reduced relative to the datagram model.
■
If a switch or a link in a connection fails, the connection is broken and a new
one will need to be established. Also, the old one needs to be torn down to free
up table storage space in the switches.
■
The issue of how a switch decides which link to forward the connection request
on has been glossed over. In essence, this is the same problem as building up the
forwarding table for datagram forwarding, which requires some sort of routing
algorithm. Routing is described in Section 4.2, and the algorithms described
there are generally applicable to routing setup requests as well as datagrams.
One of the nice aspects of virtual circuits is that by the time the host gets the goahead to send data, it knows quite a lot about the network—for example, that there
really is a route to the receiver and that the receiver is willing and able to receive data.
It is also possible to allocate resources to the virtual circuit at the time it is established.
For example, an X.25 network—a packet-switched network that uses the connectionoriented model—employs the following three-part strategy:
1
Buffers are allocated to each virtual circuit when the circuit is initialized;
2
The sliding window protocol is run between each pair of nodes along the virtual
circuit, and this protocol is augmented with flow control to keep the sending
node from overrunning the buffers allocated at the receiving node;
3
The circuit is rejected by a given node if not enough buffers are available at that
node when the connection request message is processed.
2 This is not strictly true. Some people have proposed “optimistically” sending a data packet immediately after sending
the connection request. However, most current implementations wait for connection setup to complete before sending
data.
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In doing these three things, each node is ensured of having the buffers it needs to queue
the packets that arrive on that circuit. This basic strategy is usually called hop-by-hop flow
control.
By comparison, a datagram network has no connection establishment phase,
and each switch processes each packet independently, making it less obvious how a
datagram network would allocate resources in a meaningful way. Instead, each arriving packet competes with all other packIntroduction to Congestion
ets for buffer space. If there are no free
buffers, the incoming packet must be disRecall the distinction between concarded. We observe, however, that even in
tention and congestion: Contention
a datagram-based network, a source host
occurs when multiple packets have to
often sends a sequence of packets to the
be queued at a switch because they
same destination host. It is possible for
are competing for the same output
each switch to distinguish among the set of
link, while congestion means that the
packets it currently has queued, based on
switch has so many packets queued
the source/destination pair, and thus for
that it runs out of buffer space and has
the switch to ensure that the packets beto start dropping packets. We return
longing to each source/destination pair are
to the topic of congestion in Chapreceiving a fair share of the switch’s buffers.
ter 6, after we have seen the transWe discuss this idea in much greater depth
port protocol component of the netin Chapter 6.
work architecture. At this point, howIn the virtual circuit model, we could
ever, we observe that the decision as to
imagine providing each circuit with a difwhether your network uses virtual cirferent quality of service (QoS). In this
cuits or datagrams has an impact on
setting, the term “quality of service” is
how you deal with congestion.
usually taken to mean that the network
On the one hand, suppose that
gives the user some kind of performanceeach switch allocates enough buffers
related guarantee, which in turn implies
to handle the packets belonging to
that switches set aside the resources they
each virtual circuit it supports, as is
need to meet this guarantee. For example,
done in an X.25 network. In this
the switches along a given virtual circuit
case, the network has defined away
might allocate a percentage of each outthe problem of congestion—a switch
going link’s bandwidth to that circuit. As
never encounters a situation in which
another example, a sequence of switches
it has more packets to queue than it
might ensure that packets belonging to a
has buffer space, since it does not alparticular circuit not be delayed (queued)
low the connection to be established
for more than a certain amount of time.
in the first place unless it can dedicate
We return to the topic of quality of service
enough resources to it to avoid this
in Section 6.5.
3.1 Switching and Forwarding
Figure 3.8
179
Frame Relay packet format.
situation. The problem with this approach, however, is that it is extremely
conservative—it is unlikely that all the
circuits will need to use all of their
buffers at the same time, and as a
consequence, the switch is potentially
underutilized.
On the other hand, the datagram model seemingly invites congestion—you do not know that there
is enough contention at a switch to
cause congestion until you run out of
buffers. At that point, it is too late to
prevent the congestion, and your only
choice is to try to recover from it. The
good news, of course, is that you may
be able to get better utilization out of
your switches since you are not holding buffers in reserve for a worst-case
scenario that is unlikely to happen.
As is quite often the case, nothing is strictly black and white—there
are design advantages for defining
congestion away (as the X.25 model
does) and for doing nothing about
congestion until after it happens (as
the simple datagram model does).
There are also intermediate points between these two extremes. We describe
some of these design points in Chapter 6.
The most popular examples of virtual circuit technologies are Frame Relay
and asynchronous transfer mode (ATM).
ATM has a number of interesting properties that we discuss in Section 3.3. Frame
Relay is a rather straightforward implementation of virtual circuit technology,
and its simplicity has made it extremely
popular. Many network service providers
offer Frame Relay PVC services. One
of the applications of Frame Relay is
the construction of virtual private networks (VPNs), a subject discussed in Section 4.1.8.
Frame Relay provides some basic quality of service and congestionavoidance features, but these are rather
lightweight compared to X.25 and ATM.
The Frame Relay packet format (see Figure 3.8) provides a good example of a
packet used for virtual circuit switching.
3.1.3 Source Routing
A third approach to switching that uses
neither virtual circuits nor conventional
datagrams is known as source routing. The
name derives from the fact that all the information about network topology that is
required to switch a packet across the network is provided by the source host.
There are various ways to implement source routing. One would be to
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3
Packet Switching
Figure 3.9 Source routing in a switched network (where the switch reads the
rightmost number).
assign a number to each output of each
switch and to place that number in the
header of the packet. The switching function is then very simple: For each packet
that arrives on an input, the switch would
read the port number in the header and
transmit the packet on that output. However, since there will in general be more
than one switch in the path between the
sending and the receiving host, the header
for the packet needs to contain enough information to allow every switch in the path
to determine on which output the packet
needs to be placed. One way to do this
would be to put an ordered list of switch
ports in the header and to rotate the list so
that the next switch in the path is always
at the front of the list. Figure 3.9 illustrates
this idea.
In this example, the packet needs to
traverse three switches to get from host A
Optical Switching
To a casual observer of the networking industry around the year 2000, it
might have appeared that the most interesting sort of switching was optical
switching. Indeed, optical switching
did become an important technology in the late 1990s, due to a confluence of several factors. One factor
was the commercial availability of
dense wavelength division multiplexing (DWDM) equipment, which
makes it possible to send a great deal
of information down a single fiber by
transmitting on a large number of optical wavelengths (or colors) at once.
Thus, for example, one might send
data on 100 or more different wavelengths, and each wavelength might
carry as much as 10 Gbps of data.
3.1 Switching and Forwarding
A second factor was the commercial availability of optical amplifiers. Optical signals are attenuated as
they pass through fiber, and after some
distance (about 40 km or so) they
need to be made stronger in some way.
Before optical amplifiers, it was necessary to place repeaters in the path to
recover the optical signal, convert it
to a digital electronic signal, and then
convert it back to optical again. Before you could get the data into a repeater, you would have to demultiplex
it using a DWDM terminal. Thus, a
large number of DWDM terminals
would be needed just to drive a single
fiber pair for a long distance. Optical
amplifiers, unlike repeaters, are analog
devices that boost whatever signal is
sent along the fiber, even if it is sent
on a hundred different wavelengths.
Optical amplifiers, therefore, made
DWDM gear much more attractive,
because now a pair of DWDM terminals could talk to each other when
separated by a distance of hundreds
of kilometers. Furthermore, you could
even upgrade the DWDM gear at the
ends without touching the optical amplifiers in the middle of the path,
because they will amplify 100 wavelengths as easily as 50 wavelengths.
With DWDM and optical amplifiers, it became possible to build optical networks of huge capacity. But at
least one more type of device is needed
to make these networks useful—the
optical switch. Most so-called optical
switches today actually perform their
181
to host B. At switch 1, it needs to exit
on port 1, at the next switch it needs to
exit at port 0, and at the third switch it
needs to exit at port 3. Thus, the original
header when the packet leaves host A contains the list of ports (3, 0, 1), where we
assume that each switch reads the rightmost element of the list. To make sure
that the next switch gets the appropriate information, each switch rotates the
list after it has read its own entry. Thus,
the packet header as it leaves switch 1
en route to switch 2 is now (1, 3, 0);
switch 2 performs another rotation and
sends out a packet with (0, 1, 3) in the
header. Although not shown, switch 3
performs yet another rotation, restoring
the header to what it was when host A
sent it.
There are several things to note
about this approach. First, it assumes that
host A knows enough about the topology
of the network to form a header that has
all the right directions in it for every switch
in the path. This is somewhat analogous
to the problem of building the forwarding tables in a datagram network or figuring out where to send a setup packet in
a virtual circuit network. Second, observe
that we cannot predict how big the header
needs to be, since it must be able to hold
one word of information for every switch
on the path. This implies that headers are
probably of variable length with no upper bound, unless we can predict with absolute certainty the maximum number of
switches through which a packet will ever
need to pass. Third, there are some variations on this approach. For example, rather
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Packet Switching
Figure 3.10 Three ways to handle headers for source routing: (a) rotation,
(b) stripping, and (c) pointer. The labels are read right to left.
than rotate the header, each switch could
just strip the first element as it uses it.
Rotation has an advantage over stripping,
however: Host B gets a copy of the complete header, which may help it figure
out how to get back to host A. Yet another alternative is to have the header carry
a pointer to the current “next port” entry, so that each switch just updates the
pointer rather than rotating the header;
this may be more efficient to implement.
We show these three approaches in Figure 3.10. In each case, the entry that this
switch needs to read is A, and the entry that the next switch needs to read
is B.
Source routing can be used in both
datagram networks and virtual circuit networks. For example, the Internet Protocol, which is a datagram protocol, includes
a source route option that allows selected
packets to be source routed, while the majority are switched as conventional datagrams. Source routing is also used in some
virtual circuit networks as the means to
get the initial setup request along the path
from source to destination.
switching function electronically, and
from an architectural point of view
they have more in common with the
circuit switches of the telephone network than the packet switches described in this chapter. A typical optical switch has a large number of
interfaces that understand SONET
framing, and is able to cross-connect
a SONET channel from an incoming interface to an outgoing interface.
Thus, with an optical switch, it becomes possible to provide SONET
channels from point A to point B via
point C even if there is no direct fiber
path from A to B—there just needs
to be a path from A to C, a switch at
C, and a path from C to B. In this
respect, an optical switch bears some
relationship to the switches in Figure 3.5, in that it creates the illusion
of a connection between two points
even when there is no direct physical connection between them. However, optical switches do not provide
virtual circuits, they provide “real” circuits (e.g., a SONET channel). There
3.2
Bridges and LAN Switches
183
Source routes are sometimes categorized as “strict” or “loose.” In a strict source
route, every node along the path must be specified, whereas a loose source route only
specifies a set of nodes to be traversed, without saying exactly how to get from one node
to the next. A loose source route can be thought of as a set of waypoints rather than
a completely specified route. The loose option can be helpful to limit the amount of
information that a source must obtain to create a source route. In any reasonably large
network, it is likely to be hard for a host to get the complete path information it needs
to correctly construct a strict source route to any destination. But both types of source
routes do find application in certain scenarios, one of which is described in Section 4.5.
3.2
Bridges and LAN Switches
Having discussed some of the basic ideas behind switching, we now focus more closely
on some specific switching technologies. We begin by considering a class of switch that
is used to forward packets between sharedare even some newer types of optical
media LANs such as Ethernets. Such
switch that use microscopic, electronswitches are sometimes known by the obically controlled mirrors to deflect all
vious name of LAN switches; historically
the light from one switch port to anthey have also been referred to as bridges.
other, so that there could be an uninSuppose you have a pair of Ethterrupted optical channel from point
ernets
that you want to interconnect.
A to point B. The technology behind
One approach you might try is to put
these devices is called MEMS (Microa repeater between them, as described in
electromechanical Systems).
Chapter 2. This would not be a workable
We don’t cover optical networksolution, however, if doing so exceeded
ing extensively in this book, in part
the physical limitations of the Ethernet.
because of space considerations. For
(Recall that no more than four repeaters
many practical purposes, you can think
between any pair of hosts and no more
of optical networks as a piece of the
than a total of 2,500 m in length are alinfrastructure that enables telephone
lowed.) An alternative would be to put a
companies to provide SONET links
node between the two Ethernets and have
or other types of circuits where and
the node forward frames from one Etherwhen you need them. However, it is
net to the other. This node would be in
worth noting that many of the techpromiscuous mode, accepting all frames
nologies that are discussed later in this
transmitted on either of the Ethernets,
book, such as routing protocols and
so it could forward them to the other.
multiprotocol label switching, do have
The node we have just described
application to the world of optical
is typically called a bridge, and a collecnetworking.
tion of LANs connected by one or more
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Packet Switching
bridges is usually said to form an extended LAN. In their simplest variants, bridges simply
accept LAN frames on their inputs and forward them out on all other outputs. This
simple strategy was used by early bridges, but has since been refined to make bridges a
more effective mechanism for interconnecting a set of LANs. The rest of this section fills
in the more interesting details.
Note that a bridge meets our definition of a switch from the previous section: a
multi-input, multioutput device, which transfers packets from an input to one or more
outputs. And recall that this provides a way to increase the total bandwidth of a network.
For example, while a single Ethernet segment can carry only 10 Mbps of total traffic, an
Ethernet bridge can carry as much as 10n Mbps, where n is the number of ports (inputs
and outputs) on the bridge.
3.2.1 Learning Bridges
The first optimization we can make to a bridge is to observe that it need not forward
all frames that it receives. Consider the bridge in Figure 3.11. Whenever a frame from
host A that is addressed to host B arrives on port 1, there is no need for the bridge to
forward the frame out over port 2. The question, then, is, how does a bridge come to
learn on which port the various hosts reside?
One option would be to have a human download a table into the bridge similar
to the one given in Table 3.4. Then, whenever the bridge receives a frame on port 1
that is addressed to host A, it would not forward the frame out on port 2; there would
be no need because host A would have already directly received the frame on the LAN
connected to port 1. Anytime a frame addressed to host A was received on port 2, the
bridge would forward the frame out on port 1.
Figure 3.11
Illustration of a learning bridge.
3.2
Host
Port
A
1
B
1
C
1
X
2
Y
2
Z
2
Table 3.4
Bridges and LAN Switches
185
Forwarding table maintained by a bridge.
Note that a bridge using such a table would be using the datagram (or connectionless) model of forwarding described in Section 3.1.1. Each packet carries a global address,
and the bridge decides on which output to send a packet by looking up that address in a
table.
Having a human maintain this table is quite a burden, especially considering that
there is a simple trick by which a bridge can learn this information for itself. The idea
is for each bridge to inspect the source address in all the frames it receives. Thus, when
host A sends a frame to a host on either side of the bridge, the bridge receives this frame
and records the fact that a frame from host A was just received on port 1. In this way,
the bridge can build a table just like Table 3.4.
When a bridge first boots, this table is empty; entries are added over time. Also,
a timeout is associated with each entry, and the bridge discards the entry after a specified
period of time. This is to protect against the situation in which a host—and as a consequence, its LAN address—is moved from one network to another. Thus, this table is not
necessarily complete. Should the bridge receive a frame that is addressed to a host not
currently in the table, it goes ahead and forwards the frame out on all the other ports.
In other words, this table is simply an optimization that filters out some frames; it is not
required for correctness.
Implementation
The code that implements the learning bridge algorithm is quite simple, and we sketch
it here. Structure BridgeEntry defines a single entry in the bridge’s forwarding table;
these are stored in a Map structure (which supports mapCreate, mapBind, and
MapResolve operations) to enable entries to be efficiently located when packets arrive
from sources already in the table. The constant MAX_TTL specifies how long an entry
is kept in the table before it is discarded.
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3
Packet Switching
#define BRIDGE_TAB_SIZE 1024 /* max. size of bridging
table */
#define MAX_TTL
120 /* time (in seconds) before
an entry is flushed */
typedef struct {
MacAddr
destination;
int
ifnumber;
u_short
TTL;
Binding
binding;
} BridgeEntry;
int
Map
/*
/*
/*
/*
MAC address of a node */
interface to reach it */
time to live */
binding in the Map */
numEntries = 0;
bridgeMap = mapCreate(BRIDGE_TAB_SIZE,
sizeof(BridgeEntry));
The routine that updates the forwarding table when a new packet arrives is given
by updateTable. The arguments passed are the source MAC address contained in the
packet and the interface number on which it was received. Another routine, not shown
here, is invoked at regular intervals, scans the entries in the forwarding table, and decrements the TTL (time to live) field of each entry, discarding any entries whose TTL has
reached 0. Note that the TTL is reset to MAX_TTL every time a packet arrives to refresh
an existing table entry, and that the interface on which the destination can be reached is
updated to reflect the most recently received packet.
void
updateTable (MacAddr src, int inif)
{
BridgeEntry
*b;
if (mapResolve(bridgeMap, &src, (void **)&b)
== FALSE )
{
/* this address is not in the table,
so try to add it */
if (numEntries < BRIDGE_TAB_SIZE)
{
b = NEW(BridgeEntry);
b->binding = mapBind( bridgeMap, &src, b);
/* use source address of packet as dest.
3.2
Bridges and LAN Switches
187
address in table */
b->destination = src;
numEntries++;
}
else
{
/* can’t fit this address in the table now,
so give up */
return;
}
}
/* reset TTL and use most recent input interface */
b->TTL = MAX_TTL;
b->ifnumber = inif;
}
Note that this implementation adopts a simple strategy in the case where the bridge
table has become full to capacity—it simply fails to add the new address. Recall that
completeness of the bridge table is not necessary for correct forwarding, it just optimizes
performance. If there is some entry in the table that is not currently being used, it will
eventually time out and be removed, creating space for a new entry. An alternative approach would be to invoke some sort of cache replacement algorithm on finding the
table full; for example, we might locate and remove the entry with the smallest TTL to
accommodate the new entry.
3.2.2 Spanning Tree Algorithm
The preceding strategy works just fine until the extended LAN has a loop in it, in which
case it fails in a horrible way—frames potentially loop through the extended LAN forever.
This is easy to see in the example depicted in Figure 3.12, where, for example, bridges
B1, B4, and B6 form a loop. How does an extended LAN come to have a loop in it? One
possibility is that the network is managed by more than one administrator, for example,
because it spans multiple departments in an organization. In such a setting, it is possible
that no single person knows the entire configuration of the network, meaning that a
bridge that closes a loop might be added without anyone knowing. A second, more likely
scenario is that loops are built into the network on purpose—to provide redundancy in
case of failure.
Whatever the cause, bridges must be able to correctly handle loops. This problem
is addressed by having the bridges run a distributed spanning tree algorithm. If you think
of the extended LAN as being represented by a graph that possibly has loops (cycles),
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3
Packet Switching
Figure 3.12
Extended LAN with loops.
Figure 3.13
Example of (a) a cyclic graph; (b) a corresponding spanning tree.
then a spanning tree is a subgraph of this graph that covers (spans) all the vertices, but
contains no cycles. That is, a spanning tree keeps all of the vertices of the original graph,
but throws out some of the edges. For example, Figure 3.13 shows a cyclic graph on the
left and one of possibly many spanning trees on the right.
The spanning tree algorithm, which was developed by Radia Perlman at the Digital
Equipment Corporation, is a protocol used by a set of bridges to agree upon a spanning
3.2
Bridges and LAN Switches
189
tree for a particular extended LAN. (The IEEE 802.1 specification for LAN bridges is
based on this algorithm.) In practice, this means that each bridge decides the ports over
which it is and is not willing to forward frames. In a sense, it is by removing ports from
the topology that the extended LAN is reduced to an acyclic tree.3 It is even possible that
an entire bridge will not participate in forwarding frames, which seems kind of strange
when you consider that the one reason we intentionally have loops in the network in
the first place is to provide redundancy. The algorithm is dynamic, however, meaning
that the bridges are always prepared to reconfigure themselves into a new spanning tree
should some bridge fail.
The main idea of the spanning tree is for the bridges to select the ports over which
they will forward frames. The algorithm selects ports as follows. Each bridge has a unique
identifier; for our purposes, we use the labels B1, B2, B3, and so on. The algorithm first
elects the bridge with the smallest ID as the root of the spanning tree; exactly how this
election takes place is described below. The root bridge always forwards frames out over
all of its ports. Next, each bridge computes the shortest path to the root and notes which
of its ports is on this path. This port is also selected as the bridge’s preferred path to the
root. Finally, all the bridges connected to a given LAN elect a single designated bridge that
will be responsible for forwarding frames toward the root bridge. Each LAN’s designated
bridge is the one that is closest to the root, and if two or more bridges are equally close
to the root, then the bridges’ identifiers are used to break ties; the smallest ID wins.
Of course, each bridge is connected to more than one LAN, so it participates in the
election of a designated bridge for each LAN it is connected to. In effect, this means that
each bridge decides if it is the designated bridge relative to each of its ports. The bridge
forwards frames over those ports for which it is the designated bridge.
Figure 3.14 shows the spanning tree that corresponds to the extended LAN shown
in Figure 3.12. In this example, B1 is the root bridge, since it has the smallest ID. Notice
that both B3 and B5 are connected to LAN A, but B5 is the designated bridge since
it is closer to the root. Similarly, both B5 and B7 are connected to LAN B, but in this
case, B5 is the designated bridge since it has the smaller ID; both are an equal distance
from B1.
While it is possible for a human to look at the extended LAN given in Figure 3.12
and to compute the spanning tree given in Figure 3.14 according to the rules given
above, the bridges in an extended LAN do not have the luxury of being able to see
the topology of the entire network, let alone peek inside other bridges to see their IDs.
3 Representing an extended LAN as an abstract graph is a bit awkward. Basically, you let both the bridges and the LANs
correspond to the vertices of the graph, and the ports correspond to the graph’s edges. However, the spanning tree we are
going to compute for this graph needs to span only those nodes that correspond to networks. It is possible that nodes
corresponding to bridges will be disconnected from the rest of the graph. This corresponds to a situation in which all the
ports connecting a bridge to various networks get removed by the algorithm.
190
3
Figure 3.14
Packet Switching
Spanning tree with some ports not selected.
Instead, the bridges have to exchange configuration messages with each other and then
decide whether or not they are the root or a designated bridge based on these messages.
Specifically, the configuration messages contain three pieces of information:
1
The ID for the bridge that is sending the message;
2
The ID for what the sending bridge believes to be the root bridge;
3
The distance, measured in hops, from the sending bridge to the root bridge.
Each bridge records the current “best” configuration message it has seen on each of
its ports (“best” is defined below), including both messages it has received from other
bridges and messages that it has itself transmitted.
Initially, each bridge thinks it is the root, and so it sends a configuration message
out on each of its ports identifying itself as the root and giving a distance to the root of 0.
Upon receiving a configuration message over a particular port, the bridge checks to see if
that new message is better than the current best configuration message recorded for that
port. The new configuration message is considered “better” than the currently recorded
information if
3.2
Bridges and LAN Switches
191
■
It identifies a root with a smaller ID or
■
It identifies a root with an equal ID but with a shorter distance or
■
The root ID and distance are equal, but the sending bridge has a smaller ID.
If the new message is better than the currently recorded information, the bridge discards the old information and saves the new information. However, it first adds 1 to the
distance-to-root field since the bridge is one hop farther away from the root than the
bridge that sent the message.
When a bridge receives a configuration message indicating that it is not the root
bridge—that is, a message from a bridge with a smaller ID—the bridge stops generating configuration messages on its own and instead only forwards configuration messages
from other bridges, after first adding 1 to the distance field. Likewise, when a bridge
receives a configuration message that indicates it is not the designated bridge for that
port—that is, a message from a bridge that is closer to the root or equally far from the
root but with a smaller ID—the bridge stops sending configuration messages over that
port. Thus, when the system stabilizes, only the root bridge is still generating configuration messages, and the other bridges are forwarding these messages only over ports for
which they are the designated bridge.
To make this more concrete, consider what would happen in Figure 3.14 if the
power had just been restored to the building housing this network, so that all the bridges
boot at about the same time. All the bridges would start off by claiming to be the root.
We denote a configuration message from node X in which it claims to be distance d
from root node Y as (Y , d , X ). Focusing on the activity at node B3, a sequence of
events would unfold as follows:
1
B3 receives (B2, 0, B2);
2
Since 2 < 3, B3 accepts B2 as root;
3
B3 adds one to the distance advertised by B2 (0) and thus sends (B2, 1, B3)
toward B5;
4
Meanwhile, B2 accepts B1 as root because it has the lower ID, and it sends
(B1, 1, B2) toward B3;
5
B5 accepts B1 as root and sends (B1, 1, B5) toward B3;
6
B3 accepts B1 as root, and it notes that both B2 and B5 are closer to the root
than it is. Thus, B3 stops forwarding messages on both its interfaces.
This leaves B3 with both ports not selected, as shown in Figure 3.14.
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Packet Switching
Even after the system has stabilized, the root bridge continues to send configuration messages periodically, and the other bridges continue to forward these messages
as described in the previous paragraph. Should a particular bridge fail, the downstream
bridges will not receive these configuration messages, and after waiting a specified period
of time, they will once again claim to be the root, and the algorithm just described will
kick in again to elect a new root and new designated bridges.
One important thing to notice is that although the algorithm is able to reconfigure
the spanning tree whenever a bridge fails, it is not able to forward frames over alternative
paths for the sake of routing around a congested bridge.
3.2.3 Broadcast and Multicast
The preceding discussion has focused on how bridges forward unicast frames from one
LAN to another. Since the goal of a bridge is to transparently extend a LAN across
multiple networks, and since most LANs support both broadcast and multicast, then
bridges must also support these two features. Broadcast is simple—each bridge forwards
a frame with a destination broadcast address out on each active (selected) port other than
the one on which the frame was received.
Multicast can be implemented in exactly the same way, with each host deciding
for itself whether or not to accept the message. This is exactly what is done in practice.
Notice, however, that since not all the LANs in an extended LAN necessarily have a host
that is a member of a particular multicast group, it is possible to do better. Specifically,
the spanning tree algorithm can be extended to prune networks over which multicast
frames need not be forwarded. Consider a frame sent to group M by a host on LAN A in
Figure 3.14. If there is no host on LAN J that belongs to group M, then there is no need
for bridge B4 to forward the frames over that network. On the other hand, not having a
host on LAN H that belongs to group M does not necessarily mean that bridge B1 can
avoid forwarding multicast frames onto LAN H. It all depends on whether or not there
are members of group M on LANs I and J.
How does a given bridge learn whether it should forward a multicast frame over
a given port? It learns exactly the same way that a bridge learns whether it should forward a unicast frame over a particular port—by observing the source addresses that it
receives over that port. Of course, groups are not typically the source of frames, so we
have to cheat a little. In particular, each host that is a member of group M must periodically send a frame with the address for group M in the source field of the frame
header. This frame would have as its destination address the multicast address for the
bridges.
Note that while the multicast extension just described has been proposed, it is not
widely adopted. Instead, multicast is implemented in exactly the same way as broadcast
on today’s extended LANs.
3.2
Bridges and LAN Switches
193
3.2.4 Limitations of Bridges
The bridge-based solution just described is meant to be used in only a fairly limited
setting—to connect a handful of similar LANs. The main limitations of bridges become
apparent when we consider the issues of scale and heterogeneity.
On the issue of scale, it is not realistic to connect more than a few LANs by means
of bridges, where in practice “few” typically means “tens of.” One reason for this is that
the spanning tree algorithm scales linearly, that is, there is no provision for imposing a
hierarchy on the extended LAN. A second reason is that bridges forward all broadcast
frames. While it is reasonable for all hosts within a limited setting (say, a department) to
see each other’s broadcast messages, it is unlikely that all the hosts in a larger environment
(say, a large company or university) would want to have to be bothered by each other’s
broadcast messages. Said another way, broadcast does not scale, and as a consequence,
extended LANs do not scale.
One approach to increasing the scalability of extended LANs is the virtual LAN
(VLAN). VLANs allow a single extended LAN to be partitioned into several seemingly
separate LANs. Each virtual LAN is assigned an identifier (sometimes called a color),
and packets can only travel from one segment to another if both segments have the same
identifier. This has the effect of limiting the number of segments in an extended LAN
that will receive any given broadcast packet.
We can see how VLANs work with an example. Figure 3.15 shows four hosts on
four different LAN segments. In the absence of VLANs, any broadcast packet from
any host will reach all the other hosts. Now let’s suppose that we define the segments
connected to hosts W and X as being in one VLAN, which we’ll call VLAN 100. We
also define the segments that connect to hosts Y and Z as being in VLAN 200. To do
this, we need to configure a VLAN ID on each port of bridges B1 and B2. The link
between B1 and B2 is considered to be in both VLANs.
When a packet sent by host X arrives at bridge B2, the bridge observes that it came
in a port that was configured as being in VLAN 100. It inserts a VLAN header between
Figure 3.15
Two virtual LANs share a common backbone.
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Packet Switching
the Ethernet header and its payload. The interesting part of the VLAN header is the
VLAN ID; in this case, that ID is set to 100. The bridge now applies its normal rules
for forwarding to the packet, with the extra restriction that the packet may not be sent
out an interface that is not part of VLAN 100. Thus, under no circumstances will the
packet—even a broadcast packet—be sent out the interface to host Z, which is in VLAN
200. The packet is, however, forwarded on to bridge B1, which follows the same rules,
and thus may forward the packet to host W but not to host Y.
An attractive feature of VLANs is that it is possible to change the logical topology
without moving any wires or changing any addresses. For example, if we wanted to make
the segment that connects to host Z be part of VLAN 100, and thus enable X, W, and Z
to be on the same virtual LAN, we would just need to change one piece of configuration
on bridge B2.
On the issue of heterogeneity, bridges are fairly limited in the kinds of networks
they can interconnect. In particular, bridges make use of the network’s frame header and
so can support only networks that have exactly the same format for addresses. Thus,
bridges can be used to connect Ethernets to Ethernets, 802.5 to 802.5, and Ethernets to
802.5 rings, since both networks support the same 48-bit address format. Bridges do not
readily generalize to other kinds of networks, such as ATM.4
Despite their limitations, bridges are a very important part of the complete networking picture. Their main advantage is that they allow multiple LANs to be transparently connected, that is, the networks can be connected without the end hosts having
to run any additional protocols (or even be aware, for that matter). The one potential
exception is when the hosts are expected to announce their membership in a multicast
group, as described in Section 3.2.3.
Notice, however, that this transparency can be dangerous. If a host, or more precisely, the application and transport protocol running on that host, is programmed under
the assumption that it is running on a single LAN, then inserting bridges between the
source and destination hosts can have unexpected consequences. For example, if a bridge
becomes congested, it may have to drop frames; in contrast, it is rare that a single Ethernet ever drops a frame. As another example, the latency between any pair of hosts on an
extended LAN becomes both larger and more highly variable; in contrast, the physical
limitations of a single Ethernet make the latency both small and predictable. As a final
example, it is possible (although unlikely) that frames will be reordered in an extended
LAN; in contrast, frame order is never shuffled on a single Ethernet. The bottom line is
that it is never safe to design network software under the assumption that it will run over
a single Ethernet segment. Bridges happen.
4 As we will see in Section 3.3, there are techniques to make ATM networks look more like “conventional” LANs, such as
Ethernets, and bridges do have a role in this environment.
3.3
3.3
Cell Switching (ATM)
195
Cell Switching (ATM)
Another switching technology that deserves special attention is asynchronous transfer mode
(ATM). ATM became an important technology in the 1980s and early 1990s for a variety
of reasons, not the least of which is that it was embraced by the telephone industry, which
has historically been less than active in data communications except as a supplier of links
on top of which other people have built networks. ATM also happened to be in the right
place at the right time, as a high-speed switching technology that appeared on the scene
just when shared media like Ethernet and 802.5 were starting to look a bit too slow for
many users of computer networks. In some ways ATM is a competing technology with
Ethernet switching, but the areas of application for these two technologies only partially
overlap.
ATM is a connection-oriented, packet-switched technology, which is to say, it uses
virtual circuits very much in the manner described in Section 3.1.2. In ATM terminology, the connection setup phase is called signalling. The main ATM signalling protocol
is known as Q.2931. In addition to discovering a suitable route across an ATM network,
Q.2931 is also responsible for allocating resources at the switches along the circuit. This
is done in an effort to ensure the circuit a particular quality of service. Indeed, the QoS
capabilities of ATM are one of its greatest strengths. We return to this topic in Chapter 6,
where we discuss it in the context of similar efforts to implement QoS.
When any virtual connection is set up, it is necessary to put the address of the
destination in the signalling message. In ATM, this address can be in one of several
formats, the most common ones being E.164 and NSAP (network service access point);
the details are not terribly important here, except to note that they are different from the
MAC addresses used in traditional LANs.
One thing that makes ATM really unusual is that the packets that are switched
in an ATM network are of fixed length. That length happens to be 53 bytes—5 bytes
of header followed by 48 bytes of payload—a rather interesting choice that is discussed
in more detail below. To distinguish these fixed-length packets from the more common
variable-length packets normally used in computer networks, they are given a special
name: cells. ATM may be thought of as the canonical example of cell switching.
3.3.1 Cells
All the packet-switching technologies we have looked at so far have used variable-length
packets. Variable-length packets are normally constrained to fall within some bounds.
The lower bound is usually set by the minimum amount of information that needs to
be contained in the packet, which is typically a header with no optional extensions. The
upper bound may be set by a variety of factors; the maximum FDDI packet size, for
example, determines how long each station is allowed to transmit without passing on the
token, and thus determines how long a station might have to wait for the token to reach
196
3
Packet Switching
it. Cells, in contrast, are both fixed in length and small in size. While this seems like a
simple enough design choice, there are actually a lot of factors involved, as explained in
the following paragraphs.
Cell Size
Variable-length packets have some nice characteristics. If you only have 1 byte to send
(e.g., to acknowledge the receipt of a packet), you put it in a minimum-sized packet. If
you have a large file to send, however, you break it up into as many maximum-sized packets as you need. You do not need to send any extraneous padding in the first case, and in
the second, you drive down the ratio of header to data bytes, thus increasing bandwidth
efficiency. You also minimize the total number of packets sent, thereby minimizing the
total processing incurred by per-packet operations. This can be particularly important in
obtaining high throughput, since many network devices are limited not by how many
bits per second they can process but rather by the number of packets per second.
So, why use fixed-length cells? One of the main reasons was to facilitate the implementation of hardware switches. When ATM was being created in the mid- and late
1980s, 10-Mbps Ethernet was the cutting-edge technology in terms of speed. To go
much faster, most people thought in terms of hardware. Also, in the telephone world,
people think big when they think of switches—telephone switches often serve tens of
thousands of customers. Fixed-length packets turn out to be a very helpful thing if you
want to build fast, highly scalable switches. There are two main reasons for this:
1
It is easier to build hardware to do simple jobs, and the job of processing packets
is simpler when you already know how long each one will be;
2
If all packets are the same length, then you can have lots of switching elements
all doing much the same thing in parallel, each of them taking the same time to
do its job.
This second reason, the enabling of parallelism, greatly improves the scalability of switch
designs. It would be overstating the case to say that fast parallel hardware switches can
only be built using fixed-length cells. However, it is certainly true that cells ease the task
of building such hardware and that there was a lot of knowledge available about how to
build cell switches in hardware at the time the ATM standards were being defined.
Another nice property of cells relates to the behavior of queues. Queues build up in
a switch when traffic from several inputs may be heading for a single output. In general,
once you extract a packet from a queue and start transmitting it, you need to continue
until the whole packet is transmitted; it is not practical to preempt the transmission of
a packet. The longest time that a queue output can be tied up is equal to the time it
takes to transmit a maximum-sized packet. Fixed-length cells mean that a queue output
3.3
Cell Switching (ATM)
197
is never tied up for more than the time it takes to transmit one cell, which is almost
certainly shorter than the maximum-sized packet on a variable-length packet network.
Thus, if tight control over the latency that is being experienced by cells when they pass
through a queue is important, cells provide some advantage. Of course, long queues can
still build up, and there is no getting around the fact that some cells will have to wait
their turn. What you get from cells is not much-shorter queues but potentially finer
control over the behavior of queues.
An example will help to clarify this idea. Imagine a network with variable-length
packets, where the maximum packet length is 4 KB and the link speed is 100 Mbps. The
time to transmit a maximum-sized packet is 4,096 × 8/100 = 327.68 µs. Thus, a highpriority packet that arrives just after the switch starts to transmit a 4-KB packet will have
to sit in the queue 327.68 µs waiting for access to the link. In contrast, if the switch
were forwarding 53-byte cells, the longest wait would be 53 × 8/100 = 4.24 µs. This
may not seem like a big deal, but the ability to control delay and especially to control its
variation with time (jitter) can be important for some applications.
Queues of cells also tend to be a little shorter than queues of packets, for the following reason. When a packet begins to arrive in an empty queue, it is typical for the switch
to have to wait for the whole packet to arrive before it can start transmitting the packet
on an outgoing link. This means that the link sits idle while the packet arrives. However,
if you imagine a large packet being replaced by a “train” of small cells, then as soon as
the first cell in the train has entered the queue, the switch can transmit it. Imagine in
the example above what would happen if two 4-KB packets arrived in a queue at about
the same time. The link would sit idle for 327.68 µs while these two packets arrive, and
at the end of that period we would have 8 KB in the queue. Only then could the queue
start to empty. If those same two packets were sent as trains of cells, then transmission of
the cells could start 4.24 µs after the first train started to arrive. At the end of 327.68 µs,
the link would have been active for a little over 323 µs, and there would be just over
4 KB of data left in the queue, not 8 KB as before. Shorter queues mean less delay for all
the traffic.
Having decided to use small, fixed-length packets, the next question is, what is
the right length to fix them at? If you make them too short, then the amount of header
information that needs to be carried around relative to the amount of data that fits in one
cell gets larger, so the percentage of link bandwidth that is actually used to carry data goes
down. Even more seriously, if you build a device that processes cells at some maximum
number of cells per second, then as cells get shorter, the total data rate drops in direct
proportion to cell size. An example of such a device might be a network adaptor that
reassembles cells into larger units before handing them up to the host. The performance
of such a device depends directly on cell size. On the other hand, if you make the cells too
big, then there is a problem of wasted bandwidth caused by the need to pad transmitted
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Packet Switching
data to fill a complete cell. If the cell payload size is 48 bytes and you want to send 1 byte,
you’ll need to send 47 bytes of padding. If this happens a lot, then the utilization of the
link will be very low.
Efficient link utilization is not the
only factor that influences cell size. For exA Compromise of 48 Bytes
ample, cell size has a particular effect on
The explanation for why the payload
voice traffic, and since ATM grew out of
of an ATM cell is 48 bytes is an inthe telephony community, one of the materesting one and is an excellent case
jor concerns was that it be able to carry
study in the process of standardizavoice effectively. The standard digital ention. As the ATM standard was evolvcoding of voice is done at 64 Kbps (8-bit
ing, the U.S. telephone companies
samples taken at 8 KHz). To maximize efwere pushing for a 64-byte cell size,
ficiency, you want to collect a full cell’s
while the European companies were
worth of voice samples before transmitting
advocating 32-byte cells. The reason
a cell. A sampling rate of 8 KHz means
that the Europeans wanted the smaller
that 1 byte is sampled every 125 µs, so
size was that since the countries they
the time it takes to fill an n-byte cell with
served were of a small enough size,
samples is n × 125 µs. If cells are, say,
they would not have to install echo
1,000 bytes long, it would take 125 ms
cancelers if they were able to keep the
just to collect a full cell of samples belatency induced by generating a comfore you even start to transmit it to the replete cell small enough. Thirty-twoceiver. That amount of latency starts to be
byte cells were adequate for this purquite noticeable to a human listener. Even
pose. In contrast, the United States is
considerably smaller latencies create proba large enough country that the phone
lems for voice, particularly in the form
companies had to install echo canof echoes. Echoes can be eliminated by
celers anyway, and so the larger cell
a piece of technology called an echo cansize reflected a desire to improve the
celer, but this adds cost to a telephone netheader-to-payload ratio.
work that many network operators would
Averaging is a classic form of
rather avoid.
compromise—48 bytes is simply the
All of the above factors caused a great
average of 64 bytes and 32 bytes. So as
deal of debate in the international stannot to leave the false impression that
dards bodies when ATM was being stanthis use of compromise-by-averaging
dardized, and the fact that no length was
is an isolated incident, we note that
perfect in all cases was used by those opthe seven-layer OSI model was actuposed to ATM to argue that fixed-length
ally a compromise between six and
cells were a bad idea in the first place.
eight layers.
As is so often the case with standards, the
end result was a compromise that pleased
3.3
Figure 3.16
Cell Switching (ATM)
199
ATM cell format at the UNI.
almost no one: 48 bytes was chosen as the length for the ATM cell payload. Probably the
greatest tragedy of this choice is that it is not a power of two, which means that it is quite
a mismatch to most things that computers handle, like pages and cache lines. Rather less
controversially, the header was fixed at 5 bytes. The format of an ATM cell is shown in
Figure 3.16; note that this figure shows the field lengths in bits.
Cell Format
The ATM cell actually comes in two different formats, depending on where you look in
the network. The one shown in Figure 3.16 is called the UNI (user-network interface)
format; the alternative is the NNI (network-network interface). The UNI format is used,
of course, at the user-to-network interface. This is likely to be the interface between
a telephone company and one of its customers. The network-to-network interface is
likely to be between a pair of phone companies. The only significant difference in cell
formats is that the NNI format replaces the GFC field with 4 extra bits of VPI. Clearly,
understanding all the three-letter acronyms (TLAs) is a key part of understanding ATM.
Starting from the leftmost byte of the cell (which is the first one transmitted), the
UNI cell has 4 bits for generic flow control (GFC). These bits have not been widely
used, but they were intended to have local significance at a site and could be overwritten
in the network. The basic idea behind the GFC bits was to provide a means to arbitrate
access to the link if the local site used some shared medium to connect to ATM.
The next 24 bits contain an 8-bit virtual path identifier (VPI) and a 16-bit virtual
circuit identifier (VCI). The difference between the two is explained below, but for now
it is adequate to think of them as a single 24-bit identifier that is used to identify a virtual
connection, just as in Section 3.1.2. Following the VPI/VCI is a 3-bit Type field that
has eight possible values. Four of them, when the first bit in the field is set, relate to
management functions. When that bit is clear, it means that the cell contains user data.
In this case, the second bit is the explicit forward congestion indication (EFCI) bit and
the third is the “user signalling” bit. The former can be set by a congested switch to tell an
end node that it is congested—it has its roots in the DECbit described in Section 6.4.1—
in ATM, it is used for congestion control in conjunction with the available bit rate (ABR)
service class described in Section 6.5.3. The third bit is used primarily in conjunction
with ATM Adaptation Layer 5 to delineate frames, as discussed below.
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Packet Switching
Next is a bit to indicate cell loss priority (CLP); a user or network element may set
this bit to indicate cells that should be dropped preferentially in the event of overload.
For example, a video coding application could set this bit for cells that, if dropped, would
not dramatically degrade the quality of the video. A network element might set this bit
for cells that have been transmitted by a user in excess of the amount that was negotiated.
The last byte of the header is an 8-bit CRC, known as the header error check
(HEC). It uses the CRC-8 polynomial given in Section 2.4.3 and provides error detection and single-bit error correction capability on the cell header only. Protecting the cell
header is particularly important because an error in the VCI will cause the cell to be
misdelivered.
3.3.2 Segmentation and Reassembly
Up to this point, we have assumed that a low-level protocol could just accept the packet
handed down to it by a high-level protocol, attach its own header, and pass the packet
on down. This is not possible with ATM, however, since the packets handed down from
above are often larger than 48 bytes, and thus, will not fit in the payload of an ATM
cell. The solution to this problem is to fragment the high-level message into low-level
packets at the source, transmit the individual low-level packets over the network, and
then reassemble the fragments back together at the destination. This general technique
is usually called fragmentation and reassembly. In the case of ATM, however, it is often
called segmentation and reassembly (SAR).
Segmentation is not unique to ATM, but it is much more of a problem than in a
network with a maximum packet size of, say, 1,500 bytes. To address the issue, a protocol
layer was added that sits between ATM and the variable-length packet protocols that
might use ATM, such as IP. This layer is called the ATM Adaptation Layer (AAL), and
to a first approximation, the AAL header simply contains the information needed by
the destination to reassemble the individual cells back into the original message. The
relationship between the AAL and ATM is illustrated in Figure 3.17.
Because ATM was designed to support all sorts of services, including voice, video,
and data, it was felt that different services would have different AAL needs. Thus, four
adaptation layers were originally defined: 1 and 2 were designed to support applications,
like voice, that require guaranteed bit rates, while 3 and 4 were intended to provide
support for packet data running over ATM. The idea was that AAL3 would be used by
connection-oriented packet services (such as X.25) and AAL4 would be used by connectionless services (such as IP). Eventually, the reasons for having different AALs for these
two types of service were found to be insufficient, and the AALs merged into one that is
inconveniently known as AAL3/4. Meanwhile, some perceived shortcomings in AAL3/4
caused a fifth AAL to be proposed, called AAL5. Thus, there are now four AALs: 1, 2,
3/4, and 5. The two that support computer communications are described below.
3.3
Figure 3.17
Cell Switching (ATM)
201
Segmentation and reassembly in ATM.
ATM Adaptation Layer 3/4
The main function of AAL3/4 is to provide enough information to allow variable-length
packets to be transported across the ATM network as a series of fixed-length cells. That is,
the AAL supports the segmentation and reassembly process. Since we are now working at
a new layer of the network hierarchy, convention requires us to introduce a new name for
a packet—in this case, we call it a protocol data unit (PDU). The task of segmentation/
reassembly involves two different packet formats. The first of these is the convergence
sublayer protocol data unit (CS-PDU), as depicted in Figure 3.18. The CS-PDU defines
a way of encapsulating variable-length PDUs prior to segmenting them into cells. The
PDU passed down to the AAL layer is encapsulated by adding a header and a trailer, and
the resultant CS-PDU is segmented into ATM cells.
The CS-PDU format begins with an 8-bit common part indicator (CPI), which
indicates which version of the CS-PDU format is in use. Only the value 0 is currently
defined. The next 8 bits contain the beginning tag (Btag), which is supposed to match
the end tag (Etag) for a given PDU. This protects against the situation in which the
loss of the last cell of one PDU and the first cell of another causes two PDUs to be
inadvertently joined into a single PDU and passed up to the next layer in the protocol
stack. The buffer allocation size (BASize) field is not necessarily the length of the PDU
(which appears in the trailer); it is supposed to be a hint to the reassembly process as
to how much buffer space to allocate for the reassembly. The reason for not including
Figure 3.18
ATM Adaptation Layer 3/4 packet format.
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Packet Switching
Figure 3.19
ATM cell format for AAL3/4.
the actual length here is that the sending host might not have known how long the CSPDU was when it transmitted the header. Before adding the CS-PDU trailer, the user
data is padded to one byte less than a multiple of 4 bytes, by adding up to 3 bytes of
padding. This padding, plus the 0-filled byte, ensures that the trailer is aligned on a 32bit boundary, making for more efficient processing. The CS-PDU trailer itself contains
the Etag and the real length of the PDU (Len).
In addition to the CS-PDU header and trailer, AAL3/4 specifies a header and trailer
that are carried in each cell, as depicted in Figure 3.19. Thus, the CS-PDU is actually
segmented into 44-byte chunks; an AAL3/4 header and trailer is attached to each one,
bringing it up to 48 bytes, which is then carried as the payload of an ATM cell.
The first two bits of the AAL3/4 header contain the Type field, which indicates
if this is the first cell of a CS-PDU, the last cell of a CS-PDU, a cell in the middle of a
CS-PDU, or a single-cell PDU (in which case it is both first and last). The official names
for these four conditions are shown in Table 3.5, along with the bit encodings.
Next is a 4-bit sequence number (SEQ), which is intended simply to detect cell
loss or misordering so that reassembly can be aborted. Clearly, a sequence number this
small can miss cell losses if the number of lost cells is large enough. This is followed by
a multiplexing identifier (MID), which can be used to multiplex several PDUs onto a
single connection. The 6-bit Length field shows the number of bytes of PDU that are
contained in the cell; it must equal 44 for BOM and COM cells. Finally, a 10-bit CRC
is used to detect errors anywhere in the 48-byte cell payload.
Figure 3.20 shows the entire encapsulation and segmentation process for AAL3/4.
At the top, the user data is encapsulated with the CS-PDU header and trailer. The CSValue
Name
Meaning
10
BOM
Beginning of message
00
COM
Continuation of message
01
EOM
End of message
11
SSM
Single-segment message
Table 3.5
AAL3/4 Type field.
3.3
Figure 3.20
Cell Switching (ATM)
203
Encapsulation and segmentation for AAL3/4.
PDU is then segmented into 44-byte payloads, which are encapsulated as ATM cells by
adding the AAL3/4 header and trailer as well as the 5-byte ATM header. Note that the
last cell is only partially filled whenever the CS-PDU is not an exact multiple of 44 bytes.
One thing to note about AAL3/4 is that it exacerbates the fixed per-cell overhead
that we discussed above. With 44 bytes of data to 9 bytes of header, the best possible
bandwidth utilization would be 83%. Note that the efficiency can be considerably less
than that, as illustrated by Figure 3.20, because of the CS-PDU encapsulation and the
partial filling of the last cell.
ATM Adaptation Layer 5
One thing you may have noticed in the discussion of AAL3/4 is that it seems to take a
lot of fields and thus a lot of overhead to perform the conceptually simple function of
segmentation and reassembly. This observation was, in fact, made by several people in
the early days of ATM, and numerous competing proposals arose for an AAL to support
computer communications over ATM. There was a movement, known informally as
“Back the Bit,” that argued that if we could just have 1 bit in the ATM header (as
opposed to the AAL header) to delineate the end of a frame, then segmentation and
reassembly could be accomplished without using any of the 48-byte ATM payload for
segmentation/reassembly information. This movement eventually led to the definition
of the user signalling bit described above and to the standardization of AAL5.
What AAL5 does is replace the 2-bit Type field of AAL3/4 with 1 bit of framing
information in the ATM cell header. By setting that 1 bit, we can identify the last cell of
a PDU; the next cell is assumed to be the first cell of the next PDU, and subsequent cells
are assumed to be COM cells until another cell is received with the user signalling bit
set. All the pieces of AAL3/4 that provide protection against lost, corrupt, or misordered
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Packet Switching
Figure 3.21
ATM Adaptation Layer 5 packet format.
Figure 3.22
Encapsulation and segmentation for AAL5.
cells, including the loss of an EOM cell, are provided by the AAL5 CS-PDU packet
format depicted in Figure 3.21.
The AAL5 CS-PDU consists simply of the data portion (the PDU handed down
by the higher-layer protocol) and an 8-byte trailer. To make sure that the trailer always
falls at the tail end of an ATM cell, there may be up to 47 bytes of padding between
the data and the trailer. It is necessary to force the trailer to be at the end of a cell, as
otherwise there would be no way for the entity performing reassembly of the CS-PDU
to find the trailer. The first 2 bytes of the trailer are currently reserved and must be 0.
The length field (Len) is the number of bytes carried in the PDU, not including the
trailer or any padding before the trailer. Finally, there is a 32-bit CRC.
Figure 3.22 shows the encapsulation and segmentation process for AAL5. Just like
AAL3/4, the user data is encapsulated to form a CS-PDU (although using only a trailer
in this case). The resulting PDU is then cut up into 48-byte chunks, which are carried
directly inside the payload of ATM cells without any further encapsulation.
Somewhat surprisingly, AAL5 provides almost the same functionality as AAL3/4
without using an extra 4 bytes out of every cell. For example, the CRC-32 detects lost or
misordered cells as well as bit errors in the data. In fact, having a checksum over the entire
3.3
Cell Switching (ATM)
205
PDU rather than doing it on a per-cell basis as in AAL3/4 provides stronger protection.
For example, it protects against the loss of 16 consecutive cells, an event that would not
be picked up by the sequence number checking of AAL3/4. Also, a 32-bit CRC protects
against longer burst errors than a 10-bit CRC.
The main feature missing from AAL5 is the ability to provide an additional layer
of multiplexing onto one virtual circuit using the MID. It is not clear whether this is
a significant loss. It is still possible to multiplex traffic from many applications and
higher-layer protocols onto a single VC using AAL5 by carrying a demux key of the
sort described in Section 1.3.1. It just becomes necessary to do the multiplexing on a
packet-by-packet, rather than a cell-by-cell, basis.
There are positive and negative aspects to multiplexing traffic from a lot of different applications onto a single VC. For example, if you are being charged for every virtual
circuit you set up across a network, then multiplexing traffic from lots of different applications onto one connection might be a plus. However, this approach has the drawback
that all applications will have to live with whatever quality of service (e.g., delay and
bandwidth guarantees) has been chosen for that one connection, which may mean that
some applications are not receiving appropriate service.
In general, AAL5 has been wholeheartedly embraced by the computer communications community (at least by that part of the community that has embraced ATM at
all). For example, it is the preferred AAL in the IETF for transmitting IP datagrams over
ATM. Its more efficient use of bandwidth and simple design are the main features that
make it more appealing than AAL3/4.
3.3.3 Virtual Paths
As mentioned above, ATM uses a 24-bit identifier for virtual circuits, and these circuits
operate almost exactly like the ones described in Section 3.1.2. The one twist is that the
24-bit identifier is split into two parts: an 8-bit virtual path identifier (VPI) and a 16-bit
virtual circuit identifier (VCI). This effectively creates a two-level hierarchy of virtual
connections. To understand how such a hierarchy might work, consider the following
example. (We ignore the fact that in some places there might be a network-network
interface with a different-sized VPI; just assume that 8-bit VPIs are used everywhere.)
Suppose that a corporation has two sites that connect to a public ATM network,
and that at each site the corporation has a network of ATM switches. We could imagine
establishing a virtual path between two sites using only the VPI field. Thus, the switches
in the public network would use the VPI as the only field on which to make forwarding
decisions. From their point of view, this is a virtual circuit network with 8-bit circuit
identifiers. The 16-bit VCI is of no interest to these public switches, and they neither
use the field for switching nor remap it. Within the corporate sites, however, the full
24-bit space is used for switching. Any traffic that needs to flow between the two sites is
206
3
Figure 3.23
Packet Switching
Example of a virtual path.
routed to a switch that has a connection to the public network, and its top 8 bits (the
VPI) are mapped onto the appropriate value to get the data to the other site. This idea
is illustrated in Figure 3.23. Note that the virtual path acts like a fat pipe that contains a
bundle of virtual circuits, all of which have the same 8 bits in their most significant byte.
The advantage of this approach is clear: Although there may be thousands or millions of virtual connections across the public network, the switches in the public network
behave as if there is only one connection. This means that there needs to be much less
connection-state information stored in the switches, avoiding the need for big, expensive
tables of per-VCI information.
Where Are They Now
3.3.4 Physical Layers for
ATM
While the layered approach to protocol
design might lead you to think that we
do not need to worry about what type of
point-to-point link ATM runs on top of,
this turns out not to be the case. From a
simple pragmatic point of view, when you
buy an ATM switch it comes with some
physical medium over which ATM cells
will be sent. Of course, this is also true for
other networking protocols such as 802.5
and Ethernet. Like these protocols, ATM
can also run over several different physical media and physical-layer protocols.
From early in the process of standardizing ATM, it was assumed that
ATM would run on top of a SONET
physical layer (see Section 2.3.3). While
????
ATM in the LAN
ATM grew out of the telephony community, who envisioned it as a way to
build large public networks that could
transport voice, video, and data traffic. However, it was subsequently embraced by segments of the computer
and data communications industries
as a technology to be used in LANs—
a replacement for Ethernet and 802.5.
Its popularity in this realm at a particular point in time can be attributed to
two main factors:
■
ATM is a switched technology, whereas Ethernet and
802.5 were originally envisioned as shared-media technologies.
3.3
Where Are They Now
■
????
ATM was designed to operate on links with speeds
of 155 Mbps and above,
compared to the original
10 Mbps of Ethernet and 4
or 16 Mbps of token rings.
When ATM switches first became available, these were significant
advantages over the existing solutions.
However, it should be apparent that
the distinction between shared-media
and switched networks is no longer
clear-cut. A bridge that connects a
number of shared-media networks together is also a switch, and it is possible (and now common) to connect
only one host to each segment, giving it dedicated access to that bandwidth. At the same time as ATM
switches were appearing on the scene,
high-performance Ethernet switches
became available. These devices have
large numbers of ports and high total
throughput. The 100-Mbps Ethernet
standard was defined, and so the link
speed of Ethernet—which could be
achieved over copper—began to approach that of ATM (and ultimately
surpassed it).
