3rd Edition, Chapter 5 - UMD Department of Computer Science

3rd Edition, Chapter 5 - UMD Department of Computer Science
CSMC 417
Computer Networks
Prof. Ashok K Agrawala
© 2012 Ashok Agrawala
Based on J.F Kurose and K.W. Ross
Set 5
Network Layer
1
Chapter 5: Link layer
our goals:

understand principles behind link layer
services:





error detection, correction
sharing a broadcast channel: multiple access
link layer addressing
local area networks: Ethernet, VLANs
instantiation, implementation of various link
layer technologies
Link Layer
5-2
Link layer, LANs: outline
5.1 introduction, services 5.5 link virtualization:
MPLS
5.2 error detection,
correction
5.6 data center
networking
5.3 multiple access
protocols
5.7 a day in the life of a
web request
5.4 LANs




addressing, ARP
Ethernet
switches
VLANS
Link Layer
5-3
Link layer: introduction
terminology:



hosts and routers: nodes
communication channels that
connect adjacent nodes along
communication path: links
 wired links
 wireless links
 LANs
layer-2 packet: frame,
encapsulates datagram
global ISP
data-link layer has responsibility of
transferring datagram from one node
to physically adjacent node over a link
Link Layer
5-4
Link layer: context


datagram transferred by
different link protocols over
different links:
 e.g., Ethernet on first link,
frame relay on
intermediate links, 802.11
on last link
each link protocol provides
different services
 e.g., may or may not
provide rdt over link
transportation analogy:





trip from Princeton to Lausanne
 limo: Princeton to JFK
 plane: JFK to Geneva
 train: Geneva to Lausanne
tourist = datagram
transport segment =
communication link
transportation mode = link
layer protocol
travel agent = routing
algorithm
Link Layer
5-5
Link layer services

framing, link access:
 encapsulate datagram into frame, adding header, trailer
 channel access if shared medium
 “MAC” addresses used in frame headers to identify
source, dest
• different from IP address!

reliable delivery between adjacent nodes
 we learned how to do this already (chapter 3)!
 seldom used on low bit-error link (fiber, some twisted
pair)
 wireless links: high error rates
• Q: why both link-level and end-end reliability?
Link Layer
5-6
Link layer services (more)

flow control:
 pacing between adjacent sending and receiving nodes

error detection:
 errors caused by signal attenuation, noise.
 receiver detects presence of errors:
• signals sender for retransmission or drops frame

error correction:
 receiver identifies and corrects bit error(s) without resorting to
retransmission

half-duplex and full-duplex
 with half duplex, nodes at both ends of link can transmit, but not
at same time
Link Layer
5-7
Where is the link layer implemented?




in each and every host
link layer implemented in
“adaptor” (aka network
interface card NIC) or on a
chip
 Ethernet card, 802.11
card; Ethernet chipset
 implements link, physical
layer
attaches into host’s system
buses
combination of hardware,
software, firmware
application
transport
network
link
cpu
memory
controller
link
physical
host
bus
(e.g., PCI)
physical
transmission
network adapter
card
Link Layer
5-8
Adaptors communicating
datagram
datagram
controller
controller
receiving host
sending host
datagram
frame

sending side:
 encapsulates datagram in
frame
 adds error checking bits,
rdt, flow control, etc.

receiving side
 looks for errors, rdt,
flow control, etc
 extracts datagram, passes
to upper layer at
receiving side
Link Layer
5-9
Link layer, LANs: outline
5.1 introduction, services 5.5 link virtualization:
MPLS
5.2 error detection,
correction
5.6 data center
networking
5.3 multiple access
protocols
5.7 a day in the life of a
web request
5.4 LANs




addressing, ARP
Ethernet
switches
VLANS
Link Layer 5-10
Error detection
EDC= Error Detection and Correction bits (redundancy)
D = Data protected by error checking, may include header fields
• Error detection not 100% reliable!
• protocol may miss some errors, but rarely
• larger EDC field yields better detection and correction
otherwise
Link Layer 5-11
Parity checking
single bit parity:

detect single bit
errors
two-dimensional bit parity:

detect and correct single bit errors
0
0
Link Layer 5-12
Internet checksum (review)
goal: detect “errors” (e.g., flipped bits) in transmitted packet
(note: used at transport layer only)
sender:



treat segment contents
as sequence of 16-bit
integers
checksum: addition (1’s
complement sum) of
segment contents
sender puts checksum
value into UDP
checksum field
receiver:
 compute checksum of
received segment
 check if computed
checksum equals checksum
field value:
 NO - error detected
 YES - no error detected.
But maybe errors
nonetheless?
Link Layer 5-13
Cyclic redundancy check