Initially this was not enough to
kill off ATM in the LAN. In fact,
significant effort went into developing a technology called ATM LAN
emulation, or LANE. The key challenge faced in LANE was the fact
Cell Switching (ATM)
207
it is true that standard ways of carrying
ATM cells inside a SONET frame have
been defined, and that you can buy ATMover-SONET products, the two are entirely separable. For example, you can lease
a SONET link from a phone company and
send whatever you want over it, including
variable-length packets, and this is widely
done today.
Also, you can send ATM cells over
many other physical layers instead of
SONET, and standards have been defined for these encapsulations. A notable
early physical layer for ATM was TAXI,
the physical layer used in FDDI (see Section 2.7). ATM today is widely used over
Digital Subscriber Line (DSL) links of various types, and wireless physical layers for
ATM are also defined.
When you send ATM cells over some
physical medium, the main issue is how to
find the boundaries of the ATM cells; this
is exactly the framing problem described
in Chapter 2. With SONET, there are two
easy ways to find the boundaries. One of
the overhead bytes in the SONET frame
can be used as a pointer into the SONET
payload to the start of an ATM cell. Having found the start of one cell, it is known
that the next cell starts 53 bytes further
on in the SONET payload, and so on. In
theory, you only need to read this pointer
once, but in practice, it makes sense to read
it every time the SONET overhead goes by
so that you can detect errors or resynchronize if needed.
The other way to find the boundaries of ATM cells takes advantage of the
fact that every cell has a CRC in the fifth
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3
Packet Switching
byte of the cell. Thus, if you run a CRC
calculation over the last 5 bytes received
and the answer comes out to indicate no
errors, then it is probably true that you
have just read an ATM header. If this
happens several times in a row at 53-byte
intervals, you can be pretty sure you have
found the cell boundary.
3.4
Implementation
and Performance
So far, we have talked about what a switch
must do without discussing how to do
it. There is a very simple way to build
a switch: Buy a general-purpose workstation and equip it with a number of network interfaces. Such a device, running
suitable software, can receive packets on
one of its interfaces, perform any of the
switching functions described above, and
send packets out another of its interfaces.
This is, in fact, a popular way to build
experimental switches when you want to
be able to do things like develop new
routing protocols because it offers extreme flexibility and a familiar programming environment. It is also not too far
removed from the architecture of many
low-end routers (which, as we will see in
the next chapter, have much in common
with switches).
Figure 3.24 shows a workstation
with three network interfaces used as a
switch. The figure shows a path that a
packet might take from the time it arrives
on interface 1 until it is output on interface 2. We have assumed here that the
workstation has a mechanism to move
Where Are They Now
????
that ATM doesn’t behave like a “traditional” shared media LAN like Ethernet or token ring. On traditional
LANs it is easy to implement broadcast (sending to everybody) and multicast (sending to a group). Thus, many
of the protocols that people depend
on in their LANs—for example, the
Address Resolution Protocol (ARP)
described in Section 4.1.5—depend
in turn on the ability of the LAN
to support multicast and broadcast.
However, because of its connectionoriented and switched nature, ATM
lacks a simple broadcast mechanism.
For example, how can you broadcast
to all nodes on an ATM LAN if you
don’t know all their addresses and set
up VCs to all of them?
LANE (which might be more
correctly called “shared-media emulation”) adds functionality to ATM
LANs so that anything that runs over
a shared-media LAN can operate over
an ATM LAN. While LANE might
now be considered something of a historical curiosity, it is an interesting example of how protocol layering can
work. By making the “ATM layer”
look more like an Ethernet, higherlayer protocols that worked well over
Ethernet continued to work without
modification.
To emulate the shared media behavior of traditional LANs, LANE
introduced a number of servers into
3.4
Implementation and Performance
209
data directly from an interface to its main
memory without having to be directly
Where Are They Now
copied by the CPU, that is, direct memory access (DMA) as described in Seceach LAN, called the LAN emulation 2.1.1. Once the packet is in memory,
tion configuration server (LECS), the
the CPU examines its header to determine
LAN emulation server (LES), and
on which interface the packet should be
the broadcast and unknown server
sent out. It then uses DMA to move the
(BUS). The first two servers play roles
packet out to the appropriate interface.
in bootstrapping and configuring the
Note that Figure 3.24 does not show the
network, while the BUS has the critpacket going to the CPU because the CPU
ical job of emulating the broadcast
inspects only the header of the packet; it
functionality of a traditional LAN.
does not have to read every byte of data in
The BUS is normally the root of a
the packet.
point-to-multipoint VC, with all the
The main problem with using a
other nodes on the LAN as the leaves.
workstation as a switch is that its perforThus, to send a packet to all nodes on
mance is limited by the fact that all packan ATM LAN, you need only send it
ets must pass through a single point of
to the BUS, which then forwards it on
contention: In the example shown, each
to the multipoint VC.
packet crosses the I/O bus twice and is
It should be clear that ATM
written to and read from main memLAN emulation is fairly complex, and
ory once. The upper bound on aggregate
the BUS in particular presents a scalthroughput of such a device (the total
ability bottleneck. Perhaps as a resustainable data rate summed over all insult of these factors, plus the fact that
puts) is, thus, either half the main memory
ATM ultimately offered few real adbandwidth or half the I/O bus bandwidth,
vantages over Ethernet, LANE is no
whichever is less. (Usually, it’s the I/O bus
longer widely used.
bandwidth.) For example, a workstation
with a 133-MHz, 64-bit wide I/O bus can transmit data at a peak rate of a little over
8 Gbps. Since forwarding a packet involves crossing the bus twice, the actual limit is
4 Gbps—enough to build a switch with a fair number of 100 Mbps Ethernet ports,
for example, but hardly enough for a high-end router in the core of the Internet. (We’ll
return to the subject of router implementation in Section 4.2.6.)
Moreover, this upper bound also assumes that moving data is the only problem—a
fair approximation for long packets but a bad one when packets are short. In the latter
case, the cost of processing each packet—parsing its header and deciding which output
link to transmit it on—is likely to dominate. Suppose, for example, that a workstation
can perform all the necessary processing to switch 1 million packets each second. This
is sometimes called the packet per second (pps) rate. (This number is representative of
????
210
3
Figure 3.24
Packet Switching
A workstation used as a packet switch.
what is achievable on today’s high-end PCs.) If the average packet is short, say, 64 bytes,
this would imply
Throughput = pps × (BitsPerPacket)
= 1 × 106 × 64 × 8
= 512 × 106
that is, a throughput of 512 Mbps—substantially below the range that users are demanding from their networks today. Bear in mind that this 512 Mbps would be shared by all
users connected to the switch, just as the 10 Mbps of an Ethernet is shared among all
users connected to the shared medium. Thus, for example, a 10-port switch with this
aggregate throughput would only be able to cope with an average data rate of 51.2 Mbps
on each port.
To address this problem, hardware designers have come up with a large array
of switch designs that reduce the amount of contention and provide high aggregate
throughput. Note that some contention is unavoidable: If every input has data to send
to a single output, then they cannot all send it at once. However, if data destined for
different outputs is arriving at different inputs, a well-designed switch will be able to
move data from inputs to outputs in parallel, thus increasing the aggregate throughput.
3.4.1 Ports
Most switches look conceptually similar to the one shown in Figure 3.25. They consist
of a number of input ports and output ports, and a fabric. There is usually at least one
3.4
Figure 3.25
Implementation and Performance
211
A 4 × 4 switch.
control processor in charge of the whole switch that communicates with the ports either
directly or, as shown here, via the switch fabric. The ports communicate with the outside
world. They may contain fiber-optic receivers and lasers, buffers to hold packets that are
waiting to be switched or transmitted, and often a significant amount of other circuitry
that enables the switch to function. The fabric has a very simple and well-defined job:
When presented with a packet, deliver it to the right output port.
One of the jobs of the ports, then, is to deal with the complexity of the real world
in such a way that the fabric can do its relatively simple job. For example, suppose that
this switch is supporting a virtual circuit model of communication. In general, the virtual
circuit mapping tables described in SecDefining Throughput
tion 3.1.2 are located in the ports. The
It turns out to be difficult to deports maintain lists of virtual circuit idenfine precisely the throughput of a
tifiers that are currently in use, with inswitch. Intuitively, we might think
formation about what output a packet
that if a switch has n inputs that
should be sent out on for each VCI and
each support a link speed of si , then
how the VCI needs to be remapped to
the throughput would just be the
ensure uniqueness on the outgoing link.
sum of all the si . This is actually the
Similarly, the ports of an Ethernet switch
best possible throughput that such a
store tables that map between Etherswitch could provide, but in pracnet addresses and output ports (bridge
tice almost no real switch can guarforwarding tables as described in Secantee that level of performance. One
tion 3.2). In general, when a packet is
reason for this is simple to underhanded from an input port to the fabric,
stand. Suppose that, for some period
the port has figured out where the packet
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3
Packet Switching
needs to go, and either the port sets up
the fabric accordingly by communicating
some control information to it, or it attaches enough information to the packet
itself (e.g., an output port number) to allow the fabric to do its job automatically.
Fabrics that switch packets by looking only
at the information in the packet are referred to as “self-routing,” since they require no external control to route packets.
An example of a self-routing fabric is discussed below.
The input port is the first place to
look for performance bottlenecks. The input port has to receive a steady stream of
packets, analyze information in the header
of each one to determine which output
port (or ports) the packet must be sent to,
and pass the packet on to the fabric. The
type of header analysis that it performs
can range from a simple table lookup on
a VCI to complex matching algorithms
that examine many fields in the header.
This is the type of operation that sometimes becomes a problem when the average
packet size is very small. Consider, for example, 64-byte packets arriving on a port
connected to an OC-48 (2.48-Gbps) link.
Such a port needs to process packets at a
rate of
2.48 × 109 ÷ (64 × 8) = 4.83 × 106 pps
In other words, when small packets are
arriving as fast as possible on this link
(the worst-case scenario that most ports
are engineered to handle), the input port
has approximately 200 ns to process each
packet.
of time, all the traffic arriving at the
switch needed to be sent to the same
output. As long as the bandwidth of
that output is less than the sum of the
input bandwidths, then some of the
traffic will need to be either buffered
or dropped. With this particular traffic pattern, the switch could not provide a sustained throughput higher
than the link speed of that one output. However, a switch might be able
to handle traffic arriving at the full
link speed on all inputs if it is distributed across all the outputs evenly; this
would be considered optimal.
Another factor that affects the
performance of switches is the size of
packets arriving on the inputs. For
an ATM switch, this is normally not
an issue because all “packets” (cells)
are the same length. But for Ethernet switches or IP routers, packets
of widely varying sizes are possible.
Some of the operations that a switch
must perform have a constant overhead per packet, so a switch is likely
to perform differently depending on
whether all arriving packets are very
short, very long, or mixed. For this
reason, routers or switches that forward variable-length packets are often characterized by a packet per second
(pps) rate as well as a throughput in
bits per second. The pps rate is usually
measured with minimum-sized packets. The first thing to notice about this
discussion is that the throughput of
the switch is a function of the traffic
to which it is subjected. One of the
3.4
things that switch designers spend a
lot of their time doing is trying to
come up with traffic models that approximate the behavior of real data
traffic. It turns out that it is extremely
difficult to achieve accurate models.
There are several elements to a traffic
model. The main ones are (1) when
do packets arrive, (2) what outputs are
they destined for, and (3) how big are
they.
Traffic modeling is a well-established science that has been extremely
successful in the world of telephony,
enabling telephone companies to engineer their networks to carry expected loads quite efficiently. This is
partly because the way people use the
phone network does not change that
much over time: The frequency with
which calls are placed, the amount of
time taken for a call, and the tendency
of everyone to make calls on Mother’s
Day have stayed fairly constant for
many years.5 By contrast, the rapid
evolution of computer communications, where a new application like
Napster can change the traffic patterns
almost overnight, has made effective
modeling of computer networks much
more difficult. Nevertheless, there are
some excellent books and articles on
the subject that we list at the end of
the chapter.
Implementation and Performance
213
Another key function of ports is
buffering. Observe that buffering can happen in either the input or the output port;
it can also happen within the fabric (sometimes called internal buffering). Simple input buffering has some serious limitations.
Consider an input buffer implemented as
a FIFO. As packets arrive at the switch,
they are placed in the input buffer. The
switch then tries to forward the packets at
the front of each FIFO to their appropriate output port. However, if the packets
at the front of several different input ports
are destined for the same output port at
the same time, then only one of them can
be forwarded;6 the rest must stay in their
input buffers. The drawback of this feature
is that those packets left at the front of the
input buffer prevent other packets further
back in the buffer from getting a chance
to go to their chosen outputs, even though
there may be no contention for those outputs. This phenomenon is called head-ofline blocking. A simple example of headof-line blocking is given in Figure 3.26,
where we see a packet destined for port 1
blocked behind a packet contending for
port 2. It can be shown that when traffic
is uniformly distributed among outputs,
head-of-line blocking limits the throughput of an input-buffered switch to 59%
of the theoretical maximum (which is
the sum of the link bandwidths for the
switch). Thus, the majority of switches
use either pure output buffering or a mix-
5 The advent of dial-up connections to the Internet did however cause a significant change in the average length of calls.
6 For a simple input-buffered switch, exactly one packet at a time can be sent to a given output port. It is possible to
design switches that can forward more than one packet to the same output at once, at a cost of higher switch complexity,
but there is always some upper limit on the number.
214
3
Figure 3.26
Packet Switching
Simple illustration of head-of-line blocking.
ture of internal and output buffering.
To give you a sense of the
Those that do rely on input buffers use
range of throughputs that designers
sophisticated buffer management schemes
need to be concerned about, a highto avoid head-of-line blocking.
end router used in the Internet at
Buffers actually perform a more comthe time of writing might support
plex task than just holding onto packets
10 OC-768 links for a throughput
that are waiting to be transmitted. Buffers
of approximately 400 Gbps. A 400are the main source of delay in a switch,
Gbps switch, if called upon to hanand also the place where packets are most
dle a steady stream of 64-byte packets,
likely to get dropped due to lack of space
would need a packet per second rate
to store them. The buffers therefore are
of
the main place where the quality of service
characteristics of a switch are determined.
400×109 ÷(64×8) = 781×106 pps
For example, if a certain packet has been
sent along a VC that has a guaranteed delay, it cannot afford to sit in a buffer for very long. This means that the buffers, in
general, must be managed using packet scheduling and discard algorithms that meet a
wide range of QoS requirements. We talk more about these issues in Chapter 6.
3.4.2 Fabrics
While there has been an abundance of impressive research conducted on the design of
efficient and scalable fabrics, it is sufficient for our purposes here to understand only the
high-level properties of a switch fabric. A switch fabric should be able to move packets from input ports to output ports with minimal delay and in a way that meets the
throughput goals of the switch. That usually means that fabrics display some degree of
parallelism. A high-performance fabric with n ports can often move one packet from
each of its n ports to one of the output ports at the same time. A sample of fabric types
includes the following:
■
Shared Bus. This is the type of “fabric” found in a conventional workstation
used as a switch, as described above. Because the bus bandwidth determines
3.4
Implementation and Performance
215
the throughput of the switch, high-performance switches usually have specially
designed busses rather than the standard busses found in PCs.
■
Shared Memory. In a shared memory switch, packets are written into a memory
location by an input port and then read from memory by the output ports.
Here it is the memory bandwidth that determines switch throughput, so wide
and fast memory is typically used in this sort of design. A shared memory switch
is similar in principle to the shared bus switch, except it usually uses a specially
designed, high-speed memory bus rather than an I/O bus.
■
Crossbar. A crossbar switch is a matrix of pathways that can be configured to
connect any input port to any output port. Figure 3.27 shows a 4 × 4 crossbar
switch. The main problem with crossbars is that, in their simplest form, they
require each output port to be able to accept packets from all inputs at once,
implying that each port would have a memory bandwidth equal to the total
switch throughput. In reality, more complex designs are typically used to address
this issue (see, for example, the Knockout switch and McKeown’s virtual outputbuffered approach in the Further Reading section).
■
Self-routing. As noted above, self-routing fabrics rely on some information in
the packet header to direct each packet to its correct output. Usually a special
“self-routing header” is appended to the packet by the input port after it has determined which output the packet needs to go to, as illustrated in Figure 3.28;
this extra header is removed before the packet leaves the switch. Self-routing fab-
Figure 3.27
A 4 × 4 crossbar switch.
216
3
Packet Switching
Figure 3.28 A self-routing header is applied to a packet at input to enable the fabric to
send the packet to the correct output, where it is removed. (a) Packet arrives at input
port; (b) input port attaches self-routing header to direct packet to correct output;
(c) self-routing header is removed at output port before packet leaves switch.
rics are often built from large numbers of very simple 2 × 2 switching elements
interconnected in regular patterns, such as the banyan switching fabric shown
in Figure 3.29. For some examples of self-routing fabric designs see the Further
Reading section at the end of this chapter.
Self-routing fabrics are among the most scalable approaches to fabric design, and
there has been a wealth of research on the topic, some of which is listed in the Further
3.4
Implementation and Performance
217
Figure 3.29 Routing packets through a banyan network. The 3-bit numbers represent
values in the self-routing headers of four arriving packets.
Reading section. Many self-routing fabrics resemble the one shown in Figure 3.29, consisting of regularly interconnected 2 × 2 switching elements. For example, the 2 × 2
switches in the banyan network perform a simple task: They look at 1 bit in each selfrouting header and route packets toward the upper output if it is zero or toward the
lower output if it is one. Obviously, if two packets arrive at a banyan element at the same
time and both have the bit set to the same value, then they want to be routed to the same
output and a collision will occur. Either preventing or dealing with these collisions is a
main challenge for self-routing switch design. The banyan network is a clever arrangement of 2 × 2 switching elements that routes all packets to the correct output without
collisions if the packets are presented in ascending order.
We can see how this works in an example, as shown in Figure 3.29, where the selfrouting header contains the output port number encoded in binary. The switch elements
in the first column look at the most significant bit of the output port number and route
packets to the top if that bit is a 0 or the bottom if it is a 1. Switch elements in the second
column look at the second bit in the header, and those in the last column look at the least
significant bit. You can see from this example that the packets are routed to the correct
destination port without collisions. Notice how the top outputs from the first column of
switches all lead to the top half of the network, thus getting packets with port numbers
0–3 into the right half of the network. The next column gets packets to the right quarter
of the network, and the final column gets them to the right output port. The clever part
is the way switches are arranged to avoid collisions. Part of the arrangement includes the
“perfect shuffle” wiring pattern at the start of the network. To build a complete switch
fabric around a banyan network would require additional components to sort packets
before they are presented to the banyan. The Batcher-banyan switch design is a notable
218
3
Packet Switching
example of such an approach. The Batcher network, which is also built from a regular
interconnection of 2 × 2 switching elements, sorts packets into descending order. On
leaving the Batcher network, the packets are then ready to be directed to the correct
output, with no risk of collisions, by the banyan network.
One of the interesting things about switch design is the wide range of different
types of switches that can be built using the same basic technology. For example, both
the Ethernet switches and ATM switches discussed in this chapter, as well as Internet
routers discussed in the next chapter, are all built using designs such as those outlined in
this section.
3.5
Summary
This chapter has started to look at some of the issues involved in building large scalable
networks by using switches, rather than just links, to interconnect hosts. There are several
different ways to decide how to switch packets; the two main ones are the datagram
(connectionless) model and the virtual circuit (connection-oriented) model.
An important application of switching is the interconnection of shared-media
LANs. LAN switches, or bridges, use techniques such as source address learning to
improve forwarding efficiency, and spanning tree algorithms to avoid looping. These
switches are extensively used in data centers, campuses, and corporate networks.
The most widespread uses of virtual circuit switching are in Frame Relay and ATM
switches. ATM introduces some particular challenges through the use of cells—short,
fixed-length packets. The availability of relatively high-throughput ATM switches has
contributed to the acceptance of the technology, although it has certainly not swept all
other technologies aside as some predicted. One of the main uses of ATM today is as a
multiplexing technology in DSL access networks.
Independent of the specifics of the switching technology, switches need to forward packets from inputs to outputs at a high rate, and in some circumstances, switches
need to grow to a large size to accommodate hundreds or thousands of ports. Building
switches that both scale and offer high performance at acceptable cost is complicated by
the problem of contention, and as a consequence, switches often employ special-purpose
hardware rather than being built from general-purpose workstations.
In addition to the issues of contention discussed here, we observe that the related
problem of congestion has come up throughout this chapter. We will postpone our discussion of congestion control until Chapter 6, after we have seen more of the network
architecture. We do this because it is impossible to fully appreciate congestion (both the
problem and how to address it) without understanding both what happens inside the
network (the topic of this and the next chapter) and what happens at the edges of the
network (the topic of Chapter 5).
Further Reading
219
ATM was originally envisioned
by many of its proponents as the
O P E N
I S S U E
foundation for the “Broadband InteThe Future of Switching
grated Services Digital Network,” and
it was predicted in some quarters that
ATM would displace all other networking technologies. Hosts would
acquire ATM adaptors instead of Ethernet ports, enabling “ATM to the desktop.” Phone
companies everywhere would deploy ATM, and as the technology that supports all media
types—voice, video, and data—it would remove the need for any other type of network.
It is now apparent that this scenario is unlikely to play out. The success of Ethernet switches in particular has killed off the ATM-to-the-desktop movement. Gigabit
Ethernet and 10-gigabit Ethernet technologies have successfully addressed the need for
high-speed LAN connections where ATM might once have been used. Meanwhile, the
Internet Protocol (IP) has become the dominant network layer protocol in wide area
networks. In fact one now hears ATM referred to as a “legacy protocol,” a term once
used by proponents of ATM to refer to protocols that predated it.
The more interesting question at this stage is “How far can Ethernet go?” Once
confined to local area networks, Ethernet has now become quite popular as an access
technology in metropolitan area networks. This application of Ethernet has been enabled by the ubiquity (and relatively low cost) of Ethernet switching, and increases in
the distances over which Ethernet frames can be transmitted (e.g., by sending the frames
over fiber).
One place where ATM has had continued success is in DSL access networks, where
it is typically used to connect residential customers to the Internet. However, even this
market is beginning to be addressed by Ethernet switching.
Thus, Ethernet appears likely to be the dominant switching technology of the future. As we will see in the next chapter, the chief limitation of Ethernet-based networks
is their scalability to very large numbers of nodes. Even that limitation is something that
is now being tackled by the current generation of researchers, suggesting even broader
applicability of Ethernet in the future.
F U R T H E R
R E A D I N G
The seminal paper on bridges, in particular the spanning tree algorithm, is the article by Perlman listed below. There is a wealth of survey papers on ATM; the article
by Turner, an ATM pioneer, is one of the earliest to propose the use of a cell-based
network for integrated services. The third paper describes the Sunshine switch and
220
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Packet Switching
is especially interesting because it provides insights into the important role of traffic
analysis in switch design. In particular, the Sunshine designers were among the first
to realize that cells were unlikely to arrive at a switch in a totally uncorrelated way
and thus were able to factor these correlations into their design. Finally, McKeown’s
paper describes an approach to switch design that uses cells internally but has been
used commercially as the basis for high-performance routers forwarding variable-length
packets.
■
Perlman, R. “An Algorithm for Distributed Computation of Spanning Trees in
an Extended LAN.” Proceedings of the Ninth Data Communications Symposium,
pp. 44–53, September 1985.
■
Turner, J. S. “Design of an Integrated Services Packet Network.” Proceedings of
the Ninth Data Communications Symposium, pp. 124–133, September 1985.
■
Giacopelli, J. N., et al. “Sunshine: A High-Performance Self-Routing Broadband Packet-Switched Architecture.” IEEE Journal of Selected Areas in Communications (JSAC) 9(8):1289–1298, October 1991.
■
McKeown, N. “The iSLIP Scheduling Algorithm for Input-Queued Switches.”
IEEE Transactions on Networking 7(2):188–201, April 1999.
A good general overview of bridges can be found in another work by Perlman
[Per00]. For a detailed description of many aspects of ATM, with a focus on building
real networks, we recommend the book by Ginsburg [Gin99], even though the world
has moved on somewhat since its publication. Also, as one of the key ATM standardssetting bodies, the ATM Forum, now part of the MFA Forum, produced many of the
specifications for ATM; the User Network Interface (UNI) specification, version 4.1, is
the most recent at the time of this writing. (See the live reference below.)
There have been literally thousands of papers published on switch architectures.
One early paper that explains Batcher networks well is, not surprisingly, one by Batcher
himself [Bat68]. Sorting networks are explained by Drysdale and Young [DY75], and
an interesting form of crossbar switch is described by Yeh et al. [YHA87]. A survey of
ATM switch architectures appears in Partridge [Par94], and a good overview of the performance of different switching fabrics can be found in Robertazzi [Rob93]. An example
of the design of a switch based on variable-length packets can be found in Gopal and
Guerin [GG94].
Optical networking is a rich field in its own right, with its own journals, conferences, and so on. We recommend Ramaswami and Sivarajan [RS01] as a good introductory text in that field.
Exercises
221
An excellent text to read if you want to learn about the mathematical analysis of
network performance is by Kleinrock [Kle75], one of the pioneers of the ARPANET.
Many papers have been published on the applications of queuing theory to packet
switching. We recommend the article by Paxson and Floyd [PF94] as a significant contribution focused on the Internet, and one by Leland et al. [LTWW94], a paper that introduces the important concept of “long-range dependence” and shows the inadequacy
of many traditional approaches to traffic modeling.
Finally, we recommend the following live references:
■
http://www.metroethernetforum.com: The home page of the Metro
Ethernet Forum, which promotes the use of Ethernet as a Metropolitan Area
Network technology.
■
http://www.mfaforum.org: The organization that promotes Frame Relay,
ATM, and MPLS; the site contains tutorials and specifications on all these
switching technologies.
E X E R C I S E S
1
Using the example network given in Figure 3.30, give the virtual circuit tables for all the switches after each of the following connections is established.
Assume that the sequence of connections is cumulative, that is, the first connection is still up when the second connection is established, and so on. Also
assume that the VCI assignment always picks the lowest unused VCI on each
link, starting with 0.
(a) Host A connects to host B.
(b) Host C connects to host G.
(c) Host E connects to host I.
(d) Host D connects to host B.
(e) Host F connects to host J.
(f ) Host H connects to host A.
✓
2
Using the example network given in Figure 3.30, give the virtual circuit tables for all the switches after each of the following connections is established.
Assume that the sequence of connections is cumulative, that is, the first connection is still up when the second connection is established, and so on. Also
222
3
Figure 3.30
Packet Switching
Example network for Exercises 1 and 2.
assume that the VCI assignment always picks the lowest unused VCI on each
link, starting with 0.
(a) Host D connects to host H.
(b) Host B connects to host G.
(c) Host F connects to host A.
(d) Host H connects to host C.
(e) Host I connects to host E.
(f ) Host H connects to host J.
3
For the network given in Figure 3.31, give the datagram forwarding table for
each node. The links are labeled with relative costs; your tables should forward
each packet via the lowest-cost path to its destination.
4
Give forwarding tables for switches S1–S4 in Figure 3.32. Each switch should
have a “default” routing entry, chosen to forward packets with unrecognized
Exercises
Figure 3.31
Network for Exercise 3.
Figure 3.32
Diagram for Exercise 4.
Figure 3.33
Diagram for Exercise 5.
223
destination addresses toward OUT. Any specific-destination table entries duplicated by the default entry should then be eliminated.
5
Consider the virtual circuit switches in Figure 3.33. Table 3.6 lists, for each
switch, what port, VCI (or VCI, interface) pairs are connected to other.
Connections are bidirectional. List all endpoint-to-endpoint connections.
6
In the source routing example of Section 3.1.3, the address received by B is
not reversible and doesn’t help B know how to reach A. Propose a modification
224
3
Packet Switching
Switch S1
Switch S2
Switch S3
Port VCI
Port VCI
Port VCI
Port VCI
Port VCI
Port VCI
1
2
3
1
1
1
3
3
1
3
2
1
1
1
2
3
1
2
3
2
1
2
3
1
2
1
3
2
Table 3.6
VCI tables for switches in Figure 3.33 (Exercise 5).
to the delivery mechanism that does allow for reversibility. Your mechanism
should not require giving all switches globally unique names.
7
Propose a mechanism that virtual circuit switches might use so that if one
switch loses all its state regarding connections, then a sender of packets along a
path through that switch is informed of the failure.
8
Propose a mechanism that might be used by datagram switches so that if one
switch loses all or part of its forwarding table, affected senders are informed of
the failure.
9
The virtual circuit mechanism described in Section 3.1.2 assumes that each
link is point-to-point. Extend the forwarding algorithm to work in the case
that links are shared-media connections, for example, Ethernet.
10
Suppose, in Figure 3.4, that a new link has been added, connecting switch 3
port 1 (where G is now) and switch 1 port 0 (where D is now); neither switch
is “informed” of this link. Furthermore, switch 3 mistakenly thinks that host B
is reached via port 1.
(a) What happens if host A attempts to send to host B, using datagram forwarding?
(b) What happens if host A attempts to connect to host B, using the virtual
circuit setup mechanism discussed in the text?
11
Give an example of a working virtual circuit whose path traverses some link
twice. Packets sent along this path should not, however, circulate indefinitely.
12
In Section 3.1.2, each switch chose the VCI value for the incoming link. Show
that it is also possible for each switch to choose the VCI value for the outbound
link, and that the same VCI values will be chosen by each approach. If each
Exercises
Figure 3.34
225
Network for Exercises 13 and 14.
switch chooses the outbound VCI, is it still necessary to wait one RTT before
data is sent?
✓
13
Given the extended LAN shown in Figure 3.34, indicate which ports are not
selected by the spanning tree algorithm.
14
Given the extended LAN shown in Figure 3.34, assume that bridge B1 suffers
catastrophic failure. Indicate which ports are not selected by the spanning tree
algorithm after the recovery process and a new tree has been formed.
15
Consider the arrangement of learning bridges shown in Figure 3.35. Assuming
all are initially empty, give the forwarding tables for each of the bridges B1–B4
after the following transmissions:
■
A sends to C.
■
C sends to A.
■
D sends to C.
Identify ports with the unique neighbor reached directly from that port, that
is, the ports for B1 are to be labeled “A” and “B2.”
226
3
Packet Switching
Figure 3.35
Network for Exercises 15 and 16.
Figure 3.36
Diagram for Exercise 17.
✓
16
17
As in the previous problem, consider the arrangement of learning bridges
shown in Figure 3.35. Assuming all are initially empty, give the forwarding
tables for each of the bridges B1–B4 after the following transmissions:
■
D sends to C.
■
C sends to D.
■
A sends to C.
Consider hosts X, Y, Z, W and learning bridges B1, B2, B3, with initially
empty forwarding tables, as in Figure 3.36.
(a) Suppose X sends to Z. Which bridges learn where X is? Does Y’s network
interface see this packet?
(b) Suppose Z now sends to X. Which bridges learn where Z is? Does Y’s
network interface see this packet?
Exercises
Figure 3.37
Extended LAN for Exercise 18.
Figure 3.38
Loop for Exercises 19 and 20.
227
,
(c) Suppose Y now sends to X. Which bridges learn where Y is? Does Z’s network interface see this packet?
(d) Finally, suppose Z sends to Y. Which bridges learn where Z is? Does W’s
network interface see this packet?
18
Give the spanning tree generated for the extended LAN shown in Figure 3.37,
and discuss how any ties are resolved.
19
Suppose two learning bridges B1 and B2 form a loop as shown in Figure 3.38,
and do not implement the spanning tree algorithm. Each bridge maintains a
single table of address, interface pairs.
(a) What will happen if M sends to L?
(b) Suppose a short while later L replies to M. Give a sequence of events that
leads to one packet from M and one packet from L circling the loop in
opposite directions.
20
Suppose that M in Figure 3.38 sends to itself (this normally would never happen). State what would happen, assuming
228
3
Packet Switching
(a) The bridges’ learning algorithm is to install (or update) the new sourceaddress, interface entry before searching the table for the destination address.
(b) The new source address was installed after destination address lookup.
21
Consider the extended LAN of Figure 3.12. What happens in the spanning
tree algorithm if bridge B1 does not participate and
(a) Simply forwards all spanning tree algorithm messages?
(b) Drops all spanning tree messages?
22
Suppose some repeaters (hubs), rather than bridges, are connected into a loop.
(a) What will happen when somebody transmits?
(b) Why would the spanning tree mechanism be difficult or impossible to
implement for repeaters?
(c) Propose a mechanism by which repeaters might detect loops and shut down
some ports to break the loop. Your solution is not required to work 100%
of the time.
23
Suppose a bridge has two of its ports on the same network. How might the
bridge detect and correct this?
24
What percentage of an ATM link’s total bandwidth is consumed by the ATM
cell headers? What percentage of the total bandwidth is consumed by all nonpayload bits in AAL3/4 and AAL5, when the user data is 512 bytes long?
25
Explain why AAL3/4 will not detect the loss of 16 consecutive cells of a single
PDU.
26
The IP datagram for a TCP ACK message is 40 bytes long: it contains 20 bytes
of TCP header and 20 bytes of IP header. Assume that this ACK is traversing
an ATM network that uses AAL5 to encapsulate IP packets. How many ATM
packets will it take to carry the ACK? What if AAL3/4 is used instead?
27
The CS-PDU for AAL5 contains up to 47 bytes of padding, while the AAL3/4
CS-PDU only contains up to 3 bytes of padding. Explain why the effective
bandwidth of AAL5 is always the same as, or higher than, that of AAL3/4,
given a PDU of a particular size.
Exercises
★
229
28
How reliable does an ATM connection have to be in order to maintain a loss
rate of less than one per million for a higher-level PDU of size 20 cells? Assume
AAL5.
29
Assuming the 20-cell AAL5 packet from the previous problem, suppose a final
cell is tacked on the end of the PDU, and that this cell is the XOR of all the
previous cells in the PDU. This allows recovery from any one lost cell. What
cell loss rate now would yield a net one-per-million loss rate for 20 data-cell
PDUs?
30
Recall that AAL3/4 has a CRC-10 checksum at the end of each cell, while
AAL5 has a single CRC-32 checksum at the end of the PDU. If a PDU is
carried in 12 AAL3/4 cells, then AAL3/4 devotes nearly four times as many
bits to error detection as AAL5.
(a) Suppose errors are known to come in bursts, where each burst is small
enough to be confined to a single cell. Find the probability that AAL3/4
fails to detect an error, given that it is known that exactly two cells are
affected. Do the same for three cells. Under these conditions is AAL3/4
more or less reliable than AAL5? Assume that an N -bit CRC fails to detect
an error with probability 1/2N (which is strictly true only when all errors
are equally likely).
(b) Can you think of any error distribution in which the AAL3/4 would be
more likely than AAL5 to detect an error? Do you think such circumstances
are likely?
✓
31
Cell switching methods essentially always use virtual circuit routing rather than
datagram routing. Give a specific argument why this is so.
32
Suppose a workstation has an I/O bus speed of 800 Mbps and memory bandwidth of 2 Gbps. Assuming DMA in and out of main memory, how many
interfaces to 45-Mbps T3 links could a switch based on this workstation
handle?
33
Suppose a workstation has an I/O bus speed of 1 Gbps and memory bandwidth
of 2 Gbps. Assuming DMA in and out of main memory, how many interfaces
to 45 Mbps T3 links could a switch based on this workstation handle?
34
Suppose a switch can forward packets at a rate of 100,000 per second, regardless (within limits) of size. Assuming the workstation parameters described in
230
3
Packet Switching
the previous problem, at what packet size would the bus bandwidth become
the limiting factor?
35
Suppose that a switch is designed to have both input and output FIFO buffering. As packets arrive on an input port they are inserted at the tail of the FIFO.
The switch then tries to forward the packets at the head of each FIFO to the
tail of the appropriate output FIFO.
(a) Explain under what circumstances such a switch can lose a packet destined
for an output port whose FIFO is empty.
(b) What is this behavior called?
(c) Assume that the FIFO buffering memory can be redistributed freely. Suggest a reshuffling of the buffers that avoids the above problem, and explain
why it does so.
★
36
A stage of an n × n banyan network consists of (n/2) 2 × 2 switching elements.
The first stage directs packets to the correct half of the network, the next stage
to the correct quarter, and so on, until the packet is routed to the correct output. Derive an expression for the number of 2 × 2 switching elements needed
to make an n × n banyan network. Verify your answer for n = 8.
★
37
Describe how a Batcher network works. (See the Further Reading section.)
Explain how a Batcher network can be used in combination with a banyan
network to build a switching fabric.
38
An Ethernet switch is simply a bridge that has the ability to forward some
number of packets in parallel, assuming the input and output ports are all distinct. Supposes two such N -port switches, for a large value of N , are each able
to forward individually up to three packets in parallel. They are then connected
to one another in series by joining a pair of ports, one from each switch; the
joining link is the bottleneck as it can, of course, carry only one packet at a
time.
(a) Suppose we choose two connections through this combined switch at random. What is the probability that both connections can be forwarded in
parallel? (Hint: This is the probability that at most one of the connections
crosses the link.)
(b) What if three connections are chosen at random?
Exercises
39
231
Suppose a 10-Mbps Ethernet hub (repeater) is replaced by a 10-Mbps switch,
in an environment where all traffic is between a single server and N “clients.”
Because all traffic must still traverse the server-switch link, nominally there is
no improvement in bandwidth.
(a) Would you expect any improvement in bandwidth? If so, why? Hint: See
Exercises 43 and 44 in Chapter 2.
(b) What would your answer be if the original hub were token ring rather than
Ethernet?
(c) What other advantages and drawbacks might a switch offer versus a hub?
Internetworking
Every seeming equality conceals a hierarchy.
—Mason Cooley
W
e have now seen how to build a single network using point-to-point links,
shared media, and switches. The problem is that lots of people have built
networks with these various technologies and they all want to be able to
communicate with each other, not just with the other users of a single network. This
chapter is about the problem of interconnecting different networks.
There are two important problems
that must be addressed when
P R O B L E M
connecting networks: heterogeneity and
There Is More Than One
scale. Simply stated, the problem of
Network
heterogeneity is that users on one type
of network want to be able to communicate with users on other types of
networks. To further complicate matters, establishing connectivity between hosts on two
different networks may require traversing several other networks in between, each of
which may be of yet another type. These different networks may be Ethernets, token
rings, point-to-point links, or switched networks of various kinds, and each of them is
likely to have its own addressing scheme, media access protocols, service model, and so
on. The challenge of heterogeneity is to provide a useful and fairly predictable host-tohost service over this hodgepodge of different networks. To understand the problem of
scaling, it is worth considering the growth of the Internet, which has roughly doubled in
size each year for 20 years. This sort of growth forces us to face a number of challenges.
One of these is routing: How can you find an efficient path through a network with millions, or perhaps billions, of nodes? Closely related to this is the problem of addressing,
the task of providing suitable identifiers for all those nodes.
This chapter looks at a series of approaches to interconnecting networks, and the
problems that must be solved. In doing so, we trace the evolution of the TCP/IP Internet
232
in an effort to understand the problems of heterogeneity and scale
in detail, along with the general techniques that can be applied to
them.
The first section introduces the Internet Protocol (IP) and
shows how it can be used to build a scalable, heterogeneous internetwork. This section includes a discussion of the Internet’s service model, which is the key to its ability to handle heterogeneity.
It also describes how the Internet’s hierarchical addressing scheme
has helped the Internet to scale to a relatively large size.
A central aspect of building large heterogeneous internetworks is the problem of finding efficient, loop-free paths through
the constituent networks. The second section introduces the principles of routing and explores the scaling issues of routing protocols, using some of the Internet’s routing protocols as examples.
The third section discusses several of the problems (growing
pains) that the Internet has experienced over the past several years
and introduces a variety of techniques that have been employed to
address these problems. The experience gained from using these
techniques has led to the design of a new version of IP, which is
IP version 6 (IPv6). Throughout all these discussions, we see the
importance of hierarchy in building scalable networks.
The chapter concludes by considering a pair of significant
enhancements to the Internet’s capabilities. The first, multicast,
is an enhancement of the basic service model. We show how
multicast—the ability to deliver the same packets to a group of
receivers efficiently—can be incorporated into an internet, and
we describe several of the routing protocols that have been developed to support multicast. The second enhancement, multiprotocol label switching (MPLS), modifies the forwarding mechanism
of IP networks. This modification has enabled some changes in
the way IP routing is performed and in the services offered by IP
networks.
4
234
4.1
4
Internetworking
Simple Internetworking (IP)
In the previous chapter, we saw that it was possible to build reasonably large LANs using bridges and LAN switches, but that such approaches were limited in their ability to
scale and to handle heterogeneity. In this chapter, we explore some ways to go beyond
the limitations of bridged networks, enabling us to build large, highly heterogeneous
networks with reasonably efficient routing. We refer to such networks as internetworks.
In the following sections, we make a steady progression toward larger and larger internetworks. We start with the basic functionality of the currently deployed version of the
Internet Protocol (IP), and then we examine various techniques that have been developed
to extend the scalability of the Internet in Section 4.3. This discussion culminates with a
description of IP version 6 (IPv6), also known as the next generation IP. Before delving
into the details of an internetworking protocol, however, let’s consider more carefully
what the word “internetwork” means.
4.1.1 What Is an Internetwork?
We use the term “internetwork,” or sometimes just “internet” with a lowercase i, to refer
to an arbitrary collection of networks interconnected to provide some sort of host-tohost packet delivery service. For example, a corporation with many sites might construct
a private internetwork by interconnecting the LANs at their different sites with pointto-point links leased from the phone company. When we are talking about the widely
used, global internetwork to which a large percentage of networks are now connected, we
call it the “Internet” with a capital I . In keeping with the first-principles approach of this
book, we mainly want you to learn about the principles of “lowercase i” internetworking,
but we illustrate these ideas with real-world examples from the “big I ” Internet.
Another piece of terminology that can be confusing is the difference between networks, subnetworks, and internetworks. We are going to avoid subnetworks (or subnets)
altogether until Section 4.3. For now, we use network to mean either a directly connected or a switched network of the kind that was discussed in the last two chapters.
Such a network uses one technology, such as 802.5, Ethernet, or ATM. An internetwork
is an interconnected collection of such networks. Sometimes, to avoid ambiguity, we
refer to the underlying networks that we are interconnecting as physical networks. An internet is a logical network built out of a collection of physical networks. In this context, a
collection of Ethernets connected by bridges or switches would still be viewed as a single
network.
Figure 4.1 shows an example internetwork. An internetwork is often referred to as
a network of networks because it is made up of lots of smaller networks. In this figure,
we see Ethernets, an FDDI ring, and a point-to-point link. Each of these is a singletechnology network. The nodes that interconnect the networks are called routers. They
4.1
Figure 4.1
Simple Internetworking (IP)
235
A simple internetwork. Hn = host; Rn = router.
are also sometimes called gateways, but since this term has several other connotations, we
restrict our usage to router.
The Internet Protocol is the key tool used today to build scalable, heterogeneous
internetworks. It was originally known as the Kahn-Cerf protocol after its inventors.1
One way to think of IP is that it runs on all the nodes (both hosts and routers) in a
collection of networks and defines the infrastructure that allows these nodes and networks to function as a single logical internetwork. For example, Figure 4.2 shows how
hosts H1 and H8 are logically connected by the internet in Figure 4.1, including the
protocol graph running on each node. Note that higher-level protocols, such as TCP
and UDP, typically run on top of IP on the hosts.
Most of the rest of this chapter is about various aspects of IP. While it is certainly
possible to build an internetwork that does not use IP—for example, Novell created
an internetworking protocol called IPX, which was in turn based on the XNS internet
designed by Xerox—IP is the most interesting case to study simply because of the size of
the Internet. Said another way, it is only the IP Internet that has really faced the issue of
scale, thus, it provides the best case study of a scalable internetworking protocol.
1 Robert Kahn and Vint Cerf received the A.M. Turing award, often referred to as the Nobel Prize of computer science,
in 2005 for their efforts.
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Figure 4.2 A simple internetwork, showing the protocol layers used to connect H1 to
H8 in Figure 4.1. ETH is the protocol that runs over Ethernet.
4.1.2 Service Model
A good place to start when you build an internetwork is to define its service model, that
is, the host-to-host services you want to provide. The main concern in defining a service
model for an internetwork is that we can provide a host-to-host service only if this service
can somehow be provided over each of the underlying physical networks. For example,
it would be no good deciding that our internetwork service model was going to provide
guaranteed delivery of every packet in 1 ms or less if there were underlying network
technologies that could arbitrarily delay packets. The philosophy used in defining the
IP service model, therefore, was to make it undemanding enough that just about any
network technology that might turn up in an internetwork would be able to provide the
necessary service.
The IP service model can be thought of as having two parts: an addressing scheme,
which provides a way to identify all hosts in the internetwork, and a datagram (connectionless) model of data delivery. This service model is sometimes called best effort
because, although IP makes every effort to deliver datagrams, it makes no guarantees.
We postpone a discussion of the addressing scheme for now and look first at the data
delivery model.
Datagram Delivery
The IP datagram is fundamental to the Internet Protocol. Recall from Section 3.1.1 that
a datagram is a type of packet that happens to be sent in a connectionless manner over
a network. Every datagram carries enough information to let the network forward the
packet to its correct destination; there is no need for any advance setup mechanism to
tell the network what to do when the packet arrives. You just send it, and the network
makes its best effort to get it to the desired destination. The “best-effort” part means that
if something goes wrong and the packet gets lost, corrupted, misdelivered, or in any way
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fails to reach its intended destination, the network does nothing—it made its best effort,
and that is all it has to do. It does not make any attempt to recover from the failure. This
is sometimes called an unreliable service.
Best-effort, connectionless service is about the simplest service you could ask for
from an internetwork, and this is a great strength. For example, if you provide best-effort
service over a network that provides a reliable service, then that’s fine—you end up with
a best-effort service that just happens to always deliver the packets. If, on the other hand,
you had a reliable service model over an unreliable network, you would have to put lots
of extra functionality into the routers to make up for the deficiencies of the underlying
network. Keeping the routers as simple as possible was one of the original design goals
of IP.
The ability of IP to “run over anything” is frequently cited as one of its most
important characteristics. It is noteworthy that many of the technologies over which IP
runs today did not exist when IP was invented. So far, no networking technology has
been invented that has proven too bizarre for IP; it has even been claimed that IP can
run over a network that transports messages using carrier pigeons.
Best-effort delivery does not just mean that packets can get lost. Sometimes they
can get delivered out of order, and sometimes the same packet can get delivered more
than once. The higher-level protocols or applications that run above IP need to be aware
of all these possible failure modes.
Packet Format
Clearly, a key part of the IP service model is the type of packets that can be carried. The
IP datagram, like most packets, consists of a header followed by a number of bytes of
data. The format of the header is shown in Figure 4.3. Note that we have adopted a
different style of representing packets than the one we used in previous chapters. This is
because packet formats at the internetworking layer and above, where we will be focusing
our attention for the next few chapters, are almost invariably designed to align on 32-bit
boundaries to simplify the task of processing them in software. Thus, the common way
of representing them (used in Internet Requests for Comments, for example) is to draw
them as a succession of 32-bit words. The top word is the one transmitted first, and the
leftmost byte of each word is the one transmitted first. In this representation, you can
easily recognize fields that are a multiple of 8 bits long. On the odd occasion when fields
are not an even multiple of 8 bits, you can determine the field lengths by looking at the
bit positions marked at the top of the packet.
Looking at each field in the IP header, we see that the “simple” model of best-effort
datagram delivery still has some subtle features. The Version field specifies the version
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Figure 4.3
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IPv4 packet header.
of IP. The current version of IP is 4, and it is sometimes called IPv4.2 Observe that
putting this field right at the start of the datagram makes it easy for everything else in
the packet format to be redefined in subsequent versions; the header processing software
starts off by looking at the version and then branches off to process the rest of the packet
according to the appropriate format. The next field, HLen, specifies the length of the
header in 32-bit words. When there are no options, which is most of the time, the header
is 5 words (20 bytes) long. The 8-bit type of service (TOS) field has had a number of
different definitions over the years, but its basic function is to allow packets to be treated
differently based on application needs. For example, the TOS value might determine
whether or not a packet should be placed in a special queue that receives low delay. We
discuss the use of this field (and a new name for it) in more detail in Section 6.5.3.
The next 16 bits of the header contain the Length of the datagram, including the
header. Unlike the HLen field, the Length field counts bytes rather than words. Thus,
the maximum size of an IP datagram is 65,535 bytes. The physical network over which
IP is running, however, may not support such long packets. For this reason, IP supports a
fragmentation and reassembly process. The second word of the header contains information about fragmentation, and the details of its use are presented under “Fragmentation
and Reassembly” below.
Moving on to the third word of the header, the next byte is the time to live (TTL)
field. Its name reflects its historical meaning rather than the way it is commonly used
2 The next major version of IP, which is discussed later in this chapter, has a new version number 6 and is known as IPv6.
The version number 5 was used for an experimental protocol called ST-II that was not widely used.
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today. The intent of the field is to catch packets that have been going around in routing
loops and discard them, rather than let them consume resources indefinitely. Originally,
TTL was set to a specific number of seconds that the packet would be allowed to live,
and routers along the path would decrement this field until it reached 0. However, since
it was rare for a packet to sit for as long as 1 second in a router, and routers did not
all have access to a common clock, most routers just decremented the TTL by 1 as they
forwarded the packet. Thus, it became more of a hop count than a timer, which is still
a perfectly good way to catch packets that are stuck in routing loops. One subtlety is
in the initial setting of this field by the sending host: Set it too high and packets could
circulate rather a lot before getting dropped; set it too low and they may not reach their
destination. The value 64 is the current default.
The Protocol field is simply a demultiplexing key that identifies the higher-level
protocol to which this IP packet should be passed. There are values defined for TCP (6),
UDP (17), and many other protocols that may sit above IP in the protocol graph.
The Checksum is calculated by considering the entire IP header as a sequence of
16-bit words, adding them up using ones complement arithmetic, and taking the ones
complement of the result. This is the IP checksum algorithm described in Section 2.4.
Thus, if any bit in the header is corrupted in transit, the checksum will not contain
the correct value upon receipt of the packet. Since a corrupted header may contain an
error in the destination address—and, as a result, may have been misdelivered—it makes
sense to discard any packet that fails the checksum. It should be noted that this type of
checksum does not have the same strong error detection properties as a CRC, but it is
much easier to calculate in software.
The last two required fields in the header are the SourceAddr and the DestinationAddr for the packet. The latter is the key to datagram delivery: Every packet
contains a full address for its intended destination so that forwarding decisions can be
made at each router. The source address is required to allow recipients to decide if they
want to accept the packet and to enable them to reply. IP addresses are discussed in
Section 4.1.3—for now, the important thing to know is that IP defines its own global
address space, independent of whatever physical networks it runs over. As we will see,
this is one of the keys to supporting heterogeneity.
Finally, there may be a number of options at the end of the header. The presence or absence of options may be determined by examining the header length (HLen)
field. While options are used fairly rarely, a complete IP implementation must handle
them all.
Fragmentation and Reassembly
One of the problems of providing a uniform host-to-host service model over a heterogeneous collection of networks is that each network technology tends to have its own
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idea of how large a packet can be. For example, an Ethernet can accept packets up to
1,500 bytes long, while FDDI packets may be 4,500 bytes long. This leaves two choices
for the IP service model: make sure that all IP datagrams are small enough to fit inside
one packet on any network technology, or provide a means by which packets can be fragmented and reassembled when they are too big to go over a given network technology.
The latter turns out to be a good choice, especially when you consider the fact that new
network technologies are always turning up, and IP needs to run over all of them; this
would make it hard to pick a suitably small bound on datagram size. This also means
that a host will not send needlessly small packets, which wastes bandwidth and consumes
processing resources by requiring more headers per byte of data sent. For example, two
hosts connected to FDDI networks that are interconnected by a point-to-point link
would not need to send packets small enough to fit on an Ethernet.
The central idea here is that every network type has a maximum transmission unit
(MTU), which is the largest IP datagram that it can carry in a frame. Note that this value
is smaller than the largest packet size on that network because the IP datagram needs to
fit in the payload of the link-layer frame.3
When a host sends an IP datagram, therefore, it can choose any size that it wants.
A reasonable choice is the MTU of the network to which the host is directly attached.
Then, fragmentation will only be necessary if the path to the destination includes a
network with a smaller MTU. Should the transport protocol that sits on top of IP give
IP a packet larger than the local MTU, however, then the source host must fragment it.
Fragmentation typically occurs in a router when it receives a datagram that it wants
to forward over a network that has an MTU that is smaller than the received datagram.
To enable these fragments to be reassembled at the receiving host, they all carry the same
identifier in the Ident field. This identifier is chosen by the sending host and is intended
to be unique among all the datagrams that might arrive at the destination from this
source over some reasonable time period. Since all fragments of the original datagram
contain this identifier, the reassembling host will be able to recognize those fragments
that go together. Should all the fragments not arrive at the receiving host, the host gives
up on the reassembly process and discards the fragments that did arrive. IP does not
attempt to recover from missing fragments.
To see what this all means, consider what happens when host H1 sends a datagram
to host H8 in the example internet shown in Figure 4.1. Assuming that the MTU is
1,500 bytes for the two Ethernets, 4,500 bytes for the FDDI network, and 532 bytes
for the point-to-point network, then a 1,420-byte datagram (20-byte IP header plus
1,400 bytes of data) sent from H1 makes it across the first Ethernet and the FDDI
network without fragmentation but must be fragmented into three datagrams at router
3 Note that in ATM networks, the “frame” is the CS-PDU, not the ATM cell; the fact that CS-PDUs get segmented into
cells is (fortunately) not visible to IP.
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241
Figure 4.4 IP datagrams traversing the sequence of physical networks graphed in
Figure 4.1.
R2. These three fragments are then forwarded by router R3 across the second Ethernet
to the destination host. This situation is illustrated in Figure 4.4. This figure also serves
to reinforce two important points:
1
Each fragment is itself a self-contained IP datagram that is transmitted over a
sequence of physical networks, independent of the other fragments;
2
Each IP datagram is reencapsulated for each physical network over which it
travels.
The fragmentation process can be understood in detail by looking at the header
fields of each datagram, as is done in Figure 4.5. The unfragmented packet, shown at the
top, has 1,400 bytes of data and a 20-byte IP header. When the packet arrives at router
R2, which has an MTU of 532 bytes, it has to be fragmented. A 532-byte MTU leaves
512 bytes for data after the 20-byte IP header, so the first fragment contains 512 bytes of
data. The router sets the M bit in the Flags field (see Figure 4.3), meaning that there are
more fragments to follow, and it sets the Offset to 0, since this fragment contains the
first part of the original datagram. The data carried in the second fragment starts with
the 513th byte of the original data, so the Offset field in this header is set to 64, which is
512 ÷ 8. Why the division by 8? Because the designers of IP decided that fragmentation
should always happen on 8-byte boundaries, which means that the Offset field counts
8-byte chunks, not bytes. (We leave it as an exercise for you to figure out why this design
decision was made.) The third fragment contains the last 376 bytes of data, and the offset
is now 2 × 512 ÷ 8 = 128. Since this is the last fragment, the M bit is not set.
Observe that the fragmentation process is done in such a way that it could
be repeated if a fragment arrived at another network with an even smaller MTU.
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Figure 4.5 Header fields used in IP fragmentation: (a) unfragmented packet;
(b) fragmented packets.
Fragmentation produces smaller, valid IP datagrams that can be readily reassembled into
the original datagram upon receipt, independent of the order of their arrival. Reassembly
is done at the receiving host and not at each router.
Implementation
We conclude this discussion of IP fragmentation and reassembly by giving a fragment of
code that performs reassembly. One reason we give this particular piece of code is that it
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243
is representative of a large proportion of networking software—it does little more than
tedious and unglamorous bookkeeping.
First, we define the key data structure (FragList) that is used to hold the individual fragments that arrive at the destination. Incoming fragments are saved in this data
structure until all the fragments in the original datagram have arrived, at which time they
are reassembled into a complete datagram and passed up to some higher-level protocol.
Note that each element in FragList contains either a fragment or a hole.
#define FRAGOFFMASK
#define FRAGOFFSET(fragflag)
#define INFINITE_OFFSET
0x1fff
((fragflag) & FRAGOFFMASK)
0xffff
/* structure to hold the fields that uniquely identify
fragments of the same IP datagram */
typedef struct fid {
IpHost source;
IpHost dest;
u_char prot;
u_char pad;
u_short ident;
} FragId;
typedef struct hole {
u_int
first;
u_int
last;
} Hole;
#define HOLE
#define FRAG
1
2
/* structure to hold a fragment or a hole */
typedef struct fragif {
u_char type;
union {
Hole
hole;
Msg
frag;
} u;
struct fragif *next, *prev;
} FragInfo;
/* structure to hold all the fragments and holes for a
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single IP datagram being reassembled */
typedef struct FragList {
u_short
nholes;
FragInfo
head;
/* dummy header node */
Binding
binding;
bool
gcMark; /* garbage collection flag */
} FragList;
The reassembly routine, ipReassemble, takes an incoming datagram (dg) and
the IP header for that datagram (hdr) as arguments. The third argument, fragMap, is
a Map structure (which supports mapBind, mapRemove, and MapResolve operations) used to efficiently map the incoming datagram into the appropriate FragList.
(Recall that the group of fragments that are being reassembled together are uniquely
identified by several fields in the IP header, as defined by structure FragId given above.)
The actual work done in ipReassemble is straightforward; as stated above, it
is mostly bookkeeping. First, the routine extracts the fields from the IP header that
uniquely identify the datagram to be reassembled, constructs a key from these fields,
and looks this key up in fragMap to find the appropriate FragList. If this is the first
fragment for the datagram, a new FragList must be created and initialized. Next, the
routine inserts the new fragment into this FragList. This involves comparing the sum
of the offset and length of this fragment with the offset of the next fragment in the list.
Some of this work is done in subroutine hole_create, which is given below. Finally,
ipReassemble checks to see if all the holes are filled. If all the fragments are present, it
calls the routine msgReassemble to actually reassemble the fragments into a whole
datagram and then calls deliver to pass this datagram up the protocol graph to some
high-level protocol identified as HLP.
ipReassemble(Msg *dg, IpHdr *hdr, Map fragMap)
{
FragId
fragid;
FragList
*list;
FragInfo
*fi, *prev;
Hole
*hole;
u_short
offset, len;
/* extract fragmentation info from header
(offset and fragment length) */
offset = FRAGOFFSET(hdr->frag)*8;
len = hdr->dlen - GET_HLEN(hdr) * 4;
/*
Create the unique id for this fragment */
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245
bzero((char *)&fragid, sizeof(FragId));
fragid.source = hdr->source;
fragid.dest = hdr->dest;
fragid.prot = hdr->prot;
fragid.ident = hdr->ident;
/* find reassembly list for this frag;
create one if none exists */
if (mapResolve( fragMap, &fragid, (void **)&list)
== FALSE )
{
/* first fragment of datagram need new FragList */
list = NEW(FragList);
/* insert it into the Map structure */
list->binding = mapBind( fragMap, &fragid, list );
/* initialize list with a single hole spanning
the whole datagram */
list->nholes = 1;
list->head.next = fi = NEW(FragInfo);
fi->next = 0;
fi->type = HOLE;
fi->u.hole.first = 0;
fi->u.hole.last = INFINITE_OFFSET;
}
/* mark the current FragList as ineligible
for garbage collection */
list->gcMark = FALSE;
/* walk through the FragList to find the right hole
for this frag */
prev = &list->head;
for ( fi = prev->next; fi != 0; prev = fi,
fi = fi->next )
{
if ( fi->type == FRAG )
{
continue;
}
hole = &fi->u.hole;
if ( (offset < hole->last) && ((offset + len)
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> hole->first) )
{
/* check to see if frag overlaps previously
received frags */
if ( offset < hole->first )
{
/* truncate message from left */
msgStripHdr(dg, hole->first - offset);
offset = hole->first;
}
if ( (offset + len) > hole->last )
{
/* truncate message from right */
msgTruncate(dg, hole->last - offset);
len = hole->last - offset;
}
/* now check to see if new hole(s)
need to be made */
if (((offset + len) < hole->last) &&
(hdr->frag & MOREFRAGMENTS))
{
/* creating new hole above */
hole_create(prev, fi, (offset+len),
hole->last);
list->nholes++;
}
if ( offset > hole->first )
{
/* creating new hole below */
hole_create(fi, fi->next, hole->first,
(offset));
list->nholes++;
}
/* change this FragInfo structure
to be FRAG */
list->nholes--;
fi->type = FRAG;
msgSaveCopy(&fi->u.frag, dg);
break;
} /* if found a hole */
} /* for loop */
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/* check to see if we’re done, and if so,
pass datagram up */
if ( list->nholes == 0 )
{
Msg fullMsg;
/* now have a full datagram */
for( fi = list->head.next; fi != 0;
fi = fi->next )
{
msgReassemble(&fullMsg, &fi->u.frag,
&fullMsg);
}
/* get rid of FragList and its Map entry */
mapRemove(fragMap, list->binding);
ipFreeFragList(list);
deliver(HLP, &fullMsg);
msgDestroy(&fullMsg);
}
return SUCCESS;
}
Subroutine hole_create creates a new hole in the fragment list that begins at
offset first and continues to offset last. It makes use of the utility NEW, which creates
an instance of the given structure.
static int
hole_create(FragInfo *prev, FragInfo *next,
u_int first, u_int last)
{
FragInfo
*fi;
/* creating new hole from first to last */
fi = NEW(FragInfo);
fi->type = HOLE;
fi->u.hole.first = first;
fi->u.hole.last = last;
fi->next = next;
prev->next = fi;
}
Finally, note that these routines do not capture the entire picture of reassembly.
What is not shown is a background process that periodically checks to see if there has
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been any recent activity on this datagram (it looks at field gcMark), and if not, it deletes
the corresponding FragList. IP does not attempt to recover from the situation in which
one or more of the fragments does not arrive; it simply gives up and reclaims the memory
that was being used for reassembly.
One thing to notice from this code is that IP reassembly is far from a simple process.
Note, for example, that if a single fragment is lost, the receiver will still attempt to
reassemble the datagram, and it will eventually give up and have to garbage-collect the
resources that were used to perform the failed reassembly.4 For this reason, among others,
IP fragmentation is generally considered to avoid. Hosts are now strongly encouraged to
perform “path MTU discovery,” a process by which fragmentation is avoided by sending
packets that are small enough to traverse the link with the smallest MTU in the path
from sender to receiver.
4.1.3 Global Addresses
In the above discussion of the IP service model, we mentioned that one of the things that
it provides is an addressing scheme. After all, if you want to be able to send data to any
host on any network, there needs to be a way of identifying all the hosts. Thus, we need
a global addressing scheme—one in which no two hosts have the same address. Global
uniqueness is the first property that should be provided in an addressing scheme.
Ethernet addresses are globally unique, but that alone does not suffice for an addressing scheme in a large internetwork. Ethernet addresses are also flat, which means
that they have no structure and provide very few clues to routing protocols.5 In contrast,
IP addresses are hierarchical, by which we mean that they are made up of several parts
that correspond to some sort of hierarchy in the internetwork. Specifically, IP addresses
consist of two parts: a network part and a host part. This is a fairly logical structure for
an internetwork, which is made up of many interconnected networks. The network part
of an IP address identifies the network to which the host is attached; all hosts attached
to the same network have the same network part in their IP address. The host part then
identifies each host uniquely on that particular network. Thus, in the simple internetwork of Figure 4.1, the addresses of the hosts on network 1, for example, would all have
the same network part and different host parts.
Note that the routers in Figure 4.1 are attached to two networks. They need to
have an address on each network, one for each interface. For example, router R1, which
sits between network 2 and network 3, has an IP address on the interface to network 2
that has the same network part as the hosts on network 2, and it has an IP address on the
4 As we will see in Chapter 8, getting a host to tie up resources needlessly can be the basis of a denial-of-service attack.
5 In fact, as we noted, Ethernet addresses do have a structure for the purposes of assignment—the first 24 bits identify the
manufacturer—but this provides no useful information to routing protocols since this structure has nothing to do with
network topology.
4.1
Figure 4.6
Simple Internetworking (IP)
249
IP addresses: (a) class A; (b) class B; (c) class C.
interface to network 3 that has the same network part as the hosts on network 3. Thus,
bearing in mind that a router might be implemented as a host with two network interfaces, it is more precise to think of IP addresses as belonging to interfaces than to hosts.
Now, what do these hierarchical addresses look like? Unlike some other forms of
hierarchical address, the sizes of the two parts are not the same for all addresses. Instead,
IP addresses are divided into three different classes, as shown in Figure 4.6, each of
which defines different-sized network and host parts. (There are also class D addresses
that specify a multicast group, discussed in Section 4.4, and class E addresses that are
currently unused.) In all cases, the address is 32 bits long.
The class of an IP address is identified in the most significant few bits. If the first
bit is 0, it is a class A address. If the first bit is 1 and the second is 0, it is a class B
address. If the first two bits are 1 and the third is 0, it is a class C address. Thus, of
the approximately 4 billion possible IP addresses, one-half are class A, one-quarter are
class B, and one-eighth are class C. Each class allocates a certain number of bits for the
network part of the address and the rest for the host part. Class A networks have 7 bits
for the network part and 24 bits for the host part, meaning that there can be only 126
class A networks (the values 0 and 127 are reserved), but each of them can accommodate
up to 224 − 2 (about 16 million) hosts (again, there are two reserved values). Class B
addresses allocate 14 bits for the network and 16 bits for the host, meaning that each
class B network has room for 65,534 hosts. Finally, class C addresses have only 8 bits for
the host and 21 for the network part. Therefore, a class C network can have only 256
unique host identifiers, which means only 254 attached hosts (one host identifier, 255,
is reserved for broadcast, and 0 is not a valid host number). However, the addressing
scheme supports 221 class C networks.
On the face of it, this addressing scheme has a lot of flexibility, allowing networks of
vastly different sizes to be accommodated fairly efficiently. The original idea was that the
Internet would consist of a small number of wide area networks (these would be class A
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networks), a modest number of site- (campus-) sized networks (these would be class B
networks), and a large number of LANs (these would be class C networks). However, as
we shall see in Section 4.3, additional flexibility has been needed, and some innovative
ways to provide it are now in use. Because one of these techniques actually removes the
distinction between address classes, the addressing scheme just described is now known
as “classful” addressing to distinguish it from the newer “classless” approach.
Before we look at how IP addresses get used, it is helpful to look at some practical
matters, such as how you write them down. By convention, IP addresses are written
as four decimal integers separated by dots. Each integer represents the decimal value
contained in 1 byte of the address, starting at the most significant. For example, the
address of the computer on which this sentence was typed is 171.69.210.245.
It is important not to confuse IP addresses with Internet domain names, which
are also hierarchical. Domain names tend to be ASCII strings separated by dots, such
as cs.princeton.edu. We will be talking about those in Section 9.1.3. The important
thing about IP addresses is that they are what is carried in the headers of IP packets, and
it is those addresses that are used in IP routers to make forwarding decisions.
4.1.4 Datagram Forwarding in IP
We are now ready to look at the basic mechanism by which IP routers forward datagrams in an internetwork. Recall from Chapter 3 that forwarding is the process of taking
a packet from an input and sending it out on the appropriate output, while routing is
the process of building up the tables that allow the correct output for a packet to be determined. The discussion here focuses on forwarding; we take up routing in Section 4.2.
The main points to bear in mind as we discuss the forwarding of IP datagrams are
the following:
■
Every IP datagram contains the IP address of the destination host;
■
The “network part” of an IP address uniquely identifies a single physical network
that is part of the larger Internet;
■
All hosts and routers that share the same network part of their address are connected to the same physical network and can thus communicate with each other
by sending frames over that network;
■
Every physical network that is part of the Internet has at least one router that, by
definition, is also connected to at least one other physical network; this router
can exchange packets with hosts or routers on either network.
Forwarding IP datagrams can therefore be handled in the following way. A datagram is sent from a source host to a destination host, possibly passing through several
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routers along the way. Any node, whether it is a host or a router, first tries to establish
whether it is connected to the same physical network as the destination. To do this, it
compares the network part of the destination address with the network part of the address of each of its network interfaces. (Hosts normally have only one interface, while
routers normally have two or more, since they are typically connected to two or more
networks.) If a match occurs, then that means that the destination lies on the same physical network as the interface, and the packet can be directly delivered over that network.
Section 4.1.5 explains some of the details of this process.
If the node is not connected to the same physical network as the destination node,
then it needs to send the datagram to a router. In general, each node will have a choice of
several routers, and so it needs to pick the best one, or at least one that has a reasonable
chance of getting the datagram closer to its destination. The router that it chooses is
known as the next hop router. The router finds the correct next hop by consulting its
forwarding table. The forwarding table is conceptually just a list of NetworkNum,
NextHop pairs. (As we will see below, forwarding tables in practice often contain some
additional information related to the next hop.) Normally, there is also a default router
that is used if none of the entries in the table match the destination’s network number.
For a host, it may be quite acceptable to have a default router and nothing else—this
means that all datagrams destined for hosts not on the physical network to which the
sending host is attached will be sent out through the default router.
We can describe the datagram forwarding algorithm in the following way:
if (NetworkNum of destination = NetworkNum of one of my interfaces) then
deliver packet to destination over that interface
else
if (NetworkNum of destination is in my forwarding table) then
deliver packet to NextHop route
else
deliver packet to default router
For a host with only one interface and only a default router in its forwarding table, this
simplifies to
if (NetworkNum of destination = my NetworkNum) then
deliver packet to destination directly
else
deliver packet to default router
Let’s see how this works in the example internetwork of Figure 4.1. First, suppose that H1 wants to send a datagram to H2. Since they are on the same physical
network, H1 and H2 have the same network number in their IP address. Thus, H1
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deduces that it can deliver the datagram
directly to H2 over the Ethernet. The one
issue that needs to be resolved is how H1
finds out the correct Ethernet address for
H2—this is the address resolution mechanism described in Section 4.1.5.
Now suppose H1 wants to send a
datagram to H8. Since these hosts are on
different physical networks, they have different network numbers, so H1 deduces
that it needs to send the datagram to a
router. R1 is the only choice—the default
router—so H1 sends the datagram over
the Ethernet to R1. Similarly, R1 knows
that it cannot deliver a datagram directly
to H8 because neither of R1’s interfaces
is on the same network as H8. Suppose
R1’s default router is R2; R1 then sends
the datagram to R2 over the token ring
network. Assuming R2 has the forwarding table shown in Table 4.1, it looks
up H8’s network number (network 1)
and forwards the datagram to R3. Finally, R3, since it is on the same network
as H8, forwards the datagram directly
to H8.
Note that it is possible to include the
information about directly connected networks in the forwarding table. For example, we could label the network interfaces
of router R2 as interface 0 for the pointto-point link (network 4) and interface 1
NetworkNum
NextHop
1
R3
2
R1
Table 4.1
Bridges, Switches, and Routers
It is easy to become confused about
the distinction between bridges,
switches, and routers. There is good
reason for such confusion, since at
some level, they all forward messages
from one link to another. One distinction people make is based on layering:
Bridges are link-level nodes (they forward frames from one link to another
to implement an extended LAN);
switches are network-level nodes (they
forward packets from one link to another to implement a packet-switched
network); and routers are internetlevel nodes (they forward datagrams
from one network to another to implement an internet).
The distinction between bridges
and switches is fast disappearing. For
example, we have already seen that a
multiport bridge is usually called an
Ethernet switch or LAN switch. For
this reason, bridges and switches are
often grouped together as layer 2 devices, where layer 2 in this context
means “above the physical layer, below
the internet layer.”
There remain, however, some
important
distinctions
between
LAN switches (or bridges) and ATM
Example forwarding table for router R2 in Figure 4.1.
4.1
NetworkNum
NextHop
1
R3
2
R1
3
Interface 1
4
Interface 0
Table 4.2
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253
Complete forwarding table for router R2 in Figure 4.1.
switches to learn the topology of the
whole network. This is an important distinction because knowing the
whole network topology allows the
switches to discriminate among different routes, while in contrast, the
spanning tree algorithm locks in a single tree over which messages are forwarded. It is also the case that the
spanning tree approach does not scale
as well.
What about switches and
routers? Internally, they look quite
similar (as the section on router implementation will illustrate). The key
distinction is the sort of packet they
forward: IP datagrams in the case of
routers, and layer 2 packets (Ethernet
frames or ATM cells) in the case of
switches.
One big difference between a
network built from switches and the
Internet built from routers is that
the Internet is able to accommodate heterogeneity, whereas switched
networks typically consists of homogeneous links. This support for
for the token ring (network 3). Then R2
would have the forwarding table shown in
Table 4.2.
Thus, for any network number that
R2 encounters in a packet, it knows what
to do. Either that network is directly connected to R2, in which case the packet
can be delivered to its destination over that
network, or the network is reachable via
some next hop router that R2 can reach
over a network to which it is connected. In
either case, R2 will use ARP, described below, to find the MAC address of the node
to which the packet is to be sent next.
The forwarding table used by R2 is
simple enough that it could be manually
configured. Usually, however, these tables
are more complex and would be built up
by running a routing protocol such as one
of those described in Section 4.2. Also note
that, in practice, the network numbers are
usually longer (e.g., 128.96).
We can now see how hierarchical
addressing—splitting the address into network and host parts—has improved the
scalability of a large network. Routers now
contain forwarding tables that list only a
set of network numbers, rather than all
the nodes in the network. In our simple
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▲
example, that meant that R2 could store
heterogeneity is one of the key reathe information needed to reach all the
sons why the Internet is so widely dehosts in the network (of which there were
ployed. It is also the fact that IP runs
eight) in a four-entry table. Even if there
over virtually every other protocol (inwere 100 hosts on each physical netcluding ATM and Ethernet) that now
work, R2 would still only need those same
causes those protocols to be viewed as
four entries. This is a good first step (allayer 2 technologies.
though by no means the last) in achieving
scalability.
This illustrates one of the most important principles of building scalable networks:
To achieve scalability, you need to reduce the amount of information that is stored in
each node and that is exchanged between nodes. The most common way to do that is
hierarchical aggregation. IP introduces a two-level hierarchy, with networks at the top
level and nodes at the bottom level. We have aggregated information by letting routers
deal only with reaching the right network; the information that a router needs to deliver
a datagram to any node on a given network is represented by a single aggregated piece of
information.
4.1.5 Address Translation (ARP)
In the previous section we talked about how to get IP datagrams to the right physical
network, but glossed over the issue of how to get a datagram to a particular host or
router on that network. The main issue is that IP datagrams contain IP addresses, but
the physical interface hardware on the host or router to which you want to send the
datagram only understands the addressing scheme of that particular network. Thus, we
need to translate the IP address to a link-level address that makes sense on this network
(e.g., a 48-bit Ethernet address). We can then encapsulate the IP datagram inside a frame
that contains that link-level address and send it either to the ultimate destination or to a
router that promises to forward the datagram toward the ultimate destination.
One simple way to map an IP address into a physical network address is to encode a
host’s physical address in the host part of its IP address. For example, a host with physical
address 00100001 01001001 (which has the decimal value 33 in the upper byte and
81 in the lower byte) might be given the IP address 128.96.33.81. While this solution
has been used on some networks, it is limited in that the network’s physical addresses can
be no more than 16 bits long in this example; they can be only 8 bits long on a class C
network. This clearly will not work for 48-bit Ethernet addresses.
A more general solution would be for each host to maintain a table of address pairs,
that is, the table would map IP addresses into physical addresses. While this table could
be centrally managed by a system administrator and then copied to each host on the
network, a better approach would be for each host to dynamically learn the contents
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255
of the table using the network. This can be accomplished using the Address Resolution
Protocol (ARP). The goal of ARP is to enable each host on a network to build up a table
of mappings between IP addresses and link-level addresses. Since these mappings may
change over time (e.g., because an Ethernet card in a host breaks and is replaced by a
new one with a new address), the entries are timed out periodically and removed. This
happens on the order of every 15 minutes. The set of mappings currently stored in a host
is known as the ARP cache or ARP table.
ARP takes advantage of the fact that many link-level network technologies, such as
Ethernet and token ring, support broadcast. If a host wants to send an IP datagram to a
host (or router) that it knows to be on the same network (i.e., the sending and receiving
node have the same IP network number), it first checks for a mapping in the cache. If no
mapping is found, it needs to invoke the Address Resolution Protocol over the network.
It does this by broadcasting an ARP query onto the network. This query contains the IP
address in question (the target IP address). Each host receives the query and checks to
see if it matches its IP address. If it does match, the host sends a response message that
contains its link-layer address back to the originator of the query. The originator adds
the information contained in this response to its ARP table.
The query message also includes the IP address and link-layer address of the sending host. Thus, when a host broadcasts a query message, each host on the network can
learn the sender’s link-level and IP addresses and place that information in its ARP table.
However, not every host adds this information to its ARP table. If the host already has
an entry for that host in its table, it “refreshes” this entry, that is, it resets the length of
time until it discards the entry. If that host is the target of the query, then it adds the
information about the sender to its table, even if it did not already have an entry for that
host. This is because there is a good chance that the source host is about to send it an
application-level message, and it may eventually have to send a response or ACK back to
the source; it will need the source’s physical address to do this. If a host is not the target
and does not already have an entry for the source in its ARP table, then it does not add
an entry for the source. This is because there is no reason to believe that this host will
ever need the source’s link-level address; there is no need to clutter its ARP table with
this information.
Figure 4.7 shows the ARP packet format for IP-to-Ethernet address mappings. In
fact, ARP can be used for lots of other kinds of mappings—the major differences are in
the address sizes. In addition to the IP and link-layer addresses of both sender and target,
the packet contains
■
A HardwareType field, which specifies the type of physical network (e.g.,
Ethernet);
■
A ProtocolType field, which specifies the higher-layer protocol (e.g., IP);
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Figure 4.7
Internetworking
ARP packet format for mapping IP addresses into Ethernet addresses.
■
HLen (“hardware” address length) and PLen (“protocol” address length) fields,
which specify the length of the link-layer address and higher-layer protocol address, respectively;
■
An Operation field, which specifies whether this is a request or a response;
■
The source and target hardware (Ethernet) and protocol (IP) addresses.
Note that the results of the ARP process can be added as an extra column in a
forwarding table like the one in Table 4.1. Thus, for example, when R2 needs to forward
a packet to network 2, it not only finds that the next hop is R1, but also finds the MAC
address to place on the packet to send it to R1.
ATMARP
It should be clear that if an ATM network is to operate as part of an IP internetwork, then
it too must provide a form of ARP. However, the procedure just described will clearly not
work on a simple ATM network, because it depends on the fact that ARP packets can
be broadcast to all hosts on a single network. One solution to this problem is to use the
LAN emulation procedures described in Section 3.3. Since the goal of these procedures is
to make an ATM network behave just like a shared-media LAN, which includes support
for broadcast, the effect is to reduce ARP to a previously solved problem.
There are, however, situations where it may not be desirable to treat an ATM network as an emulated LAN. In particular, LAN emulation can be quite inefficient in a
large, wide area ATM network. Recall that in an emulated LAN many packets may need
to be sent to the broadcast and unknown server, which then floods those packets to
all nodes on the emulated LAN. Clearly there are limits to how far this can scale. The
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257
problem here is that adding broadcast capabilities to an intrinsically nonbroadcast network, while useful in some circumstances, is really overkill if the only reason you need
broadcast is to enable address resolution.
For this reason, there is a different ARP procedure that may be used in an ATM
network and that does not depend on broadcast or LAN emulation. This procedure
is known as ATMARP, and is part of the classical IP over ATM model. The reason for
calling the model classical will become apparent shortly. Like LAN emulation, ATMARP
relies on the use of a server to resolve addresses—in this case, it is called an ARP server,
and its behavior is described below.
A key concept in the classical IP over ATM model is the logical IP subnet (LIS).
The LIS abstraction allows us to take one large ATM network and subdivide it into
several smaller subnets. (We define subnet precisely in Section 4.3.1, but in this case
a subnet behaves much like a single network.) All nodes on the same subnet have the
same IP network number. And just as in classical IP, two nodes (hosts or routers) that
are on the same subnet can communicate directly over the ATM network, whereas two
nodes that are on different subnets will have to communicate via one or more routers.
An example of an ATM network divided into two LISs appears in Figure 4.8. Note
that the IP address of host H1 has a network number of 10, as does the router interface that connects to the left-hand LIS, while H2 has a network number of 12, as
does the right-hand interface on the router. That is, H1 and the router connect to the
same LIS (LIS 10) while H2 is on a different subnet (LIS 12) to which the router also
connects.
An advantage of the LIS model is that we can connect a large number of hosts and
routers to a big ATM network without necessarily giving them all addresses from the
same IP network. This may make it easier to manage address assignment, for example,
in the case where not all nodes connected to the ATM network are under the control of
Figure 4.8
Logical IP subnets.
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▲
the same administrative entity. The division of the ATM network into a number of LISs
also improves scalability by limiting the number of nodes that must be supported by a
single ARP server.
The basic job of an ARP server is to enable nodes on a LIS to resolve IP addresses
to ATM addresses without using broadcast. Each node in the LIS must be configured
with the ATM address of the ARP server so that it can establish a VC to the server when
it boots. Once it has a VC to the server, the node sends a registration message to the
ARP server that contains both the IP and ATM addresses of the registering node. Thus,
the ARP server builds up a complete database of all the IP address, ATM address
pairs. Once this is in place, any node that wants to send a packet to some IP address can
ask the ARP server to provide the corresponding ATM address. Once this is received, the
sending node can use ATM signalling to set up a VC to that ATM address, and then send
the packet. Just like conventional ARP, a cache of IP-to-ATM address mappings can be
maintained. In addition, the node can keep a VC established to that ATM destination as
long as there is enough traffic flowing to justify it, thus avoiding the delay of setting up
the VC again when the next packet arrives.
An interesting consequence of the classical IP over ATM model is that two nodes
on the same ATM network cannot establish a direct VC between themselves if they are
on different subnets. This would violate the rule that communication from one subnet
to another must pass through a router. For example, host H1 and host H2 in Figure 4.8
cannot establish a direct VC under the classical model. Instead, each needs to have a VC
to router R. The simple explanation for this rule is that IP routing is known to work well
when that rule is obeyed, as it is in non-ATM networks. New techniques to work around
that rule have been developed, but they have introduced considerable complexity and
problems of robustness.
We have now seen the basic mechanisms that IP provides for dealing with both
heterogeneity and scale. On the issue of heterogeneity, IP begins by defining a best-effort
service model that makes minimal assumptions about the underlying networks; most notably, this service model is based on unreliable datagrams. IP then makes two important
additions to this starting point: (1) a common packet format (fragmentation/reassembly
is the mechanism that makes this format work over networks with different MTUs), and
(2) a global address space for identifying all hosts (ARP is the mechanism that makes this
global address space work over networks with different physical addressing schemes). On
the issue of scale, IP uses hierarchical aggregation to reduce the amount of information
needed to forward packets. Specifically, IP addresses are partitioned into network and
host components, with packets first routed toward the destination network and then
delivered to the correct host on that network.
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4.1.6 Host Configuration (DHCP)
In Section 2.6 we observed that Ethernet addresses are configured into the network adaptor by the manufacturer, and this process is managed in such a way to ensure that these
addresses are globally unique. This is clearly a sufficient condition to ensure that any collection of hosts connected to a single Ethernet (including an extended LAN) will have
unique addresses. Furthermore, uniqueness is all we ask of Ethernet addresses.
IP addresses, by contrast, must be not only unique on a given internetwork, but also
must reflect the structure of the internetwork. As noted above, they contain a network
part and a host part, and the network part must be the same for all hosts on the same
network. Thus, it is not possible for the IP address to be configured once into a host
when it is manufactured, since that would imply that the manufacturer knew which
hosts were going to end up on which networks, and it would mean that a host, once
connected to one network, could never move to another. For this reason, IP addresses
need to be reconfigurable.
In addition to an IP address, there are some other pieces of information a host needs
to have before it can start sending packets. The most notable of these is the address of a
default router—the place to which it can send packets whose destination address is not
on the same network as the sending host.
Most host operating systems provide a way for a system administrator, or even a
user, to manually configure the IP information needed by a host. However, there are
some obvious drawbacks to such manual configuration. One is that it is simply a lot of
work to configure all the hosts in a large network directly, especially when you consider
that such hosts are not reachable over a network until they are configured. Even more
importantly, the configuration process is very error-prone, since it is necessary to ensure
that every host gets the correct network number and that no two hosts receive the same IP
address. For these reasons, automated configuration methods are required. The primary
method uses a protocol known as the Dynamic Host Configuration Protocol (DHCP).
DHCP relies on the existence of a DHCP server that is responsible for providing
configuration information to hosts. There is at least one DHCP server for an administrative domain. At the simplest level, the DHCP server can function just as a centralized
repository for host configuration information. Consider, for example, the problem of administering addresses in the internetwork of a large company. DHCP saves the network
administrators from having to walk around to every host in the company with a list of
addresses and network map in hand and configuring each host manually. Instead, the
configuration information for each host could be stored in the DHCP server and automatically retrieved by each host when it is booted or connected to the network. However,
the administrator would still pick the address that each host is to receive; he would just
store that in the server. In this model, the configuration information for each host is
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stored in a table that is indexed by some form of unique client identifier, typically the
hardware address (e.g., the Ethernet address of its network adaptor).
A more sophisticated use of DHCP saves the network administrator from even
having to assign addresses to individual hosts. In this model, the DHCP server maintains
a pool of available addresses that it hands out to hosts on demand. This considerably
reduces the amount of configuration an administrator must do, since now it is only
necessary to allocate a range of IP addresses (all with the same network number) to each
network.
Since the goal of DHCP is to minimize the amount of manual configuration required for a host to function, it would rather defeat the purpose if each host had to be
configured with the address of a DHCP server. Thus, the first problem faced by DHCP
is that of server discovery.
To contact a DHCP server, a newly booted or attached host sends a DHCPDISCOVER message to a special IP address (255.255.255.255) that is an IP broadcast address. This means it will be received by all hosts and routers on that network. (Routers
do not forward such packets onto other networks, preventing broadcast to the entire
Internet.) In the simplest case, one of these nodes is the DHCP server for the network.
The server would then reply to the host that generated the discovery message (all the
other nodes would ignore it). However, it is not really desirable to require one DHCP
server on every network, because this still creates a potentially large number of servers
that need to be correctly and consistently configured. Thus, DHCP uses the concept of
a relay agent. There is at least one relay agent on each network, and it is configured with
just one piece of information: the IP address of the DHCP server. When a relay agent
receives a DHCPDISCOVER message, it unicasts it to the DHCP server and awaits the
response, which it will then send back to the requesting client. The process of relaying a
message from a host to a remote DHCP server is shown in Figure 4.9.
Figure 4.10 shows the format of a DHCP message. The message is actually sent
using a protocol called the User Datagram Protocol (UDP) that runs over IP. UDP is
discussed in detail in the next chapter, but the only interesting thing it does in this
context is to provide a demultiplexing key that says, “This is a DHCP packet.”
DHCP is derived from an earlier protocol called BOOTP, and some of the packet
fields are thus not strictly relevant to host configuration. When trying to obtain configuration information, the client puts its hardware address (e.g., its Ethernet address) in
the chaddr field. The DHCP server replies by filling in the yiaddr (“your” IP address)
field and sending it to the client. Other information such as the default router to be used
by this client can be included in the options field.
In the case where DHCP dynamically assigns IP addresses to hosts, it is clear that
hosts cannot keep addresses indefinitely, as this would eventually cause the server to
exhaust its address pool. At the same time, a host cannot be depended upon to give
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261
Figure 4.9 A DHCP relay agent receives a broadcast DHCPDISCOVER message from a
host and sends a unicast DHCPDISCOVER to the DHCP server.
Figure 4.10
DHCP packet format.
▲
back its address, since it might have crashed, been unplugged from the network, or been
turned off. Thus, DHCP allows addresses to be “leased” for some period of time. Once
the lease expires, the server is free to return that address to its pool. A host with a leased
address clearly needs to renew the lease periodically if in fact it is still connected to the
network and functioning correctly.
DHCP illustrates an important aspect of scaling: the scaling of network management. While discussions of scaling often focus on keeping the state in network devices
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from growing too fast, it is important to pay attention to growth of network management complexity. By allowing network managers to configure a range of IP addresses per
network rather than one IP address per host, DHCP improves the manageability of a
network.
Note that DHCP may also introduce some more complexity into network management, since it makes the binding between physical hosts and IP addresses much more
dynamic. This may make the network manager’s job more difficult if, for example, it
becomes necessary to locate a malfunctioning host.
4.1.7 Error Reporting (ICMP)
The next issue is how the Internet treats errors. While IP is perfectly willing to drop
datagrams when the going gets tough—for example, when a router does not know how
to forward the datagram or when one fragment of a datagram fails to arrive at the
destination—it does not necessarily fail silently. IP is always configured with a companion protocol, known as the Internet Control Message Protocol (ICMP), that defines
a collection of error messages that are sent back to the source host whenever a router or
host is unable to process an IP datagram successfully. For example, ICMP defines error
messages indicating that the destination host is unreachable (perhaps due to a link failure), that the reassembly process failed, that the TTL had reached 0, that the IP header
checksum failed, and so on.
ICMP also defines a handful of control messages that a router can send back to a
source host. One of the most useful control messages, called an ICMP-Redirect, tells the
source host that there is a better route to the destination. ICMP-Redirects are used in
the following situation. Suppose a host is connected to a network that has two routers
attached to it, called R1 and R2, where the host uses R1 as its default router. Should
R1 ever receive a datagram from the host, where based on its forwarding table it knows
that R2 would have been a better choice for a particular destination address, it sends
an ICMP-Redirect back to the host, instructing it to use R2 for all future datagrams
addressed to that destination. The host then adds this new route to its forwarding table.
4.1.8 Virtual Networks and Tunnels
We conclude our introduction to IP by considering an issue you might not have anticipated, but one that is becoming increasingly important. Our discussion up to this point
has focused on making it possible for nodes on different networks to communicate with
each other in an unrestricted way. This is usually the goal in the Internet—everybody
wants to be able to send email to everybody, and the creator of a new website wants to
reach the widest possible audience. However, there are many situations where more controlled connectivity is required. An important example of such a situation is the virtual
private network (VPN).
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263
The term VPN is heavily overused and definitions vary, but intuitively we can define a VPN by considering first the idea of a private network. Corporations with many
sites often build private networks by leasing transmission lines from the phone companies and using those lines to interconnect sites. In such a network, communication is
restricted to take place only among the sites of that corporation, which is often desirable
for security reasons. To make a private network virtual, the leased transmission lines—
which are not shared with any other corporations—would be replaced by some sort of
shared network. A virtual circuit is a very reasonable replacement for a leased line because it still provides a logical point-to-point connection between the corporation’s sites.
For example, if corporation X has a VC from site A to site B, then clearly it can send
packets between sites A and B. But there is no way that corporation Y can get its packets
delivered to site B without first establishing its own virtual circuit to site B, and the establishment of such a VC can be administratively prevented, thus preventing unwanted
connectivity between corporation X and corporation Y.
Figure 4.11(a) shows two private networks for two separate corporations. In Figure 4.11(b) they are both migrated to a virtual circuit network. The limited connectivity
of a real private network is maintained, but since the private networks now share the
same transmission facilities and switches we say that two virtual private networks have
been created.
In Figure 4.11, a Frame Relay or ATM network is used to provide the controlled
connectivity among sites. It is also possible to provide a similar function using an IP
network—an internetwork—to provide the connectivity. However, we cannot just connect the various corporations’ sites to a single internetwork because that would provide
connectivity between corporation X and corporation Y, which we wish to avoid. To solve
this problem, we need to introduce a new concept, the IP tunnel.
We can think of an IP tunnel as a virtual point-to-point link between a pair of
nodes that are actually separated by an arbitrary number of networks. The virtual link
is created within the router at the entrance to the tunnel by providing it with the IP
address of the router at the far end of the tunnel. Whenever the router at the entrance of
the tunnel wants to send a packet over this virtual link, it encapsulates the packet inside
an IP datagram. The destination address in the IP header is the address of the router at
the far end of the tunnel, while the source address is that of the encapsulating router.
In the forwarding table of the router at the entrance to the tunnel, this virtual
link looks much like a normal link. Consider, for example, the network in Figure 4.12.
A tunnel has been configured from R1 to R2 and assigned a virtual interface number
of 0. The forwarding table in R1 might therefore look like Table 4.3.
R1 has two physical interfaces. Interface 0 connects to network 1; interface 1 connects to a large internetwork and is thus the default for all traffic that does not match
something more specific in the forwarding table. In addition, R1 has a virtual interface,
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Figure 4.11 An example of virtual private networks: (a) two separate private networks;
(b) two virtual private networks sharing common switches.
Figure 4.12
A tunnel through an internetwork.
4.1
NetworkNum
NextHop
1
Interface 0
2
Virtual interface 0
Default
Interface 1
Table 4.3
Simple Internetworking (IP)
265
Forwarding table for router R1 in Figure 4.12.
which is the interface to the tunnel. Suppose R1 receives a packet from network 1 that
contains an address in network 2. The forwarding table says this packet should be sent
out virtual interface 0. In order to send a packet out this interface, the router takes the
packet, adds an IP header addressed to R2, and then proceeds to forward the packet as
if it had just been received. R2’s address is 18.5.0.1; since the network number of this
address is 18, not 1 or 2, a packet destined for R2 will be forwarded out the default
interface into the internetwork.
Once the packet leaves R1, it looks to the rest of the world like a normal IP packet
destined to R2, and it is forwarded accordingly. All the routers in the internetwork forward it using normal means, until it arrives at R2. When R2 receives the packet, it finds
that it carries its own address, so it removes the IP header and looks at the payload of the
packet. What it finds is an inner IP packet whose destination address is in network 2.
R2 now processes this packet like any other IP packet it receives. Since R2 is directly
connected to network 2, it forwards the packet on to that network. Figure 4.12 shows
the change in encapsulation of the packet as it moves across the network.
While R2 is acting as the endpoint of the tunnel, there is nothing to prevent it
from performing the normal functions of a router. For example, it might receive some
packets that are not tunneled, but which are addressed to networks that it knows how to
reach, and it would forward them in the normal way.
You might wonder why anyone would want to go to all the trouble of creating
a tunnel and changing the encapsulation of a packet as it goes across an internetwork.
One reason is security, which we will discuss in more detail in Chapter 8. Supplemented
with encryption, a tunnel can become a very private sort of link across a public network.
Another reason may be that R1 and R2 have some capabilities that are not widely available in the intervening networks, such as multicast routing. By connecting these routers
with a tunnel, we can build a virtual network in which all the routers with this capability
appear to be directly connected. This in fact is how the (MBone) multicast backbone is
built, as we will see in Section 4.4. A third reason to build tunnels is to carry packets
from protocols other than IP across an IP network. As long as the routers at either end
of the tunnel know how to handle these other protocols, the IP tunnel looks to them
like a point-to-point link over which they can send non-IP packets. Tunnels also provide
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a mechanism by which we can force a packet to be delivered to a particular place even
if its original header—the one that gets encapsulated inside the tunnel header—might
suggest that it should go somewhere else. We will see an application of this when we
consider mobile hosts in Section 4.2.5. Thus, we see that tunneling is a powerful and
quite general technique for building virtual links across internetworks.
Tunneling does have its downsides. One is that it increases the length of packets;
this might represent a significant waste of bandwidth for short packets. Longer packets
might be subject to fragmentation, which has its own set of drawbacks. There may also
be performance implications for the routers at either end of the tunnel, since they need to
do more work than normal forwarding as they add and remove the tunnel header. Finally,
there is a management cost for the administrative entity that is responsible for setting up
the tunnels and making sure they are correctly handled by the routing protocols.
4.2
Routing
▲
In both this and the previous chapter we have assumed that the switches and routers have
enough knowledge of the network topology so they can choose the right port onto which
each packet should be output. In the case of virtual circuits, routing is an issue only for
the connection request packet; all subsequent packets follow the same path as the request.
In datagram networks, including IP networks, routing is an issue for every packet. In
either case, a switch or router needs to be able to look at the packet’s destination address
and then to determine which of the output ports is the best choice to get the packet to
that address. As we saw in Section 3.1.1, the switch makes this decision by consulting a
forwarding table. The fundamental problem of routing is, how do switches and routers
acquire the information in their forwarding tables?
We restate an important distinction, which is often neglected, between forwarding and routing. Forwarding consists of taking a packet, looking at its destination address, consulting a table, and sending the packet in a direction determined by that table.
We saw several examples of forwarding in the preceding section. Routing is the process
by which forwarding tables are built. We also note that forwarding is a relatively simple and well-defined process performed locally at a node, whereas routing depends on
complex distributed algorithms that have continued to evolve throughout the history of
networking.
While the terms forwarding table and routing table are sometimes used interchangeably, we will make a distinction between them here. The forwarding table is used when
a packet is being forwarded and so must contain enough information to accomplish the
forwarding function. This means that a row in the forwarding table contains the mapping from a network number to an outgoing interface and some MAC information, such
as the Ethernet address of the next hop. The routing table, on the other hand, is the table
4.2 Routing
267
that is built up by the routing algorithms as a precursor to building the forwarding table.
It generally contains mappings from network numbers to next hops. It may also contain
information about how this information was learned, so that the router will be able to
decide when it should discard some information.
Whether the routing table and forwarding table are actually separate data structures
is something of an implementation choice, but there are numerous reasons to keep them
separate. For example, the forwarding table needs to be structured to optimize the process
of looking up a network number when forwarding a packet, while the routing table needs
to be optimized for the purpose of calculating changes in topology. In some cases, the
forwarding table may even be implemented in specialized hardware, whereas this is rarely
if ever done for the routing table. Table 4.4 provides an example of a row from each sort
of table. In this case, the routing table tells us that network number 10 is to be reached
by a next hop router with the IP address 171.69.245.10, while the forwarding table
contains the information about exactly how to forward a packet to that next hop: Send it
out interface number 0 with a MAC address of 8:0:2b:e4:b:1:2. Note that the last piece
of information is provided by the Address Resolution Protocol.
Before getting into the details of routing, we need to remind ourselves of the key
question we should be asking anytime we try to build a mechanism for the Internet:
Does this solution scale? The answer for the algorithms and protocols described in this
section is no. They are designed for networks of fairly modest size—fewer than a hundred nodes, in practice. However, the solutions we describe do serve as a building block
for a hierarchical routing infrastructure that is used in the Internet today. Specifically, the
protocols described in this section are collectively known as intradomain routing protocols, or interior gateway protocols (IGPs). To understand these terms, we need to define a
routing domain: A good working definition is an internetwork in which all the routers
are under the same administrative control (e.g., a single university campus, or the network of a single Internet service provider). The relevance of this definition will become
apparent in the next section when we look at interdomain routing protocols. For now,
Network Number
Next Hop
18
171.69.245.10
(a)
Network Number
Interface
MAC Address
18
if0
8:0:2b:e4:b:1:2
(b)
Table 4.4
Example rows from (a) routing and (b) forwarding tables.
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the important thing to keep in mind is that we are considering the problem of routing
in the context of small to midsized networks, not for a network the size of the Internet.
4.2.1 Network as a Graph
Routing is, in essence, a problem of graph theory. Figure 4.13 shows a graph representing
a network. The nodes of the graph, labeled A through F, may be either hosts, switches,
routers, or networks. For our initial discussion, we will focus on the case where the nodes
are routers. The edges of the graph correspond to the network links. Each edge has an
associated cost, which gives some indication of the desirability of sending traffic over that
link. A discussion of how edge costs are assigned is given in Section 4.2.4.6
The basic problem of routing is to find the lowest-cost path between any two
nodes, where the cost of a path equals the sum of the costs of all the edges that make
up the path. For a simple network like the one in Figure 4.13, you could imagine just
calculating all the shortest paths and loading them into some nonvolatile storage on each
node. Such a static approach has several shortcomings:
■
It does not deal with node or link failures;
■
It does not consider the addition of new nodes or links;
■
It implies that edge costs cannot change, even though we might reasonably wish
to temporarily assign a high cost to a link that is heavily loaded.
For these reasons, routing is achieved in most practical networks by running routing protocols among the nodes. These protocols provide a distributed, dynamic way
to solve the problem of finding the lowest-cost path in the presence of link and node
failures and changing edge costs. Note the word “distributed” in the last sentence: It is
Figure 4.13
Network represented as a graph.
6 In the example networks (graphs) used throughout this chapter, we use undirected edges and assign each edge a single
cost. This is actually a slight simplification. It is more accurate to make the edges directed, which typically means that
there would be a pair of edges between each node—one flowing in each direction, and each with its own edge cost.
4.2 Routing
269
difficult to make centralized solutions scalable, so all the widely used routing protocols
use distributed algorithms.
The distributed nature of routing algorithms is one of the main reasons why this
has been such a rich field of research and development—there are a lot of challenges
in making distributed algorithms work well. For example, distributed algorithms raise
the possibility that two routers will at one instant have different ideas about the shortest
path to some destination. In fact, each one may think that the other one is closer to the
destination, and decide to send packets to the other one. Clearly such packets will be
stuck in a loop until the discrepancy between the two routers is resolved, and it would
be good to resolve it as soon as possible. This is just one example of the type of problem
routing protocols must address.
To begin our analysis, we assume that the edge costs in the network are known. We
will examine the two main classes of routing protocols: distance vector and link state. In
Section 4.2.4 we return to the problem of calculating edge costs in a meaningful way.
4.2.2 Distance Vector (RIP)
The idea behind the distance-vector algorithm is suggested by its name:7 Each node constructs a one-dimensional array (a vector) containing the “distances” (costs) to all other
nodes and distributes that vector to its immediate neighbors. The starting assumption
for distance-vector routing is that each node knows the cost of the link to each of its
directly connected neighbors. A link that is down is assigned an infinite cost.
To see how a distance-vector routing algorithm works, it is easiest to consider an
example like the one depicted in Figure 4.14. In this example, the cost of each link is
set to 1, so that a least-cost path is simply the one with the fewest hops. (Since all edges
have the same cost, we do not show the costs in the graph.) We can represent each node’s
Figure 4.14
Distance-vector routing: an example network.
7 The other common name for this class of algorithm is Bellman-Ford, after its inventors.
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Distance to Reach Node
Information
Stored at Node
A
B
C
D
E
F
G
A
0
1
1
∞
1
1
∞
B
1
0
1
∞
∞
∞
∞
C
1
1
0
1
∞
∞
∞
D
∞
∞
1
0
∞
∞
1
E
1
∞
∞
∞
0
∞
∞
F
1
∞
∞
∞
∞
0
1
G
∞
∞
∞
1
∞
1
0
Table 4.5
Initial distances stored at each node (global view).
Destination
Cost
Next Hop
B
1
B
C
1
C
D
∞
—
E
1
E
F
1
F
G
∞
—
Table 4.6
Initial routing table at node A.
knowledge about the distances to all other nodes as a table like the one given in Table 4.5.
Note that each node only knows the information in one row of the table (the one that
bears its name in the left column). The global view that is presented here is not available
at any single point in the network.
We may consider each row in Table 4.5 as a list of distances from one node to
all other nodes, representing the current beliefs of that node. Initially, each node sets a
cost of 1 to its directly connected neighbors and ∞ to all other nodes. Thus, A initially
believes that it can reach B in one hop and that D is unreachable. The routing table stored
at A reflects this set of beliefs and includes the name of the next hop that A would use to
reach any reachable node. Initially, then, A’s routing table would look like Table 4.6.
The next step in distance-vector routing is that every node sends a message to its
directly connected neighbors containing its personal list of distances. For example, node
4.2 Routing
Destination
Cost
Next Hop
B
1
B
C
1
C
D
2
C
E
1
E
F
1
F
G
2
F
Table 4.7
271
Final routing table at node A.
F tells node A that it can reach node G at a cost of 1; A also knows it can reach F at a
cost of 1, so it adds these costs to get the cost of reaching G by means of F. This total
cost of 2 is less than the current cost of infinity, so A records that it can reach G at a cost
of 2 by going through F. Similarly, A learns from C that D can be reached from C at a
cost of 1; it adds this to the cost of reaching C (1) and decides that D can be reached
via C at a cost of 2, which is better than the old cost of infinity. At the same time, A
learns from C that B can be reached from C at a cost of 1, so it concludes that the cost
of reaching B via C is 2. Since this is worse than the current cost of reaching B (1), this
new information is ignored.
At this point, A can update its routing table with costs and next hops for all nodes
in the network. The result is shown in Table 4.7.
In the absence of any topology changes, it only takes a few exchanges of information between neighbors before each node has a complete routing table. The process of
getting consistent routing information to all the nodes is called convergence. Table 4.8
shows the final set of costs from each node to all other nodes when routing has converged. We must stress that there is no one node in the network that has all the information in this table—each node only knows about the contents of its own routing table.
The beauty of a distributed algorithm like this is that it enables all nodes to achieve a
consistent view of the network in the absence of any centralized authority.
There are a few details to fill in before our discussion of distance-vector routing is
complete. First, we note that there are two different circumstances under which a given
node decides to send a routing update to its neighbors. One of these circumstances is
the periodic update. In this case, each node automatically sends an update message every
so often, even if nothing has changed. This serves to let the other nodes know that this
node is still running. It also makes sure that they keep getting information that they may
need if their current routes become unviable. The frequency of these periodic updates
varies from protocol to protocol, but it is typically on the order of several seconds to
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Distance to Reach Node
Information
Stored at Node
A
B
C
D
E
F
G
A
0
1
1
2
1
1
2
B
1
0
1
2
2
2
3
C
1
1
0
1
2
2
2
D
2
2
1
0
3
2
1
E
1
2
2
3
0
2
3
F
1
2
2
2
2
0
1
G
2
3
2
1
3
1
0
Table 4.8
Final distances stored at each node (global view).
several minutes. The second mechanism, sometimes called a triggered update, happens
whenever a node receives an update from one of its neighbors that causes it to change
one of the routes in its routing table. That is, whenever a node’s routing table changes,
it sends an update to its neighbors, which may lead to a change in their tables, causing
them to send an update to their neighbors.
Now consider what happens when a link or node fails. The nodes that notice first
send new lists of distances to their neighbors, and normally the system settles down
fairly quickly to a new state. As to the question of how a node detects a failure, there
are a couple of different answers. In one approach, a node continually tests the link to
another node by sending a control packet and seeing if it receives an acknowledgment.
In another approach, a node determines that the link (or the node at the other end of
the link) is down if it does not receive the expected periodic routing update for the last
few update cycles.
To understand what happens when a node detects a link failure, consider what
happens when F detects that its link to G has failed. First, F sets its new distance to G to
infinity and passes that information along to A. Since A knows that its 2-hop path to G
is through F, A would also set its distance to G to infinity. However, with the next update
from C, A would learn that C has a 2-hop path to G. Thus, A would know that it could
reach G in 3 hops through C, which is less than infinity, and so A would update its table
accordingly. When it advertises this to F, node F would learn that it can reach G at a cost
of 4 through A, which is less than infinity, and the system would again become stable.
Unfortunately, slightly different circumstances can prevent the network from stabilizing. Suppose, for example, that the link from A to E goes down. In the next round
of updates, A advertises a distance of infinity to E, but B and C advertise a distance of 2
4.2 Routing
273
to E. Depending on the exact timing of events, the following might happen: Node B,
upon hearing that E can be reached in 2 hops from C, concludes that it can reach E
in 3 hops and advertises this to A; node A concludes that it can reach E in 4 hops and
advertises this to C; node C concludes that it can reach E in 5 hops; and so on. This cycle
stops only when the distances reach some number that is large enough to be considered
infinite. In the meantime, none of the nodes actually knows that E is unreachable, and
the routing tables for the network do not stabilize. This situation is known as the count
to infinity problem.
There are several partial solutions to this problem. The first one is to use some
relatively small number as an approximation of infinity. For example, we might decide
that the maximum number of hops to get across a certain network is never going to be
more than 16, and so we could pick 16 as the value that represents infinity. This at least
bounds the amount of time that it takes to count to infinity. Of course, it could also
present a problem if our network grew to a point where some nodes were separated by
more than 16 hops.
One technique to improve the time to stabilize routing is called split horizon. The
idea is that when a node sends a routing update to its neighbors, it does not send those
routes it learned from each neighbor back to that neighbor. For example, if B has the
route (E, 2, A) in its table, then it knows it must have learned this route from A, and
so whenever B sends a routing update to A, it does not include the route (E, 2) in that
update. In a stronger variation of split horizon, called split horizon with poison reverse, B
actually sends that route back to A, but it puts negative information in the route to ensure
that A will not eventually use B to get to E. For example, B sends the route (E, ∞) to A.
The problem with both of these techniques is that they only work for routing loops
that involve two nodes. For larger routing loops, more drastic measures are called for.
Continuing the above example, if B and C had waited for a while after hearing of the
link failure from A before advertising routes to E, they would have found that neither of
them really had a route to E. Unfortunately, this approach delays the convergence of the
protocol; speed of convergence is one of the key advantages of its competitor, link-state
routing, the subject of Section 4.2.3.
Implementation
The code that implements this algorithm is very straightforward; we give only some of
the basics here. Structure Route defines each entry in the routing table, and constant
MAX_TTL specifies how long an entry is kept in the table before it is discarded.
#define MAX_ROUTES 128 /* maximum size of routing table */
#define MAX_TTL
120 /* time (in seconds) until route
expires */
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typedef struct {
NodeAddr
Destination;
NodeAddr
NextHop;
int
Cost;
u_short
TTL;
} Route;
int
Route
/*
/*
/*
/*
address of destination */
address of next hop */
distance metric */
time to live */
numRoutes = 0;
routingTable[MAX_ROUTES];
The routine that updates the local node’s routing table based on a new route is
given by mergeRoute. Although not shown, a timer function periodically scans the
list of routes in the node’s routing table, decrements the TTL field of each route, and
discards any routes that have a time to live of 0. Notice, however, that the TTL field
is reset to MAX_TTL any time the route is reconfirmed by an update message from a
neighboring node.
void
mergeRoute (Route *new)
{
int i;
for (i = 0; i < numRoutes; ++i)
{
if (new->Destination
== routingTable[i].Destination)
{
if (new->Cost + 1 < routingTable[i].Cost)
{
/* found a better route: */
break;
} else if (new->NextHop
== routingTable[i].NextHop) {
/* metric for current next-hop
may have changed: */
break;
} else {
/* route is uninteresting--just ignore it */
return;
}
}
}
if (i == numRoutes)
4.2 Routing
275
{
/* this is a completely new route;
is there room for it? */
if (numRoutes < MAXROUTES)
{
++numRoutes;
} else {
/* can’t fit this route in table so give up */
return;
}
}
routingTable[i] = *new;
/* reset TTL */
routingTable[i].TTL = MAX_TTL;
/* account for hop to get to next node */
++routingTable[i].Cost;
}
Finally, the procedure updateRoutingTable is the main routine that calls
mergeRoute to incorporate all the routes contained in a routing update that is received from a neighboring node.
void
updateRoutingTable (Route *newRoute, int numNewRoutes)
{
int i;
for (i=0; i < numNewRoutes; ++i)
{
mergeRoute(&newRoute[i]);
}
}
Routing Information Protocol (RIP)
One of the most widely used routing protocols in IP networks is the Routing Information Protocol (RIP). Its widespread use is due in no small part to the fact that it
was distributed along with the popular Berkeley Software Distribution (BSD) version of
Unix, from which many commercial versions of Unix were derived. It is also extremely
simple. RIP is the canonical example of a routing protocol built on the distance-vector
algorithm just described.
Routing protocols in internetworks differ very slightly from the idealized graph
model described above. In an internetwork, the goal of the routers is to learn how to for-
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Figure 4.15
Example network running RIP.
Figure 4.16
RIP packet format.
ward packets to various networks. Thus, rather than advertising the cost of reaching other
routers, the routers advertise the cost of reaching networks. For example, in Figure 4.15,
router C would advertise to router A the fact that it can reach networks 2 and 3 (to
which it is directly connected) at a cost of 0; networks 5 and 6 at cost 1; and network 4
at cost 2.
We can see evidence of this in the RIP packet format in Figure 4.16. The majority
of the packet is taken up with network-address, distance pairs. However, the
principles of the routing algorithm are just the same. For example, if router A learns
from router B that network X can be reached at a lower cost via B than via the existing
4.2 Routing
277
next hop in the routing table, A updates the cost and next hop information for the
network number accordingly.
RIP is in fact a fairly straightforward implementation of distance-vector routing.
Routers running RIP send their advertisements every 30 seconds; a router also sends an
update message whenever an update from another router causes it to change its routing
table. One point of interest is that it supports multiple address families, not just IP.
The network-address part of the advertisements is actually represented as a family,
address pair. RIP version 2 (RIPv2) also has some features related to scalability that
we will discuss in the next section.