more powerful error-detection coding
view data bits, D, as a binary number
choose r+1 bit pattern (generator), G
goal: choose r CRC bits, R, such that
 <D,R> exactly divisible by G (modulo 2)
 receiver knows G, divides <D,R> by G. If non-zero remainder:
error detected!
 can detect all burst errors less than r+1 bits

widely used in practice (Ethernet, 802.11 WiFi, ATM)
Link Layer 5-14
CRC example
want:
D.2r XOR R = nG
equivalently:
D.2r = nG XOR R
equivalently:
if we divide D.2r by
G, want remainder R
to satisfy:
R = remainder[
D.2r
]
G
G
D
r=3
101000
1001 101110000
1001
101
000
1010
1001
010
000
100
000
R
1000
0000
1000
Link Layer 5-15
Cyclic Redundancy Check






Have to maximize the probability of
detecting the errors using a small
number of additional bits.
Based on powerful mathematical
formulations – theory of finite fields
Consider (n+1) bits as n degree
polynomial
3
2
C
(
x
)
=
x
+
x
+1
Message M(x) represented as
polynomial
M ( x) = x 7 + x 4 + x 3 + x1
Divisor C(x) of degree k
Send P(x) as (n+1) bits +k bits such that
P(x) is exactly divisible by C(x)
Link Layer
16
CRC Basis





Use modulo 2 arithmetic
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 they are of the same
degree
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 operation on each pair of
matching coefficients.
Link Layer
17
Error Detection – CRCs (1)

Adds bits so that transmitted frame viewed as a polynomial is evenly
divisible by a generator polynomial
Start by
adding 0s to
frame and try
dividing
Offset by any
reminder to make it
evenly divisible
Link Layer 5-18
Cyclic Redundancy Check




All single bit errors – if xk and x0 terms are
nonzero
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) has
(x+1) as a factor
Any burst error of length k bits
Link Layer
19
Common CRC Polynomials
CRC
C(x)
CRC-8
x8 + x2 + X1 + 1
CRC-10
x10 + x9 + x5 + x4 + x1 + 1
CRC-12
x12 + x11 + x3 + x2 + 1
CRC-16
x16 + x15 + x2 + 1
CRC-CCITT
x16 + x12 + x5 + 1
CRC-32
x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2
+ x1 + 1
Link Layer
20
Error Detection – CRCs (2)
Based on standard polynomials:
 Ex: Ethernet 32-bit CRC is defined by:
 Computed with simple shift/XOR circuits
Stronger detection than checksums:
 E.g., can detect all double bit errors
 Not vulnerable to systematic errors
Link Layer
Framing Methods




Byte count »
Flag bytes with byte stuffing »
Flag bits with bit stuffing »
Physical layer coding violations
• Use non-data symbol to indicate frame
Link Layer
Framing

Break sequence of bits into a frame
 Typically implemented by the network adaptor

Sentinel-based
 Delineate frame with special pattern (e.g., 01111110)
01111110
Frame contents
01111110
 Problem: what if special patterns occurs within frame?
 Solution: escaping the special characters
• E.g., sender always inserts a 0 after five 1s
• … and receiver always removes a 0 appearing after five 1s
• Bit Stuffing
 Similar to escaping special characters in C programs
Link Layer
23
Bit Oriented Protocols

Frame – a collection of bits
 No Byte boundary

SDLC – Synchronous Data Link Control
 IBM

HDLC – High-Level Data Link Control
 ISO Standard
HDLC Frame Format
Link Layer
24
Framing (Continued)

Counter-based
 Include the payload length in the header
 … instead of putting a sentinel at the end
 Problem: what if the count field gets corrupted?
• Causes receiver to think the frame ends at a different place
 Solution: catch later when doing error detection
• And wait for the next sentinel for the start of a new frame
DDCMP Frame Format
Link Layer
25
Framing – Byte count
Frame begins with a count of the number of bytes in
it
 Simple, but difficult to resynchronize after an error
Expected
case
Error
case
Link Layer
Framing – Bit stuffing
Stuffing done at the bit level:
 Frame flag has six consecutive 1s (not shown)
 On transmit, after five 1s in the data, a 0 is added
 On receive, a 0 after five 1s is deleted
Data bits
Transmitted bits
with stuffing
Link Layer
Framing – Byte stuffing
Special flag bytes delimit frames; occurrences of flags
in the data must be stuffed (escaped)
 Longer, but easy to resynchronize after error
Frame
format
Stuffing
examples
Need to
escape extra
ESCAPE bytes
too!
Link Layer
Byte-Oriented Protocols


Frame – a collection of bytes.
Examples
 BISYNC – Binary Synchronous Communication – IBM
 DDCMP – Digital Data Communication Message
Protocol
 PPP – Point-to-Point