As we will see below, it is possible to use a range of different metrics or costs for
the links in a routing protocol. RIP takes the simplest approach, with all link costs being
equal to 1, just as in our example above. Thus, it always tries to find the minimum hop
route. Valid distances are 1 through 15, with 16 representing infinity. This also limits
RIP to running on fairly small networks—those with no paths longer than 15 hops.
4.2.3 Link State (OSPF)
Link-state routing is the second major class of intradomain routing protocol. The starting
assumptions for link-state routing are rather similar to those for distance-vector routing.
Each node is assumed to be capable of finding out the state of the link to its neighbors (up or down) and the cost of each link. Again, we want to provide each node with
enough information to enable it to find the least-cost path to any destination. The basic idea behind link-state protocols is very simple: Every node knows how to reach its
directly connected neighbors, and if we make sure that the totality of this knowledge is
disseminated to every node, then every node will have enough knowledge of the network
to build a complete map of the network. This is clearly a sufficient condition (although
not a necessary one) for finding the shortest path to any point in the network. Thus,
link-state routing protocols rely on two mechanisms: reliable dissemination of link-state
information, and the calculation of routes from the sum of all the accumulated link-state
knowledge.
Reliable Flooding
Reliable flooding is the process of making sure that all the nodes participating in the
routing protocol get a copy of the link-state information from all the other nodes. As the
term “flooding” suggests, the basic idea is for a node to send its link-state information
out on all of its directly connected links, with each node that receives this information
forwarding it out on all of its links. This process continues until the information has
reached all the nodes in the network.
More precisely, each node creates an update packet, also called a link-state packet
(LSP), that contains the following information:
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■
The ID of the node that created the LSP;
■
A list of directly connected neighbors of that node, with the cost of the link to
each one;
■
A sequence number;
■
A time to live for this packet.
The first two items are needed to enable route calculation; the last two are used to make
the process of flooding the packet to all nodes reliable. Reliability includes making sure
that you have the most recent copy of the information, since there may be multiple,
contradictory LSPs from one node traversing the network. Making the flooding reliable
has proven to be quite difficult. (For example, an early version of link-state routing used
in the ARPANET caused that network to fail in 1981.)
Flooding works in the following way. First, the transmission of LSPs between adjacent routers is made reliable using acknowledgments and retransmissions just as in the
reliable link-layer protocol described in Section 2.5. However, there are several more
steps needed to reliably flood an LSP to all nodes in a network.
Consider a node X that receives a copy of an LSP that originated at some other
node Y. Note that Y may be any other router in the same routing domain as X. X checks
to see if it has already stored a copy of an LSP from Y. If not, it stores the LSP. If
it already has a copy, it compares the sequence numbers; if the new LSP has a larger
sequence number, it is assumed to be the more recent, and that LSP is stored, replacing
the old one. A smaller (or equal) sequence number would imply an LSP older (or not
newer) than the one stored, so it would be discarded and no further action would be
needed. If the received LSP was the newer one, X then sends a copy of that LSP to all
of its neighbors except the neighbor from which the LSP was just received. The fact that
the LSP is not sent back to the node from which it was received helps to bring an end
to the flooding of an LSP. Since X passes the LSP on to all its neighbors, who then turn
around and do the same thing, the most recent copy of the LSP eventually reaches all
nodes.
Figure 4.17 shows an LSP being flooded in a small network. Each node becomes
shaded as it stores the new LSP. In Figure 4.17(a) the LSP arrives at node X, which sends
it to neighbors A and C in Figure 4.17(b). A and C do not send it back to X, but send it
on to B. Since B receives two identical copies of the LSP, it will accept whichever arrived
first and ignore the second as a duplicate. It then passes the LSP onto D, who has no
neighbors to flood it to, and the process is complete.
Just as in RIP, each node generates LSPs under two circumstances. Either the expiry
of a periodic timer or a change in topology can cause a node to generate a new LSP.
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279
Figure 4.17 Flooding of link-state packets. (a) LSP arrives at node X; (b) X floods LSP
to A and C; (c) A and C flood LSP to B (but not X); (d) flooding is complete.
However, the only topology-based reason for a node to generate an LSP is if one of its
directly connected links or immediate neighbors has gone down. The failure of a link
can be detected in some cases by the link-layer protocol. The demise of a neighbor or
loss of connectivity to that neighbor can be detected using periodic “hello” packets. Each
node sends these to its immediate neighbors at defined intervals. If a sufficiently long
time passes without receipt of a “hello” from a neighbor, the link to that neighbor will
be declared down, and a new LSP will be generated to reflect this fact.
One of the important design goals of a link-state protocol’s flooding mechanism is
that the newest information must be flooded to all nodes as quickly as possible, while
old information must be removed from the network and not allowed to circulate. In
addition, it is clearly desirable to minimize the total amount of routing traffic that is sent
around the network; after all, this is just “overhead” from the perspective of those who
actually use the network for their applications. The next few paragraphs describe some
of the ways that these goals are accomplished.
One easy way to reduce overhead is to avoid generating LSPs unless absolutely
necessary. This can be done by using very long timers—often on the order of hours—for
the periodic generation of LSPs. Given that the flooding protocol is truly reliable when
topology changes, it is safe to assume that messages saying “nothing has changed” do not
need to be sent very often.
To make sure that old information is replaced by newer information, LSPs carry
sequence numbers. Each time a node generates a new LSP, it increments the sequence
number by 1. Unlike most sequence numbers used in protocols, these sequence numbers
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are not expected to wrap, so the field needs to be quite large (say, 64 bits). If a node goes
down and then comes back up, it starts with a sequence number of 0. If the node was
down for a long time, all the old LSPs for that node will have timed out (as described
below); otherwise, this node will eventually receive a copy of its own LSP with a higher
sequence number, which it can then increment and use as its own sequence number.
This will ensure that its new LSP replaces any of its old LSPs left over from before the
node went down.
LSPs also carry a time to live. This is used to ensure that old link-state information
is eventually removed from the network. A node always decrements the TTL of a newly
received LSP before flooding it to its neighbors. It also “ages” the LSP while it is stored in
the node. When the TTL reaches 0, the node refloods the LSP with a TTL of 0, which
is interpreted by all the nodes in the network as a signal to delete that LSP.
Route Calculation
Once a given node has a copy of the LSP from every other node, it is able to compute
a complete map for the topology of the network, and from this map it is able to decide
the best route to each destination. The question, then, is exactly how it calculates routes
from this information. The solution is based on a well-known algorithm from graph
theory—Dijkstra’s shortest-path algorithm.
We first define Dijkstra’s algorithm in graph-theoretic terms. Imagine that a node
takes all the LSPs it has received and constructs a graphical representation of the network,
in which N denotes the set of nodes in the graph, and l(i, j) denotes the nonnegative
cost (weight) associated with the edge between nodes i, j ∈ N , and l(i, j) = ∞ if no
edge connects i and j. In the following description, we let s ∈ N denote this node, that
is, the node executing the algorithm to find the shortest path to all the other nodes in N .
Also, the algorithm maintains the following two variables: M denotes the set of nodes
incorporated so far by the algorithm, and C(n) denotes the cost of the path from s to
each node n. Given these definitions, the algorithm is defined as follows:
M = {s}
for each n in N − {s}
C(n) = l(s, n)
while (N = M )
M = M ∪ {w} such that C(w) is the minimum for all w in (N − M )
for each n in (N − M )
C(n) = MIN(C(n), C(w) + l(w, n))
Basically, the algorithm works as follows. We start with M containing this node s
and then initialize the table of costs (the C(n)s) to other nodes using the known costs
to directly connected nodes. We then look for the node that is reachable at the lowest
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281
cost (w) and add it to M . Finally, we update the table of costs by considering the cost
of reaching nodes through w. In the last line of the algorithm, we choose a new route to
node n that goes through node w if the total cost of going from the source to w and then
following the link from w to n is less than the old route we had to n. This procedure is
repeated until all nodes are incorporated in M .
In practice, each switch computes its routing table directly from the LSPs it has
collected using a realization of Dijkstra’s algorithm called the forward search algorithm.
Specifically, each switch maintains two lists, known as Tentative and Confirmed.
Each of these lists contains a set of entries of the form (Destination, Cost, NextHop).
The algorithm works as follows:
1
Initialize the Confirmed list with an entry for myself; this entry has a cost of
0.
2
For the node just added to the Confirmed list in the previous step, call it node
Next, select its LSP.
3
For each neighbor (Neighbor) of Next, calculate the cost (Cost) to reach
this Neighbor as the sum of the cost from myself to Next and from Next to
Neighbor.
(a) If Neighbor is currently not on either the Confirmed or the Tentative
list, then add (Neighbor, Cost, NextHop) to the Tentative list, where
NextHop is the direction I go to reach Next.
(b) If Neighbor is currently on the Tentative list, and the Cost is less than
the currently listed cost for Neighbor, then replace the current entry with
(Neighbor, Cost, NextHop), where NextHop is the direction I go to
reach Next.
4
If the Tentative list is empty, stop. Otherwise, pick the entry from the Tentative list with the lowest cost, move it to the Confirmed list, and return to
step 2.
This will become a lot easier to understand when we look at an example. Consider the network depicted in Figure 4.18. Note that, unlike our previous example, this
network has a range of different edge costs. Table 4.9 traces the steps for building the
routing table for node D. We denote the two outputs of D by using the names of the
nodes to which they connect, B and C. Note the way the algorithm seems to head off on
false leads (like the 11-unit cost path to B that was the first addition to the Tentative
list) but ends up with the least-cost paths to all nodes.
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Figure 4.18
Link-state routing: an example network.
Step
Confirmed
1
(D,0,-)
2
(D,0,-)
(B,11,B)
(C,2,C)
D’s LSP says we can reach B through B at cost 11, which
is better than anything else on either list, so put it on Tentative list; same for C.
3
(D,0,-)
(C,2,C)
(B,11,B)
Put lowest-cost member of Tentative (C) onto Confirmed list. Next, examine LSP of newly confirmed
member (C).
4
(D,0,-)
(C,2,C)
(B,5,C)
(A,12,C)
Cost to reach B through C is 5, so replace (B,11,B). C’s
LSP tells us that we can reach A at cost 12.
5
(D,0,-)
(C,2,C)
(B,5,C)
(A,12,C)
Move lowest-cost member of Tentative (B) to Confirmed, then look at its LSP.
6
(D,0,-)
(C,2,C)
(B,5,C)
(A,10,C)
Since we can reach A at cost 5 through B, replace the Tentative entry.
7
(D,0,-)
(C,2,C)
(B,5,C)
(A,10,C)
Table 4.9
Tentative
Comments
Since D is the only new member of the confirmed list,
look at its LSP.
Move lowest-cost member of Tentative (A) to Confirmed, and we are all done.
Steps for building routing table for node D (Figure 4.18).
The link-state routing algorithm has many nice properties: It has been proven to
stabilize quickly, it does not generate much traffic, and it responds rapidly to topology
changes or node failures. On the downside, the amount of information stored at each
node (one LSP for every other node in the network) can be quite large. This is one of the
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283
▲
fundamental problems of routing and is an instance of the more general problem of scalability. Some solutions to both the specific problem (the amount of storage potentially
required at each node) and the general problem (scalability) will be discussed in the next
section.
Thus, the difference between the distance-vector and link-state algorithms can be
summarized as follows. In distance vector, each node talks only to its directly connected
neighbors, but it tells them everything it has learned (i.e., distance to all nodes). In link
state, each node talks to all other nodes, but it tells them only what it knows for sure
(i.e., only the state of its directly connected links).
Open Shortest Path First Protocol (OSPF)
One of the most widely used link-state routing protocols is OSPF. The first word,
“Open,” refers to the fact that it is an open, nonproprietary standard, created under
the auspices of the IETF. The “SPF” part comes from an alternative name for link-state
routing. OSPF adds quite a number of features to the basic link-state algorithm described
above, including the following:
■
Authentication of routing messages: This is a nice feature, since it is all too
common for some misconfigured host to decide that it can reach every host in
the universe at a cost of 0. When the host advertises this fact, every router in the
surrounding neighborhood updates its forwarding tables to point to that host,
and said host receives a vast amount of data that, in reality, it has no idea what
to do with. It typically drops it all, bringing the network to a halt. Such disasters
can be averted in many cases by requiring routing updates to be authenticated.
Early versions of OSPF used a simple 8-byte password for authentication. This
is not a strong enough form of authentication to prevent dedicated malicious
users, but it alleviates many problems caused by misconfiguration. (A similar
form of authentication was added to RIP in version 2.) Strong cryptographic
authentication of the sort discussed in Section 8.3 was later added.
■
Additional hierarchy: Hierarchy is one of the fundamental tools used to make
systems more scalable. OSPF introduces another layer of hierarchy into routing
by allowing a domain to be partitioned into areas. This means that a router
within a domain does not necessarily need to know how to reach every network
within that domain—it may be able to get by knowing only how to get to
the right area. Thus, there is a reduction in the amount of information that
must be transmitted to and stored in each node. We examine areas in detail in
Section 4.3.4.
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Figure 4.19
■
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OSPF header format.
Load balancing: OSPF allows multiple routes to the same place to be assigned
the same cost and will cause traffic to be distributed evenly over those routes.
There are several different types of OSPF messages, but all begin with the same
header, as shown in Figure 4.19. The Version field is currently set to 2, and the Type
field may take the values 1 through 5. The SourceAddr identifies the sender of the
message, and the AreaId is a 32-bit identifier of the area in which the node is located.
The entire packet, except the authentication data, is protected by a 16-bit checksum
using the same algorithm as the IP header (see Section 2.4). The Authentication type
is 0 if no authentication is used; otherwise it may be 1, implying a simple password is
used, or 2, which indicates that a cryptographic authentication checksum, of the sort
described in Section 8.3, is used. In the latter cases the Authentication field carries the
password or cryptographic checksum.
Of the five OSPF message types, type 1 is the “hello” message, which a router
sends to its peers to notify them that it is still alive and connected as described above.
The remaining types are used to request, send, and acknowledge the receipt of link-state
messages. The basic building block of link-state messages in OSPF is known as the linkstate advertisement (LSA). One message may contain many LSAs. We provide a few
details of the LSA here.
Like any internetwork routing protocol, OSPF must provide information about
how to reach networks. Thus, OSPF must provide a little more information than the
simple graph-based protocol described above. Specifically, a router running OSPF may
generate link-state packets that advertise one or more of the networks that are directly
connected to that router. In addition, a router that is connected to another router by
some link must advertise the cost of reaching that router over the link. These two types
of advertisements are necessary to enable all the routers in a domain to determine the cost
of reaching all networks in that domain and the appropriate next hop for each network.
4.2 Routing
Figure 4.20
285
OSPF link-state advertisement.
Figure 4.20 shows the packet format for a type 1 link-state advertisement. Type 1
LSAs advertise the cost of links between routers. Type 2 LSAs are used to advertise networks to which the advertising router is connected, while other types are used to support
additional hierarchy as described in the next section. Many fields in the LSA should be
familiar from the preceding discussion. The LS Age is the equivalent of a time to live,
except that it counts up and the LSA expires when the age reaches a defined maximum
value. The Type field tells us that this is a type 1 LSA.
In a type 1 LSA, the Link state ID and the Advertising router field are identical. Each carries a 32-bit identifier for the router that created this LSA. While a number
of assignment strategies may be used to assign this ID, it is essential that it be unique in
the routing domain and that a given router consistently uses the same router ID. One
way to pick a router ID that meets these requirements would be to pick the lowest IP
address among all the IP addresses assigned to that router. (Recall that a router may have
a different IP address on each of its interfaces.)
The LS sequence number is used exactly as described above, to detect old or
duplicate LSAs. The LS checksum is similar to others we have seen in Section 2.4
and in other protocols; it is of course used to verify that data has not been corrupted. It
covers all fields in the packet except LS Age, so that it is not necessary to recompute
a checksum every time LS Age is incremented. Length is the length in bytes of the
complete LSA.
Now we get to the actual link-state information. This is made a little complicated
by the presence of type of service (TOS) information. Ignoring that for a moment, each
link in the LSA is represented by a Link ID, some Link Data, and a metric. The
first two of these fields identify the link; a common way to do this would be to use
the router ID of the router at the far end of the link as the Link ID, and then use the
Link Data to disambiguate among multiple parallel links if necessary. The metric is of
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course the cost of the link. Type tells us something about the link, for example, if it is a
point-to-point link.
The TOS information is present to allow OSPF to choose different routes for IP
packets based on the value in their TOS field. Instead of assigning a single metric to a
link, it is possible to assign different metrics depending on the TOS value of the data. For
example, if we had a link in our network that was very good for delay-sensitive traffic, we
could give it a low metric for the TOS value representing low delay, and a high metric for
everything else. OSPF would then pick a different shortest path for those packets that
had their TOS field set to that value. It is worth noting that, at the time of writing, this
capability has not been widely deployed.8
4.2.4 Metrics
The preceding discussion assumes that link costs, or metrics, are known when we execute
the routing algorithm. In this section, we look at some ways to calculate link costs that
have proven effective in practice. One example that we have seen already, which is quite
reasonable and very simple, is to assign a cost of 1 to all links—the least-cost route
will then be the one with the fewest hops. Such an approach has several drawbacks,
however. First, it does not distinguish between links on a latency basis. Thus, a satellite
link with 250-ms latency looks just as attractive to the routing protocol as a terrestrial
link with 1-ms latency. Second, it does not distinguish between routes on a capacity
basis, making a 9.6-Kbps link look just as good as a 45-Mbps link. Finally, it does not
distinguish between links based on their current load, making it impossible to route
around overloaded links. It turns out that this last problem is the hardest because you
are trying to capture the complex and dynamic characteristics of a link in a single scalar
cost.
The ARPANET was the testing ground for a number of different approaches to
link-cost calculation. (It was also the place where the superior stability of link-state over
distance-vector routing was demonstrated; the original mechanism used distance vector
while the later version used link state.) The following discussion traces the evolution
of the ARPANET routing metric and, in so doing, explores the subtle aspects of the
problem.
The original ARPANET routing metric measured the number of packets that were
queued waiting to be transmitted on each link, meaning that a link with 10 packets
queued waiting to be transmitted was assigned a larger cost weight than a link with 5
packets queued for transmission. Using queue length as a routing metric did not work
well, however, since queue length is an artificial measure of load—it moves packets toward the shortest queue rather than toward the destination, a situation all too familiar
8 Note also that the meaning of the TOS field has changed since the OSPF specification was written. This topic is discussed
in Section 6.5.3.
4.2 Routing
287
to those of us who hop from line to line at the grocery store. Stated more precisely, the
original ARPANET routing mechanism suffered from the fact that it did not take either
the bandwidth or the latency of the link into consideration.
A second version of the ARPANET routing algorithm, sometimes called the “new
routing mechanism,” took both link bandwidth and latency into consideration and
used delay, rather than just queue length, as a measure of load. This was done as follows. First, each incoming packet was timestamped with its time of arrival at the router
(ArrivalTime); its departure time from the router (DepartTime) was also recorded.
Second, when the link-level ACK was received from the other side, the node computed
the delay for that packet as
Delay = (DepartTime − ArrivalTime) + TransmissionTime + Latency
where TransmissionTime and Latency were statically defined for the link and
captured the link’s bandwidth and latency, respectively. Notice that in this case,
DepartTime − ArrivalTime represents the amount of time the packet was delayed
(queued) in the node due to load. If the ACK did not arrive, but instead the packet timed
out, then DepartTime was reset to the time the packet was retransmitted. In this case,
DepartTime − ArrivalTime captures the reliability of the link—the more frequent
the retransmission of packets, the less reliable the link, and the more we want to avoid it.
Finally, the weight assigned to each link was derived from the average delay experienced
by the packets recently sent over that link.
Although an improvement over the original mechanism, this approach also had a
lot of problems. Under light load, it worked reasonably well, since the two static factors
of delay dominated the cost. Under heavy load, however, a congested link would start to
advertise a very high cost. This caused all the traffic to move off that link, leaving it idle,
so then it would advertise a low cost, thereby attracting back all the traffic, and so on.
The effect of this instability was that, under heavy load, many links would in fact spend
a great deal of time being idle, which is the last thing you want under heavy load.
Another problem was that the range of link values was much too large. For example,
a heavily loaded 9.6-Kbps link could look 127 times more costly than a lightly loaded
56-Kbps link. This means that the routing algorithm would choose a path with 126 hops
of lightly loaded 56-Kbps links in preference to a 1-hop 9.6-Kbps path. While shedding
some traffic from an overloaded line is a good idea, making it look so unattractive that it
loses all its traffic is excessive. Using 126 hops when 1 hop will do is in general a bad use
of network resources. Also, satellite links were unduly penalized, so that an idle 56-Kbps
satellite link looked considerably more costly than an idle 9.6-Kbps terrestrial link, even
though the former would give better performance for high-bandwidth applications.
A third approach, called the revised ARPANET routing metric, addressed these
problems. The major changes were to compress the dynamic range of the metric
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Figure 4.21
Internetworking
Revised ARPANET routing metric versus link utilization.
considerably, to account for the link type, and to smooth the variation of the metric
with time.
The smoothing was achieved by several mechanisms. First, the delay measurement
was transformed to a link utilization, and this number was averaged with the last reported
utilization to suppress sudden changes. Second, there was a hard limit on how much the
metric could change from one measurement cycle to the next. By smoothing the changes
in the cost, the likelihood that all nodes would abandon a route at once is greatly reduced.
The compression of the dynamic range was achieved by feeding the measured utilization, the link type, and the link speed into a function that is shown graphically in
Figure 4.21. Observe the following:
■
A highly loaded link never shows a cost of more than three times its cost when
idle;
■
The most expensive link is only seven times the cost of the least expensive;
■
A high-speed satellite link is more attractive than a low-speed terrestrial link;
■
Cost is a function of link utilization only at moderate to high loads.
All these factors mean that a link is much less likely to be universally abandoned, since
a threefold increase in cost is likely to make the link unattractive for some paths while
4.2 Routing
289
letting it remain the best choice for others. The slopes, offsets, and breakpoints for the
curves in Figure 4.21 were arrived at by a great deal of trial and error, and they were
carefully tuned to provide good performance.
We end our discussion of routing metrics with a dose of reality. In the majority of
real-world network deployments at the time of writing, metrics change rarely if at all, and
only under the control of a network administrator, not automatically as was described
above. The reason for this is partly that conventional wisdom now holds that dynamically
changing metrics were too unstable, even though this probably need not be true. Perhaps
more significantly, many networks today lack the great disparity of link speeds and latencies that prevailed in the ARPANET. Thus, static metrics are the norm. One common
approach to setting metrics is to use a constant multiplied by (1/link_bandwidth).
Monitoring Routing Behavior
Given the complexity of routing packets through a network of the scale of
the Internet, we might wonder how
well the system works. We know it
works some of the time because we
are able to connect to sites all over
the world. We suspect it doesn’t work
all the time, though, because sometimes we are unable to connect to
certain sites. The real problem is determining what part of the system is
at fault when our connections fail:
Has some routing machinery failed to
work properly, is the remote server too
busy, or has some link or machine simply gone down?
This is really an issue of network
management, and while there are tools
that system administrators use to keep
tabs on their own networks—for example, see the Simple Network Management Protocol (SNMP) described
in Section 9.1.4—it is a largely unresolved problem for the Internet as a
whole. In fact, the Internet has grown
4.2.5 Routing for Mobile Hosts
Looking back over the preceding discussion of how IP addressing and routing
works, you might notice that there is an
implicit assumption about the mobility of
hosts, or rather the lack of it. A host’s address consists of a network number and a
host part, and the network number tells us
which network the host is attached to. IP
routing algorithms tell the routers how to
get packets to the correct network, thus enhancing the scalability of the routing system by keeping host-specific information
out of the routers. So what would happen if a host were disconnected from one
network and connected to another? If we
didn’t change the IP address of the host,
then it would become unreachable. Any
packet destined for this host would be sent
to the network that has the appropriate
network number, but when the router(s)
on that network tried to deliver the packet
to the host, the host would not be there to
receive it.
The obvious solution to this problem
is to provide the host with a new address
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when it attaches to a new network. Techso large and complex that, even
niques such as DHCP (described in Secthough it is constructed from a collection 4.1.6) can make this a relatively
tion of man-made, largely determinsimple process. In many situations this soistic parts, we have come to view it
lution is adequate, but in others it is not.
almost as a living organism or natFor example, suppose that a user of a PC
ural phenomenon that is to be studequipped with a wireless network interied. That is, we try to understand
face is running some application while she
the Internet’s dynamic behavior by
roams the countryside. The PC might deperforming experiments on it and
tach itself from one network and attach to
proposing models that explain our
another with some frequency, but the user
observations.
would want to be oblivious to this. In parAn excellent example of this
ticular, the applications that were running
kind of study has been conducted by
when the PC was attached to network A
Vern Paxson. Paxson used the Unix
should continue to run without interruptraceroute tool to study 40,000
tion when it attaches to network B. If the
end-to-end routes between 37 InterPC simply changes its IP address in the
net sites in 1995. He was attempting
middle of running the application, the apto answer questions about how routes
plication may not continue to function
fail, how stable routes are over time,
correctly, because the remote end has no
and whether or not they are symway of knowing that it must now send the
metric. Among other things, Paxson
packets to a new IP address.
found that the likelihood of a user enThe question of whether an applicountering a serious end-to-end routcation will continue to operate correctly
ing problem was 1 in 30, and that
following a change of IP address of one
such problems usually lasted about
endpoint is a complex one. For example,
30 seconds. He also found that twoif the application is a client-server interthirds of the Internet’s routes persisted
action (such as web browsing) and the
for days or weeks, and that about oneclient’s address changes, it is likely that
third of the time the route used to
nothing would break, except perhaps a reget from host A to host B included
sponse from the server that was partially
at least one different routing domain
complete at the time of the move. By conthan the route used to get from host B
trast, a peer-to-peer application, such as a
to host A. Paxson’s overall conclusion
voiceover IP telephone call, would quite
was that Internet routing was becomlikely fail as a result of the address change
ing less and less predictable over time.
unless some special action was taken by the
application.
Ideally, we would like all applications to keep working correctly when an endpoint
moves and for this process to be reasonably transparent to the applications. The proce-
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291
dures that are designed to address this problem are usually referred to as “Mobile IP”
(which is also the name of the IETF working group that defined them).
The Mobile IP working group made some important design decisions at the outset.
In particular, it was a requirement that the solution would work without any changes to
the software of nonmobile hosts or the majority of routers in the Internet. This sort
of approach is frequently adopted in the Internet. Any new technology that requires a
majority of routers or hosts to be modified before it can work is likely to face an uphill
battle for acceptance.
While the majority of routers remain unchanged, mobility support does require
some new functionality in at least one router, known as the home agent of the mobile
node. This router is located on the “home” network of the mobile host. The mobile host
is assumed to have a permanent IP address, called its home address, which has a network
number equal to that of the home network, and thus of the home agent. This is the
address that will be used by other hosts when they send packets to the mobile host; since
it does not change, it can be used by long-lived applications as the host roams.
In many cases, a second router with enhanced functionality, the foreign agent, is also
required. This router is located on a network to which the mobile node attaches itself
when it is away from its home network. We will consider first the operation of mobile IP
when a foreign agent is used. An example network with both home and foreign agents is
shown in Figure 4.22.
Both home and foreign agents periodically announce their presence on the networks to which they are attached using agent advertisement messages. A mobile host
may also solicit an advertisement when it attaches to a new network. The advertisement
by the home agent enables a mobile host to learn the address of its home agent before it
leaves its home network. When the mobile host attaches to a foreign network, it hears
an advertisement from a foreign agent and registers with the agent, providing the address
of its home agent. The foreign agent then contacts the home agent, providing a care-of
address. This is usually the IP address of the foreign agent.
Figure 4.22
Mobile host and mobility agents.
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At this point, we can see that any host that tries to send a packet to the mobile host
will send it with a destination address equal to the home address of that node. Normal
IP forwarding will cause that packet to arrive on the home network of the mobile node,
on which the home agent is sitting. Thus, we can divide the problem of delivering the
packet to the mobile node into three parts:
1
How does the home agent intercept a packet that is destined for the mobile
node?
2
How does the home agent then deliver the packet to the foreign agent?
3
How does the foreign agent deliver the packet to the mobile node?
The first problem might look easy if you just look at Figure 4.22, in which the
home agent is clearly the only path between the sending host and the home network,
and thus must receive packets that are destined to the mobile node. But what if the
sending node were on network 18, or what if there were another router connected to
network 18 that tried to deliver the packet without its passing through the home agent?
To address this problem, the home agent actually impersonates the mobile node, using
a technique called proxy ARP. This works just like ARP as described in Section 4.1.5,
except that the home agent inserts the IP address of the mobile node, rather than its
own, in the ARP messages. It uses its own hardware address, so that all the nodes on
the same network learn to associate the hardware address of the home agent with the
IP address of the mobile node. One subtle aspect of this process is the fact that ARP
information may be cached in other nodes on the network. To make sure that these
caches are invalidated in a timely way, the home agent issues an ARP message as soon
as the mobile node registers with a foreign agent. Because the ARP message is not a
response to a normal ARP request, it is termed a gratuitous ARP.
The second problem is the delivery of the intercepted packet to the foreign agent.
Here we use the tunneling technique described in Section 4.1.8. The home agent simply
“wraps” the packet inside an IP header that is destined for the foreign agent and transmits
it into the internetwork. All the intervening routers just see an IP packet destined for the
IP address of the foreign agent. Another way of looking at this is that an IP tunnel is
established between the home agent and the foreign agent, and the home agent just
drops packets destined for the mobile node into that tunnel.
When a packet finally arrives at the foreign agent, it strips the extra IP header and
finds inside an IP packet destined for the mobile node. Clearly the foreign agent cannot
treat this like any old IP packet because this would cause it to send it back to the home
network. Instead, it has to recognize the address as that of a registered mobile node. It
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then delivers the packet to the hardware address of the mobile node (e.g., its Ethernet
address), which was learned as part of the registration process.
One observation that can be made about these procedures is that it is possible for
the foreign agent and the mobile node to be in the same box, that is, a mobile node
can perform the foreign agent function itself. To make this work, however, the mobile
node must be able to dynamically acquire an IP address that is located in the address
space of the foreign network. This address will then be used as the care-of address. In
our example, this would have to be an address with a network number of 12. We have
already seen one way in which a host can dynamically acquire a correct IP address using
DHCP (Section 4.1.6). This approach has the desirable feature of allowing mobile nodes
to attach to networks that don’t have foreign agents; thus, mobility can be achieved with
only the addition of a home agent and some new software on the mobile node (assuming
DHCP is used on the foreign network).
What about traffic in the other direction (i.e., from mobile node to fixed node)?
This turns out to be much easier. The mobile node just puts the IP address of the fixed
node in the destination field of its IP packets, while putting its permanent address in the
source field, and the packets are forwarded to the fixed node using normal means. Of
course, if both nodes in a conversation are mobile, then the procedures described above
are used in each direction.
Route Optimization in Mobile IP
There is one significant drawback to the above approach, which may be familiar to users
of cellular telephones. The route from sending node to mobile node can be significantly
suboptimal. One of the most extreme examples is when a mobile node and the sending
node are on the same network, but the home network for the mobile node is on the far
side of the Internet. The sending node addresses all packets to the home network; they
traverse the Internet to reach the home agent, which then tunnels them back across the
Internet to reach the foreign agent. Clearly it would be nice if the sending node could
find out that the mobile node is actually on the same network and deliver the packet
directly. In the more general case, the goal is to deliver packets as directly as possible from
sending node to mobile node without passing through a home agent. This is sometimes
referred to as the triangle routing problem since the path from sender to mobile node via
home agent takes two sides of a triangle, rather than the third side that is the direct path.
The basic idea behind the solution to triangle routing is to let the sending node
know the care-of address of the mobile node. The sending node can then create its
own tunnel to the foreign agent. This is treated as an optimization of the process just
described. If the sender has been equipped with the necessary software to learn the careof address and create its own tunnel, then the route can be optimized; if not, packets just
follow the suboptimal route.
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When a home agent sees a packet destined for one of the mobile nodes that it
supports, it can deduce that the sender is not using the optimal route. Therefore, it
sends a binding update message back to the source, in addition to forwarding the data
packet to the foreign agent. The source, if capable, uses this binding update to create an
entry in a binding cache, which consists of a list of mappings from mobile node addresses
to care-of addresses. The next time this source has a data packet to send to that mobile
node, it will find the binding in the cache and can tunnel the packet directly to the
foreign agent.
There is an obvious problem with this scheme, which is that the binding cache may
become out-of-date if the mobile host moves to a new network. If an out-of-date cache
entry is used, the foreign agent will receive tunneled packets for a mobile node that is no
longer registered on its network. In this case, it sends a binding warning message back
to the sender to tell it to stop using this cache entry. This scheme works only in the case
where the foreign agent is not the mobile node itself, however. For this reason, cache
entries need to be deleted after some period of time; the exact amount is specified in the
binding update message.
Mobile routing provides some interesting security challenges. For example, an attacker wishing to intercept the packets destined to some other node in an internetwork
could contact the home agent for that node and announce itself as the new foreign agent
for the node. Thus, it is clear that some authentication mechanisms are required. We
discuss such mechanisms in Chapter 8.
Finally, we note that there are many open issues in mobile networking. The security
and performance aspects of mobile networks might require routing algorithms to take
account of several factors when finding a route to a mobile host; for example, it might be
desirable to find a route that doesn’t pass through some untrusted network. There is also
the problem of ad hoc mobile networks—enabling a group of mobile nodes to form a
network in the absence of any fixed nodes. These continue to be areas of active research.
4.2.6 Router Implementation
In Section 3.4 we saw a variety of ways to build a switch, ranging from a general-purpose
workstation with a suitable number of network interfaces to some sophisticated hardware
designs. In general, the same range of options are available for building routers, many of
which look something like Figure 4.23. The control processor is responsible for running
the routing protocols discussed above, among other things, and generally acts as the
central point of control of the router. The switching fabric transfers packets from one
port to another, just as in a switch; and the ports provide a range of functionality to
allow the router to interface to links of various types (e.g., Ethernet or SONET).
A few points are worth noting about router design and how it differs from switch
design. First, routers must be designed to handle variable-length packets, a constraint that
4.2 Routing
Figure 4.23
295
Block diagram of a router.
does not apply to ATM switches but is certainly applicable to Ethernet or Frame Relay
switches. It turns out that many high-performance routers are designed using a switching
fabric that is cell-based. In such cases the ports must be able to convert variable length
packets into cells and back again. This is very much like the standard ATM segmentation
and reassembly (SAR) problem described in Section 3.3.2.
Another consequence of the variable length of IP datagrams is that it can be harder
to characterize the performance of a router than a switch that forwards only cells. Routers
can usually forward a certain number of packets per second, and this implies that the
total throughput in bits per second depends on packet size. Router designers generally
have to make a choice as to what packet length they will support at line rate. That is, if
(pps) packets per second is the rate at which packets arriving on a particular port can be
forwarded, and linerate is the physical speed of the port in bits per second, then there
will be some packetsize in bits such that:
packetsize × pps = linerate
This is the packet size at which the router can forward at line rate; it is likely to be
able to sustain line rate for longer packets but not for shorter packets. Sometimes a
designer might decide that the right packet size to support is 40 bytes, since that is
the minimum size of an IP packet that has a TCP header attached. Another choice
might be the expected average packet size, which can be determined by studying traces
of network traffic. For example, measurements of the Internet backbone suggest that the
average IP packet is around 300 bytes long. However, such a router would fall behind
and perhaps start dropping packets when faced with a long sequence of short packets,
which is statistically likely from time to time and also very possible if the router is subject
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to an active attack (see Chapter 8). Design decisions of this type depend heavily on cost
considerations and the intended application of the router.
When it comes to the task of forwarding IP packets, routers can be broadly characterized as having either a centralized or distributed forwarding model. In the centralized
model, the IP forwarding algorithm, outlined earlier in this chapter, is done in a single
processing engine that handles the traffic from all ports. In the distributed model, there
are several processing engines, perhaps one per port, or more often one per line card
where a line card may serve one or more physical ports. Each model has advantages and
disadvantages. All things being equal, a distributed forwarding model should be able to
forward more packets per second through the router as a whole, because there is more
processing power in total. But a distributed model also complicates the software architecture, because each forwarding engine typically needs its own copy of the forwarding
table, and thus it is necessary for the control processor to ensure that the forwarding
tables are updated consistently and in a timely manner.
Another aspect of router implementation that is significantly different than that of
switches is the IP forwarding algorithm itself. In bridges and most ATM switches, the
forwarding algorithm simply involves looking up a fixed length identifier (MAC address
or VCI) in a table, finding the correct output port in the table, and sending the packet
to that port. We have already seen in Section 4.1.4 that the IP forwarding algorithm
is a little more complicated than that, in part because of the need to decide whether a
particular IP address is directly reachable out to an interface of this router or whether
the packet needs to be sent to another router. We also saw that the relevant number
of bits that need to be examined when forwarding a packet is not fixed but variable,
depending on whether the address in question is from a class A, B, or C network. As we
will see in the next section, the situation is even more complicated in today’s Internet,
where “classless” addressing is the norm, and the number of bits that must be examined
to make the forwarding decision can be anything from 1 to 32 bits.
Because of the relatively high complexity of the IP forwarding algorithm, there have
been periods of time when it seemed IP routers might be running up against fundamental upper limits of performance. However, as we discuss in the Further Reading section
of this chapter, there have been many innovative approaches to IP forwarding developed
over the years, and at the time of writing there are commercial routers that can forward
40 Gbps of IP traffic per interface. By combining many such high-performance IP forwarding engines with the sort of very scalable switch fabrics discussed in Section 3.4, it
has now become possible to build routers with many terabits of total throughput. That
is more than enough to see us through the next few years of growth in Internet traffic.
Another technology of interest in the field of router implementation is the network
processor. A network processor is intended to be a device that is just about as programmable as a standard workstation or PC processor, but that is more highly optimized for
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networking tasks. For example, a network processor might have instructions that are particularly well suited to performing lookups on IP addresses, or calculating checksums on
IP datagrams. Such devices could be used in routers and other networking devices (e.g.,
firewalls).
One of the interesting and ongoing debates about network processors is whether
they can do a better job than the alternatives. For example, given the continuous and
remarkable improvements in performance of conventional processors, and the huge industry that drives those improvements, can network processors keep up? And can a device
that strives for generality do as good a job as a custom-designed ASIC that does nothing
except, say, IP forwarding? Part of the answer to questions like these depend on what
you mean by “do a better job.” For example, there will always be trade-offs to be made
between cost of hardware, time to market, performance, and flexibility—the ability to
change the features supported by a router after it is built. We will see in the rest of this
chapter and in later chapters just how diverse the requirements for router functionality
can be. It is safe to assume that a wide range of router designs will exist for the foreseeable
future and that network processors will have some role to play.
4.3
Global Internet
At this point, we have seen how to connect a heterogeneous collection of networks to create an internetwork and how to use the simple hierarchy of the IP address to make routing in an internet somewhat scalable. We say “somewhat” scalable because even though
each router does not need to know about all the hosts connected to the internet, it does,
in the model described so far, need to know about all the networks connected to the
internet. Today’s Internet has tens of thousands of networks connected to it. Routing
protocols such as those we have just discussed do not scale to those kinds of numbers.
This section looks at a variety of techniques that greatly improve scalability and that have
enabled the Internet to grow as far as it has.
Before getting to these techniques, we need to have a general picture in our heads of
what the global Internet looks like. It is not just a random interconnection of Ethernets,
but instead it takes on a shape that reflects the fact that it interconnects many different
organizations. Figure 4.24 gives a simple depiction of the state of the Internet in 1990.
Since that time, the Internet’s topology has grown much more complex than this figure
suggests—we present a more accurate picture of the current Internet in Section 4.3.3
and Figure 4.29—but this picture will do for now.
One of the salient features of this topology is that it consists of “end user” sites
(e.g., Stanford University) that connect to “service provider” networks (e.g., BARRNET
was a provider network that served sites in the San Francisco Bay area). In 1990, many
providers served a limited geographic region and were thus known as regional networks.
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The tree structure of the Internet in 1990.
The regional networks were, in turn, connected by a nationwide backbone. In 1990,
this backbone was funded by the National Science Foundation (NSF) and was therefore called the NSFNET backbone. Although the detail is not shown in this figure, the
provider networks are typically built from a large number of point-to-point links (e.g.,
DS-3 or OC-3 links) that connect to routers; similarly, each end user site is typically not
a single network, but instead consists of multiple physical networks connected by routers
and bridges.
Notice in Figure 4.24 that each provider and end user is likely to be an administratively independent entity. This has some significant consequences on routing. For
example, it is quite likely that different providers will have different ideas about the best
routing protocol to use within their network, and on how metrics should be assigned to
links in their network. Because of this independence, each provider’s network is usually
a single autonomous system (AS). We will define this term more precisely in Section 4.3.3,
but for now it is adequate to think of an AS as a network that is administered independently of other ASs.
The fact that the Internet has a discernible structure can be used to our advantage
as we tackle the problem of scalability. In fact, we need to deal with two related scaling
issues. The first is the scalability of routing. We need to find ways to minimize the
number of network numbers that get carried around in routing protocols and stored in
the routing tables of routers. The second is address utilization, that is, making sure that
the IP address space does not get consumed too quickly.
Throughout this section we will see the principle of hierarchy used again and again
to improve scalability. We begin with subnetting, which primarily deals with address
space utilization. Next we introduce classless routing or supernetting, which tackles both
address utilization and routing scalability. We then look at how hierarchy can be used
to improve the scalability of routing, both through interdomain routing and within a
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single domain. Our final subsection looks at the emerging standards for IP version 6, the
invention of which was largely the result of scalability concerns.
4.3.1 Subnetting
The original intent of IP addresses was that the network part would uniquely identify
exactly one physical network. It turns out that this approach has a couple of drawbacks.
Imagine a large campus that has lots of internal networks and that decides to connect
to the Internet. For every network, no matter how small, the site needs at least a class C
network address. Even worse, for any network with more than 255 hosts, they need a
class B address. This may not seem like a big deal, and indeed it wasn’t when the Internet
was first envisioned, but there are only a finite number of network numbers, and there
are far fewer class B addresses than class C’s. Class B addresses tend to be in particularly
high demand because you never know if your network might expand beyond 255 nodes,
so it is easier to use a class B address from the start than to have to renumber every
host when you run out of room on a class C network. The problem we observe here is
address assignment inefficiency: A network with two nodes uses an entire class C network
address, thereby wasting 253 perfectly useful addresses; a class B network with slightly
more than 255 hosts wastes over 64,000 addresses.
Assigning one network number per physical network, therefore, uses up the IP address space potentially much faster than we would like. While we would need to connect
over 4 billion hosts to use up all the valid addresses, we only need to connect 214 (about
16,000) class B networks before that part of the address space runs out. Therefore, we
would like to find some way to use the network numbers more efficiently.
Assigning many network numbers has another drawback that becomes apparent
when you think about routing. Recall that the amount of state that is stored in a node
participating in a routing protocol is proportional to the number of other nodes, and
that routing in an internet consists of building up forwarding tables that tell a router
how to reach different networks. Thus, the more network numbers there are in use, the
bigger the forwarding tables get. Big forwarding tables add cost to routers, and they are
potentially slower to search than smaller tables for a given technology, so they degrade
router performance. This provides another motivation for assigning network numbers
carefully.
Subnetting provides an elegantly simple way to reduce the total number of network
numbers that are assigned. The idea is to take a single IP network number and allocate
the IP addresses with that network number to several physical networks, which are now
referred to as subnets. Several things need to be done to make this work. First, the subnets
should be close to each other. This is because at a distant point in the Internet, they will
all look like a single network, having only one network number between them. This
means that a router will only be able to select one route to reach any of the subnets,
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so they had better all be in the same general direction. A perfect situation in which to
use subnetting is a large campus or corporation that has many physical networks. From
outside the campus, all you need to know to reach any subnet inside the campus is where
the campus connects to the rest of the Internet. This is often at a single point, so one
entry in your forwarding table will suffice. Even if there are multiple points at which the
campus is connected to the rest of the Internet, knowing how to get to one point in the
campus network is still a good start.
The mechanism by which a single network number can be shared among multiple
networks involves configuring all the nodes on each subnet with a subnet mask. With
simple IP addresses, all hosts on the same network must have the same network number.
The subnet mask enables us to introduce a subnet number; all hosts on the same physical
network will have the same subnet number, which means that hosts may be on different
physical networks but share a single network number.
What the subnet mask effectively does is introduce another level of hierarchy into
the IP address. For example, suppose that we want to share a single class B address among
several physical networks. We could use a subnet mask of 255.255.255.0. (Subnet masks
are written down just like IP addresses; this mask is therefore all 1s in the upper 24 bits
and 0s in the lower 8 bits.) In effect, this means that the top 24 bits (where the mask has
1s) are now defined to be the network number, and the lower 8 bits (where the mask has
0s) are the host number. Since the top 16 bits identify the network in a class B address,
we may now think of the address as having not two parts but three: a network part, a
subnet part, and a host part. That is, we have divided what used to be the host part into
a subnet part and a host part. This is shown in Figure 4.25.
What subnetting means to a host is that it is now configured with both an IP
address and a subnet mask for the subnet to which it is attached. For example, host
H1 in Figure 4.26 is configured with an address of 128.96.34.15 and a subnet mask of
Figure 4.25
Subnet addressing.
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Figure 4.26
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301
An example of subnetting.
255.255.255.128. (All hosts on a given subnet are configured with the same mask; i.e.,
there is exactly one subnet mask per subnet.) The bitwise AND of these two numbers
defines the subnet number of the host and of all other hosts on the same subnet. In
this case, 128.96.34.15 AND 255.255.255.128 equals 128.96.34.0, so this is the subnet
number for the topmost subnet in the figure.
When the host wants to send a packet to a certain IP address, the first thing it
does is to perform a bitwise AND between its own subnet mask and the destination IP
address. If the result equals the subnet number of the sending host, then it knows that the
destination host is on the same subnet and the packet can be delivered directly over the
subnet. If the results are not equal, the packet needs to be sent to a router to be forwarded
to another subnet. For example, if H1 is sending to H2, then H1 ANDs its subnet mask
(255.255.255.128) with the address for H2 (128.96.34.139) to obtain 128.96.34.128.
This does not match the subnet number for H1 (128.96.34.0) so H1 knows that H2 is
on a different subnet. Since H1 cannot deliver the packet to H2 directly over the subnet,
it sends the packet to its default router R1.
Note that ARP is largely unaffected by the change in address structure. Once a host
or router figures out which node it needs to deliver a packet to on one of the networks to
which it is attached, it performs ARP to find the MAC address for that node if necessary.
The job of a router also changes when we introduce subnetting. Recall that,
for simple IP, a router has a forwarding table that consists of entries of the form
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SubnetNumber
SubnetMask
NextHop
128.96.34.0
255.255.255.128
Interface 0
128.96.34.128
255.255.255.128
Interface 1
128.96.33.0
255.255.255.0
R2
Table 4.10
Example forwarding table with subnetting for Figure 4.26.
NetworkNum, NextHop. To support subnetting, the table must now hold entries
of the form SubnetNumber, SubnetMask, NextHop. To find the right entry in
the table, the router ANDs the packet’s destination address with the SubnetMask for
each entry in turn; if the result matches the SubnetNumber of the entry, then this is
the right entry to use, and it forwards the packet to the next hop router indicated. In the
example network of Figure 4.26, router R1 would have the entries shown in Table 4.10.
Continuing with the example of a datagram from H1 being sent to H2, R1
would AND H2’s address (128.96.34.139) with the subnet mask of the first entry
(255.255.255.128) and compare the result (128.96.34.128) with the network number
for that entry (128.96.34.0). Since this is not a match, it proceeds to the next entry. This
time a match does occur, so R1 delivers the datagram to H2 using interface 1, which is
the interface connected to the same network as H2.
We can now describe the datagram forwarding algorithm in the following way:
D = destination IP address
for each forwarding table entry SubnetNumber, SubnetMask, NextHop
D1 = SubnetMask & D
if D1 = SubnetNumber
if NextHop is an interface
deliver datagram directly to destination
else
deliver datagram to NextHop (a router)
Although not shown in this example, a default router would usually be included in
the table and would be used if no explicit matches were found. We note in passing
that a naive implementation of this algorithm—one involving repeated ANDing of the
destination address with a subnet mask that may not be different every time, and a linear
table search—would be very inefficient.
A few fine points about subnetting need to be mentioned. We have already seen
that the subnet mask does not need to align with a byte boundary, with the example
mask of 255.255.255.128 (25 1s followed by 7 0s) used above. More confusingly, it is
not even necessary for all the 1s in a subnet mask to be contiguous. For example, it
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would be quite possible to use a subnet mask of 255.255.1.0. All of the mechanisms
described above should continue to work, but now you can’t look at a contiguous part
of the IP address and say, “That is the subnet number.” This makes administration more
difficult. It may also fail to work with implementations that assume that no one would
use noncontiguous masks, and so it is not recommended in practice.
We can also put multiple subnets on a single physical network. The effect of this
would be to force hosts on the same network to talk to each other through a router, which
might be useful for administrative purposes; for example, to provide isolation between
different departments sharing a LAN.
A third point to which we have alluded is that different parts of the internet see
the world differently. From outside our hypothetical campus, routers see a single network. In the example above, routers outside the campus see the collection of networks
in Figure 4.26 as just the network 128.96, and they keep one entry in their forwarding
tables to tell them how to reach it. Routers within the campus, however, need to be able
to route packets to the right subnet. Thus, not all parts of the internet see exactly the
same routing information. The next section takes a closer look at how the propagation
of routing information is done in the Internet.
The bottom line is that subnetting helps solve our scalability problems in two ways.
First, it improves our address assignment efficiency by letting us not use up an entire
class C or class B address every time we add a new physical network. Second, it helps
us aggregate information. From a reasonable distance, a complex collection of physical
networks can be made to look like a single network, so that the amount of information
that routers need to store to deliver datagrams to those networks can be reduced.
4.3.2 Classless Routing (CIDR)
Classless interdomain routing (CIDR, pronounced “cider”) is a technique that addresses
two scaling concerns in the Internet: the growth of backbone routing tables as more
and more network numbers need to be stored in them, and the potential for the 32bit IP address space to be exhausted well before the 4 billionth host is attached to the
Internet. We have already mentioned the problem that would cause this address space
exhaustion: address assignment inefficiency. The inefficiency arises because the IP address
structure, with class A, B, and C addresses, forces us to hand out network address space
in fixed-sized chunks of three very different sizes. A network with two hosts needs a
class C address, giving an address assignment efficiency of 2/255 = 0.78%; a network
with 256 hosts needs a class B address, for an efficiency of only 256/65,535 = 0.39%.
Even though subnetting can help us to assign addresses carefully, it does not get around
the fact that any autonomous system with more than 255 hosts, or an expectation of
eventually having that many, wants a class B address.
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As it turns out, exhaustion of the IP address space centers on exhaustion of the
class B network numbers. One way to deal with that would seem to be saying no to any
AS that requests a class B address unless they can show a need for something close to
64K addresses, and instead giving them an appropriate number of class C addresses to
cover the expected number of hosts. Since we would now be handing out address space
in chunks of 256 addresses at a time, we could more accurately match the amount of
address space consumed to the size of the AS. For any AS with at least 256 hosts (which
means the majority of ASs), we can guarantee an address utilization of at least 50%, and
typically much more.
This solution, however, raises a problem that is at least as serious: excessive storage
requirements at the routers. If a single AS has, say, 16 class C network numbers assigned
to it, that means every Internet backbone router needs 16 entries in its routing tables
for that AS. This is true even if the path to every one of those networks is the same.
If we had assigned a class B address to the AS, the same routing information could be
stored in one table entry. However, our address assignment efficiency would then be only
16 × 255/65,536 = 6.2%.
CIDR, therefore, tries to balance the desire to minimize the number of routes that
a router needs to know against the need to hand out addresses efficiently. To do this,
CIDR helps us to aggregate routes. That is, it lets us use a single entry in a forwarding table to tell us how to reach a lot of different networks. As you may have guessed from the
name, it does this by breaking the rigid boundaries between address classes. To understand how this works, consider our hypothetical AS with 16 class C network numbers.
Instead of handing out 16 addresses at random, we can hand out a block of contiguous class C addresses. Suppose we assign the class C network numbers from 192.4.16
through 192.4.31. Observe that the top 20 bits of all the addresses in this range are the
same (11000000 00000100 0001). Thus, what we have effectively created is a 20-bit
network number—something that is between a class B network number and a class C
number in terms of the number of hosts that it can support. In other words, we get both
the high address efficiency of handing out addresses in chunks smaller than a class B network, and a single network prefix that can be used in forwarding tables. Observe that for
this scheme to work, we need to hand out blocks of class C addresses that share a common prefix, which means that each block must contain a number of class C networks
that is a power of two.
CIDR requires a new type of notation to represent network numbers, or prefixes as
they are known, because the prefixes can be of any length. The convention is to place a
/X after the prefix where X is the prefix length in bits. So, for the example above, the 20bit prefix for all the networks 192.4.16 through 192.4.31 is represented as 192.4.16/20.
By contrast, if we wanted to represent a single class C network number, which is 24
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bits long, we would write it 192.4.16/24. Today with CIDR being the norm it is more
common to hear people talk about “slash 24” prefixes than class C networks.
All we need now to make CIDR solve our problems is a routing protocol that can
deal with these “classless” addresses, which means that it must understand that a network
number may be of any length. Modern routing protocols (such as BGP-4, described
below) do exactly that. The network numbers that are carried in such a routing protocol
are represented simply by length, value pairs, where the length gives the number of
bits in the network prefix—20 in the above example. Note that representing a network
address in this way is similar to the mask, value approach used in subnetting, as long
as masks consist of contiguous bits starting from the most significant bit. Also note that
we used subnetting to share one address among multiple physical networks, while CIDR
aims to collapse the multiple addresses that would be assigned to a single AS onto one
address. The similarity between the two approaches is reflected in the original name for
CIDR—supernetting.
In fact, the ability to aggregate routes in the way that we have just shown is only the
first step. Imagine an Internet service provider network whose primary job is to provide
Internet connectivity to a large number of corporations and campuses (customers). If we
assign prefixes to the customers in such a way that many different customer networks
connected to the provider network share a common, shorter address prefix, then we can
get even greater aggregation of routes. Consider the example in Figure 4.27. Assume that
eight customers served by the provider network have each been assigned adjacent 24-bit
Figure 4.27
Route aggregation with CIDR.
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network prefixes. Those prefixes all start with the same 21 bits. Since all of the customer
are reachable through the same provider network, it can advertise a single route to all of
them by just advertising the common 21-bit prefix they share. And it can do this even
if not all the 24-bit prefixes have been handed out, as long as the provider ultimately
will have the right to hand out those prefixes to a customer. One way to accomplish that
is to assign a portion of address space to the provider in advance, and then to let the
network provider assign addresses from that space to its customers as needed. Note that,
in contrast to this simple example, there is no need for all customer prefixes to be the
same length.
IP Forwarding Revisited
In all our discussion of IP forwarding so far, we have assumed that we could find the
network number in a packet and then look up that number in a forwarding table. However, now that we have introduced CIDR, we need to reexamine this assumption. CIDR
means that prefixes may be of any length, from 2 to 32 bits. Furthermore, it is sometimes
possible to have prefixes in the forwarding table that “overlap,” in the sense that some
addresses may match more than one prefix. For example, we might find both 171.69 (a
16-bit prefix) and 171.69.10 (a 24-bit prefix) in the forwarding table of a single router. In
this case, a packet destined to, say, 171.69.10.5, clearly matches both prefixes. The rule
in this case is based on the principle of “longest match,” that is, the packet matches the
longest prefix, which would be 171.69.10 in this example. On the other hand, a packet
destined to 171.69.20.5 would match 171.69 and not 171.69.10, and in the absence of
any other matching entry in the routing table, 171.69 would be the longest match.
The task of efficiently finding the longest match between an IP address and the
variable-length prefixes in a forwarding table has been a fruitful field of research in recent
years, and the Further Reading section of this chapter provides some references. The most
well-known algorithm uses an approach known as a PATRICIA tree, which was actually
developed well in advance of CIDR.
4.3.3 Interdomain Routing (BGP)
At the beginning of this section we introduced the notion that the Internet is organized
as autonomous systems, each of which is under the control of a single administrative
entity. A corporation’s complex internal network might be a single AS, as may the network of a single Internet service provider. Figure 4.28 shows a simple network with two
autonomous systems.
The basic idea behind autonomous systems is to provide an additional way to hierarchically aggregate routing information in a large internet, thus improving scalability.
We now divide the routing problem into two parts: routing within a single autonomous
system and routing between autonomous systems. Since another name for autonomous
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A network with two autonomous systems.
systems in the Internet is routing domains, we refer to the two parts of the routing problem as interdomain routing and intradomain routing. In addition to improving scalability, the AS model decouples the intradomain routing that takes place in one AS from that
taking place in another. Thus, each AS can run whatever intradomain routing protocols
it chooses. It can even use static routes or multiple protocols if desired. The interdomain
routing problem is then one of having different ASs share reachability information—
descriptions of the set of IP addresses that can be reached via a given AS—with each
other.
Perhaps the most important challenge of interdomain routing today is the need for
each AS to determine its own routing policies. A simple example routing policy implemented at a particular AS might look like this: Whenever possible, I prefer to send traffic
via AS X than via AS Y, but I’ll use AS Y if it is the only path, and I never want to carry
traffic from AS X to AS Y or vice versa. Such a policy would be typical when I have paid
money to both AS X and AS Y to connect my AS to the rest of the Internet, and AS X is
my preferred provider of connectivity with AS Y being the fallback. Because I view both
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AS X and AS Y as providers (and presumably I paid them to play this role), I don’t expect to help them out by carrying traffic between them across my network (this is called
transit traffic). The more ASs I connect to, the more complex policies I might have, especially when you consider backbone providers, who may interconnect with dozens of
other providers and hundreds of customers, and have different economic arrangements
(which affect routing policies) with each one.
A key design goal of interdomain routing is that policies like the example above,
and much more complex ones, should be supported by the interdomain routing system.
To make the problem harder, I need to be able to implement such a policy without any
help from other ASs, and in the face of possible misconfiguration or malicious behavior
by other ASs.
There have been two major interdomain routing protocols in the recent history of
the Internet. The first was the Exterior Gateway Protocol (EGP). EGP had a number of
limitations, perhaps the most severe of which was that it constrained the topology of the
Internet rather significantly. EGP basically forced a treelike topology onto the Internet, or
to be more precise, it was designed when the Internet had a treelike topology, such as that
illustrated in Figure 4.24. EGP did not allow for the topology to become more general.
Note that in this simple treelike structure, there is a single backbone, and autonomous
systems are connected only as parents and children and not as peers.
The replacement for EGP is the Border Gateway Protocol (BGP), which is in its
fourth version at the time of this writing (BGP-4). BGP is also known for being rather
complex. This section presents the highlights of BGP-4.
As a starting position, BGP assumes that the Internet is an arbitrarily interconnected set of ASs. This model is clearly general enough to accommodate nontreestructured internetworks, like the simplified picture of today’s multibackbone Internet
shown in Figure 4.29.9
Unlike the simple tree-structured Internet shown in Figure 4.24, today’s Internet
consists of an interconnection of multiple backbone networks (they are usually called
service provider networks, and they are operated by private companies rather than the
government), and sites are connected to each other in arbitrary ways. Some large corporations connect directly to one or more of the backbones, while others connect to smaller,
nonbackbone service providers. Many service providers exist mainly to provide service
to “consumers” (i.e., individuals with PCs in their homes), and these providers must also
connect to the backbone providers. Often many providers arrange to interconnect with
each other at a single “peering point.” In short, it is hard to discern much structure at all
in today’s Internet.
9 In an interesting stretch of metaphor, the Internet now has multiple backbones, having had only one for most of its early
life. The authors know of no other animal that has this characteristic.
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Today’s multibackbone Internet.
Given this rough sketch of the Internet, if we define local traffic as traffic that
originates at or terminates on nodes within an AS, and transit traffic as traffic that passes
through an AS, we can classify ASs into three types:
■
Stub AS: an AS that has only a single connection to one other AS; such an AS
will only carry local traffic. The small corporation in Figure 4.29 is an example
of a stub AS.
■
Multihomed AS: an AS that has connections to more than one other AS but that
refuses to carry transit traffic; for example, the large corporation at the top of
Figure 4.29.
■
Transit AS: an AS that has connections to more than one other AS and that is
designed to carry both transit and local traffic, such as the backbone providers
in Figure 4.29.
Whereas the discussion of routing in Section 4.2 focused on finding optimal paths
based on minimizing some sort of link metric, the goals of interdomain routing are rather
more complex. First, it is necessary to find some path to the intended destination that is
loop-free. Second, paths must be compliant with the policies of the various ASs along the
path—and as we have already seen, those policies might be almost arbitrarily complex.
Thus, while intradomain focuses on a well-defined problem of optimizing the scalar
cost of the path, intradomain focuses on finding the best, nonlooping, policy-compliant
path—a much more complex optimization problem.
There are additional factors that make interdomain routing hard. The first is simply a matter of scale. An Internet backbone router must be able to forward any packet
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destined anywhere in the Internet. That means having a routing table that will provide
a match for any valid IP address. While CIDR has helped to control the number of distinct prefixes that are carried in the Internet’s backbone routing, there is inevitably a lot
of routing information to pass around—on the order of 200,000 prefixes at the time of
writing.
A second challenge in interdomain routing arises from the autonomous nature of
the domains. Note that each domain may run its own interior routing protocols, and
use any scheme they choose to assign metrics to paths. This means that it is impossible
to calculate meaningful path costs for a path that crosses multiple ASs. A cost of 1,000
across one provider might imply a great path, but it might mean an unacceptably bad
one from another provider. As a result, interdomain routing advertises only reachability.
The concept of reachability is basically a statement that “you can reach this network
through this AS.” This means that for interdomain routing to pick an optimal path is
essentially impossible.
The third challenge involves the issue of trust. Provider A might be unwilling to
believe certain advertisements from provider B for fear that provider B will advertise
erroneous routing information. For example, trusting provider B when he advertises a
great route to anywhere in the Internet can be a disastrous choice if provider B turns out
to have made a mistake configuring his routers or to have insufficient capacity to carry
the traffic.
The issue of trust is closely related to the need to support complex policies as
noted above. For example, I might be willing to trust a particular provider only when he
advertises reachability to certain prefixes, and thus I would have a policy that says “use
AS X to reach only prefixes p and q, if and only if AS X advertises reachability to those
prefixes.”
When configuring BGP, the administrator of each AS picks at least one node to
be a “BGP speaker,” which is essentially a spokesperson for the entire AS. That BGP
speaker establishes BGP sessions to other BGP speakers in other ASs. These sessions are
used to exchange reachability information among ASs.
In addition to the BGP speakers, the AS has one or more border gateways, which
need not be the same as the speakers. The border gateways are the routers through which
packets enter and leave the AS. In our simple example in Figure 4.28, routers R2 and R4
would be border gateways. Note that we have avoided using the word “gateway” until
this point because it tends to be confusing. We can’t avoid it here, given the name of
the protocol we are describing. The important point to understand here is that, in the
context of interdomain routing, a border gateway is simply an IP router that is charged
with the task of forwarding packets between ASs.
BGP does not belong to either of the two main classes of routing protocols
(distance-vector and link-state protocols) described in Section 4.2. Unlike these pro-
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Example of a network running BGP.
tocols, BGP advertises complete paths as an enumerated list of ASs to reach a particular
network. This is necessary to enable the sorts of policy decisions described above to be
made in accordance with the wishes of a particular AS. It also enables routing loops to
be readily detected.
To see how this works, consider the example network in Figure 4.30. Assume that
the providers are transit networks, while the customer networks are stubs. A BGP speaker
for the AS of provider A (AS 2) would be able to advertise reachability information
for each of the network numbers assigned to customers P and Q. Thus, it would say,
in effect, “The networks 128.96, 192.4.153, 192.4.32, and 192.4.3 can be reached directly from AS 2.” The backbone network, on receiving this advertisement, can advertise,
“The networks 128.96, 192.4.153, 192.4.32, and 192.4.3 can be reached along the path
AS 1, AS 2.” Similarly, it could advertise, “The networks 192.12.69, 192.4.54, and
192.4.23 can be reached along the path AS 1, AS 3.”
An important job of BGP is to prevent the establishment of looping paths. For
example, consider three interconnected ASs, 1, 2, and 3. Suppose AS 1 learns that it
can reach network 10.0.1 through AS 2, so it advertises this fact to AS 3, who in turn
advertises it back to AS 2. AS 2 could now decide that AS 3 was the place to send packets
destined for 10.0.1; AS 3 sends them to AS 1; AS 1 sends them back to AS 2; and they
would loop forever. This is prevented by carrying the complete AS path in the routing
messages. In this case, the advertisement received by AS 2 from AS 3 would contain an
AS path of AS 3, AS 1, AS 2. AS 2 sees itself in this path, and thus concludes that this
is not a useful path for it to use.
It should be apparent that the AS numbers carried in BGP need to be unique.
For example, AS 2 can only recognize itself in the AS path in the above example if no
other AS identifies itself in the same way. AS numbers are 16-bit numbers assigned by
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a central authority to assure uniqueness. While 16 bits only allows about 65,000 ASs,
which might not seem like a lot, we note that stub ASs do not need a unique AS number,
and this covers the overwhelming majority of nonprovider networks.
We should note that a given AS will only advertise routes that it considers good
enough for itself. That is, if a BGP speaker has a choice of several different routes to a
destination, it will choose the best one according to its own local policies, and then that
will be the route it advertises. Furthermore, a BGP speaker is under no obligation to
advertise any route to a destination, even if it has one. This is how an AS can implement
a policy of not providing transit—by refusing to advertise routes to prefixes that are not
contained within that AS, even if it knows how to reach them.
In addition to advertising paths, BGP speakers need to be able to cancel previously
advertised paths if a critical link or node on a path goes down. This is done with a
form of negative advertisement known as a withdrawn route. Both positive and negative
reachability information are carried in a BGP update message, the format of which is
shown in Figure 4.31. (Note that the fields in this figure are multiples of 16 bits, unlike
other packet formats in this chapter.)
One point to note about BGP-4 is that it was designed to cope with the classless
addresses described in Section 4.3.2. This means that the “networks” that are advertised
in BGP are actually prefixes of any length. Thus, the updates contain both the prefix itself
and its length in bits. When writing these down, it is common to write prefix/length.
For example, a CIDR prefix that begins 192.4.16 and is 20 bits long would be written
as 192.4.16/20.
Figure 4.31
BGP-4 update packet format.
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A final point to note is that BGP is defined to run on top of TCP, the reliable
transport protocol described in Section 5.2. Because BGP speakers can count on TCP
to be reliable, this means that any information that has been sent from one speaker to
another does not need to be sent again. Thus, as long as nothing has changed, a BGP
speaker can simply send an occasional “keep alive” message that says, in effect “I’m still
here and nothing has changed.” If that router were to crash, it would stop sending the
keep alives, and the other routers that had learned routes from it would know that those
routes were no longer valid.
We will not delve further into the details of BGP-4, except to point out that all
the protocol does is specify how reachability information should be exchanged among
autonomous systems. BGP speakers obtain enough information by this exchange to calculate loop-free routes to all reachable networks, but how they choose the “best” routes
is largely left to the policies of the AS.
Let’s return to the real question: How does all this help us to build scalable networks? First, the number of nodes participating in BGP is on the order of the number
of ASs, which is much smaller than the number of networks. Second, finding a good
interdomain route is only a matter of finding a path to the right border router, of which
there are only a few per AS. Thus, we have neatly subdivided the routing problem into
manageable parts, once again using a new level of hierarchy to increase scalability. The
complexity of interdomain routing is now on the order of the number of ASs, and the
complexity of intradomain routing is on the order of the number of networks in a single
AS.
Integrating Interdomain and Intradomain Routing
While the preceding discussion illustrates how a BGP speaker learns interdomain routing
information, the question still remains as to how all the other routers in a domain get
this information. There are several ways this problem can be addressed.
Let’s start with a very simple situation, which is also very common. In the case of a
stub AS that only connects to other ASs at a single point, the border router is clearly the
only choice for all routes that are outside the AS. Such a router can “inject” a default route
into the intradomain routing protocol. In effect, this is a statement that any network that
has not been explicitly advertised in the intradomain protocol is reachable through the
border router. Recall from the discussion of IP forwarding in Section 4.1 that the default
entry in the forwarding table comes after all the more specific entries, and it matches
anything that failed to match a specific entry.
The next step up in complexity is to have the border routers inject specific routes
they have learned from outside the AS. Consider, for example, the border router of a
provider AS that connects to a customer AS. That router could learn that the network
prefix 192.4.54/24 is located inside the customer AS, either through BGP or because the
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information is configured into the border router. It could inject a route to that prefix into
the routing protocol running inside the provider AS. This would be an advertisement of
the sort “I have a link to 192.4.54/24 of cost X.” This would cause other routers in the
provider AS to learn that this border router is the place to send packets destined for that
prefix.
The final level of complexity comes in backbone networks, which learn so much
routing information from BGP that it becomes too costly to inject it into the intradomain protocol. For example, if a border router wants to inject 10,000 prefixes that it
learned about from another AS, it will have to send very big link-state packets to the
other routers in that AS, and their shortest-path calculations are going to become very
complex. For this reason, the routers in a backbone network use a variant of BGP called
interior BGP (iBGP) to effectively redistribute the information that is learned by the
BGP speakers at the edges of the AS to all the other routers in the AS. (The other variant
of BGP, discussed above, runs between ASs and is called exterior BGP or eBGP.) iBGP
enables any router in the AS to learn the best border router to use when sending a packet
to any address. At the same time, each router in the AS keeps track of how to get to each
border router using a conventional intradomain protocol with no injected information.
By combining these two sets of information, each router in the AS is able to determine
the appropriate next hop for all prefixes.
To see how this all works, consider the simple example network, representing a
single AS, in Figure 4.32. There are three border routers, A, D, and E, that speak eBGP
to other ASs and learn how to reach various prefixes. These three border routers communicate with other, and with the interior routers B and C, by building a mesh of
iBGP sessions among all the routers in the AS. Let’s now focus in on how router B
builds up its complete view of how to forward packets to any prefix. Now look at the
table at the top left of Figure 4.33 that shows the information that router B learns from
its iBGP sessions. It learns that some prefixes are best reached via router A, some via
D, and some via E. At the same time, all the routers in the AS are also running some
intradomain routing protocol such as RIP or OSPF. (A generic term for intradomain
protocols is IGP—interior gateway protocol.) From this completely separate protocol,
B learns how to reach other nodes inside the domain, as shown in the top right table.
For example, to reach router E, B needs to send packets toward router C. Finally, in
the bottom table, B puts the whole picture together, combining the information about
external prefixes learned from iBGP with the information about interior routes to the
border routers learned from the IGP. Thus, if a prefix like 18.0/16 is reachable via border
router E, and the best interior path to E is via C, then it follows that any packet destined
for 18.0/16 should be forwarded toward C. In this way, any router in the AS can build
up a complete routing table for any prefix that is reachable via some border router of
the AS.
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Figure 4.32 Example of interdomain and intradomain routing. All routers run iBGP and
an intradomain routing protocol. Border routers (A, D, E) also run eBGP to other ASs.
Figure 4.33
BGP routing table, IGP routing table, and combined table at router B.
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4.3.4 Routing Areas
As if we didn’t already have enough hierarchy, link-state intradomain routing protocols
provide a means to partition a routing domain into subdomains called areas. (The terminology varies somewhat among protocols—we use the OSPF terminology here.) By
adding this extra level of hierarchy, we enable single domains to grow larger without
overburdening the intradomain routing protocols.
An area is a set of routers that are administratively configured to exchange link-state
information with each other. There is one special area—the backbone area, also known
as area 0. An example of a routing domain divided into areas is shown in Figure 4.34.
Routers R1, R2, and R3 are members of the backbone area. They are also members of at
least one nonbackbone area; R1 is actually a member of both area 1 and area 2. A router
that is a member of both the backbone area and a nonbackbone area is an area border
router (ABR). Note that these are distinct from the routers that are at the edge of an AS,
which are referred to as AS border routers for clarity.
Routing within a single area is exactly as described in Section 4.2.3. All the routers
in the area send link-state advertisements to each other, and thus develop a complete,
consistent map of the area. However, the link-state advertisements of routers that are not
area border routers do not leave the area in which they originated. This has the effect
of making the flooding and route calculation processes considerably more scalable. For
example, router R4 in area 3 will never see a link-state advertisement from router R8 in
area 1. As a consequence, it will know nothing about the detailed topology of areas other
than its own.
Figure 4.34
A domain divided into areas.
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How, then, does a router in one area determine the right next hop for a packet
destined to a network in another area? The answer to this becomes clear if we imagine
the path of a packet that has to travel from one nonbackbone area to another as being
split into three parts. First, it travels from its source network to the backbone area, then it
crosses the backbone, then it travels from backbone to destination network. To make this
work, the area border routers summarize routing information that they have learned from
one area and make it available in their advertisements to other areas. For example, R1
receives link-state advertisements from all the routers in area 1 and can thus determine
the cost of reaching any network in area 1. When R1 sends link-state advertisements
into area 0, it advertises the costs of reaching the networks in area 1 much as if all those
networks were directly connected to R1. This enables all the area 0 routers to learn
the cost to reach all networks in area 1. The area border routers then summarize this
information and advertise it into the nonbackbone areas. Thus, all routers learn how to
reach all networks in the domain.
Note that in the case of area 2, there are two ABRs, and that routers in area 2 will
thus have to make a choice as to which one they use to reach the backbone. This is easy
enough, since both R1 and R2 will be advertising costs to various networks, so that it will
become clear which is the better choice as the routers in area 2 run their shortest-path
algorithm. For example, it is pretty clear that R1 is going to be a better choice than R2
for destinations in area 1.
When dividing a domain into areas, the network administrator makes a trade-off
between scalability and optimality of routing. The use of areas forces all packets traveling
from one area to another to go via the backbone area, even if a shorter path might have
been available. For example, even if R4 and R5 were directly connected, packets would
not flow between them because they are in different nonbackbone areas. It turns out that
the need for scalability is often more important than the need to use the absolute shortest
path.
This illustrates an important principle in network design. There is frequently a
trade-off between some sort of optimality and scalability. When hierarchy is introduced,
information is hidden from some nodes in the network, hindering their ability to make
perfectly optimal decisions. However, information hiding is essential to scalability, since
it saves all nodes from having global knowledge. It is invariably true in large networks
that scalability is a more pressing design goal than perfect optimality.
Finally, we note that there is a trick by which network administrators can more
flexibly decide which routers go in area 0. This trick uses the idea of a virtual link between routers. Such a virtual link is obtained by configuring a router that is not directly
connected to area 0 to exchange backbone routing information with a router that is.
For example, a virtual link could be configured from R8 to R1, thus making R8 part of
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the backbone. R8 would now participate in link-state advertisement flooding with the
other routers in area 0. The cost of the virtual link from R8 to R1 is determined by the
exchange of routing information that takes place in area 1. This technique can help to
improve the optimality of routing.
4.3.5 IP Version 6 (IPv6)
In many respects, the motivation for a new version of IP is the same as the motivation
for the techniques described so far in this section: to deal with scaling problems caused
by the Internet’s massive growth. Subnetting and CIDR have helped to contain the rate
at which the Internet address space is being consumed (the address depletion problem)
and have also helped to control the growth of routing table information needed in the
Internet’s routers (the routing information problem). However, there will come a point
at which these techniques are no longer adequate. In particular, it is virtually impossible
to achieve 100% address utilization efficiency, so the address space will be exhausted
well before the 4 billionth host is connected to the Internet. Even if we were able to
use all 4 billion addresses, it’s not too hard to imagine ways that that number could be
exhausted, such as the assignment of IP addresses to mobile phones, televisions, or other
household appliances. All of these possibilities argue that a bigger address space than that
provided by 32 bits will eventually be needed.
Historical Perspective
The IETF began looking at the problem of expanding the IP address space in 1991,
and several alternatives were proposed. Since the IP address is carried in the header of
every IP packet, increasing the size of the address dictates a change in the packet header.
This means a new version of the Internet Protocol, and as a consequence, a need for new
software for every host and router in the Internet. This is clearly not a trivial matter—it
is a major change that needs to be thought about very carefully.
The effort to define a new version of IP was known as IP Next Generation, or
IPng. As the work progressed, an official IP version number was assigned, so IPng is now
known as IPv6. Note that the version of IP discussed so far in this chapter is version 4
(IPv4). The apparent discontinuity in numbering is the result of version number 5 being
used for an experimental protocol some years ago.
The significance of the change to a new version of IP caused a snowball effect. The
general feeling among network designers was that if you are going to make a change of
this magnitude, you might as well fix as many other things in IP as possible at the same
time. Consequently, the IETF solicited white papers from anyone who cared to write
one, asking for input on the features that might be desired in a new version of IP. In
addition to the need to accommodate scalable routing and addressing, some of the other
wish list items for IPng were
4.3
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■
Support for real-time services;
■
Security support;
■
Autoconfiguration (i.e., the ability of hosts to automatically configure themselves with such information as their own IP address and domain name);
■
Enhanced routing functionality, including support for mobile hosts.
It is interesting to note that while many of these features were absent from IPv4 at
the time IPv6 was being designed, support for all of them has made its way into IPv4 in
recent years, often using similar techniques in both protocols. It can be argued that the
freedom to think of IPv6 as a clean slate facilitated the design of new capabilities for IP
that were then retrofitted into IPv4.
In addition to the wish list, one absolutely nonnegotiable feature for IPng was that
there must be a transition plan to move from the current version of IP (version 4) to
the new version. With the Internet being so large and having no centralized control, it
would be completely impossible to have a flag day on which everyone shut down their
hosts and routers and installed a new version of IP. Thus, there will probably be a long
transition period in which some hosts and routers will run IPv4 only, some will run IPv4
and IPv6, and some will run IPv6 only.
The IETF appointed a committee called the IPng Directorate to collect all the
inputs on IPng requirements and to evaluate proposals for a protocol to become IPng.
Over the life of this committee there were a number of proposals, some of which merged
with other proposals, and eventually one was chosen by the Directorate to be the basis
for IPng. That proposal was called Simple Internet Protocol Plus (SIPP). SIPP originally
called for a doubling of the IP address size to 64 bits. When the Directorate selected
SIPP, they stipulated several changes, one of which was another doubling of the address
to 128 bits (16 bytes). It was around this time that the version number 6 was assigned.
The rest of this section describes some of the main features of IPv6. At the time of this
writing, most of the key specifications for IPv6 are Proposed or Draft Standards in the
IETF.
Addresses and Routing
First and foremost, IPv6 provides a 128-bit address space, as opposed to the 32 bits
of version 4. Thus, while version 4 can potentially address 4 billion nodes if address
assignment efficiency reaches 100%, IPv6 can address 3.4 × 1038 nodes, again assuming
100% efficiency. As we have seen, though, 100% efficiency in address assignment is not
likely. Some analysis of other addressing schemes, such as those of the French and U.S.
telephone networks, as well as that of IPv4, have turned up some empirical numbers for
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address assignment efficiency. Based on the most pessimistic estimates of efficiency drawn
from this study, the IPv6 address space is predicted to provide over 1,500 addresses per
square foot of the earth’s surface, which certainly seems like it should serve us well even
when toasters on Venus have IP addresses.
Address Space Allocation
Drawing on the effectiveness of CIDR in IPv4, IPv6 addresses are also classless, but the
address space is still subdivided in various ways based on the leading bits. Rather than
specifying different address classes, the leading bits specify different uses of the IPv6
address. The current assignment of prefixes is listed in Table 4.11.
This allocation of the address space warrants a little discussion. First, the entire
functionality of IPv4’s three main address classes (A, B, and C) is contained inside the
“everything else” range. Global unicast addresses, as we will see shortly, are a lot like
classless IPv4 addresses, only much longer. These are the main ones of interest at this
point, with over 99% of the total IPv6 address space available to this important form
of address. (At the time of writing, IPv6 unicast addresses are being allocated from the
block that begins 001, with the remaining address space—about 87%—being reserved
for future use.)
The multicast address space is (obviously) for multicast, thereby serving the same
role as class D addresses in IPv4. Note that multicast addresses are easy to distinguish—
they start with a byte of all 1s. We will see how these addresses are used in Section 4.4.
The idea behind link local use addresses is to enable a host to construct an address
that will work on the network to which it is connected without being concerned about
global uniqueness of the address. This may be useful for autoconfiguration, as we will
see below. Similarly, the site local use addresses are intended to allow valid addresses to
be constructed on a site (e.g., a private corporate network) that is not connected to the
larger Internet; again, global uniqueness need not be an issue.
Prefix
Use
00. . . 0 (128 bits)
Unspecified
00. . . 1 (128 bits)
Loopback
1111 1111
Multicast addresses
1111 1110 10
Link local unicast
1111 1110 11
Site local unicast
Everything else
Global unicast
Table 4.11
Address prefix assignments for IPv6.
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Within the global unicast address space are some important special types of addresses. A node may be assigned an IPv4-compatible IPv6 address by zero-extending a
32-bit IPv4 address to 128 bits. A node that is only capable of understanding IPv4 can be
assigned an IPv4-mapped IPv6 address by prefixing the 32-bit IPv4 address with 2 bytes
of all 1s and then zero-extending the result to 128 bits. These two special address types
have uses in the IPv4-to-IPv6 transition (see the sidebar on this topic).
Address Notation
Just as with IPv4, there is some special notation for writing down IPv6 addresses. The
standard representation is x:x:x:x:x:x:x:x where each “x” is a hexadecimal representation of a 16-bit piece of the address. An example would be
47CD:1234:4422:ACO2:0022:1234:A456:0124
Any IPv6 address can be written using this notation. Since there are a few special types
of IPv6 addresses, there are some special notations that may be helpful in certain circumstances. For example, an address with a large number of contiguous 0s can be written
more compactly by omitting all the 0 fields. Thus,
47CD:0000:0000:0000:0000:0000:A456:0124
could be written
47CD::A456:0124
Clearly, this form of shorthand can only be used for one set of contiguous 0s in an address
to avoid ambiguity.
Since there are two types of IPv6 addresses that contain an embedded IPv4 address,
these have their own special notation that makes extraction of the IPv4 address easier. For
example, the IPv4-mapped IPv6 address of a host whose IPv4 address was 128.96.33.81
could be written as
::FFFF:128.96.33.81
That is, the last 32 bits are written in IPv4 notation, rather than as a pair of hexadecimal
numbers separated by a colon. Note that the double colon at the front indicates the
leading 0s.
Global Unicast Addresses
By far the most important sort of addressing that IPv6 must provide is plain old unicast
addressing. It must do this in a way that supports the rapid rate of addition of new
hosts to the Internet and that allows routing to be done in a scalable way as the number
of physical networks in the Internet grows. Thus, at the heart of IPv6 is the unicast
address allocation plan that determines how unicast addresses will be assigned to service
providers, autonomous systems, networks, hosts, and routers.
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In fact, the address allocation plan that is proposed for IPv6 unicast addresses is
extremely similar to that being deployed with CIDR in IPv4. To understand how it
works and how it provides scalability, it is helpful to define some new terms. We may
think of a nontransit AS (i.e., a stub or multihomed AS) as a subscriber, and we may think
of a transit AS as a provider. Furthermore, we may subdivide providers into direct and
indirect. The former are directly connected to subscribers. The latter primarily connect
other providers, are not connected directly to subscribers, and are often known
Transition from IPv4 to IPv6
as backbone networks.
The most important idea behind the
With this set of definitions, we can
transition from IPv4 to IPv6 is that
see that the Internet is not just an arbithe Internet is far too big and detrarily interconnected set of ASs; it has
centralized to have a flag day—one
some intrinsic hierarchy. The difficulty is
specified day on which every host and
in making use of this hierarchy without
router is upgraded from IPv4 to IPv6.
inventing mechanisms that fail when the
Thus, IPv6 needs to be deployed inhierarchy is not strictly observed, as hapcrementally in such a way that hosts
pened with EGP. For example, the distincand routers that only understand IPv4
tion between direct and indirect providers
can continue to function for as long
becomes blurred when a subscriber conas possible. Ideally, IPv4 nodes should
nects to a backbone or when a direct
be able to talk to other IPv4 nodes
provider starts connecting to many other
and some set of other IPv6-capable
providers.
nodes indefinitely. Also, IPv6 hosts
As with CIDR, the goal of the IPv6
should be capable of talking to other
address allocation plan is to provide agIPv6 nodes even when some of the ingregation of routing information to refrastructure between them may only
duce the burden on intradomain routers.
support IPv4. Two major mechanisms
Again, the key idea is to use an address
have been defined to help this transiprefix—a set of contiguous bits at the
tion: dual-stack operation and tunnelmost significant end of the address—to
ing.
aggregate reachability information to a
The idea of dual stacks is fairly
large number of networks and even to a
straightforward: IPv6 nodes run both
large number of ASs. The main way to
IPv6 and IPv4 and use the Version
achieve this is to assign an address prefix
field to decide which stack should
to a direct provider and then for that diprocess an arriving packet. In this case,
rect provider to assign longer prefixes that
the IPv6 address could be unrelated
begin with that prefix to its subscribers.
to the IPv4 address, or it could be
This is exactly what we observed in Figthe “IPv4-mapped IPv6 address” deure 4.27. Thus, a provider can advertise
scribed earlier in this section.
a single prefix for all of its subscribers.
4.3
Global Internet
323
Of course, the drawback is that if a site decides to change providers, it will need
to obtain a new address prefix and renumber all the nodes in the site. This could be a
colossal undertaking, enough to dissuade most people from ever changing providers. For
this reason, there is ongoing research on other addressing schemes, such as geographic
addressing, in which a site’s address is a function of its location rather than the provider
to which it attaches. At present, however,
The basic tunneling technique,
provider-based addressing is necessary to
in which an IP packet is sent as the
make routing work efficiently.
payload of another IP packet, was deNote that while IPv6 address assignscribed in Section 4.1. For IPv6 tranment is essentially equivalent to the way
sition, tunneling is used to send an
address assignment has happened in IPv4
IPv6 packet over a piece of the netsince the introduction of CIDR, IPv6 has
work that only understands IPv4. This
the significant advantage of not having a
means that the IPv6 packet is encaplarge installed base of assigned addresses to
sulated within an IPv4 header that has
fit into its plans.
the address of the tunnel endpoint in
One question is whether it makes
its header, is transmitted across the
sense for hierarchical aggregation to take
IPv4-only piece of network, and then
place at other levels in the hierarchy. For
is decapsulated at the endpoint. The
example, should all providers obtain their
endpoint could be either a router or a
address prefixes from within a prefix alhost; in either case, it must be IPv6located to the backbone to which they
capable to be able to process the IPv6
connect? Given that most providers conpacket after decapsulation. If the endnect to multiple backbones, this probably
point is a host with an IPv4-mapped
doesn’t make sense. Also, since the numIPv6 address, then tunneling can be
ber of providers is much smaller than the
done automatically, by extracting the
number of sites, the benefits of aggregating
IPv4 address from the IPv6 address
at this level are much less.
and using it to form the IPv4 header.
One place where aggregation may
Otherwise, the tunnel must be configmake sense is at the national or contiured manually. In this case, the encapnental level. Continental boundaries form
sulating node needs to know the IPv4
natural divisions in the Internet topology,
address of the other end of the tunand if all addresses in Europe, for example,
nel, since it cannot be extracted from
had a common prefix, then a great deal of
the IPv6 header. From the perspective
aggregation could be done, so that most
of IPv6, the other end of the tunnel
routers in other continents would only
looks like a regular IPv6 node that is
need one routing table entry for all netjust one hop away, even though there
works with the Europe prefix. Providers in
may be many hops of IPv4 infrastrucEurope would all select their prefixes such
ture between the tunnel endpoints.
that they began with the European prefix.
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4
Figure 4.35
Internetworking
An IPv6 provider-based unicast address.
Using this scheme, an IPv6 address might look like Figure 4.35. The RegistryID might
be an identifier assigned to a European address registry, with different IDs assigned to
other continents or countries. Note that prefixes would be of different lengths under this
scenario. For example, a provider with few customers could have a longer prefix (and
thus less total address space available) than one with many customers.
One tricky situation could occur when a subscriber is connected to more than one
provider. Which prefix should the subscriber use for his site? There is no perfect solution
to the problem. For example, suppose a subscriber is connected to two providers X and Y.
If the subscriber takes his prefix from X, then Y has to advertise a prefix that has no
relationship to its other subscribers and that as a consequence cannot be aggregated. If
the subscriber numbers part of his AS with the prefix of X and part with the prefix of Y, he
runs the risk of having half his site become unreachable if the connection to one provider
goes down. One solution that works fairly well if X and Y have a lot of subscribers
in common is for them to have three prefixes between them: one for subscribers of X
only, one for subscribers of Y only, and one for the sites that are subscribers of both
X and Y.
Packet Format
Despite the fact that IPv6 extends IPv4 in several ways, its header format is actually
simpler. This simplicity is due to a concerted effort to remove unnecessary functionality
from the protocol. Figure 4.36 shows the result. (For comparison with IPv4, see the
header format shown in Figure 4.3.)
As with many headers, this one starts with a Version field, which is set to 6 for
IPv6. The Version field is in the same place relative to the start of the header as IPv4’s
Version field so that header-processing software can immediately decide which header
format to look for. The TrafficClass and FlowLabel fields both relate to quality of
service issues, as discussed in Section 6.5.
The PayloadLen field gives the length of the packet, excluding the IPv6 header,
measured in bytes. The NextHeader field cleverly replaces both the IP options and
the Protocol field of IPv4. If options are required, then they are carried in one or
more special headers following the IP header, and this is indicated by the value of the
NextHeader field. If there are no special headers, the NextHeader field is the demux
4.3
Figure 4.36
Global Internet
325
IPv6 packet header.
key identifying the higher-level protocol running over IP (e.g., TCP or UDP), that is, it
serves the same purpose as the IPv4 Protocol field. Also, fragmentation is now handled
as an optional header, which means that the fragmentation-related fields of IPv4 are not
included in the IPv6 header. The HopLimit field is simply the TTL of IPv4, renamed
to reflect the way it is actually used.
Finally, the bulk of the header is taken up with the source and destination addresses,
each of which is 16 bytes (128 bits) long. Thus, the IPv6 header is always 40 bytes long.
Considering that IPv6 addresses are four times longer than those of IPv4, this compares
quite well with the IPv4 header, which is 20 bytes long in the absence of options.
The way that IPv6 handles options is quite an improvement over IPv4. In IPv4,
if any options were present, every router had to parse the entire options field to see
if any of the options were relevant. This is because the options were all buried at the
end of the IP header, as an unordered collection of type, length, value tuples.
In contrast, IPv6 treats options as extension headers that must, if present, appear in a
specific order. This means that each router can quickly determine if any of the options
are relevant to it; in most cases, they will not be. Usually this can be determined by
just looking at the NextHeader field. The end result is that option processing is much
more efficient in IPv6, which is an important factor in router performance. In addition,
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4
Figure 4.37
Internetworking
IPv6 fragmentation extension header.
the new formatting of options as extension headers means that they can be of arbitrary
length, whereas in IPv4 they were limited to 44 bytes at most. We will see how some of
the options are used below.
Each option has its own type of extension header. The type of each extension
header is identified by the value of the NextHeader field in the header that precedes
it, and each extension header contains a NextHeader field to identify the header following it. The last extension header will be followed by a transport-layer header (e.g.,
TCP) and in this case the value of the NextHeader field is the same as the value of the
Protocol field would be in an IPv4 header. Thus, the NextHeader field does double
duty; it may either identify the type of extension header to follow, or, in the last extension
header, it serves as a demux key to identify the higher-layer protocol running over IPv6.
Consider the example of the fragmentation header, shown in Figure 4.37. This
header provides functionality similar to the fragmentation fields in the IPv4 header described in Section 4.1.2, but it is only present if fragmentation is necessary. Assuming it is
the only extension header present, then the NextHeader field of the IPv6 header would
contain the value 44, which is the value assigned to indicate the fragmentation header.
The NextHeader field of the fragmentation header itself contains a value describing
the header that follows it. Again, assuming no other extension headers are present, then
the next header might be the TCP header, which results in NextHeader containing the
value 6, just as the Protocol field would in IPv4. If the fragmentation header were followed by, say, an authentication header, then the fragmentation header’s NextHeader
field would contain the value 51.
Autoconfiguration
While the Internet’s growth has been impressive, one factor that has inhibited faster acceptance of the technology is the fact that getting connected to the Internet has typically
required a fair amount of system administration expertise. In particular, every host that
is connected to the Internet needs to be configured with a certain minimum amount
of information, such as a valid IP address, a subnet mask for the link to which it attaches, and the address of a name server. Thus, it has not been possible to unpack a
new computer and connect it to the Internet without some preconfiguration. One goal
4.3
Global Internet
327
of IPv6, therefore, is to provide support for autoconfiguration, sometimes referred to as
“plug-and-play” operation.
As we saw in Section 4.1.6, autoconfiguration is possible for IPv4, but it depends
on the existence of a server that is configured to hand out addresses and other configuration information to DHCP clients. The longer address format in IPv6 helps provide a
useful, new form of autoconfiguration called stateless autoconfiguration, which does not
require a server.
Recall that IPv6 unicast addresses are hierarchical, and that the least significant
portion is the interface ID. Thus, we can subdivide the autoconfiguration problem into
two parts:
1
Obtain an interface ID that is unique on the link to which the host is attached;
2
Obtain the correct address prefix for this subnet.
Network Address Translation
While IPv6 was motivated by a concern that increased usage of IP would
lead to exhaustion of the address
space, another technology has become
popular as a way to conserve IP address space. That technology is network address translation (NAT), and
it is possible that its widespread use
will significantly delay the need to deploy IPv6. NAT is often viewed as “architecturally impure,” but it is also a
fact of networking life that cannot be
ignored.
The basic idea behind NAT is
that all the hosts that might communicate with each other over the
Internet do not need to have globally unique addresses. Instead,
a host could be assigned a “private
address” that is not necessarily
globally unique, but is unique
within some more limited scope;
for example, within the corporate
network where the host resides.
The first part turns out to be rather
easy, since every host on a link must have a
unique link-level address. For example, all
hosts on an Ethernet have a unique 48-bit
Ethernet address. This can be turned into
a valid link local use address by adding the
appropriate prefix from Table 4.11 (1111
1110 10) followed by enough 0s to make
up 128 bits. For some devices—for example, printers or hosts on a small routerless
network that do not connect to any other
networks—this address may be perfectly
adequate. Those devices that need a globally valid address depend on a router on
the same link to periodically advertise the
appropriate prefix for the link. Clearly, this
requires that the router be configured with
the correct address prefix, and that this
prefix be chosen in such a way that there
is enough space at the end (e.g., 48 bits) to
attach an appropriate link-level address.
The ability to embed link-level addresses as long as 48 bits into IPv6 addresses was one of the reasons for choosing
such a large address size. Not only does
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128 bits allow the embedding, but it
leaves plenty of space for the multilevel
hierarchy of addressing that we discussed
above.
Advanced Routing Capabilities
Another of IPv6’s extension headers is
the routing header. In the absence of this
header, routing for IPv6 differs very little
from that of IPv4 under CIDR. The routing header contains a list of IPv6 addresses
that represent nodes or topological areas
that the packet should visit en route to its
destination. A topological area may be, for
example, a backbone provider’s network.
Specifying that packets must visit this network would be a way of implementing
provider selection on a packet-by-packet
basis. Thus, a host could say that it wants
some packets to go through a provider
that is cheap, others through a provider
that provides high reliability, and still others through a provider that the host trusts
to provide security.
To provide the ability to specify
topological entities rather than individual nodes, IPv6 defines an anycast address. An anycast address is assigned to
a set of interfaces, and packets sent to
that address will go to the “nearest” of
those interfaces, with nearest being determined by the routing protocols. For
example, all the routers of a backbone
provider could be assigned a single anycast address, which would be used in the
routing header.
The anycast address and the routing
header are also expected to be used to provide enhanced routing support to mobile
The class A network number 10 is often used for this purpose, since that
network number was assigned to the
ARPANET and is no longer in use
as a globally unique address. As long
as the host communicates only with
other hosts in the corporate network,
a locally unique address is sufficient. If
it should want to communicate with
a host outside the corporate network,
it does so via a “NAT box”—a device that is able to translate from the
private address used by the host to
some globally unique address that is
assigned to the NAT box. Since it’s
likely that a small subset of the hosts
in the corporation need the services
of the NAT box at any one time, the
NAT box might be able to get by with
a small pool of globally unique addresses, much smaller than the number of addresses that would be needed
if every host in the corporation had a
globally unique address.
So, we can imagine a NAT box
receiving IP packets from a host inside the corporation and translating
the IP source address from some private address (say, 10.0.1.5) to a globally unique address (say, 171.69.210.
246). When packets come back from
the remote host addressed to 171.69.
210.246, the NAT box translates the
destination address to 10.0.1.5 and
forwards the packet on toward the
host.
The chief drawback of NAT is
that it breaks a key assumption of the
4.4
IP service model—that all nodes have
globally unique addresses. It turns out
that lots of applications and protocols
rely on this assumption. In particular, many protocols that run over IP
(e.g., application protocols) carry IP
addresses in their messages. These addresses also need to be translated by
a NAT box if the higher-layer protocol is to work properly, and thus
NAT boxes become much more complex than simple IP header translators.
They potentially need to understand
an ever-growing number of higherlayer protocols. This in turn presents
an obstacle to deployment of new
applications.
It is probably safe to say that
networks would be better off without
NAT, but its disappearance seems unlikely. While widespread deployment
of IPv6 would probably help, NAT is
now popular for a range of other reasons beyond its original purpose. For
example, it becomes easier to switch
providers if your entire internal network has (private) IP addresses that
bear no relation to the provider’s address space. And while NAT boxes
cannot be considered a true solution
to security threats, the fact that the
addresses behind a NAT box are not
globally meaningful provides a level
of protection against simple attacks.
It will be interesting to see how NAT
fares in the future as IPv6 deployment
gathers momentum.
Multicast
329
hosts. The detailed mechanisms for providing this support are still being defined.
Other Features
As mentioned at the beginning of this
section, the primary motivation behind
the development of IPv6 was to support
the continued growth of the Internet.
Once the IP header had to be changed
for the sake of the addresses, however,
the door was open for a wide variety
of other changes, two of which we have
just described—autoconfiguration and
source-directed routing. IPv6 includes
several additional features, most of which
are covered elsewhere in this book—
mobility is discussed in Section 4.2.5, network security is the topic of Chapter 8,
and a new service model proposed for
the Internet is described in Section 6.5.
It is interesting to note that, in most of
these areas, the IPv4 and IPv6 capabilities have become virtually indistinguishable, so that the main driver for IPv6 remains the need for larger addresses.
4.4
Multicast
As we saw in Chapter 2, multiaccess networks like Ethernet and token rings implement multicast in hardware. There are,
however, applications that need a broader
multicasting capability that is effective at
the scale of internetworks. For example,
when a radio station is broadcast over the
Internet, the same data must be sent to
all the hosts where a user has tuned in to
that station. In that example, the communication is one-to-many. Other examples
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of one-to-many applications include transmitting the same news, current stock prices,
or software updates to multiple hosts. There are also applications whose communication
is many-to-many, such as multimedia teleconferencing, online multiplayer gaming, or
distributed simulations. In such cases, members of a group receive data from multiple
senders, typically each other. From any particular sender, they all receive the same data.
Normal IP communication, in which each packet must be addressed and sent to a
single host, is not well-suited to such applications. If an application has data to send to
a group, it would have to send a separate packet with the identical data to each member
of the group. This redundancy consumes more bandwidth than necessary. Furthermore,
the redundant traffic is not distributed evenly but rather is focused around the sending
host, and may easily exceed the capacity of the sending host and the nearby networks and
routers. Another problem is that the application would have to keep track of all the IP
addresses to send to. For many, perhaps most, applications, that set of IP addresses could
be constantly changing, for example, as listeners tune into an Internet radio station and
other listeners turn it off.
To better support many-to-many and one-to-many communication, IP provides an
IP-level multicast analogous to the link-level multicast provided by multiaccess networks
like Ethernet and token rings as we saw in Chapter 2. Now that we are introducing the
concept of multicast for IP, we also need a term for the “traditional” one-to-one service
of IP that has been the focus of this chapter so far: that service is referred to as unicast.
The basic IP multicast model is a many-to-many model based on multicast groups,
where each group has its own IP multicast address. The hosts that are members of a
group receive copies of any packets sent to that group’s multicast address. A host can be
in multiple groups, and it can join and leave groups freely by telling its local router using
a protocol that we will discuss shortly. Thus, while we think of unicast addresses as being
associated with a node or an interface, multicast addresses are associated with an abstract
group, the membership of which changes dynamically over time. Further, the original
IP multicast service model allows any host to send multicast traffic to a group; it doesn’t
have to be a member of the group, and there may be any number of such senders to a
given group.
Using IP multicast to send the identical packet to each member of the group, a
host sends a single copy of the packet addressed to the group’s multicast address. The
sending host doesn’t need to know the individual unicast IP address of each member of
the group because, as we will see, that knowledge is distributed among the routers in
the internetwork. Similarly, the sending host doesn’t need to send multiple copies of the
packet because the routers will make copies whenever they have to forward the packet
over more than one link. Compared to using unicast IP to deliver the same packets to
many receivers, IP multicast is more scalable because it eliminates the redundant traffic
4.4
Multicast
331
(packets) that would have been sent many times over the same links, especially those near
to the sending host.
IP’s original many-to-many multicast has been supplemented with support for a
form of one-to-many multicast. In this model of one-to-many multicast, called sourcespecific multicast (SSM), a receiving host specifies both a multicast group and a specific
sending host. The receiving host would then receive multicasts addressed to the specified
group, but only if they are from the specified sender. Many Internet multicast applications (e.g., radio broadcasts) fit the SSM model. To contrast it with SSM, IP’s original
many-to-many model is sometimes referred to as any source multicast (ASM).
A host signals its desire to join or leave a multicast group by communicating with
its local router using a special protocol for just that purpose. In IPv4, that protocol is
Internet Group Management Protocol (IGMP); in IPv6, it is Multicast Listener Discovery
(MLD). The router then has the responsibility for making multicast behave correctly
with regard to that host. Because a host may fail to leave a multicast group when it
should (after a crash or other failure, for example), the router periodically polls the LAN
to determine which groups are still of interest to the attached hosts.
4.4.1 Multicast Addresses
IP has a subrange of its address space reserved for multicast addresses. In IPv4, these
addresses are assigned in the class D address space, and IPv6 also has a portion of its
address space (see Table 4.11) reserved for multicast group addresses. Some subranges
of the multicast ranges are reserved for intradomain multicast, so they can be reused
independently by different domains.
Thus, there are 28 bits of possible multicast addresses in IPv4 when we ignore the
prefix shared by all multicast addresses. This presents a problem when attempting to take
advantage of hardware multicasting on a LAN. Let’s take the case of Ethernet. Ethernet
multicast addresses have only 23 bits when we ignore their shared prefix. In other words,
to take advantage of Ethernet multicasting, IP has to map 28-bit IP multicast addresses
into 23-bit Ethernet multicast addresses. This is implemented by taking the low-order
23 bits of any IP multicast address to use as its Ethernet multicast address, and ignoring
the high-order 5 bits. Thus, 32 (25 ) IP addresses map into each one of the Ethernet
addresses.
When a host on an Ethernet joins an IP multicast group, it configures its Ethernet interface to receive any packets with the corresponding Ethernet multicast address.
Unfortunately, this causes the receiving host to receive not only the multicast traffic it desired, but also traffic sent to any of the other 31 IP multicast groups that map to the same
Ethernet address, if they are routed to that Ethernet. Therefore, IP at the receiving host
must examine the IP header of any multicast packet to determine whether the packet
really belongs to the desired group. In summary, the mismatch of multicast address sizes
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means that multicast traffic may place a burden on hosts that are not even interested in
the group to which the traffic was sent. Fortunately, in some switched networks (such
as switched Ethernet) this problem can be mitigated by schemes wherein the switches
recognize unwanted packets and discard them.
One perplexing question is how senders and receivers learn which multicast addresses to use in the first place. This is normally handled by out-of-band means, and
there are some quite sophisticated tools to enable group addresses to be advertised on the
Internet. One example is sdr, discussed in Section 9.3.1.
4.4.2 Multicast Routing (DVMRP, PIM, MSDP)
A router’s unicast forwarding tables indicate, for any IP address, which link to use to forward the unicast packet. To support multicast, a router must additionally have multicast
forwarding tables that indicate, based on multicast address, which links—possibly more
than one—to use to forward the multicast packet (the router duplicates the packet if it is
to be forwarded over multiple links). Thus, where unicast forwarding tables collectively
specify a set of paths, multicast forwarding tables collectively specify a set of trees: multicast distribution trees. Furthermore, to support source-specific multicast (and, it turns
out, for some types of any source multicast), the multicast forwarding tables must indicate which links to use based on the combination of multicast address and the (unicast)
IP address of the source, again specifying a set of trees.
Multicast routing is the process by which the multicast distribution trees are determined or, more concretely, the process by which the multicast forwarding tables are
built. As with unicast routing, it is not enough that a multicast routing protocol “work”;
it must also scale reasonably well as the network grows, and it must accommodate the
autonomy of different routing domains.
DVMRP
Distance-vector routing, which we discussed in Section 4.2.2 for unicast, can be extended
to support multicast. The resulting protocol is called Distance Vector Multicast Routing
Protocol, or DVMRP. DVMRP was the first multicast routing protocol to see widespread
use.
Recall that, in the distance-vector algorithm, each router maintains a table of
Destination, Cost, NextHop tuples, and exchanges a list of Destination, Cost
pairs with its directly connected neighbors. Extending this algorithm to support multicast is a two-stage process. First, we create a broadcast mechanism that allows a packet to
be forwarded to all the networks on the internet. Second, we need to refine this mechanism so that it prunes back networks that do not have hosts that belong to the multicast
group. Consequently, DVMRP is one of several multicast routing protocols described as
flood-and-prune protocols.
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Multicast
333
Given a unicast routing table, each router knows that the current shortest path to
a given destination goes through NextHop. Thus, whenever it receives a multicast
packet from source S, the router forwards the packet on all outgoing links (except the
one on which the packet arrived) if and only if the packet arrived over the link that is on
the shortest path to S (i.e., the packet came from the NextHop associated with S in the
routing table). This strategy effectively floods packets outward from S, but does not loop
packets back toward S.
There are two major shortcomings to this approach. The first is that it truly floods
the network; it has no provision for avoiding LANs that have no members in the multicast group. We address this problem below. The second limitation is that a given packet
will be forwarded over a LAN by each of the routers connected to that LAN. This is
due to the forwarding strategy of flooding packets on all links other than the one on
which the packet arrived, without regard to whether or not those links are part of the
shortest-path tree rooted at the source.
The solution to this second limitation is to eliminate the duplicate broadcast packets that are generated when more than one router is connected to a given LAN. One way
to do this is to designate one router as the “parent” router for each link, relative to the
source, where only the parent router is allowed to forward multicast packets from that
source over the LAN. The router that has the shortest path to source S is selected as the
parent; a tie between two routers would be broken according to which router has the
smallest address. A given router can learn if it is the parent for the LAN (again relative
to each possible source) based upon the distance-vector messages it exchanges with its
neighbors.
Notice that this refinement requires that each router keep, for each source, a bit for
each of its incident links indicating whether or not it is the parent for that source/link
pair. Keep in mind that in an internet setting, a “source” is a network, not a host, since an
internet router is only interested in forwarding packets between networks. The resulting
mechanism is sometimes called reverse path broadcast (RPB) or reverse path forwarding
(RPF). The path is “reverse” because we are considering the shortest path toward the
source when making our forwarding decisions, as compared to unicast routing, which
looks for the shortest path to a given destination.
The RPB mechanism just described implements shortest-path broadcast. We now
want to prune the set of networks that receives each packet addressed to group G to exclude those that have no hosts that are members of G. This can be accomplished in two
stages. First, we need to recognize when a leaf network has no group members. Determining that a network is a leaf is easy—if the parent router as described above is the only
router on the network, then the network is a leaf. Determining if any group members
reside on the network is accomplished by having each host that is a member of group G
periodically announce this fact over the network, as described in our earlier description
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of link-state multicast. The router then uses this information to decide whether or not
to forward a multicast packet addressed to G over this LAN.
The second stage is to propagate this “no members of G here” information up
the shortest-path tree. This is done by having the router augment the Destination,
Cost pairs it sends to its neighbors with the set of groups for which the leaf network is
interested in receiving multicast packets. This information can then be propagated from
router to router, so that for each of its links, a given router knows for what groups it
should forward multicast packets.
Note that including all of this information in the routing update is a fairly expensive thing to do. In practice, therefore, this information is exchanged only when some
source starts sending packets to that group. In other words, the strategy is to use RPB,
which adds a small amount of overhead to the basic distance-vector algorithm, until a
particular multicast address becomes active. At that time, routers that are not interested
in receiving packets addressed to that group speak up, and that information is propagated
to the other routers.
PIM-SM
Protocol-independent multicast, or PIM, was developed in response to the scaling problems of earlier multicast routing protocols. In particular, it was recognized that the existing protocols did not scale well in environments where a relatively small proportion
of routers want to receive traffic for a certain group. For example, broadcasting traffic
to all routers until they explicitly ask to be removed from the distribution is not a good
design choice if most routers don’t want to receive the traffic in the first place. This situation is sufficiently common that PIM divides the problem space into sparse mode and
dense mode, where sparse and dense refer to the proportion of routers that will want the
multicast. PIM dense mode (PIM-DM) uses a flood-and-prune algorithm like DVMRP,
and suffers from the same scalability problem. PIM sparse mode (PIM-SM) has become
the dominant multicast routing protocol and is the focus of our discussion here. The
“protocol-independent” aspect of PIM, by the way, refers to the fact that, unlike earlier protocols such as DVMRP, PIM does not depend on any particular sort of unicast
routing—it can be used with any unicast routing protocol, as we will see below.
In PIM-SM, routers explicitly join the multicast distribution tree using PIM protocol messages known as Join messages. Note the contrast to DVMRP’s approach of
creating a broadcast tree first and then pruning the uninterested routers. The question
that arises is where to send those Join messages because, after all, any host (and any
number of hosts) could send to the multicast group. To address this, PIM-SM assigns
to each group a special router known as the rendezvous point (RP). In general, a number
of routers in a domain are configured to be candidate RPs, and PIM-SM defines a set
of procedures by which all the routers in a domain can agree on the router to use as the
4.4
Multicast
335
RP for a given group. These procedures are rather complex, as they must deal with a
wide variety of scenarios, such as the failure of a candidate RP and the partitioning of a
domain into two separate networks due to a number of link or node failures. For the rest
of this discussion, we assume that all routers in a domain know the unicast IP address of
the RP for a given group.
A multicast forwarding tree is built as a result of routers sending Join messages to
the RP. PIM-SM allows two types of tree to be constructed: a shared tree, which may
be used by all senders, and a source-specific tree, which may be used only by a specific
sending host. The normal mode of operation creates the shared tree first, followed by
one or more source-specific trees if there is enough traffic to warrant it. Because building
trees installs state in the routers along the tree, it is important that the default is to have
only one tree for a group, not one for every sender to a group.
When a router sends a Join message toward the RP for a group G, it is sent using
normal IP unicast transmission. This is illustrated in Figure 4.38(a), in which router R4
is sending a Join to the rendezvous point for some group. The initial Join message is
“wildcarded,” that is, it applies to all senders. A Join message clearly must pass through
some sequence of routers before reaching the RP (e.g., R2). Each router along the path
looks at the Join and creates a forwarding table entry for the shared tree, called a (*, G)
entry (* meaning “all senders”). To create the forwarding table entry, it looks at the
interface on which the Join arrived and marks that interface as one on which it should
forward data packets for this group. It then determines which interface it will use to
forward the Join toward the RP. This will be the only acceptable interface for incoming
packets sent to this group. It then forwards the Join toward the RP. Eventually, the
message arrives at the RP, completing the construction of the tree branch. The shared
tree thus constructed is shown as a solid line from the RP to R4 in Figure 4.38(a).
As more routers send Joins toward the RP, they cause new branches to be added
to the tree, as illustrated in Figure 4.38(b). Note that in this case, the Join only needs to
travel to R2, which can add the new branch to the tree simply by adding a new outgoing
interface to the forwarding table entry created for this group. R2 need not forward the
Join on to the RP. Note also that the end result of this process is to build a tree whose
root is the RP.