Sentinel Based – Use special character as marker
 BISYNC
• SYN and SOH
• STX and ETX
• DLE as escape character. - Character Stuffing
Link Layer
29
Frame Structure
PPP Frame Format
BISYNC Frame Format
Link Layer
30
Clock-Based Framing (SONET)

Clock-based
 Make each frame a fixed size
 No ambiguity about start and end of frame
 But, may be wasteful

Synchronous Optical Network (SONET)
 Slowest speed link STS-1 – 51.84 Mbps ( 810*8*8K)
 Frame – 9 rows of 90 bytes
• First 3 bytes of each row are overhead
• First two bytes of a frame contain a special bit pattern – to
mark the start of the frame – check for it every 810 bytes
Link Layer
31
Sonet Frame
Link Layer
32
Elementary Data Link Protocols




Link layer environment »
Utopian Simplex Protocol »
Stop-and-Wait Protocol for Error-free channel »
Stop-and-Wait Protocol for Noisy channel »
Link Layer
Link layer environment (1)
Commonly implemented as NICs and OS drivers;
network layer (IP) is often OS software
Link Layer
Link layer environment (2)

Link layer protocol implementations use library functions
 See code (protocol.h) for more details
Group
Library Function
Description
Network
layer
from_network_layer(&packet)
to_network_layer(&packet)
enable_network_layer()
disable_network_layer()
Take a packet from network layer to send
Deliver a received packet to network layer
Let network cause “ready” events
Prevent network “ready” events
Physical
layer
from_physical_layer(&frame)
to_physical_layer(&frame)
Get an incoming frame from physical layer
Pass an outgoing frame to physical layer
Events &
timers
wait_for_event(&event)
start_timer(seq_nr)
stop_timer(seq_nr)
start_ack_timer()
stop_ack_timer()
Wait for a packet / frame / timer event
Start a countdown timer running
Stop a countdown timer from running
Start the ACK countdown timer
Stop the ACK countdown timer
Link Layer 5-35
Protocol Definitions
Continued 
Some definitions needed in the protocols to follow.
These are located in the file protocol.h.
Link Layer
36
Protocol
Definitions
(ctd.)
Some definitions
needed in the
protocols to follow.
These are located in
the file protocol.h.
Link Layer
37
Utopian Simplex Protocol
An optimistic protocol (p1) to get us started
 Assumes no errors, and receiver as fast as sender
 Considers one-way data transfer
}
Sender loops blasting frames
Receiver loops eating frames
 That’s it, no error or flow control …
Link Layer
Utopian Simplex Protocol (1)
...
A utopian simplex protocol.
Link Layer 5-39
Utopian Simplex Protocol (2)
A utopian simplex protocol.
Link Layer 5-40
Reliable Transmission

Transfer frames without errors
 Error Correction
 Error Detection
 Discard frames with error



Acknowledgements and Timeouts
Retransmission
ARQ – Automatic Repeat Request
Link Layer
41
Stop and Wait with 1-bit Seq No
Link Layer
42
Stop and Wait Protocols


Simple
Low Throughput
 One Frame per RTT

Increase throughput by having more frames in
flight
 Sliding Window Protocol
Link Layer
43
Stop and Wait
Duplicate
Frames
Link Layer
44
Stop-and-Wait – Error-free channel
Protocol (p2) ensures sender can’t outpace receiver:
 Receiver returns a dummy frame (ack) when ready
 Only one frame out at a time – called stop-and-wait
 We added flow control!
Sender waits to for ack after
passing frame to physical
layer
Receiver sends ack after passing
frame to network layer
Link Layer
Stop-and-Wait – Noisy channel (1)
ARQ (Automatic Repeat reQuest) adds error
control
 Receiver acks frames that are correctly delivered
 Sender sets timer and resends frame if no ack)
For correctness, frames and acks must be numbered
 Else receiver can’t tell retransmission (due to lost ack
or early timer) from new frame
 For stop-and-wait, 2 numbers (1 bit) are sufficient
Link Layer
Stop-and-Wait – Noisy channel (2)
{
Sender loop (p3):
Send frame (or retransmission)
Set timer for retransmission
Wait for ack or timeout
If a good ack then set up for
the next frame to send (else
the old frame will be
retransmitted)
Link Layer
Stop-and-Wait – Noisy channel (3)
Receiver loop (p3):
Wait for a frame
If it’s new then
take it and
advance expected
frame
Ack current frame
Link Layer
Sliding Window Protocols




Sliding Window concept »
One-bit Sliding Window »
Go-Back-N »
Selective Repeat »
Link Layer
Sliding Window concept (1)
Sender maintains window of frames it can send
 Needs to buffer them for possible retransmission
 Window advances with next acknowledgements
Receiver maintains window of frames it can
receive
 Needs to keep buffer space for arrivals
 Window advances with in-order arrivals
Link Layer
Sliding Window Protocol