At this point, suppose a host wishes to send a message to the group. To do so, it
constructs a packet with the appropriate multicast group address as its destination and
sends it to a router on its local network known as the designated router (DR). Suppose the
DR is R1 in Figure 4.38. There is no state for this multicast group between R1 and the
RP at this point, so instead of simply forwarding the multicast packet, R1 “tunnels” it
to the RP. That is, R1 encapsulates the multicast packet inside a PIM Register message
that it sends to the unicast IP address of the RP. Just like a tunnel endpoint of the sort
described in Section 4.1.8, the RP receives the packet addressed to it, looks at the payload
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Figure 4.38 PIM operation. (a) R4 sends Join to RP and joins shared tree; (b) R5 joins
shared tree; (c) RP builds source-specific tree to R1 by sending Join to R1; (d) R4 and R5
build source-specific tree to R1 by sending Joins to R1.
of the Register message, and finds inside an IP packet addressed to the multicast address
of this group. The RP, of course, does know what to do with such a packet—it sends it
out onto the shared tree of which the RP is the root. In the example of Figure 4.38, this
means that the RP sends the packet on to R2, which is able to forward it on to R4 and
R5. The complete delivery of a packet from R1 to R4 and R5 is shown in Figure 4.39.
We see the tunneled packet travel from R1 to the RP with an extra IP header containing
the unicast address of RP, and then the multicast packet addressed to G making its way
along the shared tree to R4 and R5.
At this point, we might be tempted to declare success, since all hosts can send
to all receivers this way. However, there is some bandwidth inefficiency and processing
cost in the encapsulation and decapsulation of packets on the way to the RP, so the RP
4.4
Multicast
337
Figure 4.39 Delivery of a packet along a shared tree. R1 tunnels the packet to the RP,
which forwards it along the shared tree to R4 and R5.
forces knowledge about this group into the intervening routers so that tunneling can be
avoided. It sends a Join message toward the sending host (Figure 4.38(c)). As this Join
travels toward the host, it causes the routers along the path (R3) to learn about the group,
so that it will be possible for the DR to send the packet to the group as “native” (i.e., not
tunneled) multicast packets.
An important detail to note at this stage is that the Join message sent by the RP
to the sending host is specific to that sender, whereas the previous ones sent by R4 and
R5 applied to all senders. Thus, the effect of the new Join is to create a sender-specific
state in the routers between the identified source and the RP. This is referred to as (S, G)
state, since it applies to one sender to one group, and contrasts with the (*, G) state
that was installed between the receivers and the RP that applies to all senders. Thus, in
Figure 4.38(c), we see a source-specific route from R1 to the RP (indicated by the dashed
line) and a tree that is valid for all senders from the RP to the receivers (indicated by the
solid line).
The next possible optimization is to replace the entire shared tree with a sourcespecific tree. This is desirable because the path from sender to receiver via the RP might
be significantly longer than the shortest possible path. This again is likely to be triggered
by a high data rate being observed from some sender. In this case, the router at the downstream end of the tree—say, R4 in our example—sends a source-specific Join toward the
source. As it follows the shortest path toward the source, the routers along the way create
(S, G) state for this tree, and the result is a tree that has its root at the source, rather than
the RP. Assuming both R4 and R5 made the switch to the source-specific tree, we would
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end up with the tree shown in Figure 4.38(d). Note that this tree no longer involves the
RP at all. We have removed the shared tree from this picture to simplify the diagram,
but in reality all routers with receivers for a group must stay on the shared tree in case
new senders show up.
We can now see why PIM is protocol independent. All of its mechanisms for building and maintaining trees take advantage of unicast routing without depending on any
particular unicast routing protocol. The formation of trees is entirely determined by the
paths that Join messages follow, which is determined by the choice of shortest paths
made by unicast routing. Thus, to be precise, PIM is “unicast routing protocol independent,” as compared to DVMRP. Note that PIM is very much bound up with the Internet
Protocol—it is not protocol independent in terms of network-layer protocols.
The design of PIM-SM again illustrates the challenges in building scalable networks, and how scalability is sometimes pitted against some sort of optimality. The
shared tree is certainly more scalable than a source-specific tree, in the sense that it
reduces the total state in routers to be on the order of the number of groups rather
than the number of senders times the number of groups. However, the source-specific
tree is likely to be necessary to achieve efficient routing and effective use of link bandwidth.
Interdomain Multicast (MSDP)
PIM-SM has some significant shortcomings when it comes to interdomain multicast. In
particular, the existence of a single RP for a group goes against the principle that domains are autonomous. For a given multicast group, all the participating domains would
be dependent on the domain where the RP is located. Furthermore, if there is a particular multicast group for which a sender and some receivers shared a single domain, the
multicast traffic would still have to be routed initially from the sender to those receivers
via whatever domain has the RP for that multicast group. Consequently, the PIM-SM
protocol is typically not used across domains, only within a domain.
To extend multicast across domains using PIM-SM, Multicast Source Discovery
Protocol (MSDP) was devised. MSDP is used to connect different domains—each running PIM-SM internally, with its own RPs—by connecting the RPs of the different
domains. Each RP has one or more MSDP peer RPs in other domains. Each pair of
MSDP peers is connected by a TCP connection (Section 5.2) over which the MSDP
protocol runs. Together, all the MSDP peers for a given multicast group form a loose
mesh that is used as a broadcast network. MSDP messages are broadcast through the
mesh of peer RPs using the reverse path broadcast algorithm that we discussed in the
context of DVMRP.
What information does MSDP broadcast through the mesh of RPs? Not group
membership information; when a host joins a group, the furthest that information will
4.4
Multicast
339
Figure 4.40 MSDP operation. (a) The source SR sends a Register to its domain’s RP,
RP1; then RP1 sends a source-specific Join to SR, and an MSDP Source Active to its
MSDP peer in Domain B, RP2; then RP2 sends a source-specific Join to SR. (b) As a
result, RP1 and RP2 are in the source-specific tree for source SR.
flow is its own domain’s RP. Instead, it is source—multicast sender—information. Each
RP knows the sources in its own domain because it receives a Register message whenever a new source arises. Each RP periodically uses MSDP to broadcast Source Active
messages to its peers, giving the IP address of the source, the multicast group address,
and the IP address of the originating RP.
If an MSDP peer RP that receives one of these broadcasts has active receivers for
that multicast group, it sends a source-specific Join, on that RP’s own behalf, to the
source host, as shown in Figure 4.40(a). The Join message builds a branch of the sourcespecific tree to this RP, as shown in Figure 4.40(b). The result is that every RP that is
part of the MSDP network and has active receivers for a particular multicast group is
added to the source-specific tree of the new source. When an RP receives a multicast
from the source, the RP uses its shared tree to forward the multicast to the receivers in
its domain.
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Source-Specific Multicast (PIM-SSM)
The original service model of PIM was, like earlier multicast protocols, a many-to-many
model. Receivers joined a group, and any host could send to the group. However, it was
recognized in the late 1990s that it might be useful to add a one-to-many model. Lots
of multicast applications, after all, have only one legitimate sender, such as the speaker
at a conference being sent over the Internet. We already saw that PIM-SM can create
source-specific shortest path trees as an optimization after using the shared tree initially.
In the original PIM design, this optimization was invisible to hosts—only routers joined
source-specific trees. However, once the need for a one-to-many service model was recognized, it was decided to make the source-specific routing capability of PIM-SM explicitly
available to hosts. It turns out that this mainly required changes to IGMP and its IPv6
analog, MLD, rather than PIM itself. The newly-exposed capability is now known as
PIM source-specific multicast (PIM-SSM).
PIM-SSM introduces a new concept, the channel, which is the combination of a
source address S and a group address G. The group address G looks just like a normal
IP multicast address, and both IPv4 and IPv6 have allocated subranges of the multicast
address space for SSM. To use PIM-SSM, a host specifies both the group and the source
in an IGMP Membership Report message to its local router. That router then sends a
PIM-SM source-specific Join message toward the source, thereby adding a branch to
itself in the source-specific tree, just as was described above for “normal” PIM-SM, but
bypassing the whole shared-tree stage. Since the tree that results is source-specific, only
the designated source can send packets on that tree.
The introduction of PIM-SSM has provided some significant benefits, particularly
since there is relatively high demand for one-to-many multicasting:
■
Multicasts travel more directly to receivers.
■
The address of a channel is effectively a multicast group address plus a source
address. Therefore, given that a certain range of multicast group addresses will be
used for SSM exclusively, multiple domains can use the same multicast group
address independently and without conflict, as long as they use it only with
sources in their own domains.
■
Since only the specified source can send to an SSM group, there is less risk
of attacks based on malicious hosts overwhelming the routers or receivers with
bogus multicast traffic.
■
PIM-SSM can be used across domains exactly as it is used within a domain,
without reliance on anything like MSDP.
SSM, therefore, is quite a useful addition to the multicast service model.
4.4
Multicast
341
Bidirectional Trees (BIDIR-PIM)
We round off our discussion of multicast with another enhancement to PIM known
as Bidirectional PIM. BIDIR-PIM is a recent variant of PIM-SM that is well-suited to
many-to-many multicasting within a domain, especially when senders and receivers to a
group may be the same, as in a multiparty videoconference for example. As in PIM-SM,
would-be receivers join groups by sendWhere Are They Now
ing IGMP Membership Report messages
(which must not be source-specific), and
The Fate of Multicast Protocols
a shared tree rooted at an RP is used
A number of IP multicast protocols
to forward multicast packets to receivers.
have fallen by the wayside since the
Unlike PIM-SM, however, the shared tree
1991 publication of Steve Deering’s
also has branches to the sources. That
doctoral thesis, “Multicast Routing
wouldn’t make any sense with PIM-SM’s
in a Datagram Network.” In most
unidirectional tree, but BIDIR-PIM’s
cases, their downfall had something to
trees are bidirectional—a router that redo with scaling. The most successful
ceives a multicast packet from a downearly multicast protocol was DVMRP,
stream branch can forward it both up the
which we discussed at the start of the
tree and down other branches. The route
section. The Multicast Open Shortfollowed to deliver a packet to any particest Path First (MOSPF) protocol was
ular receiver goes only as far up the tree as
based on the OSPF unicast routing
necessary before going down the branch
protocol. PIM dense mode (PIMto that receiver. See the multicast route
DM) has some similarity to DVMRP,
from R1 to R2 in Figure 4.41(b) for an
in that it also uses a flood-and-prune
example. R4 forwards a multicast packet
approach; at the same time it is like
downstream to R2 at the same time that
PIM-SM in being independent of the
it forwards a copy of the same packet upunicast routing protocol used. All of
stream to R5.
these protocols are more appropriate
A surprising aspect of BIDIR-PIM
to a “dense” domain (i.e., one with a
is that there need not actually be an
high proportion of routers interested
RP. All that is needed is a routable adin the multicast). These protocols all
dress, which is known as an RP adappeared relatively early in the hisdress even though it need not be the
tory of multicast, before some of the
address of an RP or anything at all.
scaling challenges were fully apparent.
How can this be? A Join from a reAlthough they would still make sense
ceiver is forwarded toward the RP address
within a domain for multicast groups
until it reaches a router with an interexpected to be of “dense” interest, they
face on the link where the RP address
are rarely used today, in part because
would reside, where the Join terminates.
????
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Figure 4.41 BIDIR-PIM operation. (a) R2 and R3 send Joins toward the RP address that
terminate when they reach a router on the RP address’s link. (b) A multicast packet
from R1 is forwarded upstream to the RP address’s link, and downstream wherever it
intersects a group member branch.
Figure 4.41(a) shows a Join from R2
terminating at R5, and a Join from
R3 terminating at R6. The upstream
forwarding of a multicast packet similarly flows toward the RP address until
it reaches a router with an interface on
the link where the RP address would reside, but then the router forwards the
multicast packet onto that link as the final step of upstream forwarding, ensuring that all other routers on that link
receive the packet. Figure 4.41(b) illustrates the flow of multicast traffic originating at R1.
BIDIR-PIM cannot thus far be used
across domains. On the other hand, it has
Where Are They Now
????
the routers usually must support PIMSM anyway.
Core-based trees (CBT) was another approach to multicast that was
proposed at about the same time as
PIM. The IETF was initially unable to
choose between the two approaches,
and both PIM and CBT were advanced as “experimental” protocols.
However, PIM was more widely adopted by industry, and the main technical contributions of CBT—shared
trees and bidirectional trees—were
4.5
Where Are They Now
????
ultimately incorporated into PIM-SM
and BIDIR-PIM, respectively.
Border Gateway Multicast Protocol (BGMP) also uses the concept of a
bidirectional shared tree. In BGMP’s
case, however, the nodes of the tree
are domains, with one of the domains
as the root. In other words, BGMP
is like MSDP in tying together domains to support interdomain multicasts. Unlike MSDP, the domains are
free to choose their own intradomain
protocols. BGMP was proposed at the
IETF, and just a few years ago BGMP
was expected to replace MSDP as the
dominant interdomain routing protocol. BGMP is quite complex, however, and it requires the existence of a
protocol that assigns ranges of multicast addresses to domains, in order for BGMP to know which domain is the root for a given address.
Consequently, there have been, it appears, no implementations of BGMP,
let alone deployments, at the time of
writing.
Multiprotocol Label Switching
343
several advantages over PIM-SM for manyto-many multicast within a domain:
There is no source registration
process because the routers already know how to route a multicast packet toward the RP address.
The routes are more direct than
those that use PIM-SM’s shared
tree because they go only as far up
the tree as necessary, not all the
way to the RP.
Bidirectional trees use much less
state than the source-specific trees
of PIM-SM because there is never
any source-specific state. (On
the other hand, the routes will
be longer than those of sourcespecific trees.)
The RP cannot be a bottleneck, and indeed no actual RP is
needed.
One conclusion to draw from the
fact that there are so many different approaches to multicast just within PIM is
that multicast is a difficult problem space
in which to find optimal solutions. You
need to decide which criteria you want to optimize (bandwidth usage, router state, path
length, etc.) and what sort of application you are trying to support (one-to-many, manyto-many, etc.) before you can make a choice of the “best” multicast mode for the task.
4.5
Multiprotocol Label Switching
We conclude our discussion of IP by describing an idea that was originally viewed as
a way to improve the performance of the Internet. The idea, called multiprotocol label
switching (MPLS), tries to combine some of the properties of virtual circuits with the
flexibility and robustness of datagrams. On the one hand, MPLS is very much associated
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with the Internet Protocol’s datagram-based architecture—it relies on IP addresses and IP
routing protocols to do its job. On the other hand, MPLS-enabled routers also forward
packets by examining relatively short, fixed-length labels, and these labels have local
scope, just like in a virtual circuit network. It is perhaps this marriage of two seemingly
opposed technologies that has caused MPLS to have a somewhat mixed reception in the
Internet engineering community.
Before looking at how MPLS works, it is reasonable to ask, “What is it good for?”
Many claims have been made for MPLS, but there are three main things that it is used
for today:
■
To enable IP capabilities on devices that do not have the capability to forward
IP datagrams in the normal manner;
■
To forward IP packets along “explicit routes”—precalculated routes that don’t
necessarily match those that normal IP routing protocols would select;
■
To support certain types of virtual private network services.
It is worth noting that one of the original goals—improving performance—is not on
the list. This has a lot to do with the advances that have been made in forwarding algorithms for IP routers in recent years, and with the complex set of factors beyond header
processing that determine performance.
The best way to understand how MPLS works is to look at some examples of its
use. In the next three sections we will look at examples to illustrate the three applications
of MPLS mentioned above.
4.5.1 Destination-Based Forwarding
One of the earliest publications to introduce the idea of attaching labels to IP packets
was a paper by Chandranmenon and Vargese that described an idea called “threaded indices.” A very similar idea is now implemented in MPLS-enabled routers. The following
example shows how this idea works.
Consider the network in Figure 4.42. Each of the two routers on the far right
(R3 and R4) has one connected network, with prefixes 18.1.1/24 and 18.3.3/24. The
remaining routers (R1 and R2) have routing tables that indicate which outgoing interface
each router would use when forwarding packets to one of those two networks.
When MPLS is enabled on a router, the router allocates a label for each prefix
in its routing table, and advertises both the label and the prefix that it represents to
its neighboring routers. This advertisement is carried in the Label Distribution Protocol.
This is illustrated in Figure 4.43. Router R2 has allocated the label value 15 for the prefix
18.1.1 and the label value 16 for the prefix 18.3.3. These labels can be chosen at the
4.5
Figure 4.42
Multiprotocol Label Switching
345
Routing tables in example network.
convenience of the allocating router, and can be thought of as indices into the routing
table. After allocating the labels, R2 advertises the label bindings to its neighbors; in this
case, we see R2 advertising a binding between the label 15 and the prefix 18.1.1 to R1.
The meaning of such an advertisement is that R2 has said, in effect, “please attach the
label 15 to all packets sent to me that are destined to prefix 18.1.1.” R1 stores the label
in a table alongside the prefix that it represents as the “remote” or “outgoing” label for
any packets that it sends to that prefix.
In Figure 4.43(c), we see another label advertisement from router R3 to R2 for the
prefix 18.1.1, and R2 places the “remote” label that it learned from R3 in the appropriate
place in its table.
At this point, we can look at what happens when a packet is forwarded in this
network. Suppose a packet destined to the IP address 18.1.1.5 arrives from the left to
router R1. R1 in this case is referred to as a label edge router (LER); an LER performs
a complete IP lookup on arriving IP packets, and then applies labels to them as a result
of the lookup. In this case, R1 would see that 18.1.1.5 matches the prefix 18.1.1 in its
forwarding table, and that this entry contains both an outgoing interface and a remote
label value. R1 therefore attaches the remote label 15 to the packet before sending it.
When the packet arrives at R2, R2 looks only at the label in the packet, not the IP
address. The forwarding table at R2 indicates that packets arriving with a label value of
15 should be sent out interface 1, and that it should carry the label value 24, as advertised
by router R3. R2 therefore rewrites, or swaps, the label, and forwards it on to R3.
What has been accomplished by all this application and swapping of labels? Observe that when R2 forwarded the packet in this example, it never actually needed to
examine the IP address. Instead, R2 looked only at the incoming label. Thus, we have
replaced the normal IP destination address lookup with a label lookup. To understand
why this is significant, it helps to recall that although IP addresses are always the same
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Figure 4.43 (a) R2 allocates labels and advertises bindings to R1. (b) R1 stores the
received labels in a table. (c) R3 advertises another binding, and R2 stores the received
label in a table.
length, IP prefixes are of variable length, and the IP destination address lookup algorithm
needs to find the longest match; the longest prefix that matches the high-order bits in the
IP address of the packet being forwarded. By contrast, the label forwarding mechanism
just described is an exact match algorithm. It is possible to implement a very simple exact
match algorithm, for example, by using the label as an index into an array, where each
element in the array is one line in the forwarding table.
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347
Note that while the forwarding algorithm has been changed from longest match
to exact match, the routing algorithm can be any standard IP routing algorithm (e.g.,
OSPF). The path that a packet will follow in this environment is the exact same path
that it would have followed if MPLS were not involved—the path chosen by the IP
routing algorithms. All that has changed is the forwarding algorithm.
An important fundamental concept of MPLS is illustrated by this example. Every
MPLS label is associated with a forwarding equivalence class (FEC)—a set of packets that
are to receive the same forwarding treatment in a particular router. In this example, each
prefix in the routing table is an FEC. That is, all packets that match the prefix 18.1.1—
no matter what the low-order bits of the IP address are—get forwarded along the same
path. Thus, each router can allocate one label that maps to 18.1.1, and any packet that
contains an IP address whose high-order bits match that prefix can be forwarded using
that label.
As we will see in the subsequent examples, FECs are a very powerful and flexible
concept. FECs can be formed using almost any criteria; for example, all the packets
corresponding to a particular customer could be considered to be in the same FEC.
Returning to the example at hand, we observe that changing the forwarding algorithm from normal IP forwarding to label-swapping has an important consequence:
devices that previously didn’t know how to forward IP packets can be used to forward
IP traffic in an MPLS network. The most notable early application of this result was to
ATM switches, which can support MPLS without any changes to their forwarding hardware. ATM switches support the label-swapping forwarding algorithm just described,
and by providing these switches with IP routing protocols and a method to distribute label bindings, they could be turned into label switching routers (LSRs)—devices that run
IP control protocols but use the label-switching forwarding algorithm. More recently,
the same idea has been applied to optical switches of the sort described in Section 3.1.2.
Before we consider the purported benefits of turning an ATM switch into an LSR,
we should tie up some loose ends. We have said that labels are “attached” to packets,
but where exactly are they attached? The answer depends on the type of link on which
packets are carried. Two common methods for carrying labels on packets are shown in
Figure 4.44. When IP packets are carried as complete frames, as they are on most link
types including Ethernet, token ring, and PPP, the label is inserted as a “shim” between
the layer 2 header and the IP (or other layer 3) header, as shown in the lower part of the
figure. However, if an ATM switch is to function as an MPLS LSR, then the label needs
to be in a place where the switch can use it, and that means it needs to be in the ATM
cell header, exactly where one would normally find the VCI and VPI fields.
Having now devised a scheme by which an ATM switch can function as an LSR,
what have we gained? One thing to note is that we could now build a network that used
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Figure 4.44 (a) Label on an ATM-encapsulated packet; (b) label on a frameencapsulated packet.
What Layer Is MPLS?
There have been many debates about
where MPLS belongs in the layered
protocol architectures presented in
Section 1.3. Since the MPLS header
is normally found between the layer 3
and the layer 2 headers in a packet, it
is sometimes referred to as a layer 2.5
protocol. Some people argue that,
since IP packets are encapsulated inside MPLS headers, MPLS must be
“below” IP, making it a layer 2 protocol. Others argue that, since the
control protocols for MPLS are, in
large part, the same protocols as IP—
MPLS uses IP routing protocols and
IP addressing—then MPLS must be
at the same layer as IP (i.e., layer 3).
As we noted in Section 1.3, layered
architectures are useful tools but they
may not always exactly describe the
real world, and MPLS is a good example of where strictly layerist views may
be difficult to reconcile with reality.
a mixture of conventional IP routers,
label edge routers, and ATM switches
functioning as LSRs, and they would all
use the same routing protocols. To understand the benefits of using the same
protocols, consider the alternative. In Figure 4.45(a) we see a set of routers interconnected by virtual circuits over an ATM
network, a configuration called an “overlay” network. At one point in time, networks of this type were often built because
commercially-available ATM switches supported higher total throughput than routers. Today, networks like this are less
common because routers have caught up
with and even surpassed ATM switches.
However, these networks still exist because
of the significant installed base of ATM
switches in network backbones, which in
turn is partly a result of ATM’s ability
to support a range of capabilities such
as circuit emulation and virtual circuit
services.
In an overlay network, each router
would potentially be connected to each
of the other routers by a virtual circuit,
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349
Figure 4.45 (a) Routers connect to each other using an “overlay” of virtual circuits.
(b) Routers peer directly with LSRs.
but in this case for clarity we have just shown the circuits from R1 to all of its peer
routers. R1 has five routing neighbors and needs to exchange routing protocol messages with all of them—we say that R1 has five routing adjacencies. By contrast, in
Figure 4.45(b), the ATM switches have been replaced with LSRs. There are no longer
virtual circuits interconnecting the routers. Thus, R1 has only one adjacency, with LSR1.
In large networks, running MPLS on the switches leads to a significant reduction in
the number of adjacencies that each router must maintain, and can greatly reduce the
amount of work that the routers have to do to keep each other informed of topology
changes.
A second benefit of running the same routing protocols on edge routers and on the
LSRs is that the edge routers now have a full view of the topology of the network. This
means that if some link or node fails inside the network, the edge routers will have a
better chance of picking a good new path than if the ATM switches rerouted the affected
VCs without the knowledge of the edge routers.
Note that the step of replacing ATM switches with LSRs is actually achieved by
changing the protocols running on the switches, but typically no change to the forward-
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ing hardware is needed. That is, an ATM switch can often be converted to an MPLS LSR
by upgrading only its software. Furthermore, an MPLS LSR might continue to support
standard ATM capabilities at the same time as it runs the MPLS control protocols, in
what is referred to as “ships in the night” mode.
More recently, the idea of running IP control protocols on devices that are unable
to forward IP packets natively has been extended to optical switches and TDM devices
such as SONET multiplexors. This is known as generalized MPLS or GMPLS. Part of
the motivation for GMPLS was to provide routers with topological knowledge of an
optical network, just as in the ATM case. Even more important was the fact that there
were no standard protocols for controlling optical devices, and so MPLS seemed like a
natural fit for that job.
4.5.2 Explicit Routing
In Section 3.1.3 we introduced the concept of source routing. IP has a source routing
option, but it is not widely used for several reasons, including the fact that only a limited
number of hops can be specified, and because it is usualy processed outside the “fast
path” on most routers.
MPLS provides a convenient way to add capabilities similar to source routing to
IP networks, although the capability is more often called explicit routing rather than
source routing. One reason for the distinction is that it usually isn’t the real source of the
packet that picks the route. More often it is one of the routers inside a service provider’s
network. Figure 4.46 shows an example of how the explicit routing capability of MPLS
might be applied. This sort of network is often called a fish network because of its shape
(the routers R1 and R2 form the tail; R7 is at the head).
Suppose that the operator of the network in Figure 4.46 has determined that any
traffic flowing from R1 to R7 should follow the path R1-R3-R6-R7, and that any traffic
going from R2 to R7 should follow the path R2-R3-R4-R7. One reason for such a choice
would be to make good use of the capacity available along the two distinct paths from R3
to R7. We can think of the R1-to-R7 traffic as constituting one forwarding equivalence
Figure 4.46
A network requiring explicit routing.
4.5
Multiprotocol Label Switching
351
class, and the R2-to-R7 traffic constitutes a second FEC. Forwarding traffic in these
two classes along different paths is difficult with normal IP routing, because R3 doesn’t
normally look at where traffic came from in making its forwarding decisions.
Because MPLS uses label swapping to forward packets, it is easy enough to achieve
the desired routing if the routers are MPLS-enabled. If R1 and R2 attach distinct labels to
packets before sending them to R3—thus identifying them as being in different FECs—
then R3 can forward packets from R1 and R2 along different paths. The question that
then arises is, how do all the routers in the network agree on what labels to use and how
to forward packets with particular labels? Clearly, we can’t use the same procedures as
described in the preceding section to distribute labels, because those procedures establish
labels that cause packets to follow the normal paths picked by IP routing, which is exactly
what we are trying to avoid. Instead, a new mechanism is needed. It turns out that the
protocol used for this task is the Resource Reservation Protocol (RSVP). We’ll talk more
about this protocol in Section 6.5.2, but for now it suffices to say that it is possible to
send an RSVP message along an explicitly specified path (e.g., R1-R3-R6-R7) and use
it to set up label forwarding table entries all along that path. This is very similar to the
process of establishing a virtual circuit described in Section 3.5.
One of the applications of explicit routing is traffic engineering, which refers to the
task of ensuring that sufficient resources are available in a network to meet the demands
placed on it. Controlling exactly on which paths the traffic flows is an important part
of traffic engineering. Explicit routing can also help to make networks more resilient
in the face of failure, using a capability called fast reroute. For example, it is possible to
precalculate a path from router A to router B that explicitly avoids a certain link L. In the
event that link L fails, router A could send all traffic destined to B down the precalculated
path. The combination of precalculation of the backup path and the explicit routing of
packets along the path means that A doesn’t need to wait for routing protocol packets to
make their way across the network or for routing algorithms to be executed by various
other nodes in the network. In certain circumstances, this can significantly reduce the
time taken to reroute packets around a point of failure.
One final point to note about explicit routing is that explicit routes need not be
calculated by a network operator as in the above example. There are a range of algorithms that routers can use to calculate explicit routes automatically. The most common
of these is called constrained shortest path first (CSPF), which is like the link-state algorithms described in Section 4.2.3, but which also takes constraints into account. For
example, if it was required to find a path from R1 to R7 in that could carry an offered
load of 100 Mbps, we could say that the “constraint” is that each link must have at
least 100 Mbps of available capacity. CSPF addresses this sort of problem. More details
on CSPF, and the applications of explicit routing, are provided in the Further Reading
section.
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4.5.3 Virtual Private Networks and Tunnels
We first talked about virtual private networks (VPNs) in Section 4.1.8, and we noted
that one way to build them was using tunnels. It turns out that MPLS can be thought of
as a way to build tunnels, and this makes it suitable for building VPNs of various types.
The simplest form of MPLS VPN to understand is a “layer 2” VPN. In this type
of VPN, MPLS is used to tunnel layer 2 data (such as Ethernet frames or ATM cells)
across a network of MPLS-enabled routers. Recall from Section 4.1.8 that one reason
for tunnels is to provide some sort of network service (such as multicast) that is not
supported by some routers in the network. The same logic applies here: IP routers are
not ATM switches, so you cannot provide an ATM virtual circuit service across a network of conventional routers. However, if you had a pair of routers interconnected by a
tunnel, they could send ATM cells across the tunnel and emulate an ATM circuit. The
term for this technique within the IETF is pseudowire emulation. Figure 4.47 illustrates
the idea.
We have already seen how IP tunnels are built: The router at the entrance of the
tunnel wraps the data to be tunneled in an IP header (the tunnel header), which represents the address of the router at the far end of the tunnel, and sends the data like
any other IP packet. The receiving router receives the packet with its own address in
the header, strips the tunnel header, and finds the data that was tunneled, which it then
processes. Exactly what it does with that data depends on what it is. For example, if it
were another IP packet, it would then be forwarded on like a normal IP packet. However, it need not be an IP packet, as long as the receiving router knows what to do with
non-IP packets. We’ll return to the issue of how to handle non-IP data in a moment.
An MPLS tunnel is not too different from an IP tunnel, except that the tunnel
header consists of an MPLS header rather than an IP header. Looking back to our first
example, in Figure 4.43, we saw that router R1 attached a label (15) to every packet
that it sent toward prefix 18.1.1. Such a packet would then follow the path R1-R2R3, which each router in the path examining only the MPLS label. Thus, we observe
Figure 4.47
An ATM circuit is emulated by a tunnel.
4.5
Multiprotocol Label Switching
353
that there was no requirement that R1 only send IP packets along this path—any data
could be wrapped up in the MPLS header and it would follow the same path, because
the intervening routers never look beyond the MPLS header. In this regard, an MPLS
header is just like an IP tunnel header.10 The only issue with sending non-IP traffic along
a tunnel, MPLS or otherwise, is this: What to do with non-IP traffic when it reaches the
end of the tunnel? The general solution is to carry some sort of demultiplexing identifier
in the tunnel payload that tells the router at the end of the tunnel what to do. It turns
out that an MPLS label is a perfect fit for such an identifier. An example will make this
clear.
Let’s assume we want to tunnel ATM cells from one router to another across a
network of MPLS-enabled routers, as in Figure 4.47. Further, we assume that the goal
is to emulate an ATM virtual circuit, that is, cells arrive at the entrance, or head, of the
tunnel on a certain input port with a certain VCI and should leave the tail end of the
tunnel on a certain output port and potentially different VCI. This can be accomplished
by configuring the head and tail routers as follows:
■
The head router needs to be configured with the incoming port, the incoming
VCI, the “demultiplexing label” for this emulated circuit, and the address of the
tunnel end router;
■
The tail end router needs to be configured with the outgoing port, the outgoing
VCI, and the demultiplexing label.
Once the routers are provided with this information, we can see how an ATM cell would
be forwarded. Figure 4.48 illustrates the steps.
Figure 4.48
Forwarding ATM cells along a tunnel.
10 Note, however, that an MPLS header is only 4 bytes long, compared to 20 for an IP header, which implies a bandwidth
savings when MPLS is used.
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An ATM cell arrives on the designated input port with the appropriate VCI value (101 in this
example).
2
The head router attaches the demultiplexing label that identifies
the emulated circuit.
3
The head router then attaches a
second label, which is the tunnel label that will get the packet
to the tail router. This label is
learned by mechanisms just like
those described in Section 4.5.1.
4
Routers between the head and
tail forward the packet using
only the tunnel label.
5
The tail router removes the tunnel label, finds the demultiplexing label, and recognizes the
emulated circuit.
6
The tail router modifies the
ATM VCI to the correct value
(202 in this case) and sends it out
the correct port.
One item in this example that
might be surprising is that the packet has
two labels attached to it. This is one of the
interesting features of MPLS—labels may
be stacked on a packet to any depth. This
provides some useful scaling capabilities.
In this example, it enables a single tunnel
to carry a potentially large number of emulated circuits.
The same techniques described here
can be applied to emulate many other
Where Are They Now
????
Deployment of MPLS
Originally conceived as a technology
that would operate within the network
of individual service providers, MPLS
remains hidden from most consumer
and academic users of the Internet today. However, it is now sufficiently
popular among service providers that
it has become almost mandatory for
high-end router manufacturers to include MPLS capabilities in their products. The widespread success of MPLS
is a relatively well-kept secret, at least
to students and researchers focused on
the public Internet.
Two main applications of MPLS
account for most of its deployment.
The layer 3 VPN application described in this section is the “killer
application” for MPLS. Almost every
service provider in the world now offers an MPLS-based layer 3 VPN service. This is often run on routers that
are essentially separate from the Internet, since the main use of layer 3
VPNs is to provider “private” IP service to corporations, not to provide
global Internet connectivity. Some
providers do run their Internet service and VPN service over a common
backbone, however.
The second popular usage of
MPLS is explicit routing, either for
traffic engineering, fast reroute, or
both. Unlike the layer 3 VPN service,
which is explicitly marketed to end
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355
layer 2 services, including Frame Relay and
Ethernet. It is worth noting that virtually
identical capabilities can be provided using
customers, explicit routing is an inIP tunnels; the main advantage of MPLS
ternal capability that providers use to
here is the shorter tunnel header.
improve the reliability of their netBefore MPLS was used to tunnel
works or reduce the cost. Providers
layer 2 services, it was also being used
do not usually publicize details of
to support layer 3 VPNs. We won’t go
their internal network designs, makinto the details of layer 3 VPNs, which
ing it more difficult to determine how
are quite complex—see the Further Readmany providers actually use this teching section for some good sources of more
nology. It is clear that the explicit
information—but we will note that they
routing features of MPLS are used by
represent one of the most popular uses of
fewer providers than the VPN feaMPLS today. Layer 3 VPNs also use stacks
tures, but nevertheless there is eviof MPLS labels to tunnel packets across
dence of significant usage, especially
an IP network. However, the packets that
when bandwidth is expensive or when
are tunneled are themselves IP packets—
there is a strong desire to maintain
hence the name layer 3 VPNs. In a layer
low levels of congestion (e.g., to sup3 VPN, a single service provider operport real-time services).
ates a network of MPLS-enabled routers,
and provides a “virtually private” IP network service to any number of distinct customers. That is, each customer of the provider
has some number of sites, and the service provider creates the illusion for each customer that there are no other customers on the network. The customer sees an IP
network interconnecting his own sites, and no other sites. This means that each customer is isolated from all other customers in terms of both routing and addressing.
Customer A can’t sent packets directly to customer B, and vice versa.11 Customer A
can even use IP addresses that have also been used by customer B. The basic idea
is illustrated in Figure 4.49. As in layer 2 VPNs, MPLS is used to tunnel packets
from one site to another. However, the configuration of the tunnels is performed automatically by some fairly elaborate use of BGP, which is beyond the scope of this
book.
In summary, MPLS is a rather versatile tool that has been applied to a wide range of
different networking problems. It combines the label-swapping forwarding mechanism
that is normally associated with virtual circuit networks with the routing and control
Where Are They Now
????
11 Customer A in fact usually can send data to customer B in some restricted way. Most likely, both customer A and
customer B have some connection to the global Internet, and thus it is probably possible for customer A to send email
messages, for example, to the mail server inside customer B’s network. The “privacy” offered by a VPN prevents customer A
from having unrestricted access to all the machines and subnets inside customer B’s network.
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Figure 4.49 Example of a layer 3 VPN. Customers A and B each obtain a virtually
private IP service for a single provider.
protocols of IP datagram networks to produce a class of network that is somewhere
between the two conventional extremes. This extends the capabilities of IP networks to
enable, among other things, more precise control of routing and the support of a range
of VPN services.
4.6
Summary
The main theme of this chapter was how to build big networks by interconnecting
smaller networks. We looked at bridging in the last chapter, but it is a technique
that is mostly used to interconnect a small to moderate number of similar networks.
What bridging does not do well is tackle the two closely related problems of building very large networks: heterogeneity and scale. The Internet Protocol is the key
tool for dealing with these problems, and it provided most of the examples for this
chapter.
IP tackles heterogeneity by defining a simple, common service model for an internetwork, which is based on the best-effort delivery of IP datagrams. An important part
of the service model is the global addressing scheme, which enables any two nodes in an
4.6
Summary
357
internetwork to uniquely identify each other for the purposes of exchanging data. The
IP service model is simple enough to be supported by any known networking technology, and the ARP mechanism is used to translate global IP addresses into local link-layer
addresses.
A crucial aspect of the operation of an internetwork is the determination of efficient routes to any destination in the internet. Internet routing algorithms solve this
problem in a distributed fashion; this chapter introduced the two major classes of
algorithms—link-state and distance-vector—along with examples of their application
(RIP and OSPF). We also examined the extensions to IP routing that will support mobile hosts.
We then saw a succession of scaling problems and the ways that IP deals with
them. The major scaling issues are the efficient use of address space and the growth of
routing tables as the Internet grows. The hierarchical IP address format, with its network and host parts, gives us one level of hierarchy to manage scale. Subnetting lets us
make more efficient use of network numbers and helps consolidate routing information;
in effect, it adds one more level of hierarchy to the address. Classless routing (CIDR)
lets us introduce more levels of hierarchy and achieve further routing aggregation. Autonomous systems allow us to partition the routing problem into two parts, interdomain
and intradomain routing, each of which is much smaller than the total routing problem would be. These mechanisms have enabled today’s Internet to sustain remarkable
growth.
Eventually, all of these mechanisms will be unable to keep up with the Internet’s
growth, and a new address format will be needed. This will require a new IP datagram
format and a new version of the protocol. Originally known as Next Generation IP
(IPng), this new protocol is now known as IPv6, and it provides a 128-bit address with
CIDR-like addressing and routing. While many new capabilities have been claimed for
IPv6, its main advantage remains its ability to support an extremely large number of
addressable devices.
Finally, we looked at two enhancements to the original IP datagram model. The
first, multicast, enables efficient delivery of the same data to groups of receivers. As with
unicast, many of the challenges in multicast relate to scaling, and a number of different
protocols and multicast modes have been developed to optimize scaling and routing in
different environments. The second enhancement, MPLS, brings some of the aspects of
virtual circuit networks to IP, and has been widely used to extend the capabilities of IP.
Applications of MPLS range from traffic engineering to the support of virtual private
networks over the Internet.
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More than 15 years have elapsed
since the shortage of IPv4 address
O P E N
I S S U E
space became serious enough to warDeployment of IPv6
rant proposals for a new version of IP.
The original IPv6 specification is now
more than 10 years old. IPv6-capable
host operating systems are now widely
available and the major router vendors offer varying degrees of support for IPv6 in their
products. Yet the deployment of IPv6 in the Internet can only be described as embryonic. It is worth wondering when deployment is likely to begin in earnest, and what will
cause it.
One reason why IPv6 has not been needed sooner is because of the extensive use of NAT (network address translation, described earlier in this chapter). As
providers viewed IPv4 addresses as a scarce resource, they handed out fewer of them
to their customers, or charged for the number of addresses used; customers responded
by hiding many of their devices behind a NAT box and a single IPv4 address. For
example, it is likely that most home networks with more than one IP-capable device have some sort of NAT in the network to conserve addresses. So one factor
that might drive IPv6 deployment would be applications that don’t work well with
NAT. While client-server applications work reasonably well when the client’s address is “hidden” behind a NAT box, peer-to-peer applications fare less well. Examples of applications that would work better without NAT and would therefore benefit from more liberal address allocation policies are multiplayer gaming and IP telephony.
Obtaining blocks of IPv4 addresses has been getting more difficult for years, and
this is particularly noticeable in countries outside the United States. As the difficulty
increases, the incentive for providers to start offering IPv6 addresses to their customers
also rises. At the same time, for existing providers, offering IPv6 is a substantial additional cost, because they don’t get to stop supporting IPv4 when they start to offer
IPv6. This means, for example, that the size of a provider’s routing tables can only increase initially, because they need to carry all the existing IPv4 prefixes plus new IPv6
prefixes.
At the moment, IPv6 deployment is happening primarily in research networks.
A few service providers are starting to offer it (often with some incentive from national
governments). It seems hard to imagine that the Internet can continue to grow indefinitely without IPv6 seeing some more significant deployments, but it also seems likely
that the overwhelming majority of hosts and networks will be IPv4-only for several more
years at least.
Further Reading
F U R T H E R
359
R E A D I N G
Not surprisingly, there have been countless papers written on various aspects of the
Internet. Of these, we recommend two as must reading: The paper by Cerf and Kahn
is the one that originally introduced the TCP/IP architecture and is worth reading just
for its historical perspective; the paper by Bradner and Mankin gives an informative
overview on how the rapidly growing Internet has stressed the scalability of the original
architecture, ultimately resulting in the next generation IP. The paper by Paxson describes
a study of how routers behave in the Internet. It also happens to be a good example of
how researchers are now studying the dynamic behavior of the Internet. The final paper
discusses multicast, presenting the approach to multicast originally used on the MBone.
■
Cerf, V., and R. Kahn. “A Protocol for Packet Network Intercommunication.”
IEEE Transactions on Communications COM-22(5):637–648, May 1974.
■
Bradner, S., and A. Mankin. “The Recommendation for the Next Generation
IP Protocol.” Request for Comments 1752, January 1995.
■
Paxson, V. “End-to-End Routing Behavior in the Internet.” SIGCOMM ’96,
pp. 25–38, August 1996.
■
Deering, S., and D. Cheriton. “Multicast Routing in Datagram Internetworks
and Extended LANs.” ACM Transactions on Computer Systems 8(2):85–110,
May 1990.
Beyond these papers, Perlman gives an excellent explanation of routing in an internet, including coverage of both bridges and routers [Per00]. Also, the book by Lynch and
Rose gives general information on the scalability of the Internet [Cha93]. Some interesting experimental studies of the behavior of Internet routing are presented in Labovitz et
al. [LAAJ00].
Many of the techniques and protocols developed to help the Internet scale are
described in RFCs: Subnetting is described in Mogul and Postel [MP85], CIDR is described in Fuller and Li [FL06], RIP is defined in Hedrick [Hed88] and Mogul and Postel [MP94], OSPF is defined in Moy [Moy98], and BGP-4 is defined in Rekhter et al.
[RLH06]. The OSPF specification, at over 200 pages, is one of the longer RFCs around,
but also contains an unusual wealth of detail about how to implement a protocol. A collection of RFCs related to IPv6 can be found in Bradner and Mankin [BM95], and
the most recent IPv6 spec is by Deering and Hinden [DH98]. The reasons to avoid IP
fragmentation are examined in Kent and Mogul [KM87] and the Path MTU discovery
technique is described in Mogul and Deering [MD90]. Protocol Independent Multicast
(PIM) is described in Deering et al. [DEF+ 96] and [EFH+ 98]. MSDP is described in
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[FM03] and PIM-SSM in [Bha03]. [Wil00] and [HC99] are both very readable introductions to multicast with interesting historical details.
There has been a lot of work developing algorithms that can be used by routers
to do fast lookup of IP addresses. (Recall that the problem is that the router needs to
match the longest prefix in the forwarding table.) PATRICIA trees are one of the first
algorithms applied to this problem [Mor68]. More recent work is reported in [DBSP97],
[WVTP97], [LS98], and [SVSW98]. For an overview of how these algorithms can be
used to build a high-speed router, see Partridge et al. [Par98].
Multiprotocol label switching and the related protocols that fed its development
are described in Chandranmenon and Varghese [CV95], Rekhter et al. [RDR+ 97], and
Davie and Rekhter [DR00]. The latter reference describes many applications of MPLS
such as traffic engineering, fast recovery from network failures, and virtual private networks. [RR06] provides the specification of MPLS/BGP VPNs, a form of layer 3 VPN
that can be provided over MPLS networks.
Finally, we recommend the following live references:
■
http://www.ietf.org: The IETF home page, from which you can get RFCs,
internet drafts, and working group charters.
■
http://www.isoc.org/internet/history/: A collection of links related to Internet history, including some articles written by the original researchers who
built the Internet.
E X E R C I S E S
1
What aspect of IP addresses makes it necessary to have one address per network interface, rather than just one per host? In light of your answer, why
does IP tolerate point-to-point interfaces that have nonunique addresses or no
addresses?
2
Why does the Offset field in the IP header measure the offset in 8-byte units?
(Hint: Recall that the Offset field is 13 bits long.)
3
Some signalling errors can cause entire ranges of bits in a packet to be overwritten by all 0s or all 1s. Suppose all the bits in the packet including the Internet
checksum are overwritten. Could a packet with all 0s or all 1s be a legal IPv4
packet? Will the Internet checksum catch that error? Why or why not?
4
Suppose a TCP message that contains 2,048 bytes of data and 20 bytes of TCP
header is passed to IP for delivery across two networks of the Internet (i.e.,
from the source host to a router to the destination host). The first network
Exercises
361
uses 14-byte headers and has an MTU of 1,024 bytes; the second uses 8-byte
headers with an MTU of 512 bytes. Each network’s MTU gives the size of
the largest IP datagram that can be carried in a link layer frame. Give the sizes
and offsets of the sequence of fragments delivered to the network layer at the
destination host. Assume all IP headers are 20 bytes.
✓
5
Path MTU is the smallest MTU of any link on the current path (route) between two hosts. Assume we could discover the Path MTU of the path used
in the previous exercise, and that we use this value as the MTU for all the path
segments. Give the sizes and offsets of the sequence of fragments delivered to
the network layer at the destination host.
★
6
Suppose an IP packet is fragmented into 10 fragments, each with a 1% (independent) probability of loss. To a reasonable approximation, this means there
is a 10% chance of losing the whole packet due to loss of a fragment. What
is the probability of net loss of the whole packet if the packet is transmitted
twice,
(a) Assuming all fragments received must have been part of the same transmission?
(b) Assuming any given fragment may have been part of either transmission?
(c) Explain how use of the Ident field might be applicable here.
7
Suppose the fragments of Figure 4.5(b) all pass through another router onto
a link with an MTU of 380 bytes, not counting the link header. Show the
fragments produced. If the packet were originally fragmented for this MTU,
how many fragments would be produced?
8
What is the maximum bandwidth at which an IP host can send 576-byte packets without having the Ident field wrap around within 60 seconds? Suppose
IP’s maximum segment lifetime (MSL) is 60 seconds, that is, delayed packets can arrive up to 60 seconds late but no later. What might happen if this
bandwidth were exceeded?
9
ATM AAL3/4 uses fields Btag/Etag, BASize/Len, Type, SEQ, MID,
Length, and CRC-10 to implement fragmentation into cells. IPv4 uses
Ident, Offset, and the M bit in Flags, among others. What is the IP analog,
if any, for each AAL3/4 field? Does each IP field listed here have an AAL3/4
analog? How well do these fields correspond?
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10
Why do you think IPv4 has fragment reassembly done at the endpoint, rather
than at the next router? Why do you think IPv6 abandoned fragmentation
entirely? (Hint: Think about the differences between IP-layer fragmentation
and link-layer fragmentation.)
11
Having ARP table entries time out after 10 to 15 minutes is an attempt at a
reasonable compromise. Describe the problems that can occur if the timeout
value is too small or too large.
12
IP currently uses 32-bit addresses. If we could redesign IP to use the 6-byte
MAC address instead of the 32-bit address, would we be able to eliminate the
need for ARP? Explain why or why not.
13
Suppose hosts A and B have been assigned the same IP address on the same
Ethernet, on which ARP is used. B starts up after A. What will happen to
A’s existing connections? Explain how “self-ARP” (querying the network on
start-up for one’s own IP address) might help with this problem.
14
Suppose an IP implementation adheres literally to the following algorithm on
receipt of a packet, P, destined for IP address D:
if (Ethernet address for D is in ARP cache)
send P
else
send out an ARP query for D
put P into a queue until the response comes back
(a) If the IP layer receives a burst of packets destined for D, how might this
algorithm waste resources unnecessarily?
(b) Sketch an improved version.
(c) Suppose we simply drop P, after sending out a query, when cache lookup
fails. How would this behave? (Some early ARP implementations allegedly
did this.)
15
For the network given in Figure 4.50, give global distance-vector tables like
those of Tables 4.5 and 4.8 when
(a) Each node knows only the distances to its immediate neighbors.
(b) Each node has reported the information it had in the preceding step to its
immediate neighbors.
Exercises
Figure 4.50
Network for Exercises 15, 17, and 20.
Figure 4.51
Network for Exercise 16.
363
(c) Step (b) happens a second time.
✓
16
For the network given in Figure 4.51, give global distance-vector tables like
those of Tables 4.5 and 4.8 when
(a) Each node knows only the distances to its immediate neighbors.
(b) Each node has reported the information it had in the preceding step to its
immediate neighbors.
(c) Step (b) happens a second time.
17
For the network given in Figure 4.50, show how the link-state algorithm builds
the routing table for node D.
18
Suppose we have the forwarding tables shown in Table 4.12 for nodes A and
F, in a network where all links have cost 1. Give a diagram of the smallest
network consistent with these tables.
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A
F
Node
Cost
NextHop
Node
Cost
NextHop
B
1
B
A
3
E
C
2
B
B
2
C
D
1
D
C
1
C
E
2
B
D
2
E
F
3
D
E
1
E
Table 4.12
Forwarding tables for Exercise 18.
A
F
Node
Cost
NextHop
Node
Cost
NextHop
B
1
B
A
2
C
C
1
C
B
3
C
D
2
B
C
1
C
E
3
C
D
2
C
F
2
C
E
1
E
Table 4.13
✓
Forwarding tables for Exercise 19.
19
Suppose we have the forwarding tables shown in Table 4.13 for nodes A and
F, in a network where all links have cost 1. Give a diagram of the smallest
network consistent with these tables.
20
For the network in Figure 4.50, suppose the forwarding tables are all established as in Exercise 15 and then the C–E link fails. Give
(a) The tables of A, B, D, and F after C and E have reported the news.
(b) The tables of A and D after their next mutual exchange.
(c) The table of C after A exchanges with it.
21
Suppose a router has built up the routing table shown in Table 4.14. The
router can deliver packets directly over interfaces 0 and 1, or it can forward
packets to routers R2, R3, or R4. Describe what the router does with a packet
addressed to each of the following destinations:
Exercises
SubnetNumber
SubnetMask
NextHop
128.96.39.0
255.255.255.128
Interface 0
128.96.39.128
255.255.255.128
Interface 1
128.96.40.0
255.255.255.128
R2
192.4.153.0
255.255.255.192
R3
default
Table 4.14
R4
Routing table for Exercise 21.
SubnetNumber
SubnetMask
NextHop
128.96.170.0
255.255.254.0
Interface 0
128.96.168.0
255.255.254.0
Interface 1
128.96.166.0
255.255.254.0
R2
128.96.164.0
255.255.252.0
R3
default
Table 4.15
365
R4
Routing table for Exercise 22.
(a) 128.96.39.10.
(b) 128.96.40.12.
(c) 128.96.40.151.
(d) 192.4.153.17.
(e) 192.4.153.90.
✓
22
Suppose a router has built up the routing table shown in Table 4.15. The
router can deliver packets directly over interfaces 0 and 1, or it can forward
packets to routers R2, R3, or R4. Assume the router does the longest prefix
match. Describe what the router does with a packet addressed to each of the
following destinations:
(a) 128.96.171.92.
(b) 128.96.167.151.
(c) 128.96.163.151.
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Figure 4.52
Simple network for Exercise 23.
(d) 128.96.169.192.
(e) 128.96.165.121.
★
23
Consider the simple network in Figure 4.52, in which A and B exchange
distance-vector routing information. All links have cost 1. Suppose the A–E
link fails.
(a) Give a sequence of routing table updates that leads to a routing loop between A and B.
(b) Estimate the probability of the scenario in (a), assuming A and B send out
routing updates at random times, each at the same average rate.
(c) Estimate the probability of a loop forming if A broadcasts an updated
report within 1 second of discovering the A–E failure, and B broadcasts
every 60 seconds uniformly.
24
Consider the situation involving the creation of a routing loop in the network
of Figure 4.14 when the A–E link goes down. List all sequences of table updates among A, B, and C, pertaining to destination E, that lead to the loop.
Assume that table updates are done one at a time, that the split-horizon technique is observed by all participants, and that A sends its initial report of
E’s unreachability to B before C. You may ignore updates that don’t result in
changes.
25
Suppose a set of routers all use the split-horizon technique; we consider here
under what circumstances it makes a difference if they use poison reverse in
addition.
(a) Show that poison reverse makes no difference in the evolution of the routing loop in the two examples described in Section 4.2.2, given that the
hosts involved use split horizon.
(b) Suppose split-horizon routers A and B somehow reach a state in which
they forward traffic for a given destination X toward each other. Describe
how this situation will evolve with and without the use of poison reverse.
Exercises
Figure 4.53
Networks for Exercise 26.
Figure 4.54
Network for Exercise 27.
367
(c) Give a sequence of events that leads A and B to a looped state as in (b), even
if poison reverse is used. (Hint: Suppose B and A connect through a very
slow link. They each reach X through a third node, C, and simultaneously
advertise their routes to each other.)
26
Hold down is another distance-vector loop-avoidance technique, whereby hosts
ignore updates for a period of time until link failure news has had a chance to
propagate. Consider the networks in Figure 4.53, where all links have cost 1
except E–D with cost 10. Suppose that the E–A link breaks and B reports
its loop-forming E route to A immediately afterwards (this is the false route,
via A). Specify the details of a hold-down interpretation, and use this to describe the evolution of the routing loop in both networks. To what extent can
hold down prevent the loop in the EAB network without delaying the discovery of the alternative route in the EABD network?
27
Consider the network in Figure 4.54, using link-state routing. Suppose the
B–F link fails, and the following then occur in sequence:
(a) Node H is added to the right side with a connection to G.
(b) Node D is added to the left side with a connection to C.
(c) A new link D–A is added.
The failed B–F link is now restored. Describe what link-state packets will flood
back and forth. Assume that the initial sequence number at all nodes is 1, and
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Internetworking
Figure 4.55
Network for Exercise 28.
Figure 4.56
Network for Exercise 29.
Figure 4.57
Network for Exercise 30.
that no packets time out, and that both ends of a link use the same sequence
number in their LSP for that link, greater than any sequence number either
used before.
✓
28
Give the steps as in Table 4.9 in the forward search algorithm as it builds the
routing database for node A in the network shown in Figure 4.55.
29
Give the steps as in Table 4.9 in the forward search algorithm as it builds the
routing database for node A in the network shown in Figure 4.56.
30
Suppose that nodes in the network shown in Figure 4.57 participate in linkstate routing, and C receives contradictory LSPs: One from A arrives claiming
the A–B link is down, but one from B arrives claiming the A–B link is up.
(a) How could this happen?
(b) What should C do? What can C expect?
Do not assume that LSPs contain any synchronized timestamp.
Exercises
Figure 4.58
31
369
Network for Exercise 31.
Consider the network shown in Figure 4.58, in which horizontal lines represent transit providers and numbered vertical lines are interprovider links.
(a) How many routes to P could provider Q’s BGP speakers receive?
(b) Suppose Q and P adopt the policy that outbound traffic is routed to the
closest link to the destination’s provider, thus minimizing their own cost.
What paths will traffic from host A to host B and from host B to host A
take?
(c) What could Q do to have the B−→A traffic use the closer link 1?
(d) What could Q do to have the B−→A traffic pass through R?
★
32
Give an example of an arrangement of routers grouped into autonomous systems so that the path with the fewest hops from a point A to another point B
crosses the same AS twice. Explain what BGP would do with this situation.
33
Let A be the number of autonomous systems on the Internet, and let D (for
diameter) be the maximum AS path length.
(a) Give a connectivity √
model for which D is of order log A and another for
which D is of order A.
(b) Assuming each AS number is 2 bytes and each network number is 4 bytes,
give an estimate for the amount of data a BGP speaker must receive to
keep track of the AS path to every network. Express your answer in terms
of A, D, and the number of networks N .
34
Suppose IP routers learned about IP networks and subnets the way Ethernet
learning bridges learn about hosts: by noting the appearance of new ones, and
the interface by which they arrive. Compare this with existing distance-vector
router learning
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Internetworking
(a) For a leaf site with a single attachment to the Internet.
(b) For internal use at an organization that did not connect to the Internet.
Assume that routers only receive new-network notices from other routers, and
that the originating routers receive their IP network information via configuration.
35
IP hosts that are not designated routers are required to drop packets misaddressed to them, even if they would otherwise be able to forward them correctly. In the absence of this requirement, what would happen if a packet addressed to IP address A were inadvertently broadcast at the link layer? What
other justifications for this requirement can you think of?
36
Read the man page or other documentation for the Unix/Windows utility
netstat. Use netstat to display the current IP routing table on your host.
Explain the purpose of each entry. What is the practical minimum number of
entries?
37
Use the Unix utility traceroute (Windows tracert) to determine how
many hops it is from your host to other hosts in the Internet (e.g.,
cs.princeton.edu or www.cisco.com). How many routers do you traverse just to get out of your local site? Read the man page or other documentation for traceroute and explain how it is implemented.
38
What will happen if traceroute is used to find the path to an unassigned
address? Does it matter if the network portion or only the host portion is
unassigned?
39
A site is shown in Figure 4.59. R1 and R2 are routers; R2 connects to the
outside world. Individual LANs are Ethernets. RB is a bridge-router; it routes
traffic addressed to it and acts as a bridge for other traffic. Subnetting is used
inside the site; ARP is used on each subnet. Unfortunately, host A has been
misconfigured and doesn’t use subnets. Which of B, C, D can A reach?
40
An organization has a class C network 200.1.1 and wants to form subnets for
four departments, with hosts as follows:
A
72 hosts
B
35 hosts
C
20 hosts
D
18 hosts
Exercises
Figure 4.59
Site for Exercise 39.
Figure 4.60
Network for Exercise 41.
371
There are 145 hosts in all.
(a) Give a possible arrangement of subnet masks to make this possible.
(b) Suggest what the organization might do if department D grows to 34
hosts.
41
Suppose hosts A and B are on an Ethernet LAN with class C IP network
address 200.0.0. It is desired to attach a host C to the network via a direct
connection to B (see Figure 4.60). Explain how to do this with subnets; give
sample subnet assignments. Assume that an additional network address is not
available. What does this do to the size of the Ethernet LAN?
42
An alternative method for connecting host C in Exercise 41 is to use proxy
ARP and routing: B agrees to route traffic to and from C, and also answers
ARP queries for C received over the Ethernet.
(a) Give all packets sent, with physical addresses, as A uses ARP to locate and
then send one packet to C.
(b) Give B’s routing table. What peculiarity must it contain?
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Net/MaskLength
NextHop
C4.50.0.0/12
A
C4.5E.10.0/20
B
C4.60.0.0/12
C
C4.68.0.0/14
D
80.0.0.0/1
E
40.0.0.0/2
F
00.0.0.0/2
G
Table 4.16
Routing table for Exercise 45.
43
Propose a plausible addressing plan for IPv6 that runs out of bits. Specifically,
provide a diagram such as Figure 4.35, perhaps with additional ID fields, that
adds up to more than 128 bits, together with plausible justifications for the
size of each field. You may assume fields are divided on byte boundaries, and
that the InterfaceID is 64 bits. (Hint: Consider fields that would approach
maximum allocation only under unusual circumstances. Can you do this if the
InterfaceID is 48 bits?)
44
Suppose two subnets share the same physical LAN; hosts on each subnet will
see the other subnet’s broadcast packets.
(a) How will DHCP fare if two servers, one for each subnet, coexist on the
shared LAN? What problems might (do!) arise?
(b) Will ARP be affected by such sharing?
45
Table 4.16 is a routing table using CIDR. Address bytes are in hexadecimal.
The notation “/12” in C4.50.0.0/12 denotes a netmask with 12 leading 1 bits,
that is, FF.F0.0.0. Note that the last three entries cover every address and thus
serve in lieu of a default route. State to what next hop the following will be
delivered.
(a) C4.5E.13.87.
(b) C4.5E.22.09.
(c) C3.41.80.02.
Exercises
Net/MaskLength
NextHop
C4.5E.2.0/23
A
C4.5E.4.0/22
B
C4.5E.C0.0/19
C
C4.5E.40.0/18
D
C4.4C.0.0/14
E
C0.0.0.0/2
F
80.0.0.0/1
G
Table 4.17
373
Routing table for Exercise 46.
(d) 5E.43.91.12.
(e) C4.6D.31.2E.
(f ) C4.6B.31.2E.
✓
46
Table 4.17 is a routing table using CIDR. Address bytes are in hexadecimal.
The notation “/12” in C4.50.0.0/12 denotes a netmask with 12 leading 1 bits,
that is, FF.F0.0.0. State to what next hop the following will be delivered.
(a) C4.4B.31.2E.
(b) C4.5E.05.09.
(c) C4.4D.31.2E.
(d) C4.5E.03.87.
(e) C4.5E.7F.12.
(f ) C4.5E.D1.02.
47
Suppose P, Q, and R are network service providers, with respective CIDR
address allocations (using the notation of Exercise 45) C1.0.0.0/8, C2.0.0.0/8,
and C3.0.0.0/8. Each provider’s customers initially receive address allocations
that are a subset of the provider’s. P has the following customers:
PA, with allocation C1.A3.0.0/16;
PB, with allocation C1.B0.0.0/12.
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Q has the following customers:
QA, with allocation C2.0A.10.0/20;
QB, with allocation C2.0B.0.0/16.
Assume there are no other providers or customers.
(a) Give routing tables for P, Q, and R assuming each provider connects to
the other two.
(b) Now assume P is connected to Q and Q is connected to R, but P and R
are not directly connected. Give tables for P and R.
(c) Suppose customer PA acquires a direct link to Q, and QA acquires a direct
link to P, in addition to existing links. Give tables for P and Q, ignoring R.
48
In the previous problem, assume each provider connects to the other two.
Suppose customer PA switches to provider Q and customer QB switches to
provider R. Use the CIDR longest-match rule to give routing tables for all
three providers that allow PA and QB to switch without renumbering.
49
Suppose most of the Internet used some form of geographical addressing, but
that a large international organization has a single IP network address and
routes its internal traffic over its own links.
(a) Explain the routing inefficiency for the organization’s inbound traffic inherent in this situation.
(b) Explain how the organization might solve this problem for outbound
traffic.
(c) For your method above to work for inbound traffic, what would have to
happen?
(d) Suppose the large organization now changes its addressing to separate geographical addresses for each office. What will its internal routing structure
have to look like if internal traffic is still to be routed internally?
50
The telephone system uses geographical addressing. Why do you think this
wasn’t adopted as a matter of course by the Internet?
Exercises
51
375
Suppose a site A is multihomed, in that it has two Internet connections from
two different providers, P and Q. Provider-based addressing as in Exercise 47 is
used, and A takes its address assignment from P. Q has a CIDR longest-match
routing entry for A.
(a) Describe what inbound traffic might flow on the A–Q connection. Consider cases where Q does and does not advertise A to the world using BGP.
(b) What is the minimum advertising of its route to A that Q must do in
order for all inbound traffic to reach A via Q if the P–A link breaks?
(c) What problems must be overcome if A is to use both links for its outbound
traffic?
52
An ISP with a class B address is working with a new company to allocate
it a portion of address space based on CIDR. The new company needs IP
addresses for machines in three divisions of its corporate network: Engineering,
Marketing, and Sales. These divisions plan to grow as follows: Engineering has
five machines as of the start of year 1 and intends to add one machine every
week; Marketing will never need more than 16 machines; and Sales needs
one machine for every two clients. As of the start of year 1, the company
has no clients but the sales model indicates that by the start of year 2, the
company will have six clients and each week thereafter gets one new client
with probability 60%, loses one client with probability 20%, or maintains the
same number with probability 20%.
(a) What address range would be required to support the company’s growth
plans for at least 7 years if Marketing uses all 16 of its addresses and the
Sales and Engineering plans behave as expected?
(b) How long would this address assignment last? At the time when the company runs out of address space, how would the addresses be assigned to
the three groups?
(c) If CIDR addressing were not available for the 7-year plan, what options
would the new company have in terms of getting address space?
★
53
Propose a lookup algorithm for a CIDR forwarding table that does not require
a linear search of the entire table to find the longest match.
54
Suppose a network N within a larger organization A acquires its own direct
connection to an Internet service provider, in addition to an existing connec-
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tion via A. Let R1 be the router connecting N to its own provider, and let R2
be the router connecting N to the rest of A.
(a) Assuming N remains a subnet of A, how should R1 and R2 be configured?
What limitations would still exist with N’s use of its separate connection?
Would A be prevented from using N’s connection? Specify your configuration in terms of what R1 and R2 should advertise, and with what paths.
Assume a BGP-like mechanism is available.
(b) Now suppose N gets its own network number; how does this change your
answer in (a)?
(c) Describe a router configuration that would allow A to use N’s link when
its own link is down.
✓
55
How do routers determine that an incoming IP packet is to be multicast?
56
Suppose a multicast group is intended to be private to a particular routing domain. Can an IP multicast address be assigned to the group without consulting
with other domains with no risk of conflicts?
57
Under what conditions could a nonrouter host on an Ethernet receive an IP
multicast packet for a multicast group it has not joined?
58
Consider the example internet shown in Figure 4.61, in which sources D and
E send packets to multicast group G, whose members are shaded in gray. Show
the shortest-path multicast trees for each source.
59
Consider the example internet shown in Figure 4.62 in which sources S1 and
S2 send packets to multicast group G, whose members are shaded in gray.
Show the shortest-path multicast trees for each source.
60
Suppose host A is sending to a multicast group; the recipients are leaf nodes of
a tree rooted at A with depth N and with each nonleaf node having k children;
there are thus kN recipients.
(a) How many individual link transmissions are involved if A sends a multicast message to all recipients?
(b) How many individual link transmissions are involved if A sends unicast
messages to each individual recipient?
Exercises
Figure 4.61
377
Example internet for Exercise 58.
(c) Suppose A sends to all recipients, but some messages are lost and retransmission is necessary. Unicast retransmissions to what fraction of the recipients is equivalent, in terms of individual link transmissions, to a multicast
retransmission to all recipients?
61
The existing Internet depends in many respects on participants being good
“network citizens”—cooperating above and beyond adherence to standard
protocols.
(a) In the PIM-SM scheme, who determines when to create a source-specific
tree? How might this be a problem?
(b) In the PIM-SSM scheme, who determines when to create a source-specific
tree? Why is this presumably not a problem?
378
4
Internetworking
Figure 4.62
62
Example internet for Exercise 59.
(a) Draw an example internetwork where the BIDIR-PIM route from a
source’s router to a group member’s router is longer than the PIM-SM
source-specific route.
(b) Draw an example where they are the same.
63
Determine whether or not the following IPv6 address notations are correct:
(a) ::0F53:6382:AB00:67DB:BB27:7332.
(b) 7803:42F2:::88EC:D4BA:B75D:11CD.
(c) ::4BA8:95CC::DB97:4EAB.
(d) 74DC::02BA.
(e) ::00FF:128.112.92.116.
Exercises
379
64
Determine if your site is connected to the MBone. If so, investigate and experiment with any MBone tools, such as sdr, vat, and vic.
65
MPLS labels are usually 20 bits long. Explain why this provides enough labels
when MPLS is used for destination-based forwarding.
66
MPLS has sometimes been claimed to improve router performance. Explain
why this might be true, and suggest reasons why in practice this may not be
the case.
67
Assume that it takes 32 bits to carry each MPLS label that is added to a packet
when the “shim” header of Figure 4.44(b) is used.
(a) How many additional bytes are needed to tunnel a packet using the MPLS
techniques described in Section 4.5.3?
(b) How many additional bytes are needed, at a minimum, to tunnel a packet
using an additional IP header as described in Section 4.1.8?
(c) Calculate the efficiency of bandwidth usage for each of the two tunneling
approaches when the average packet size is 300 bytes. Repeat for 64-byte
packets. Bandwidth efficiency is defined as (payload bytes carried) ÷ (total
bytes carried).
68
RFC 791 describes the Internet Protocol, and includes two options for source
routing. Describe three disadvantages of using IP source route options compared to using MPLS for explicit routing. (Hint: The IP header including
options may be at most 15 words long.)
End-to-End Protocols
Victory is the beautiful, bright coloured flower. Transport is the
stem without which it could never have blossomed.
—Winston Churchill
T
he previous three chapters have described various technologies that can be
used to connect together a collection of computers: direct links (including
LAN technologies like Ethernet and token ring), packet-switched networks
(including cell-based networks like ATM), and internetworks. The next problem is
to turn this host-to-host packet delivery service into a process-to-process communication channel. This is the role played
P R O B L E M
by the transport level of the network
architecture, which, because it supGetting Processes to
ports communication between the
Communicate
end application programs, is sometimes called the end-to-end protocol.
Two forces shape the end-to-end protocol. From above, the application-level
processes that use its services have certain requirements. The following list itemizes some
of the common properties that a transport protocol can be expected to provide:
380
■
Guarantees message delivery.
■
Delivers messages in the same order they are sent.
■
Delivers at most one copy of each message.
■
Supports arbitrarily large messages.
■
Supports synchronization between the sender and the receiver.
■
Allows the receiver to apply flow control to the sender.
■
Supports multiple application processes on each host.
Note that this list does not include all the functionality that application processes might want from the network. For example, it
does not include security, which is typically provided by protocols
that sit above the transport level.
From below, the underlying network upon which the transport protocol operates has certain limitations in the level of service it can provide. Some of the more typical limitations of the
network are that it may
■
Drop messages.
■
Reorder messages.
■
Deliver duplicate copies of a given message.
■
Limit messages to some finite size.
■
Deliver messages after an arbitrarily long delay.
Such a network is said to provide a best-effort level of service, as
exemplified by the Internet.
The challenge, therefore, is to develop algorithms that turn
the less-than-desirable properties of the underlying network into
the high level of service required by application programs. Different transport protocols employ different combinations of these algorithms. This chapter looks at these algorithms in the context of
four representative services—a simple asynchronous demultiplexing service, a reliable byte-stream service, a request/reply service,
and a service for real-time applications.
In the case of the demultiplexing and byte-stream services,
we use the Internet’s UDP and TCP protocols, respectively, to
illustrate how these services are provided in practice. In the case
of a request/reply service, we discuss the role it plays in a Remote
Procedure Call (RPC) service, and what features that entails. This
discussion is capped off with a description of two widely used
RPC protocols, SunRPC and DCE-RPC.
Real-time applications make particular demands on the
transport protocol, such as the need to carry timing information
that allows audio or video samples to be played back at the appropriate point in time. The protocol that is most widely used for
this purpose is the Real-time Transport Protocol (RTP), which we
examine here. Finally, the chapter concludes with a section that
discusses the performance of the different transport protocols.
5
382
5.1
5
End-to-End Protocols
Simple Demultiplexer (UDP)
The simplest possible transport protocol is one that extends the host-to-host delivery service of the underlying network into a process-to-process communication service. There
are likely to be many processes running on any given host, so the protocol needs to
add a level of demultiplexing, thereby allowing multiple application processes on each
host to share the network. Aside from this requirement, the transport protocol adds no
other functionality to the best-effort service provided by the underlying network. The
Internet’s User Datagram Protocol (UDP) is an example of such a transport protocol.
The only interesting issue in such a protocol is the form of the address used to
identify the target process. Although it is possible for processes to directly identify each
other with an OS-assigned process ID (pid), such an approach is only practical in a
closed distributed system in which a single OS runs on all hosts and assigns each process
a unique ID. A more common approach, and the one used by UDP, is for processes
to indirectly identify each other using an abstract locater, often called a port or mailbox.
The basic idea is for a source process to send a message to a port and for the destination
process to receive the message from a port.
The header for an end-to-end protocol that implements this demultiplexing function typically contains an identifier (port) for both the sender (source) and the receiver
(destination) of the message. For example, the UDP header is given in Figure 5.1. Notice
that the UDP port field is only 16 bits long. This means that there are up to 64K possible ports, clearly not enough to identify all the processes on all the hosts in the Internet.
Fortunately, ports are not interpreted across the entire Internet, but only on a single host.
That is, a process is really identified by a port on some particular host—a port, host
pair. In fact, this pair constitutes the demultiplexing key for the UDP protocol.
The next issue is how a process learns the port for the process to which it wants
to send a message. Typically, a client process initiates a message exchange with a server
process. Once a client has contacted a server, the server knows the client’s port (it was
contained in the message header) and can reply to it. The real problem, therefore, is how
Figure 5.1
Format for UDP header.
5.1
Simple Demultiplexer (UDP)
383
the client learns the server’s port in the first place. A common approach is for the server
to accept messages at a well-known port. That is, each server receives its messages at some
fixed port that is widely published, much like the emergency telephone service available
at the well-known phone number 911. In the Internet, for example, the domain name
server (DNS) receives messages at well-known port 53 on each host, the mail service
listens for messages at port 25, and the Unix talk program accepts messages at wellknown port 517, and so on. This mapping is published periodically in an RFC and is
available on most Unix systems in file /etc/services. Sometimes a well-known port is
just the starting point for communication: The client and server use the well-known port
to agree on some other port that they will use for subsequent communication, leaving
the well-known port free for other clients.
An alternative strategy is to generalize this idea, so that there is only a single wellknown port—the one at which the “port mapper” service accepts messages. A client
would send a message to the port mapper’s well-known port asking for the port it should
use to talk to the “whatever” service, and the port mapper returns the appropriate port.
This strategy makes it easy to change the port associated with different services over time,
and for each host to use a different port for the same service.
As just mentioned, a port is purely an abstraction. Exactly how it is implemented
differs from system to system, or more precisely, from OS to OS. For example, the socket
API described in Chapter 1 is an example implementation of ports. Typically, a port is
implemented by a message queue, as illustrated in Figure 5.2. When a message arrives,
the protocol (e.g., UDP) appends the message to the end of the queue. Should the queue
be full, the message is discarded. There is no flow-control mechanism that tells the sender
to slow down. When an application process wants to receive a message, one is removed
from the front of the queue. If the queue is empty, the process blocks until a message
becomes available.
Finally, although UDP does not implement flow control or reliable/ordered delivery, it does a little more work than to simply demultiplex messages to some application
process—it also ensures the correctness of the message by the use of a checksum. (The
UDP checksum is optional in the current Internet, but it will become mandatory with
IPv6.) UDP computes its checksum over the UDP header, the contents of the message
body, and something called the pseudoheader. The pseudoheader consists of three fields
from the IP header—protocol number, source IP address, and destination IP address—
plus the UDP length field. (Yes, the UDP length field is included twice in the checksum
calculation.) UDP uses the same checksum algorithm as IP, as defined in Section 2.4.2.
The motivation behind having the pseudoheader is to verify that this message has been
delivered between the correct two endpoints. For example, if the destination IP address
was modified while the packet was in transit, causing the packet to be misdelivered, this
fact would be detected by the UDP checksum.
384
5
Figure 5.2
5.2
End-to-End Protocols
UDP message queue.
Reliable Byte Stream (TCP)
In contrast to a simple demultiplexing protocol like UDP, a more sophisticated transport
protocol is one that offers a reliable, connection-oriented, byte-stream service. Such a service has proven useful to a wide assortment of applications because it frees the application
from having to worry about missing or reordered data. The Internet’s Transmission Control Protocol (TCP) is probably the most widely used protocol of this type; it is also the
most carefully tuned. It is for these two reasons that this section studies TCP in detail,
although we identify and discuss alternative design choices at the end of the section.
In terms of the properties of transport protocols given in the problem statement
at the start of this chapter, TCP guarantees the reliable, in-order delivery of a stream of
bytes. It is a full-duplex protocol, meaning that each TCP connection supports a pair of
byte streams, one flowing in each direction. It also includes a flow-control mechanism for
each of these byte streams that allows the receiver to limit how much data the sender can
transmit at a given time. Finally, like UDP, TCP supports a demultiplexing mechanism
that allows multiple application programs on any given host to simultaneously carry on
a conversation with their peers. In addition to the above features, TCP also implements
a highly-tuned congestion-control mechanism. The idea of this mechanism is to throttle
how fast TCP sends data, not for the sake of keeping the sender from overrunning the
5.2
Reliable Byte Stream (TCP)
385
▲
receiver, but to keep the sender from overloading the network. A description of TCP’s
congestion-control mechanism is postponed until Chapter 6, where we discuss it in the
larger context of how network resources are fairly allocated.
Since many people confuse congestion control and flow control, we restate the
difference. Flow control involves preventing senders from overrunning the capacity of
receivers. Congestion control involves preventing too much data from being injected into
the network, thereby causing switches or links to become overloaded. Thus, flow control is an end-to-end issue, while congestion control is concerned with how hosts and
networks interact.
5.2.1 End-to-End Issues
At the heart of TCP is the sliding window algorithm. Even though this is the same
basic algorithm we saw in Section 2.5.2, because TCP runs over the Internet rather than
a point-to-point link, there are many important differences. This subsection identifies
these differences and explains how they complicate TCP. The following subsections then
describe how TCP addresses these, and other complications.
First, whereas the sliding window algorithm presented in Section 2.5.2 runs over a
single physical link that always connects the same two computers, TCP supports logical
connections between processes that are running on any two computers in the Internet.
This means that TCP needs an explicit connection establishment phase during which
the two sides of the connection agree to exchange data with each other. This difference is
analogous to having to dial up the other party rather than having a dedicated phone line.
TCP also has an explicit connection teardown phase. One of the things that happens
during connection establishment is that the two parties establish some shared state to
enable the sliding window algorithm to begin. Connection teardown is needed so each
host knows it is OK to free this state.
Second, whereas a single physical link that always connects the same two computers
has a fixed RTT, TCP connections are likely to have widely different round-trip times.
For example, a TCP connection between a host in San Francisco and a host in Boston,
which are separated by several thousand kilometers, might have an RTT of 100 ms, while
a TCP connection between two hosts in the same room, only a few meters apart, might
have an RTT of only 1 ms. The same TCP protocol must be able to support both of
these connections. To make matters worse, the TCP connection between hosts in San
Francisco and Boston might have an RTT of 100 ms at 3 A.M., but an RTT of 500 ms at
3 P.M. Variations in the RTT are even possible during a single TCP connection that lasts
only a few minutes. What this means to the sliding window algorithm is that the timeout
mechanism that triggers retransmissions must be adaptive. (Certainly, the timeout for a
point-to-point link must be a settable parameter, but it is not necessary to adapt this
timer for a particular pair of nodes.)
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5
End-to-End Protocols
A third difference is that packets may be reordered as they cross the Internet, but
this is not possible on a point-to-point link where the first packet put into one end of
the link must be the first to appear at the other end. Packets that are slightly out of order
do not cause a problem since the sliding window algorithm can reorder packets correctly
using the sequence number. The real issue is how far out-of-order packets can get, or
said another way, how late a packet can arrive at the destination. In the worst case, a
packet can be delayed in the Internet until IP’s time to live (TTL) field expires, at which
time the packet is discarded (and hence there is no danger of it arriving late). Knowing
that IP throws packets away after their TTL expires, TCP assumes that each packet has a
maximum lifetime. The exact lifetime, known as the maximum segment lifetime (MSL), is
an engineering choice. The current recommended setting is 120 seconds. Keep in mind
that IP does not directly enforce this 120-second value; it is simply a conservative estimate that TCP makes of how long a packet might live in the Internet. The implication
is significant—TCP has to be prepared for very old packets to suddenly show up at the
receiver, potentially confusing the sliding window algorithm.
Fourth, the computers connected to a point-to-point link are generally engineered
to support the link. For example, if a link’s delay × bandwidth product is computed
to be 8 KB—meaning that a window size is selected to allow up to 8 KB of data to be
unacknowledged at a given time—then it is likely that the computers at either end of
the link have the ability to buffer up to 8 KB of data. Designing the system otherwise
would be silly. On the other hand, almost any kind of computer can be connected to the
Internet, making the amount of resources dedicated to any one TCP connection highly
variable, especially considering that any one host can potentially support hundreds of
TCP connections at the same time. This means that TCP must include a mechanism
that each side uses to “learn” what resources (e.g., how much buffer space) the other side
is able to apply to the connection. This is the flow control issue.
Fifth, because the transmitting side of a directly connected link cannot send any
faster than the bandwidth of the link allows, and only one host is pumping data into
the link, it is not possible to unknowingly congest the link. Said another way, the load
on the link is visible in the form of a queue of packets at the sender. In contrast, the
sending side of a TCP connection has no idea what links will be traversed to reach the
destination. For example, the sending machine might be directly connected to a relatively
fast Ethernet—and so, capable of sending data at a rate of 100 Mbps—but somewhere
out in the middle of the network, a 1.5-Mbps T1 link must be traversed. And to make
matters worse, data being generated by many different sources might be trying to traverse
this same slow link. This leads to the problem of network congestion. Discussion of this
topic is delayed until Chapter 6.
We conclude this discussion of end-to-end issues by comparing TCP’s approach to
providing a reliable/ordered delivery service with the approach used by X.25 networks.
5.2
Reliable Byte Stream (TCP)
387
▲
In TCP, the underlying IP network is assumed to be unreliable and to deliver messages
out of order; TCP uses the sliding window algorithm on an end-to-end basis to provide
reliable/ordered delivery. In contrast, X.25 networks use the sliding window protocol
within the network, on a hop-by-hop basis. The assumption behind this approach is
that if messages are delivered reliably and in order between each pair of nodes along the
path between the source host and the destination host, then the end-to-end service also
guarantees reliable/ordered delivery.
The problem with this latter approach is that a sequence of hop-by-hop guarantees
does not necessarily add up to an end-to-end guarantee. First, if a heterogeneous link
(say, an Ethernet) is added to one end of the path, then there is no guarantee that this
hop will preserve the same service as the other hops. Second, just because the sliding window protocol guarantees that messages are delivered correctly from node A to node B,
and then from node B to node C, it does not guarantee that node B behaves perfectly.
For example, network nodes have been known to introduce errors into messages while
transferring them from an input buffer to an output buffer. They have also been known
to accidentally reorder messages. As a consequence of these small windows of vulnerability, it is still necessary to provide true end-to-end checks to guarantee reliable/ordered
service, even though the lower levels of the system also implement that functionality.
This discussion serves to illustrate one of the most important principles in system
design—the end-to-end argument. In a nutshell, the end-to-end argument says that a
function (in our example, providing reliable/ordered delivery) should not be provided in
the lower levels of the system unless it can be completely and correctly implemented at
that level. Therefore, this rule argues in favor of the TCP/IP approach. This rule is not
absolute, however. It does allow for functions to be incompletely provided at a low level
as a performance optimization. This is why it is perfectly consistent with the end-to-end
argument to perform error detection (e.g., CRC) on a hop-by-hop basis; detecting and
retransmitting a single corrupt packet across one hop is preferable to having to retransmit
an entire file end-to-end.
5.2.2 Segment Format
TCP is a byte-oriented protocol, which means that the sender writes bytes into a TCP
connection and the receiver reads bytes out of the TCP connection. Although “byte
stream” describes the service TCP offers to application processes, TCP does not itself
transmit individual bytes over the Internet. Instead, TCP on the source host buffers
enough bytes from the sending process to fill a reasonably sized packet and then sends
this packet to its peer on the destination host. TCP on the destination host then empties
the contents of the packet into a receive buffer, and the receiving process reads from this
buffer at its leisure. This situation is illustrated in Figure 5.3, which, for simplicity, shows
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5
End-to-End Protocols
Figure 5.3
How TCP manages a byte stream.
Figure 5.4
TCP header format.
data flowing in only one direction. Remember that, in general, a single TCP connection
supports byte streams flowing in both directions.
The packets exchanged between TCP peers in Figure 5.3 are called segments, since
each one carries a segment of the byte stream. Each TCP segment contains the header
schematically depicted in Figure 5.4. The relevance of most of these fields will become
apparent throughout this section. For now, we simply introduce them.
The SrcPort and DstPort fields identify the source and destination ports, respectively, just as in UDP. These two fields, plus the source and destination IP addresses,
5.2
Reliable Byte Stream (TCP)
389
Figure 5.5 Simplified illustration (showing only one direction) of the TCP process,
with data flow in one direction and ACKs in the other.
combine to uniquely identify each TCP connection. That is, TCP’s demux key is given
by the 4-tuple
SrcPort, SrcIPAddr, DstPort, DstIPAddr
Note that because TCP connections come and go, it is possible for a connection between
a particular pair of ports to be established, used to send and receive data, and closed, and
then at a later time for the same pair of ports to be involved in a second connection. We
sometimes refer to this situation as two different incarnations of the same connection.
The Acknowledgment, SequenceNum, and AdvertisedWindow fields
are all involved in TCP’s sliding window algorithm. Because TCP is a byte-oriented protocol, each byte of data has a sequence number; the SequenceNum field contains the
sequence number for the first byte of data carried in that segment. The Acknowledgment and AdvertisedWindow fields carry information about the flow of data going
in the other direction. To simplify our discussion, we ignore the fact that data can flow
in both directions, and we concentrate on data that has a particular SequenceNum
flowing in one direction and Acknowledgment and AdvertisedWindow values
flowing in the opposite direction, as illustrated in Figure 5.5. The use of these three
fields is described more fully in Section 5.2.4.
The 6-bit Flags field is used to relay control information between TCP peers. The
possible flags include SYN, FIN, RESET, PUSH, URG, and ACK. The SYN and FIN
flags are used when establishing and terminating a TCP connection, respectively. Their
use is described in Section 5.2.3. The ACK flag is set any time the Acknowledgment
field is valid, implying that the receiver should pay attention to it. The URG flag signifies that this segment contains urgent data. When this flag is set, the UrgPtr field
indicates where the nonurgent data contained in this segment begins. The urgent data is
contained at the front of the segment body, up to and including a value of UrgPtr bytes
into the segment. The PUSH flag signifies that the sender invoked the push operation,
which indicates to the receiving side of TCP that it should notify the receiving process
of this fact. We discuss these last two features more in Section 5.2.7. Finally, the RESET
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End-to-End Protocols
flag signifies that the receiver has become confused—for example, because it received a
segment it did not expect to receive—and so wants to abort the connection.
Finally, the Checksum field is used in exactly the same way as for UDP—it is
computed over the TCP header, the TCP data, and the pseudoheader, which is made
up of the source address, destination address, and length fields from the IP header. The
checksum is required for TCP in both IPv4 and IPv6. Also, since the TCP header is of
variable length (options can be attached after the mandatory fields), a HdrLen field is
included that gives the length of the header in 32-bit words. This field is also known as
the Offset field, since it measures the offset from the start of the packet to the start of
the data.
5.2.3 Connection Establishment and Termination
A TCP connection begins with a client (caller) doing an active open to a server (callee).
Assuming that the server had earlier done a passive open, the two sides engage in an
exchange of messages to establish the connection. (Recall from Chapter 1 that a party
wanting to initiate a connection performs an active open, while a party willing to accept
a connection does a passive open.) Only after this connection establishment phase is over
do the two sides begin sending data. Likewise, as soon as a participant is done sending
data, it closes one direction of the connection, which causes TCP to initiate a round of
connection termination messages. Notice that while connection setup is an asymmetric
activity (one side does a passive open and the other side does an active open) connection
teardown is symmetric (each side has to close the connection independently).1 Therefore,
it is possible for one side to have done a close, meaning that it can no longer send data,
but for the other side to keep the other half of the bidirectional connection open and to
continue sending data.
Three-Way Handshake
The algorithm used by TCP to establish and terminate a connection is called a three-way
handshake. We first describe the basic algorithm and then show how it is used by TCP.
The three-way handshake involves the exchange of three messages between the client and
the server, as illustrated by the timeline given in Figure 5.6.
The idea is that two parties want to agree on a set of parameters, which, in the
case of opening a TCP connection, are the starting sequence numbers the two sides
plan to use for their respective byte streams. In general, the parameters might be any
facts that each side wants the other to know about. First, the client (the active participant) sends a segment to the server (the passive participant) stating the initial sequence number it plans to use (Flags = SYN, SequenceNum = x). The server
1 To be more precise, connection setup can be symmetric, with both sides trying to open the connection at the same time,
but the common case is for one side to do an active open and the other side to do a passive open.
5.2
Figure 5.6
Reliable Byte Stream (TCP)
391
Timeline for three-way handshake algorithm.
then responds with a single segment that both acknowledges the client’s sequence number (Flags = ACK, Ack = x + 1) and states its own beginning sequence number
(Flags = SYN, SequenceNum = y). That is, both the SYN and ACK bits are set in
the Flags field of this second message. Finally, the client responds with a third segment
that acknowledges the server’s sequence number (Flags = ACK, Ack = y + 1). The
reason that each side acknowledges a sequence number that is one larger than the one
sent is that the Acknowledgment field actually identifies the “next sequence number
expected,” thereby implicitly acknowledging all earlier sequence numbers. Although not
shown in this timeline, a timer is scheduled for each of the first two segments, and if the
expected response is not received, the segment is retransmitted.
You may be asking yourself why the client and server have to exchange starting
sequence numbers with each other at connection setup time. It would be simpler if each
side simply started at some “well-known” sequence number, such as 0. In fact, the TCP
specification requires that each side of a connection select an initial starting sequence
number at random. The reason for this is to protect against two incarnations of the same
connection reusing the same sequence numbers too soon; that is, while there is still a
chance that a segment from an earlier incarnation of a connection might interfere with
a later incarnation of the connection.
State-Transition Diagram
TCP is complex enough that its specification includes a state-transition diagram. A copy
of this diagram is given in Figure 5.7. This diagram shows only the states involved in
opening a connection (everything above ESTABLISHED) and in closing a connection (everything below ESTABLISHED). Everything that goes on while a connection
is open—that is, the operation of the sliding window algorithm—is hidden in the ESTABLISHED state.
392
5
Figure 5.7
End-to-End Protocols
TCP state-transition diagram.
TCP’s state-transition diagram is fairly easy to understand. Each circle denotes a
state that one end of a TCP connection can find itself in. All connections start in the
CLOSED state. As the connection progresses, the connection moves from state to state
according to the arcs. Each arc is labeled with a tag of the form event/action. Thus, if a
connection is in the LISTEN state and a SYN segment arrives (i.e., a segment with the
SYN flag set), the connection makes a transition to the SYN_RCVD state and takes the
action of replying with an ACK + SYN segment.
Notice that two kinds of events trigger a state transition: (1) a segment arrives from
the peer (e.g., the event on the arc from LISTEN to SYN_RCVD), or (2) the local
application process invokes an operation on TCP (e.g., the active open event on the arc
from CLOSE to SYN_SENT). In other words, TCP’s state-transition diagram effectively
defines the semantics of both its peer-to-peer interface and its service interface, as defined
in Section 1.3.1. The syntax of these two interfaces is given by the segment format (as
5.2
Reliable Byte Stream (TCP)
393
illustrated in Figure 5.4), and by some application programming interface (an example
of which is given in Section 1.4.1), respectively.
Now let’s trace the typical transitions taken through the diagram in Figure 5.7.
Keep in mind that at each end of the connection, TCP makes different transitions from
state to state. When opening a connection, the server first invokes a passive open operation on TCP, which causes TCP to move to the LISTEN state. At some later time, the
client does an active open, which causes its end of the connection to send a SYN segment
to the server and to move to the SYN_SENT state. When the SYN segment arrives at
the server, it moves to the SYN_RCVD state and responds with a SYN + ACK segment.
The arrival of this segment causes the client to move to the ESTABLISHED state and to
send an ACK back to the server. When this ACK arrives, the server finally moves to the
ESTABLISHED state. In other words, we have just traced the three-way handshake.
There are three things to notice about the connection establishment half of the
state-transition diagram. First, if the client’s ACK to the server is lost, corresponding to
the third leg of the three-way handshake, then the connection still functions correctly.
This is because the client side is already in the ESTABLISHED state, so the local application process can start sending data to the other end. Each of these data segments will
have the ACK flag set, and the correct value in the Acknowledgment field, so the
server will move to the ESTABLISHED state when the first data segment arrives. This
is actually an important point about TCP—every segment reports what sequence number the sender is expecting to see next, even if this repeats the same sequence number
contained in one or more previous segments.
The second thing to notice about the state-transition diagram is that there is a
funny transition out of the LISTEN state whenever the local process invokes a send
operation on TCP. That is, it is possible for a passive participant to identify both ends of
the connection (i.e., itself and the remote participant that it is willing to have connect
to it), and then for it to change its mind about waiting for the other side and instead
actively establish the connection. To the best of our knowledge, this is a feature of TCP
that no application process actually takes advantage of.
The final thing to notice about the diagram is the arcs that are not shown. Specifically, most of the states that involve sending a segment to the other side also schedule
a timeout that eventually causes the segment to be resent if the expected response does
not happen. These retransmissions are not depicted in the state-transition diagram. If
after several tries the expected response does not arrive, TCP gives up and returns to the
CLOSED state.
Turning our attention now to the process of terminating a connection, the important thing to keep in mind is that the application process on both sides of the connection
must independently close its half of the connection. If only one side closes the connection, then this means it has no more data to send, but it is still available to receive data
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from the other side. This complicates the state-transition diagram because it must account for the possibility that the two sides invoke the close operator at the same time, as
well as the possibility that first one side invokes close and then, at some later time, the
other side invokes close. Thus, on any one side there are three combinations of transitions that get a connection from the ESTABLISHED state to the CLOSED state:
■
This side closes first: ESTABLISHED → FIN_WAIT_1 → FIN_WAIT_2 →
TIME_WAIT → CLOSED.
■
The other side closes first: ESTABLISHED → CLOSE_WAIT → LAST_ACK
→ CLOSED.
■
Both sides close at the same time: ESTABLISHED → FIN_WAIT_1 →
CLOSING → TIME_WAIT → CLOSED.
There is actually a fourth, although rare, sequence of transitions that leads to the
CLOSED state; it follows the arc from FIN_WAIT_1 to TIME_WAIT. We leave it as
an exercise for you to figure out what combination of circumstances leads to this fourth
possibility.
The main thing to recognize about connection teardown is that a connection in
the TIME_WAIT state cannot move to the CLOSED state until it has waited for two
times the maximum amount of time an IP datagram might live in the Internet (i.e.,
120 seconds). The reason for this is that while the local side of the connection has sent
an ACK in response to the other side’s FIN segment, it does not know that the ACK was
successfully delivered. As a consequence, the other side might retransmit its FIN segment, and this second FIN segment might be delayed in the network. If the connection
were allowed to move directly to the CLOSED state, then another pair of application
processes might come along and open the same connection (i.e., use the same pair of port
numbers), and the delayed FIN segment from the earlier incarnation of the connection
would immediately initiate the termination of the later incarnation of that connection.
5.2.4 Sliding Window Revisited
We are now ready to discuss TCP’s variant of the sliding window algorithm, which serves
several purposes: (1) it guarantees the reliable delivery of data, (2) it ensures that data is
delivered in order, and (3) it enforces flow control between the sender and the receiver.
TCP’s use of the sliding window algorithm is the same as we saw in Section 2.5.2 in the
case of the first two of these three functions. Where TCP differs from the earlier algorithm is that it folds the flow-control function in as well. In particular, rather than having
a fixed-size sliding window, the receiver advertises a window size to the sender. This is
5.2
Figure 5.8
Reliable Byte Stream (TCP)
395
Relationship between TCP send buffer (a) and receive buffer (b).
done using the AdvertisedWindow field in the TCP header. The sender is then limited to having no more than a value of AdvertisedWindow bytes of unacknowledged
data at any given time. The receiver selects a suitable value for AdvertisedWindow
based on the amount of memory allocated to the connection for the purpose of buffering
data. The idea is to keep the sender from overrunning the receiver’s buffer. We discuss
this at greater length below.
Reliable and Ordered Delivery
To see how the sending and receiving sides of TCP interact with each other to implement
reliable and ordered delivery, consider the situation illustrated in Figure 5.8. TCP on the
sending side maintains a send buffer. This buffer is used to store data that has been
sent but not yet acknowledged, as well as data that has been written by the sending
application, but not transmitted. On the receiving side, TCP maintains a receive buffer.
This buffer holds data that arrives out of order, as well as data that is in the correct order
(i.e., there are no missing bytes earlier in the stream) but that the application process has
not yet had the chance to read.
To make the following discussion simpler to follow, we initially ignore the fact
that both the buffers and the sequence numbers are of some finite size, and hence will
eventually wrap around. Also, we do not distinguish between a pointer into a buffer
where a particular byte of data is stored and the sequence number for that byte.
Looking first at the sending side, three pointers are maintained into the send
buffer, each with an obvious meaning: LastByteAcked, LastByteSent, and LastByteWritten. Clearly,
LastByteAcked ≤ LastByteSent
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since the receiver cannot have acknowledged a byte that has not yet been sent, and
LastByteSent ≤ LastByteWritten
since TCP cannot send a byte that the application process has not yet written. Also note
that none of the bytes to the left of LastByteAcked need to be saved in the buffer
because they have already been acknowledged, and none of the bytes to the right of
LastByteWritten need to be buffered because they have not yet been generated.
A similar set of pointers (sequence numbers) are maintained on the receiving side:
LastByteRead, NextByteExpected, and LastByteRcvd. The inequalities are a
little less intuitive, however, because of the problem of out-of-order delivery. The first
relationship
LastByteRead < NextByteExpected
is true because a byte cannot be read by the application until it is received and all preceding bytes have also been received. NextByteExpected points to the byte immediately
after the latest byte to meet this criterion. Second,
NextByteExpected ≤ LastByteRcvd + 1
since, if data has arrived in order, NextByteExpected points to the byte after LastByteRcvd, whereas if data has arrived out of order, NextByteExpected points to
the start of the first gap in the data, as in Figure 5.8. Note that bytes to the left of
LastByteRead need not be buffered because they have already been read by the local application process, and bytes to the right of LastByteRcvd need not be buffered
because they have not yet arrived.
Flow Control
Most of the above discussion is similar to that found in Section 2.5.2; the only real
difference is that this time we elaborated on the fact that the sending and receiving
application processes are filling and emptying their local buffer, respectively. (The earlier
discussion glossed over the fact that data arriving from an upstream node was filling the
send buffer, and data being transmitted to a downstream node was emptying the receive
buffer.)
You should make sure you understand this much before proceeding because now
comes the point where the two algorithms differ more significantly. In what follows, we
reintroduce the fact that both buffers are of some finite size, denoted MaxSendBuffer
and MaxRcvBuffer, although we don’t worry about the details of how they are implemented. In other words, we are only interested in the number of bytes being buffered,
not in where those bytes are actually stored.
5.2
Reliable Byte Stream (TCP)
397
Recall that in a sliding window protocol, the size of the window sets the amount of
data that can be sent without waiting for acknowledgment from the receiver. Thus, the
receiver throttles the sender by advertising a window that is no larger than the amount
of data that it can buffer. Observe that TCP on the receive side must keep
LastByteRcvd − LastByteRead ≤ MaxRcvBuffer
to avoid overflowing its buffer. It therefore advertises a window size of
AdvertisedWindow
= MaxRcvBuffer − ((NextByteExpected − 1) − LastByteRead)
which represents the amount of free space remaining in its buffer. As data arrives, the
receiver acknowledges it as long as all the preceding bytes have also arrived. In addition,
LastByteRcvd moves to the right (is incremented), meaning that the advertised window potentially shrinks. Whether or not it shrinks depends on how fast the local application process is consuming data. If the local process is reading data just as fast as it arrives
(causing LastByteRead to be incremented at the same rate as LastByteRcvd), then
the advertised window stays open (i.e., AdvertisedWindow = MaxRcvBuffer). If,
however, the receiving process falls behind, perhaps because it performs a very expensive
operation on each byte of data that it reads, then the advertised window grows smaller
with every segment that arrives, until it eventually goes to 0.
TCP on the send side must then adhere to the advertised window it gets from the
receiver. This means that at any given time, it must ensure that
LastByteSent − LastByteAcked ≤ AdvertisedWindow
Said another way, the sender computes an effective window that limits how much data it
can send:
EffectiveWindow = AdvertisedWindow−(LastByteSent−LastByteAcked)
Clearly, EffectiveWindow must be greater than 0 before the source can send more
data. It is possible, therefore, that a segment arrives acknowledging x bytes, thereby allowing the sender to increment LastByteAcked by x, but because the receiving process
was not reading any data, the advertised window is now x bytes smaller than the time
before. In such a situation, the sender would be able to free buffer space, but not to send
any more data.
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All the while this is going on, the send side must also make sure that the local
application process does not overflow the send buffer; that is, that
LastByteWritten − LastByteAcked ≤ MaxSendBuffer
If the sending process tries to write y bytes to TCP, but
(LastByteWritten − LastByteAcked) + y > MaxSendBuffer
▲
then TCP blocks the sending process and does not allow it to generate more data.
It is now possible to understand how a slow receiving process ultimately stops a
fast sending process. First, the receive buffer fills up, which means the advertised window
shrinks to 0. An advertised window of 0 means that the sending side cannot transmit any
data, even though data it has previously sent has been successfully acknowledged. Finally,
not being able to transmit any data means that the send buffer fills up, which ultimately
causes TCP to block the sending process. As soon as the receiving process starts to read
data again, the receive-side TCP is able to open its window back up, which allows the
send-side TCP to transmit data out of its buffer. When this data is eventually acknowledged, LastByteAcked is incremented, the buffer space holding this acknowledged
data becomes free, and the sending process is unblocked and allowed to proceed.
There is only one remaining detail that must be resolved—how does the sending
side know that the advertised window is no longer 0? As mentioned above, TCP always
sends a segment in response to a received data segment, and this response contains the
latest values for the Acknowledge and AdvertisedWindow fields, even if these
values have not changed since the last time they were sent. The problem is this. Once
the receive side has advertised a window size of 0, the sender is not permitted to send
any more data, which means it has no way to discover that the advertised window is no
longer 0 at some time in the future. TCP on the receive side does not spontaneously
send nondata segments; it only sends them in response to an arriving data segment.
TCP deals with this situation as follows. Whenever the other side advertises a window size of 0, the sending side persists in sending a segment with 1 byte of data every so
often. It knows that this data will probably not be accepted, but it tries anyway, because
each of these 1-byte segments triggers a response that contains the current advertised
window. Eventually, one of these 1-byte probes triggers a response that reports a nonzero
advertised window.
Note that the reason the sending side periodically sends this probe segment is that
TCP is designed to make the receive side as simple as possible—it simply responds to
segments from the sender, and it never initiates any activity on its own. This is an example of a well-recognized (although not universally applied) protocol design rule, which,
5.2
Reliable Byte Stream (TCP)
399
for lack of a better name, we call the smart sender/dumb receiver rule. Recall that we saw
another example of this rule when we discussed the use of NAKs in Section 2.5.2.
Protecting Against Wraparound
This subsection and the next consider the size of the SequenceNum and AdvertisedWindow fields and the implications of their sizes on TCP’s correctness and performance. TCP’s SequenceNum field is 32 bits long, and its AdvertisedWindow
field is 16 bits long, meaning that TCP has easily satisfied the requirement of the sliding
window algorithm that the sequence number space be twice as big as the window size:
232 2 × 216 . However, this requirement is not the interesting thing about these two
fields. Consider each field in turn.
The relevance of the 32-bit sequence number space is that the sequence number
used on a given connection might wrap around—a byte with sequence number x could
be sent at one time, and then at a later time a second byte with the same sequence number
x might be sent. Once again, we assume that packets cannot survive in the Internet for
longer than the recommended MSL. Thus, we currently need to make sure that the
sequence number does not wrap around within a 120-second period of time. Whether
or not this happens depends on how fast data can be transmitted over the Internet;
that is, how fast the 32-bit sequence number space can be consumed. (This discussion
assumes that we are trying to consume the sequence number space as fast as possible, but
of course we will be if we are doing our job of keeping the pipe full.) Table 5.1 shows
how long it takes for the sequence number to wrap around on networks with various
bandwidths.
As you can see, the 32-bit sequence number space is adequate for most situations of
today’s networks, but given that OC-192 links exist in the Internet backbone, and that
some servers now come with Gigabit Ethernet interfaces, it is getting close to the point
Bandwidth
Time until Wraparound
T1 (1.5 Mbps)
6.4 hours
Ethernet (10 Mbps)
57 minutes
T3 (45 Mbps)
13 minutes
Fast Ethernet (100 Mbps)
6 minutes
OC-3 (155 Mbps)
4 minutes
OC-12 (622 Mbps)
55 seconds
OC-48 (2.5 Gbps)
14 seconds
Table 5.1
Time until 32-bit sequence number space wraps around.
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Bandwidth
Delay × Bandwidth Product
T1 (1.5 Mbps)
18 KB
Ethernet (10 Mbps)
122 KB
T3 (45 Mbps)
549 KB
Fast Ethernet (100 Mbps)
1.2 MB
OC-3 (155 Mbps)
1.8 MB
OC-12 (622 Mbps)
7.4 MB
OC-48 (2.5 Gbps)
29.6 MB
Table 5.2
Required window size for 100-ms RTT.
where 32 bits is too small. Fortunately, the IETF has already worked out an extension to
TCP that effectively extends the sequence number space to protect against the sequence
number wrapping around. This and related extensions are described in Section 5.2.8.
Keeping the Pipe Full
The relevance of the 16-bit AdvertisedWindow field is that it must be big enough to
allow the sender to keep the pipe full. Clearly, the receiver is free to not open the window
as large as the AdvertisedWindow field allows; we are interested in the situation in
which the receiver has enough buffer space to handle as much data as the largest possible
AdvertisedWindow allows.
In this case, it is not just the network bandwidth but the delay × bandwidth product that dictates how big the AdvertisedWindow field needs to be—the window
needs to be opened far enough to allow a full delay × bandwidth product’s worth of data
to be transmitted. Assuming an RTT of 100 ms (a typical number for a cross-country
connection in the United States), Table 5.2 gives the delay × bandwidth product for
several network technologies.
As you can see, TCP’s AdvertisedWindow field is in even worse shape than its
SequenceNum field—it is not big enough to handle even a T3 connection across the
continental United States, since a 16-bit field allows us to advertise a window of only
64 KB. The very same TCP extension mentioned above (see also Section 5.2.8) provides
a mechanism for effectively increasing the size of the advertised window.
5.2.5 Triggering Transmission
We next consider a surprisingly subtle issue: how TCP decides to transmit a segment. As
described earlier, TCP supports a byte-stream abstraction; that is, application programs
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401
write bytes into the stream, and it is up to TCP to decide that it has enough bytes to
send a segment. What factors govern this decision?
If we ignore the possibility of flow control—that is, we assume the window is
wide open, as would be the case when a connection first starts—then TCP has three
mechanisms to trigger the transmission of a segment. First, TCP maintains a variable,
typically called the maximum segment size (MSS), and it sends a segment as soon as it
has collected MSS bytes from the sending process. MSS is usually set to the size of the
largest segment TCP can send without causing the local IP to fragment. That is, MSS
is set to the MTU of the directly connected network, minus the size of the TCP and IP
headers. The second thing that triggers TCP to transmit a segment is that the sending
process has explicitly asked it to do so. Specifically, TCP supports a push operation, and
the sending process invokes this operation to effectively flush the buffer of unsent bytes.
The final trigger for transmitting a segment is that a timer fires; the resulting segment
contains as many bytes as are currently buffered for transmission. However, as we will
soon see, this “timer” isn’t exactly what you expect.
Silly Window Syndrome
Of course, we can’t just ignore flow control, which plays an obvious role in throttling
the sender. If the sender has MSS bytes of data to send and the window is open at least
that much, then the sender transmits a full segment. Suppose, however, that the sender
is accumulating bytes to send, but the window is currently closed. Now suppose an ACK
arrives that effectively opens the window enough for the sender to transmit, say, MSS/2
bytes. Should the sender transmit a half-full segment, or wait for the window to open to a
full MSS? The original specification was silent on this point, and early implementations
of TCP decided to go ahead and transmit a half-full segment. After all, there is no telling
how long it will be before the window opens further.
It turns out that the strategy of aggressively taking advantage of any available window leads to a situation now known as the silly window syndrome. Figure 5.9 helps visualize what happens. If you think of a TCP stream as a conveyer belt with “full” containers
(data segments) going in one direction and empty containers (ACKs) going in the reverse
direction, then MSS-sized segments correspond to large containers and 1-byte segments
correspond to very small containers. If the sender aggressively fills an empty container
as soon as it arrives, then any small container introduced into the system remains in the
system indefinitely. That is, it is immediately filled and emptied at each end, and never
coalesced with adjacent containers to create larger containers. This scenario was discovered when early implementations of TCP regularly found themselves filling the network
with tiny segments.
Note that the silly window syndrome is only a problem when either the sender
transmits a small segment or the receiver opens the window a small amount. If neither
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Figure 5.9
End-to-End Protocols
Silly window syndrome.
of these happens, then the small container is never introduced into the stream. It’s not
possible to outlaw sending small segments; for example, the application might do a push
after sending a single byte. It is possible, however, to keep the receiver from introducing
a small container (i.e., a small open window). The rule is that after advertising a zero
window, the receiver must wait for space equal to an MSS before it advertises an open
window.
Since we can’t eliminate the possibility of a small container being introduced into
the stream, we also need mechanisms to coalesce them. The receiver can do this by
delaying ACKs—sending one combined ACK rather than multiple smaller ones—but
this is only a partial solution because the receiver has no way of knowing how long it is
safe to delay waiting either for another segment to arrive or for the application to read
more data (thus opening the window). The ultimate solution falls to the sender, which
brings us back to our original issue: When does the TCP sender decide to transmit a
segment?
Nagle’s Algorithm
Returning to the TCP sender, if there is data to send but the window is open less than
MSS, then we may want to wait some amount of time before sending the available data,
but the question is, how long? If we wait too long, then we hurt interactive applications
like Telnet. If we don’t wait long enough, then we risk sending a bunch of tiny packets
and falling into the silly window syndrome. The answer is to introduce a timer, and to
transmit when the timer expires.
While we could use a clock-based timer—for example, one that fires every
100 ms—Nagle introduced an elegant self-clocking solution. The idea is that as long
as TCP has any data in flight, the sender will eventually receive an ACK. This ACK can
be treated like a timer firing, triggering the transmission of more data. Nagle’s algorithm
provides a simple, unified rule for deciding when to transmit:
When the application produces data to send
if both the available data and the window ≥ MSS
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403
send a full segment
else
if there is unACKed data in flight
buffer the new data until an ACK arrives
else
send all the new data now
In other words, it’s always OK to send a full segment if the window allows. It’s also alright
to immediately send a small amount of data if there are currently no segments in transit,
but if there is anything in flight, the sender must wait for an ACK before transmitting
the next segment. Thus, an interactive application like Telnet that continually writes
one byte at a time will send data at a rate of one segment per RTT. Some segments will
contain a single byte, while others will contain as many bytes as the user was able to type
in one round-trip time. Because some applications cannot afford such a delay for each
write it does to a TCP connection, the socket interface allows the application to turn
off Nagel’s algorithm by setting the TCP_NODELAY option. Setting this option means
that data is transmitted as soon as possible.
5.2.6 Adaptive Retransmission
Because TCP guarantees the reliable delivery of data, it retransmits each segment if an
ACK is not received in a certain period of time. TCP sets this timeout as a function of the
RTT it expects between the two ends of the connection. Unfortunately, given the range
of possible RTTs between any pair of hosts in the Internet, as well as the variation in RTT
between the same two hosts over time, choosing an appropriate timeout value is not that
easy. To address this problem, TCP uses an adaptive retransmission mechanism. We now
describe this mechanism and how it has evolved over time as the Internet community
has gained more experience using TCP.
Original Algorithm
We begin with a simple algorithm for computing a timeout value between a pair of hosts.
This is the algorithm that was originally described in the TCP specification—and the
following description presents it in those terms—but it could be used by any end-to-end
protocol.
The idea is to keep a running average of the RTT and then to compute the timeout
as a function of this RTT. Specifically, every time TCP sends a data segment, it records
the time. When an ACK for that segment arrives, TCP reads the time again, and then
takes the difference between these two times as a SampleRTT. TCP then computes
an EstimatedRTT as a weighted average between the previous estimate and this new
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Figure 5.10 Associating the ACK with (a) original transmission versus
(b) retransmission.
sample. That is,
EstimatedRTT = α × EstimatedRTT + (1 − α) × SampleRTT
The parameter α is selected to smooth the EstimatedRTT. A small α tracks changes in
the RTT but is perhaps too heavily influenced by temporary fluctuations. On the other
hand, a large α is more stable but perhaps not quick enough to adapt to real changes.
The original TCP specification recommended a setting of α between 0.8 and 0.9. TCP
then uses EstimatedRTT to compute the timeout in a rather conservative way:
TimeOut = 2 × EstimatedRTT
Karn/Partridge Algorithm
After several years of use on the Internet, a rather obvious flaw was discovered in this
simple algorithm. The problem was that an ACK does not really acknowledge a transmission; it actually acknowledges the receipt of data. In other words, whenever a segment
is retransmitted and then an ACK arrives at the sender, it is impossible to determine if
this ACK should be associated with the first or the second transmission of the segment
for the purpose of measuring the sample RTT. It is necessary to know which transmission to associate it with so as to compute an accurate SampleRTT. As illustrated in
Figure 5.10, if you assume that the ACK is for the original transmission but it was really
for the second, then the SampleRTT is too large (a), while if you assume that the ACK
is for the second transmission but it was actually for the first, then the SampleRTT is
too small (b).
The solution, which was proposed in 1987, is surprisingly simple. Whenever TCP
retransmits a segment, it stops taking samples of the RTT; it only measures SampleRTT
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405
for segments that have been sent only once. This solution is known as the Karn/Partridge
algorithm, after its inventors. Their proposed fix also includes a second small change
to TCP’s timeout mechanism. Each time TCP retransmits, it sets the next timeout to
be twice the last timeout, rather than basing it on the last EstimatedRTT. That is,
Karn and Partridge proposed that TCP use exponential backoff, similar to what the
Ethernet does. The motivation for using exponential backoff is simple: Congestion is
the most likely cause of lost segments, meaning that the TCP source should not react
too aggressively to a timeout. In fact, the more times the connection times out, the more
cautious the source should become. We will see this idea again, embodied in a much
more sophisticated mechanism, in Chapter 6.
Jacobson/Karels Algorithm
The Karn/Partridge algorithm was introduced at a time when the Internet was suffering
from high levels of network congestion. Their approach was designed to fix some of the
causes of that congestion, and although it was an improvement, the congestion was not
eliminated. The following year (1988), two other researchers—Jacobson and Karels—
proposed a more drastic change to TCP to battle congestion. The bulk of that proposed
change is described in Chapter 6. Here, we focus on the aspect of that proposal that is
related to deciding when to time out and retransmit a segment.
As an aside, it should be clear how the timeout mechanism is related to
congestion—if you time out too soon, you may unnecessarily retransmit a segment,
which only adds to the load on the network. As we will see in Chapter 6, the other reason for needing an accurate timeout value is that a timeout is taken to imply congestion,
which triggers a congestion-control mechanism. Finally, note that there is nothing about
the Jacobson/Karels timeout computation that is specific to TCP. It could be used by any
end-to-end protocol.
The main problem with the original computation is that it does not take the variance of the sample RTTs into account. Intuitively, if the variation among samples is
small, then the EstimatedRTT can be better trusted and there is no reason for multiplying this estimate by 2 to compute the timeout. On the other hand, a large variance
in the samples suggests that the timeout value should not be too tightly coupled to the
EstimatedRTT.
In the new approach, the sender measures a new SampleRTT as before. It then
folds this new sample into the timeout calculation as follows:
Difference = SampleRTT − EstimatedRTT
EstimatedRTT = EstimatedRTT + (δ × Difference)
Deviation = Deviation + δ(|Difference| − Deviation)
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where δ is a fraction between 0 and 1. That is, we calculate both the mean RTT and the
variation in that mean.
TCP then computes the timeout value as a function of both EstimatedRTT and
Deviation as follows:
TimeOut = µ × EstimatedRTT + φ × Deviation
where based on experience, µ is typically set to 1 and φ is set to 4. Thus, when the
variance is small, TimeOut is close to EstimatedRTT; a large variance causes the
Deviation term to dominate the calculation.
Implementation
There are two items of note regarding the implementation of timeouts in TCP. The first
is that it is possible to implement the calculation for EstimatedRTT and Deviation
without using floating-point arithmetic. Instead, the whole calculation is scaled by 2n ,
with δ selected to be 1/2n . This allows us to do integer arithmetic, implementing multiplication and division using shifts, thereby achieving higher performance. The resulting
calculation is given by the following code fragment, where n = 3 (i.e., δ = 1/8). Note
that EstimatedRTT and Deviation are stored in their scaled-up forms, while the value
of SampleRTT at the start of the code and of TimeOut at the end are real, unscaled
values. If you find the code hard to follow, you might want to try plugging some real
numbers into it and verifying that it gives the same results as the equations above.
{
SampleRTT -= (EstimatedRTT >> 3);
EstimatedRTT += SampleRTT;
if (SampleRTT < 0)
SampleRTT = -SampleRTT;
SampleRTT -= (Deviation >> 3);
Deviation += SampleRTT;
TimeOut = (EstimatedRTT >> 3) + (Deviation >> 1);
}
The second point of note is that the Jacobson/Karels algorithm is only as good as
the clock used to read the current time. On typical Unix implementations at the time,
the clock granularity was as large as 500 ms, which is significantly larger than the average
cross-country RTT of somewhere between 100 and 200 ms. To make matters worse, the
Unix implementation of TCP only checked to see if a timeout should happen every time
this 500-ms clock ticked, and would only take a sample of the round-trip time once per
RTT. The combination of these two factors could mean that a timeout would happen
1 second after the segment was transmitted. Once again, the extensions to TCP include
a mechanism that makes this RTT calculation a bit more precise.
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407
All of the retransmission algorithms we have discussed are based on acknowledgment timeouts, which indicate that a segment has probably been lost. Note that a timeout does not, however, tell the sender whether any segments it sent after the lost segment
were successfully received. That is because TCP acknowledgments are cumulative; they
identify only the last segment that was received without any preceding gaps. The reception of segments that occur after a gap grows more frequent as faster networks lead to
larger windows. If ACKs also told the sender which subsequent segments, if any, had
been received, then the sender could be more intelligent about which segments it retransmits, draw better conclusions about the state of congestion, and make better RTT
estimates. A TCP extension supporting this is described in Section 5.2.8.
5.2.7 Record Boundaries
Since TCP is a byte-stream protocol, the number of bytes written by the sender are
not necessarily the same as the number of bytes read by the receiver. For example, the
application might write 8 bytes, then 2 bytes, then 20 bytes to a TCP connection, while
on the receiving side, the application reads 5 bytes at a time inside a loop that iterates
6 times. TCP does not interject record boundaries between the 8th and 9th bytes, nor
between the 10th and 11th bytes. This is in contrast to a message-oriented protocol such
as UDP, in which the message that is sent is exactly the same length as the message that
is received.
Even though TCP is a byte-stream protocol, it has two different features that can be
used by the sender to insert record boundaries into this byte stream, thereby informing
the receiver how to break the stream of bytes into records. (Being able to mark record
boundaries is useful, for example, in many database applications.) Both of these features
were originally included in TCP for completely different reasons; they have only come
to be used for this purpose over time.
The first mechanism is the urgent data feature, as implemented by the URG flag
and the UrgPtr field in the TCP header. Originally, the urgent data mechanism was
designed to allow the sending application to send out-of-band data to its peer. By “out
of band” we mean data that is separate from the normal flow of data (e.g., a command
to interrupt an operation already underway). This out-of-band data was identified in
the segment using the UrgPtr field and was to be delivered to the receiving process as
soon as it arrived, even if that meant delivering it before data with an earlier sequence
number. Over time, however, this feature has not been used, so instead of signifying
“urgent” data, it has come to be used to signify “special” data, such as a record marker.
This use has developed because, as with the push operation, TCP on the receiving side
must inform the application that “urgent data” has arrived. That is, the urgent data in
itself is not important, it is the fact that the sending process can effectively send a signal
to the receiver that is important.
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The second mechanism for inserting end-of-record markers into a byte is the push
operation. Originally, this mechanism was designed to allow the sending process to tell
TCP that it should send (flush) whatever bytes it had collected to its peer. The push
operation can be used to implement record boundaries because the specification says
that TCP must send whatever data it has buffered at the source when the application
says push, and optionally, TCP at the destination notifies the application whenever an
incoming segment has the PUSH flag set. If the receiving side supports this option (the
socket interface does not), then the push operation can be used to break the TCP stream
into records.
Of course, the application program is always free to insert record boundaries without any assistance from TCP. For example, it can send a field that indicates the length of
a record that is to follow, or it can insert its own record boundary markers into the data
stream.
5.2.8 TCP Extensions
We have mentioned at four different points in this section that there are now extensions
to TCP that help to mitigate some problem that TCP is facing as the underlying network
gets faster. These extensions are designed to have as small an impact on TCP as possible.
In particular, they are realized as options that can be added to the TCP header. (We
glossed over this point earlier, but the reason that the TCP header has a HdrLen field is
that the header can be of variable length; the variable part of the TCP header contains
the options that have been added.) The significance of adding these extensions as options
rather than changing the core of the TCP header is that hosts can still communicate using
TCP even if they do not implement the options. Hosts that do implement the optional
extensions, however, can take advantage of them. The two sides agree that they will use
the options during TCP’s connection establishment phase.
The first extension helps to improve TCP’s timeout mechanism. Instead of measuring the RTT using a coarse-grained event, TCP can read the actual system clock when
it is about to send a segment, and put this time—think of it as a 32-bit timestamp—in
the segment’s header. The receiver then echoes this timestamp back to the sender in its
acknowledgment, and the sender subtracts this timestamp from the current time to measure the RTT. In essence, the timestamp option provides a convenient place for TCP to
“store” the record of when a segment was transmitted; it stores the time in the segment
itself. Note that the endpoints in the connection do not need synchronized clocks, since
the timestamp is written and read at the same end of the connection.
The second extension addresses the problem of TCP’s 32-bit SequenceNum
field wrapping around too soon on a high-speed network. Rather than define a new 64bit sequence number field, TCP uses the 32-bit timestamp just described to effectively
extend the sequence number space. In other words, TCP decides whether to accept or
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Reliable Byte Stream (TCP)
409
reject a segment based on a 64-bit identifier that has the SequenceNum field in the
low-order 32 bits and the timestamp in the high-order 32 bits. Since the timestamp is
always increasing, it serves to distinguish between two different incarnations of the same
sequence number. Note that the timestamp is being used in this setting only to protect
against wraparound; it is not treated as part of the sequence number for the purpose of
ordering or acknowledging data.
The third extension allows TCP to advertise a larger window, thereby allowing it to
fill larger delay × bandwidth pipes that are made possible by high-speed networks. This
extension involves an option that defines a scaling factor for the advertised window. That
is, rather than interpreting the number that appears in the AdvertisedWindow field
as indicating how many bytes the sender is allowed to have unacknowledged, this option
allows the two sides of TCP to agree that the AdvertisedWindow field counts larger
chunks (e.g., how many 16-byte units of data the sender can have unacknowledged). In
other words, the window scaling option specifies how many bits each side should leftshift the AdvertisedWindow field before using its contents to compute an effective
window.
The fourth extension allows TCP to augment its cumulative acknowledgment with
selective acknowledgments of any additional segments that have been received but aren’t
contiguous with all previously received segments. This is the selective acknowledgment, or
SACK, option. When the SACK option is used, the receiver continues to acknowledge
segments normally—the meaning of the Acknowledge field does not change—but it
also uses optional fields in the header to acknowledge any additional blocks of received
data. This allows the sender to retransmit just the segments that are missing according
to the selective acknowledgment.
Without SACK, there are only two reasonable strategies for a sender. The pessimistic strategy responds to a timeout by retransmitting not just the segment that timed
out, but any segments transmitted subsequently. In effect, the pessimistic strategy assumes the worst: that all those segments were lost. The disadvantage of the pessimistic
strategy is that it may unnecessarily retransmit segments that were successfully received
the first time. The other strategy is the optimistic strategy, which responds to a timeout
by retransmitting only the segment that timed out. In effect, the optimistic scenario assumes the rosiest scenario: that only the one segment has been lost. The disadvantage of
the optimistic strategy is that it is very slow, unnecessarily, when a series of consecutive
segments has been lost, as might happen when there is congestion. It is slow because each
segment’s loss is not discovered until the sender receives an ACK for its retransmission
of the previous segment. So it consumes one RTT per segment until it has retransmitted
all the segments in the lost series. With the SACK option, a better strategy is available to
the sender: retransmit just the segments that fill the gaps between the segments that have
been selectively acknowledged.
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5.2.9 Alternative Design Choices
Although TCP has proven to be a robust protocol that satisfies the needs of a wide range
of applications, the design space for transport protocols is quite large. TCP is, by no
means, the only valid point in that design space. We conclude our discussion of TCP
by considering alternative design choices. While we offer an explanation for why TCP’s
designers made the choices they did, we leave it to you to decide if there might be a place
for alternative transport protocols.
First, we have suggested from the very first chapter of this book that there are at
least two interesting classes of transport protocols: stream-oriented protocols like TCP
and request/reply protocols like RPC. In other words, we have implicitly divided the
design space in half and placed TCP squarely in the stream-oriented half of the world.
We could further divide the stream-oriented protocols into two groups—reliable and
unreliable—with the former containing TCP and the latter being more suitable for interactive video applications that would rather drop a frame than incur the delay associated
with a retransmission.
This exercise in building a transport protocol taxonomy is interesting and could
be continued in greater and greater detail, but the world isn’t as black and white as we
might like. Consider the suitability of TCP as a transport protocol for request/reply applications, for example. TCP is a full-duplex protocol, so it would be easy to open a
TCP connection between the client and server, send the request message in one direction, and send the reply message in the other direction. There are two complications,
however. The first is that TCP is a byte-oriented protocol rather than a message-oriented
protocol, and request/reply applications always deal with messages. (We explore the issue of bytes versus messages in greater detail in a moment.) The second complication
is that in those situations where both the request message and the reply message fit in
a single network packet, a well-designed request/reply protocol needs only two packets
to implement the exchange, whereas TCP would need at least nine: three to establish
the connection, two for the message exchange, and four to tear down the connection.
Of course, if the request or reply messages are large enough to require multiple network
packets (e.g., it might take 100 packets to send a 100,000-byte reply message), then the
overhead of setting up and tearing down the connection is inconsequential. In other
words, it isn’t always the case that a particular protocol cannot support a certain functionality; it’s sometimes the case that one design is more efficient than another under
particular circumstances.
Second, as just suggested, you might question why TCP chose to provide a reliable
byte-stream service rather than a reliable message-stream service; messages would be the
natural choice for a database application that wants to exchange records. There are two
answers to this question. The first is that a message-oriented protocol must, by definition,
establish an upper bound on message sizes. After all, an infinitely long message is a
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411
byte stream. For any message size that a protocol selects, there will be applications that
want to send larger messages, rendering the transport protocol useless and forcing the
application to implement its own transportlike services. The second reason is that while
message-oriented protocols are definitely more appropriate for applications that want to
send records to each other, you can easily insert record boundaries into a byte stream to
implement this functionality, as described in Section 5.2.7.
Third, TCP chose to implement explicit setup/teardown phases, but this is not
required. In the case of connection setup, it would certainly be possible to send all necessary connection parameters along with the first data message. TCP elected to take a
more conservative approach that gives the receiver the opportunity to reject the connection before any data arrives. In the case of teardown, we could quietly close a connection
that has been inactive for a long period of time, but this would complicate applications
like Telnet that want to keep a connection alive for weeks at a time; such applications
would be forced to send out-of-band “keep alive” messages to keep the connection state
at the other end from disappearing.
Finally, TCP is a window-based protocol, but this is not the only possibility. The alternative is a rate-based design, in which the receiver tells the sender the rate—expressed
in either bytes or packets per second—at which it is willing to accept incoming data.
For example, the receiver might inform the sender that it can accommodate 100 packets
a second. There is an interesting duality between windows and rate, since the number
of packets (bytes) in the window, divided by the RTT, is exactly the rate. For example,
a window size of 10 packets and a 100-ms RTT implies that the sender is allowed to
transmit at a rate of 100 packets a second. It is by increasing or decreasing the advertised window size that the receiver is effectively raising or lowering the rate at which the
sender can transmit. In TCP, this information is fed back to the sender in the AdvertisedWindow field of the ACK for every segment. One of the key issues in a rate-based
protocol is how often the desired rate—which may change over time—is relayed back to
the source: Is it for every packet, once per RTT, or only when the rate changes? While we
have just now considered window versus rate in the context of flow control, it is an even
more hotly contested issue in the context of congestion control, which we will discuss in
Chapter 6.
5.3
Remote Procedure Call
As discussed in Chapter 1, one common pattern of communication used by application
programs is the request/reply paradigm, also called message transaction: a client sends a
request message to a server, and the server responds with a reply message, with the client
blocking (suspending execution) to wait for the reply. Figure 5.11 illustrates the basic
interaction between the client and server in such a message transaction.
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Figure 5.11
End-to-End Protocols
Timeline for RPC.
A transport protocol that supports the request/reply paradigm is much more than
a UDP message going in one direction followed by a UDP message going in the other
direction. It needs to deal with correctly identifying processes on remote hosts and correlating requests with responses. It may also need to overcome some or all of the limitations of the underlying network outlined in the problem statement at the beginning
of this chapter. While TCP overcomes these limitations by providing a reliable bytestream service, it doesn’t match the request/reply paradigm very well either—going to
the trouble to establish a TCP connection just to exchange a pair of messages seems like
overkill. This section describes a third category of transport protocol, called Remote Procedure Call (RPC), that more closely matches the needs of an application involved in a
request/reply message exchange.
5.3.1 RPC Fundamentals
RPC is actually more than just a protocol—it is a popular mechanism for structuring
distributed systems. RPC is popular because it is based on the semantics of a local procedure call—the application program makes a call into a procedure without regard for
whether it is local or remote and blocks until the call returns. An application developer
can be largely unaware of whether the procedure is local or remote, simplifying his task
considerably. When the procedures being called are actually methods of remote objects in
an object-oriented language, RPC is known as remote method invocation (RMI). While
the RPC concept is simple, there are two main problems that make it more complicated
than local procedure calls:
■
The network between the calling process and the called process has much more
complex properties than the backplane of a computer. For example, it is likely
to limit message sizes and has a tendency to lose and reorder messages.
5.3
■
Remote Procedure Call
413
The computers on which the calling and called processes run may have significantly different architectures and data representation formats.
Thus, a complete RPC mechanism actually involves two major components:
1
A protocol that manages the messages sent between the client and the server
processes and that deals with the potentially undesirable properties of the underlying network.
2
Programming language and compiler support to package the arguments into a
request message on the client machine and then to translate this message back
into the arguments on the server machine, and likewise with the return value
(this piece of the RPC mechanism is usually called a stub compiler).
Figure 5.12 schematically depicts what happens when a client invokes a remote
procedure. First, the client calls a local stub for the procedure, passing it the arguments
required by the procedure. This stub hides the fact that the procedure is remote by
Figure 5.12
Complete RPC mechanism.
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End-to-End Protocols
translating the arguments into a request message and then invoking an RPC protocol to
send the request message to the server machine. At the server, the RPC protocol delivers
the request message to the server stub (sometimes called a skeleton), which translates it
into the arguments to the procedure and then calls the local procedure. After the server
procedure completes, it returns the answer to the server stub, which packages this return
value in a reply message that it hands off to the RPC protocol for transmission back
to the client. The RPC protocol on the client passes this message up to the client stub,
which translates it into a return value that it returns to the client program.
This section considers just the protocol-related aspects of an RPC mechanism. That
is, it ignores the stubs and focuses instead on the RPC protocol, sometimes referred
to as a request/reply protocol, that transmits messages between client and server. The
transformation of arguments into messages and vice versa is covered in Chapter 7.
The term RPC refers to a type of
protocol rather than a specific standard
What Layer Is RPC?
like TCP, so specific RPC protocols vary
Once again, the “What layer is this?”
in the functions they perform. And unlike
issue raises its ugly head. To many
TCP, which is the dominant reliable bytepeople, especially those who adhere to
stream protocol, there is no one domia strictly layerist view of protocol arnant RPC protocol. Thus, in this section
chitecture, RPC is implemented on
we will talk more about alternative design
top of a transport protocol (usually
choices than previously.
UDP) and so cannot itself (by definition) be a transport protocol. It is
Identifiers in RPC
certainly valid, however, to argue that
Two functions that must be performed by
the Internet should have an RPC proany RPC protocol are:
tocol, since RPC offers a process-toprocess service that is fundamentally
■ Provide a name space for
different from that offered by TCP
uniquely identifying the proceand UDP. The usual response to such
dure to be called.
a suggestion, however, is that the In■ Match each reply message to the
ternet architecture does not prohibit
corresponding request message.
network designers from implementing
their own RPC protocol on top of
The first problem has some similarities
UDP. Whichever side of the issue of
to the problem of identifying nodes in a
whether the Internet should have an
network, something that we saw in previofficial RPC protocol you support, the
ous chapters (e.g., IP addresses in Chapimportant point is that the way you
ter 4). One of the design choices when
implement RPC in the Internet aridentifying things is whether to make this
chitecture says nothing about whether
namespace flat or hierarchical.
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415
A flat namespace would simply assign a unique, unstructured identifier (e.g., an
integer) to each procedure, and this number would be carried in a single field in an RPC
request message. This would require some kind of central coordination to avoid assigning the same procedure number to two different procedures. Alternatively, the protocol
could implement a hierarchical name space, analogous to that used for file pathnames,
which requires only that a file’s “basename” be unique within its directory. This approach
potentially simplifies the job of ensuring uniqueness of procedure names. A hierarchical
namespace for RPC could be implemented by defining a set of fields in the request
message format, one for each level of naming in, say, a two- or three-level hierarchical
namespace.
The key to matching a reply message to the corresponding request is to uniquely
identify request-replies pairs using a message ID field. A reply message had its message ID
field set to the same value as the request message. When the client RPC module receives
the reply, it uses the message ID to search
for the corresponding outstanding request.
RPC should be considered a transport
To make the RPC transaction appear like a
protocol or not.
local procedure call to the caller, the caller
Interestingly, there are other peois blocked (e.g., by using a semaphore) unple who believe that RPC is the most
til the reply message is received. When the
interesting protocol in the world and
reply is received, the blocked caller is identhat TCP/IP is just what you do when
tified based on the request number in the
you want to go “off site.” This is
reply, the remote procedure’s return value
the predominant view of the operis obtained from the reply, and the caller is
ating systems community, which has
unblocked so that it can return with that
built countless OS kernels for disreturn value.
tributed systems that contain exactly
One of the recurrent challenges in
one protocol—you guessed it, RPC—
RPC is dealing with unexpected responses,
running on top of a network device
and we see this with message IDs. For exdriver.
ample, consider the following pathological
Our position is that any pro(but realistic) situation. A client machine
tocol that offers process-to-process
sends a request message with a message ID
service, as opposed to node-to-node
of 0, then crashes and reboots, and then
or host-to-host service, qualifies as a
sends an unrelated request message, also
transport protocol. Thus, RPC is a
with a message ID of 0. The server may
transport protocol and, in fact, can be
not have been aware that the client crashed
implemented on top of other protoand rebooted, and upon seeing a request
cols that are themselves valid transport
message with a message ID of 0, acknowlprotocols.
edges it and discards it as a duplicate. The
client never gets a response to the request.
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5
Figure 5.13
End-to-End Protocols
Simple timeline for a reliable RPC protocol.
One way to eliminate this problem is to use a boot ID. A machine’s boot ID is a
number that is incremented each time the machine reboots; this number is read from
disk, incremented, and written back to disk during the machine’s start-up procedure.
This number is then put in every message sent by that host. If a message is received with
an old message ID but a new boot ID, it is recognized as a new message. In effect, the
message ID and boot ID combine to form a unique ID for each transaction.
Overcoming Network Limitations
RPC protocols often perform additional functions to deal with the fact that networks
are not perfect channels. Two such functions are:
■
Provide reliable message delivery.
■
Support large message sizes through fragmentation and reassembly.
An RPC protocol might implement reliability because the underlying protocols
(e.g., UDP/IP) do not provide it, or perhaps to recover more quickly or efficiently from
failures that otherwise would eventually be repaired by underlying protocols. An RPC
protocol can implement reliability using acknowledgments and timeouts, similarly to
TCP. The basic algorithm is straightforward, as illustrated by the timeline given in Figure 5.13. The client sends a request message and the server acknowledges it. Then, after
executing the procedure, the server sends a reply message and the client acknowledges
the reply.
Either a message carrying data (a request message or a reply message) or the ACK
sent to acknowledge that message may be lost in the network. To account for this possibility, both client and server save a copy of each message they send until an ACK for it
5.3
Figure 5.14
Remote Procedure Call
417
Timeline for a reliable RPC protocol using implicit acknowledgment.
has arrived. Each side also sets a RETRANSMIT timer and resends the message should
this timer expire. Both sides reset this timer and try again some agreed-upon number of
times before giving up and freeing the message.
If an RPC client receives a reply message, clearly the corresponding request message must have been received by the server. Hence the reply message itself is an implicit
acknowledgment, and any additional acknowledgment from the server is not logically
necessary. Similarly, a request message could implicitly acknowledge the preceding reply message—assuming the protocol makes request-reply transactions sequential, so that
one transaction must complete before the next begins. Unfortunately, this sequentiality
would severely limit RPC performance.
A way out of this predicament is for the RPC protocol to implement a channel abstraction. Within a given channel, request/reply transactions are sequential—there can
be only one transaction active on a given channel at any given time—but there can be
multiple channels. Each message includes a channel ID field to indicate which channel
the message belongs to. A request message in a given channel would implicitly acknowledge the previous reply in that channel, if it hadn’t already been acknowledged. An application program can open multiple channels to a server if it wants to have more than
one request/reply transaction between them at the same time (the application would
need multiple threads). As illustrated in Figure 5.14, the reply message serves to acknowledge the request message, and a subsequent request acknowledges the preceding
reply. Note that we saw a very similar approach—called concurrent logical channels—
in Section 2.5.3 as a way to improve on the performance of a stop-and-wait reliability
mechanism.
Another complication that RPC must address is that the server may take an arbitrarily long time to produce the result, and worse yet, it may crash before generating
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End-to-End Protocols
the reply. Keep in mind that we are talking about the period of time after the server has
acknowledged the request but before it has sent the reply. To help the client distinguish
between a slow server and a dead server, the RPC’s client side periodically sends an “Are
you alive?” message to the server, and the server side responds with an ACK. Alternatively, the server could send “I am still alive” messages to the client without the client
having first solicited them. The client-initiated approach is more scalable because it puts
more of the per-client burden (managing the timeout timer) on the clients.
RPC reliability may include the property known as at-most-once semantics. This
means that for every request message that the client sends, at most one copy of that
message is delivered to the server. Each time the client calls a remote procedure, that
procedure is invoked at most one time on the server machine. We say “at most once”
rather than “exactly once” because it is always possible that either the network or the
server machine has failed, making it impossible to deliver even one copy of the request
message.
To implement at-most-once semantics, RPC on the server side must recognize duplicate requests (and ignore them), even if it has already successfully replied to the original request. Hence it must maintain some state information that identifies past requests.
One approach is to identify requests using sequence numbers, so a server need only remember the most recent sequence number. Unfortunately, this would limit an RPC to
one outstanding request (to a given server) at a time, since one request must be completed
before the request with the next sequence number can be transmitted. Once again, channels provide a solution. The server could recognize duplicate requests by remembering
the current sequence number for each channel, without limiting the client to one request
at a time.
As obvious as at-most-once sounds, not all RPC protocols support this behavior.
Some support a semantics that is facetiously called zero-or-more semantics; that is, each
invocation on a client results in the remote procedure being invoked zero or more times.
It is not hard to understand how this would cause problems for a remote procedure
that changed some local state variable (e.g., incremented a counter) or that had some
externally visible side effect (e.g., launched a missile) each time it was invoked. On the
other hand, if the remote procedure being invoked is idempotent—multiple invocations
have the same effect as just one—then the RPC mechanism need not support at-mostonce semantics; a simpler (possibly faster) implementation will suffice.
As was the case with reliability, the two reasons why an RPC protocol might implement message fragmentation and reassembly are (1) it is not provided by the underlying protocol stack, or (2) it can be implemented more efficiently by the RPC protocol.
Consider the case where RPC is implemented on top of UDP/IP and relies on IP for
fragmentation and reassembly. If even one fragment of a message fails to arrive within a
certain amount of time, IP discards the fragments that did arrive and the message is effec-
5.3
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419
tively lost. Eventually the RPC protocol (assuming it implements reliability) would time
out and retransmit the message. In contrast, consider an RPC protocol that implements
its own fragmentation and reassembly and aggressively ACKs or NACKs individual fragments. Lost fragments would be more quickly detected and retransmitted, and only the
lost fragments would be retransmitted, not the whole message.
Synchronous versus Asynchronous Protocols
One way to characterize a protocol is by whether it is synchronous or asynchronous. The
precise meaning of these terms depends on where in the protocol hierarchy you use them.
At the transport layer, it is most accurate to think of them as defining the extremes of
a spectrum rather than as two mutually exclusive alternatives. The key attribute of any
point along the spectrum is how much the sending process knows after the operation to
send a message returns. In other words, if we assume that an application program invokes
a send operation on a transport protocol, then exactly what does the application know
about the success of the operation when the send operation returns?
At the asynchronous end of the spectrum, the application knows absolutely nothing
when send returns. It not only doesn’t know if the message was received by its peer,
but it doesn’t even know for sure that the message has successfully left the local machine.
At the synchronous end of the spectrum, the send operation typically returns a reply
message. That is, the application not only knows that the message it sent was received by
its peer, but it knows that the peer has returned an answer. Thus, synchronous protocols
implement the request/reply abstraction, while asynchronous protocols are used if the
sender wants to be able to transmit many messages without having to wait for a response.
Using this definition, RPC protocols are obviously synchronous protocols.
Although we have not discussed them in this chapter, there are interesting points
between these two extremes. For example, the transport protocol might implement send
so that it blocks (does not return) until the message has been successfully received at
the remote machine, but returns before the sender’s peer on that machine has actually
processed and responded to it. This is sometimes called a reliable datagram protocol.
5.3.2 RPC Implementations (SunRPC, DCE)
We now turn our discussion to some example implementations of RPC protocols. These
will serve to highlight some of the different design decisions that protocol designers have
made. Our first example is SunRPC, a widely used RPC protocol also known as ONC
RPC (Open Network Computing RPC). Our second example, which we will refer to
as DCE-RPC, is part of the Distributed Computing Environment (DCE). DCE is a
set of standards and software for building distributed systems that was defined by the
Open Software Foundation (OSF), a consortium of computer companies that originally
included IBM, the Digital Equipment Corporation, and Hewlett-Packard; today OSF
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Figure 5.15
End-to-End Protocols
Protocol graph for SunRPC on top of UDP.
goes by the name Open Group. These two examples represent interesting alternative
design choices in the RPC solution space.
SunRPC
SunRPC became a de facto standard thanks to its wide distribution with Sun workstations and to the central role it plays in Sun’s popular Network File System (NFS). The
IETF subsequently adopted it as a standard Internet protocol under the name ONC
RPC.
SunRPC can be implemented over several different transport protocols. Figure 5.15
illustrates the protocol graph when SunRPC is implemented on UDP. As we noted earlier
in this section, a strict layerist might frown on the idea of running a transport protocol
over a transport protocol, or argue that RPC must be something other than a transport
protocol since it appears “above” the transport layer. Pragmatically, the design decision
to run RPC over an existing transport layer makes quite a lot of sense, as will be apparent
in the following discussion.
SunRPC uses two-tier identifiers to identify remote procedures: a 32-bit program
number and a 32-bit procedure number. (There is also a 32-bit version number, but
we ignore that in the following discussion.) For example, the NFS server has been assigned program number x00100003, and within this program, getattr is procedure
1, setattr is procedure 2, read is procedure 6, write is procedure 8, and so on. The
program number and procedure number are transmitted in the SunRPC request message’s header, whose fields are shown in Figure 5.16. The server—which may support
several program numbers—is responsible for calling the specified procedure of the specified program. A SunRPC request really represents a request to call the specified program
and procedure on the particular machine to which the request was sent, even though the
same program number may be implemented on other machines in the same network.
5.3
Figure 5.16
Remote Procedure Call
421
SunRPC header formats: (a) request; (b) reply.
Thus, the address of the server’s machine (e.g., an IP address) is an implicit third tier of
the RPC address.
Different program numbers may belong to different servers on the same machine.
These different servers have different transport layer demux keys (e.g., UDP ports), most
of which are not well-known numbers but instead are assigned dynamically. These demux keys are called transport selectors. How can a SunRPC client that wants to talk to a
particular program determine which transport selector to use to reach the corresponding server? The solution is to assign a well-known address to just one program on the
remote machine, and let that program handle the task of telling clients which transport selector to use to reach any other program on the machine. The original version of
this SunRPC program is called the Port Mapper, and it supports only UDP and TCP
as underlying protocols. Its program number is x00100000 and its well-known port
is 111. RPCBIND, which evolved from the Port Mapper, supports arbitrary underlying
transport protocols. As each SunRPC server starts, it calls an RPCBIND registration
procedure on the server’s own home machine to register its transport selector and the
program numbers that it supports. A remote client can then call an RPCBIND lookup
procedure to look up the transport selector for a particular program number.
To make this more concrete, consider an example using the Port Mapper with
UDP. To send a request message to NFS’s read procedure, a client first sends a request
message to the Port Mapper at well-known UDP port 111, asking that procedure 3 be
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invoked to map program number x00100003 to the UDP port where the NFS program currently resides.2 The client then sends a SunRPC request message with program
number x00100003 and procedure number 6 to this UDP port, and the SunRPC
module listening at that port calls the NFS read procedure. The client also caches the
program-to-port number mapping so that it need not go back to the Port Mapper each
time it wants to talk to the NFS program.
To match up a reply message with the corresponding request, so that the result of
the RPC can be returned to the correct caller, both request and reply message headers
include an XID (transaction ID) field, as in Figure 5.16. An XID is a unique transaction
id, used only by one request and the corresponding reply. After the server has successfully
replied to a given request, it does not remember the XID. Because of this, SunRPC
cannot guarantee at-most-once semantics.
The details of SunRPC’s semantics depend on underlying the transport protocol.
It does not implement its own reliability, so it is only reliable if the underlying transport
is reliable. (Of course, any application that runs over SunRPC may also choose to implement its own reliability mechanisms above the level of SunRPC if it chooses.) The ability
to send request and reply messages that are larger than the network MTU is also dependent on the underlying transport. In other words, SunRPC does not make any attempt
to improve on the underlying transport when it comes to reliability and message size.
Since SunRPC can run over many different transport protocols, this gives it considerable
flexibility without complicating the design of the RPC protocol itself.
Returning to the SunRPC header format of Figure 5.16, the request message contains variable-length Credentials and Verifier fields, both of which are used by the
client to authenticate itself to the server; that is, to give evidence that the client has the
right to invoke the server. How a client authenticates itself to a server is a general issue that must be addressed by any protocol that wants to provide a reasonable level of
security. This topic is discussed in more detail in the next chapter.
DCE-RPC
DCE-RPC is the RPC protocol at the core of the DCE system and was the basis of
the RPC mechanism underlying Microsoft’s DCOM and ActiveX. It can be used with
the Network Data Representation (NDR) stub compiler described in Chapter 7, but
it also serves as the underlying RPC protocol for the Common Object Request Broker
Architecture (CORBA), which is an industry-wide standard for building distributed,
object-oriented systems.
DCE-RPC, like SunRPC, can be implemented on top of several transport protocols including UDP and TCP. It is also similar to SunRPC in that it defines a two-level
2 In practice, NFS is such an important program that it has been given its own well-known UDP port, but for the purposes
of illustration we’re pretending that’s not the case.
5.3
Figure 5.17
Remote Procedure Call
423
Typical DCE-RPC message exchange.
addressing scheme: the transport protocol demultiplexes to the correct server, DCERPC dispatches to a particular procedure exported by that server, and clients consult an
“endpoint mapping service” (similar to SunRPC’s Port Mapper) to learn how to reach
a particular server. Unlike SunRPC, however, DCE-RPC implements at-most-once call
semantics. (In truth, DCE-RPC supports multiple call semantics, including an idempotent semantics similar to SunRPC’s, but at-most-once is the default behavior.) There
are some other differences between the two approaches, which we will highlight in the
following paragraphs.
Figure 5.17 gives a timeline for the typical exchange of messages, where each message is labeled by its DCE-RPC type. The client sends a Request message, the server
eventually replies with a Response message, and the client acknowledges (Ack) the response. Instead of the server acknowledging the request messages, however, the client periodically sends a Ping message to the server, which responds with a Working message
to indicate that the remote procedure is still in progress. If the server’s reply is received
reasonably quickly, no Pings are sent. Although not shown in the figure, other message
types are also supported. For example, the client can send a Quit message to the server,
asking it to abort an earlier call that is still in progress; the server responds with a Quack
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(quit acknowledgment) message. Also, the server can respond to a Request message
with a Reject message (indicating that a call has been rejected), and it can respond to a
Ping message with a Nocall message (indicating that the server has never heard of the
caller).
Each request/reply transaction in DCE-RPC takes place in the context of an activity. An activity is a logical request/reply channel between a pair of participants. At any
given time, there can be only one message transaction active on a given channel. Like
the concurrent logical channel approach described above and in Section 2.5.3, the application programs have to open multiple channels if they want to have more than one
request/reply transaction between them at the same time. The activity to which a message
belongs is identified by the message’s ActivityId field. A SequenceNum field then
distinguishes between calls made as part of the same activity; it serves the same purpose
as SunRPC’s XID (transaction ID) field. Unlike SunRPC, DCE-RPC keeps track of the
last sequence number used as part of a particular activity, so as to ensure at-most-once
semantics. To distinguish between replies sent before and after a server machine reboots,
DCE-RPC uses a ServerBoot field to hold the machine’s boot ID.
Another design choice made in DCE-RPC that differs from SunRPC is the support of fragmentation and reassembly in the RPC protocol. As noted above, even if
an underlying protocol such as IP provides fragmentation/reassembly, a more sophisticated algorithm implemented as part of RPC can result in quicker recovery and reduced
bandwidth consumption when fragments are lost. The FragmentNum field uniquely
identifies each fragment that makes up a given request or reply message. Each DCERPC fragment is assigned a unique fragment number (e.g., 0, 1, 2, 3, and so on). Both
the client and server implement a selective acknowledgment mechanism, which works
as follows. (We describe the mechanism in terms of a client sending a fragmented request message to the server; the same mechanism applies when a server sends a fragment
response to the client.)
First, each fragment that makes up the request message contains both a unique
FragmentNum, and a flag indicating whether this packet is a fragment of a call (frag)
or the last fragment of a call (last_frag); request messages that fit in a single packet carry
a no_frag flag. The server knows it has received the complete request message when it
has the last_frag packet and there are no gaps in the fragment numbers. Second, in
response to each arriving fragment, the server sends a Fack (fragment acknowledgment)
message to the client. This acknowledgment identifies the highest fragment number that
the server has successfully received. In other words, the acknowledgment is cumulative,
much like in TCP. In addition, however, the server selectively acknowledges any higher
fragment numbers it has received out of order. It does so with a bit-vector that identifies
these out-of-order fragments relative to the highest in-order fragment it has received.
Finally, the client responds by retransmitting the missing fragments.
5.3
Figure 5.18
Remote Procedure Call
425
Fragmentation with selective acknowledgments.
Figure 5.18 illustrates how this all works. Suppose the server has successfully received fragments up through number 20, plus fragments 23, 25, and 26. The server
responds with a Fack that identifies fragment 20 as the highest in-order fragment, plus
a bit-vector (SelAck) with the third (23 = 20 + 3), fifth (25 = 20 + 5), and sixth
(26 = 20 + 6) bits turned on. So as to support an (almost) arbitrarily long bit-vector,
the size of the vector (measured in 32-bit words) is given in the SelAckLen field.
Given DCE-RPC’s support for very large messages—the FragmentNum field
is 16 bits long, meaning it can support 64K fragments—it is not appropriate for the
protocol to blast all the fragments that make up a message as fast as it can since doing
so might overrun the receiver. Instead, DCE-RPC implements a flow-control algorithm
that is very similar to TCP’s. Specifically, each Fack message not only acknowledges
received fragments, but it also informs the sender of how many fragments it may now
send. This is the purpose of the WindowSize field in Figure 5.18, which serves exactly
the same purpose as TCP’s AdvertisedWindow field except it counts fragments rather
than bytes. DCE-RPC also implements a congestion-control mechanism that is similar
to TCP’s, which we will see in Chapter 6. Given the complexity of congestion control, it
is perhaps not surprising that some RPC protocols avoid it by avoiding fragmentation.
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In summary, designers have quite a range of options open to them when designing an RPC protocol. SunRPC takes the more minimalist approach and adds relatively
little to the underlying transport beyond the essentials of locating the right procedure
and identifying messages. DCE-RPC adds more functionality, with the possibility of
improved performance in some environments at the cost of greater complexity.
5.4
Transport for Real-Time Applications (RTP)
In the early days of packet switching, most applications were concerned with the movement of data: accessing remote computing resources, transferring files, sending email,
and so on. However, at least as early as 1981, experiments were underway to carry “realtime” traffic, such as digitized voice samples, over packet networks. We call an application
“real-time” when it has strong requirements for the timely delivery of information. Internet telephony or “Voiceover IP” (VOIP) is a classic example of a real-time application,
because you can’t easily carry on a conversation with someone if it takes more than a
fraction of a second to get a response. As we will see shortly, real-time applications place
some specific demands on the transport protocol that are not well met by the protocols
discussed so far in this chapter.
Multimedia applications—those that involve video, audio, and data—are sometimes divided into two classes: interactive applications and streaming applications. A relatively early and popular example of the former class is vat, a multiparty audioconferencing tool that is often used over networks supporting IP multicast. The control
panel for a typical vat conference is shown in Figure 5.19. Internet telephony is also a
class of interactive application, and probably the most widely used one at the time of
writing. Another interactive conferencing application is vic, the videoconferencing tool
discussed in Chapter 1 and illustrated in Figure 1.1. These are the sort of applications
with the most stringent real-time requirements.
Streaming applications typically deliver audio or video streams from a server
to a client, and are typified by such commercial products as Real Audio. Because
of the lack of human-to-human interaction, they place less stringent real-time requirements on the underlying protocols. While such applications are not strictly realtime, however, they still have enough in common with interactive multimedia applications to warrant consideration of a common protocol for both types of application.
It should by now be apparent that designers of a transport protocol for real-time
and multimedia applications face a real challenge in defining the requirements broadly
enough to meet the needs of very different applications. They must also pay attention to
the interactions among different applications; for example, the synchronization of audio
and video streams. We will see how these concerns affected the design of RTP below.
5.4
Transport for Real-Time Applications (RTP)
Figure 5.19
User interface of a vat audioconference.
Figure 5.20
Protocol stack for multimedia applications using RTP.
427
Much of RTP actually derives from protocol functionality that was originally embedded in the application vat. Newer versions of vat (and many other applications)
run over RTP. RTP can run over many lower-layer protocols, but commonly runs over
UDP. That leads to the protocol stack shown in Figure 5.20. Note that we are therefore
running a transport protocol over a transport protocol. There is no rule against that, and
in fact it makes a lot of sense, since UDP provides such a minimal level of functionality,
and the basic demultiplexing based on port numbers happens to be just what RTP needs
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as a starting point. So rather than recreate port numbers in RTP, RTP outsources the
demultiplexing function to UDP.
5.4.1 Requirements
The most basic requirement for a general-purpose multimedia protocol is that it allows
similar applications to interoperate with each other. For example, it should be possible for two independently implemented audioconferencing applications to talk to each
other. This immediately suggests that the applications had better use the same method of
encoding and compressing voice; otherwise, the data sent by one party will be incomprehensible to the receiving party. Since there are quite a few different coding schemes for
voice, each with its own trade-offs between quality, bandwidth requirements, and computational cost, it would probably be a bad idea to decree that only one such scheme can
be used. Instead, our protocol should provide a way that a sender can tell a receiver which
coding scheme it wants to use, and possibly negotiate until a scheme that is available to
both parties is identified.
Just as with audio, there are many different video coding schemes.3 Thus, we see
that the first common function that RTP can provide is the ability to communicate that
choice of coding scheme. Note that this also serves to identify the type of application
(e.g., audio or video); once we know what coding algorithm is being used, we know
what type of data is being encoded as well.
Another important requirement for RTP is to enable the recipient of a data stream
to determine the timing relationship among the received data. As we will see in Section 6.5, real-time applications need to place received data into a playback buffer to
smooth out the jitter that may have been introduced into the data stream during transmission across the network. Thus, some sort of timestamping of the data will be necessary
to enable the receiver to play it back at the appropriate time.
Related to the timing of a single media stream is the issue of synchronization of
multiple media in a conference. The obvious example of this would be to synchronize an
audio and video stream that are originating from the same sender. As we will see below,
this is a slightly more complex problem than playback time determination for a single
stream.
Another important function to be provided is an indication of packet loss. Note
that an application with tight latency bounds generally cannot use a reliable transport
like TCP because retransmission of data to correct for loss would probably cause the
packet to arrive too late to be useful. Thus, the application must be able to deal with
missing packets, and the first step in dealing with them is noticing that they are in fact
missing. As an example, a video application using MPEG encoding will need to take
3 Audio and video coding and compression are discussed in more detail in Chapter 7.
5.4
Transport for Real-Time Applications (RTP)
429
different actions when a packet is lost, depending on whether the packet came from an
I frame, a B frame, or a P frame since these different frame types carry different amounts
of information (see Section 7.2.3 for more details).
Since multimedia applications generally do not run over TCP, they also miss out on
the congestion avoidance features of TCP (as described in Section 6.3). Yet many multimedia applications are capable of responding to congestion, for example, by changing
the parameters of the coding algorithm to reduce the bandwidth consumed. Clearly, to
make this work, the receiver needs to notify the sender that losses are occurring so that
the sender can adjust its coding parameters.
Another common function across multimedia applications is the concept of frame
boundary indication. A frame in this context is application-specific. For example, it may
be helpful to notify a video application that a certain set of packets correspond to a single
frame. In an audio application it is helpful to mark the beginning of a “talkspurt,” which
is a collection of sounds or words followed by silence. The receiver can then identify
the silences between talkspurts and use them as opportunities to move the playback
point. This follows the observation that slight shortening or lengthening of the spaces
between words are not perceptible to users, whereas shortening or lengthening the words
themselves is both perceptible and annoying.
A final function that we might want to put into the protocol is some way of identifying senders that is more user-friendly than an IP address. Tools such as vat and vic
can display strings such as Joe User ([email protected]) on their control panels,
and thus the application protocol should support the association of such a string with a
data stream.
In addition to the functionality that is required from our protocol, we note an additional requirement: It should make reasonably efficient use of bandwidth. Put another
way, we don’t want to introduce a lot of extra bits that need to be sent with every packet
in the form of a long header. The reason for this is that audio packets, which are one of
the most common types of multimedia data, tend to be small, so as to reduce the time
it takes to fill them with samples. Long audio packets would mean high latency due to
packetization, which has a negative effect on the perceived quality of conversations. (Recall that this was one of the factors in choosing the length of ATM cells.) Since the data
packets themselves are short, a large header would mean that a relatively large amount
of link bandwidth would be used by headers, thus reducing the available capacity for
“useful” data. We will see several aspects of the design of RTP that have been influenced
by the necessity of keeping the header short.
5.4.2 RTP Details
Now that we have seen the rather long list of requirements for our transport protocol for
multimedia, we turn to the details of the protocol that has been specified to meet those
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▲
requirements. This protocol, RTP, was developed in the IETF and is in widespread use.
The RTP standard actually defines a pair of protocols, RTP and the Real-time Transport
Control Protocol (RTCP). The former is used for the exchange of multimedia data, while
the latter is used to periodically send control information associated with a certain data
flow. When running over UDP, the RTP data stream and the associated RTCP control
stream use consecutive transport-layer ports. The RTP data uses an even port number
and the RTCP control information uses the next higher (odd) port number.
Because RTP is designed to support a wide variety of applications, it provides a
flexible mechanism by which new applications can be developed without repeatedly revising the RTP protocol itself. For each class of application (e.g., audio), RTP defines
a profile and one or more formats. The profile provides a range of information that ensures a common understanding of the fields in the RTP header for that application class,
as will be apparent when we examine the header in detail. The format specification explains how the data that follows the RTP header is to be interpreted. For example, the
RTP header might just be followed by a sequence of bytes, each of which represents a
single audio sample taken from a defined interval after the previous one. Alternatively,
the format of the data might be much more complex; an MPEG-encoded video stream,
for example, would need to have a good deal of structure to represent all the different
types of information.
The design of RTP embodies an architectural principle known as Application Level
Framing (ALF). This principle was put forward by Clark and Tennenhouse in 1990 as
a new way to design protocols for emerging multimedia applications. They recognized
that these new applications were unlikely to be well served by existing protocols such as
TCP, and that furthermore they might not be well served by any sort of “one-size-fits-all”
protocol. At the heart of this principle is the belief that an application understands its
own needs best. For example, an MPEG video application knows how best to recover
from lost frames, and how to react differently depending on what type of frame is lost.
The same application also understands best how to segment the data for transmission—
for example, it’s better to send the data from different frames in different datagrams,
so that a lost packet only corrupts a single frame, not two. It is for this reason that
RTP leaves so many of the protocol details to the profile and format documents that are
specific to an application.
Header Format
Figure 5.21 shows the header format used by RTP. The first 12 bytes are always present,
whereas the contributing source identifiers are only used in certain circumstances. After this header there may be optional header extensions, as described below. Finally, the
header is followed by the RTP payload, the format of which is determined by the application. The intention of this header is that it contain only the fields that are likely to be
5.4
Figure 5.21
Transport for Real-Time Applications (RTP)
431
RTP header format.
used by many different applications, since anything that is very specific to a single application would be more efficiently carried in the RTP payload for that application only.
The first two bits are a version identifier, which contains the value 2 in the RTP
version deployed at the time of writing. You might think that the designers of the protocol were rather bold to think that 2 bits would be enough to contain all future versions of
RTP, but recall that bits are at a premium in the RTP header. Furthermore, the use of profiles for different applications makes it less likely that many revisions to the base RTP protocol would be needed. In any case, if it turns out that another version of RTP is needed
beyond version 2, it would be possible to consider a change to the header format so that
more than one future version would be possible. For example, a new RTP header with the
value 3 in the version field could have a “subversion” field somewhere else in the header.
The next bit is the “padding” (P) bit, which is set in circumstances in which the
RTP payload has been padded for some reason. RTP data might be padded to fill up
a block of a certain size as required by an encryption algorithm, for example. In such a
case, the complete length of the RTP header, data, and padding would be conveyed by
the lower-layer protocol header (e.g., the UDP header) and the last byte of the padding
would contain a count of how many bytes should be ignored. This is illustrated in Figure 5.22. Note that this approach to padding removes any need for a length field in the
RTP header (thus serving the goal of keeping the header short); in the common case of
no padding, the length is deduced from the lower-layer protocol.
The extension (X) bit is used to indicate the presence of an extension header, which
would be defined for a specific application and follow the main header. Such headers are
rarely used, since it is generally possible to define a payload-specific header as part of the
payload format definition for a particular application.
The X bit is followed by a 4-bit field that counts the number of “contributing
sources,” if any are included in the header. Contributing sources are discussed below.
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Figure 5.22
End-to-End Protocols
Padding of an RTP packet.
We noted above the frequent need for some sort of frame indication; this is provided by the marker bit, which could be set at the beginning of a talkspurt, for example.
The 7-bit payload type field follows; it indicates what type of multimedia data is carried
in this packet. One possible use of this field would be to enable an application to switch
from one coding scheme to another based on information about resource availability in
the network or feedback on application quality. The exact usage of the marker bit and
the payload type is determined by the application profile.
Note that the payload type is generally not used as a demultiplexing key to direct data to different applications (or to different streams within a single application; for
example, the audio and video stream for a videoconference). This is because such demultiplexing is typically provided at a lower layer (e.g., by UDP, as described in Section 5.1).
Thus, two media streams using RTP would typically use different UDP port numbers.
The sequence number is used to enable the receiver of an RTP stream to detect
missing and misordered packets. The sender simply increments the value by one for
each transmitted packet. Note that RTP does not do anything when it detects a lost
packet, in contrast to TCP, which both corrects for the loss (by retransmission) and
interprets the loss as a congestion indication (which may cause it to reduce its window
size). Rather, it is left to the application to decide what to do when a packet is lost
because this decision is likely to be highly application-dependent. For example, a video
application might decide that the best thing to do when a packet is lost is to replay the
last frame that was correctly received. Some applications might also decide to modify
their coding algorithms to reduce bandwidth needs in response to loss, but this is not a
function of RTP. It would not be sensible for RTP to decide that the sending rate should
be reduced, as this might make the application useless.
The function of the timestamp field is to enable the receiver to play back samples
at the appropriate intervals and to enable different media streams to be synchronized.
Because different applications may require different granularities of timing, RTP itself
does not specify the units in which time is measured. Instead, the timestamp is just a
counter of “ticks,” where the time between ticks is dependent on the encoding in use.
For example, an audio application that samples data once every 125 µs could use that
value as its clock resolution. The clock granularity is one of the details that is specified
in the RTP profile or payload format for an application.
5.4
Transport for Real-Time Applications (RTP)
433
The timestamp value in the packet is a number representing the time at which the
first sample in the packet was generated. The timestamp is not a reflection of the time of
day; only the differences between timestamps are relevant. For example, if the sampling
interval is 125 µs and the first sample in packet n + 1 was generated 10 ms after the first
sample in packet n, then the number of sampling instants between these two samples is
TimeBetweenPackets ÷ TimePerSample = (10 × 10−3 ) ÷ (125 × 10−6 )
= 80
Assuming the clock granularity is the same as the sampling interval, then the timestamp
in packet n + 1 would be greater than that in packet n by 80. Note that fewer than 80
samples might have been sent due to compression techniques such as silence detection,
and yet the timestamp allows the receiver to play back the samples with the correct
temporal relationship.
The synchronization source (SSRC) is a 32-bit number that uniquely identifies a
single source of an RTP stream. In a given multimedia conference, each sender picks a
random SSRC and is expected to resolve conflicts in the unlikely event that two sources
pick the same value. By making the source identifier something other than the network or transport address of the source, RTP ensures independence from the lower-layer
protocol. It also enables a single node with multiple sources (e.g., several cameras) to
distinguish those sources. When a single node generates different media streams (e.g.,
audio and video), it is not required to use the same SSRC in each stream, as there are
mechanisms in RTCP (described below) to allow intermedia synchronization.
The contributing source (CSRC) is used only when a number of RTP streams
pass through a “mixer.” A mixer can be used to reduce the bandwidth requirements for
a conference by receiving data from many sources and sending it as a single stream.
For example, the audio streams from several concurrent speakers could be decoded and
recoded as a single audio stream. In this case, the mixer lists itself as the synchronization
source but also lists the contributing sources—the SSRC values of the speakers who
contributed to the packet in question.
5.4.3 Control Protocol
RTCP provides a control stream that is associated with a data stream for a multimedia
application. This control stream provides three main functions:
1
Feedback on the performance of the application and the network;
2
A way to correlate and synchronize different media streams that have come from
the same sender;
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End-to-End Protocols
A way to convey the identity of a sender for display on a user interface (e.g., the
vat interface shown in Figure 5.19).
The first function may be useful for rate-adaptive applications, which may use
performance data to decide to use a more aggressive compression scheme to reduce congestion, or to send a higher-quality stream when there is little congestion. It can also be
useful in diagnosing network problems.
You might think that the second function is already provided by the synchronization source ID of RTP, but in fact it is not. As already noted, multiple cameras from a
single node might have different SSRC values. Furthermore, there is no requirement that
an audio and video stream from the same node use the same SSRC. Because collisions of
SSRC values may occur, it may be necessary to change the SSRC value of a stream. To
deal with this problem, RTCP uses the concept of a canonical name (CNAME) that is
assigned to a sender, which is then associated with the various SSRC values that might
be used by that sender using RTCP mechanisms.
Simply correlating two streams is only part of the problem of intermedia synchronization. Because different streams may have completely different clocks (with different
granularities and even different amounts of inaccuracy or drift), there needs to be a way
to accurately synchronize streams with each other. RTCP addresses this problem.
RTCP defines a number of different packet types, including
■
Sender reports, which enable active senders to a session to report transmission
and reception statistics;
■
Receiver reports, which receivers who are not senders use to report reception
statistics;
■
Source descriptions, which carry CNAMEs and other sender description
information;
■
Application-specific control packets.
These different RTCP packet types are sent over the lower-layer protocol, which,
as we have noted, is typically UDP. Several RTCP packets can be packed into a single
PDU of the lower-level protocol. It is required that at least two RTCP packets are sent in
every lower-level PDU: One of these is a report packet; the other is a source description
packet. Other packets may be included up to the size limits imposed by the