Sender assigns a sequence number –SeqNum
Sender maintains three variables:
 Send Window Size – SWS
 Last Ack Received – LAR
 Last Frame Sent – LFS



Invariant LFS – LAR < SWS
When ACK arrives sender movers LAR to the
right and thereby allowing the sender to transmit
another frame
Associate a timer with each frame it transmits
Link Layer
51
Sliding Window Protocol

Receiver maintains three variables:
 Receiver Window Size – RWS
 Largest acceptable Frame Number – LAF
 Last Frame Received – LFR


Invariant LAF –LFR < RWR
When frame with SEQNum arrives
 If SeqNum < LFR or SeqNum >LAF discard the
frame
 If LFR < SeqNum < LAF then accept the frame

SeqNumtoAck – largest seq no not yet acked.
 Send this as ack.
Link Layer
52
Sliding Window
Sliding Window on Sender
Timeline
Sliding window on Receiver
Link Layer
53
Sliding Window concept (2)
A sliding window advancing at the sender and
receiver
 Ex: window size is 1, with a 3-bit sequence number.
Sender
Receive
r
At the start
First
frame is
sent
First
frame is
received
Sender gets
first ack
Link Layer
Sliding Window concept (3)
Larger windows enable pipelining for efficient link
use
 Stop-and-wait (w=1) is inefficient for long links
 Best window (w) depends on bandwidth-delay (BD)
 Want w ≥ 2BD+1 to ensure high link utilization
Pipelining leads to different choices for
errors/buffering
 We will consider Go-Back-N and Selective Repeat
Link Layer
One-Bit Sliding Window (1)

Transfers data in both directions with stop-and-wait
 Piggybacks acks on reverse data frames for efficiency
 Handles transmission errors, flow control, early timers
{
Each node is sender
and receiver (p4):
Prepare first frame
Launch it, and set
timer
...
Link Layer
5-56
One-Bit Sliding Window (2)
...
Wait for frame or timeout
If a frame with new
data then deliver it
If an ack for last send
then prepare for next
data frame
(Otherwise it was a timeout)
Send next data frame or
retransmit old one; ack
the last data we
received
Link Layer
5-57
One-Bit Sliding Window (3)

Two scenarios show subtle interactions exist in p4:
• Simultaneous start [right] causes correct but slow operation compared
to normal [left] due to duplicate transmissions.
Time
Notation is (seq, ack, frame number). Asterisk indicates frame accepted by network layer .
(a) Normal case
(b) Correct, but poor
performance
Link Layer
5-58
Go-Back-N (1)
Receiver only accepts/acks frames that arrive in
order:
 Discards frames that follow a missing/errored frame
 Sender times out and resends all outstanding frames
Link Layer
Go-Back-N (2)
Tradeoff made for Go-Back-N:
 Simple strategy for receiver; needs only 1 frame
 Wastes link bandwidth for errors with large
windows; entire window is retransmitted
Link Layer
Selective Repeat (1)
Receiver accepts frames anywhere in receive
window
 Cumulative ack indicates highest in-order frame
 NAK (negative ack) causes sender retransmission of a
missing frame before a timeout resends window
Link Layer
Selective Repeat (2)
Tradeoff made for Selective Repeat:
 More complex than Go-Back-N due to buffering at
receiver and multiple timers at sender
 More efficient use of link bandwidth as only lost
frames are resent (with low error rates)
Implemented as p6 (see code in book)
Link Layer
Selective Repeat (3)
For correctness, we require:
 Sequence numbers (s) at least twice the window (w)
Error case (s=8, w=7) –
too few sequence numbers
Originals
Correct (s=8, w=4) – enough
sequence numbers
Retransmits
New receive window overlaps
old – retransmits ambiguous
Originals
Retransmits
New and old receive window
don’t overlap – no ambiguity
Link Layer
Example Data Link Protocols
 Packet over SONET »
 PPP (Point-to-Point Protocol) »
 ADSL (Asymmetric Digital Subscriber Loop) »
Link Layer
Packet over SONET
Packet over SONET is the method used to carry IP
packets over SONET optical fiber links
 Uses PPP (Point-to-Point Protocol) for framing
Protocol stacks
PPP frames may be split
over SONET payloads
Link Layer
Packet over SONET (2)
PPP Features
1. Separate packets, error detection
2. Link Control Protocol
3. Network Control Protocol
Link Layer 5-66
PPP (1)
PPP (Point-to-Point Protocol) is a general method
for delivering packets across links
 Framing uses a flag (0x7E) and byte stuffing
 “Unnumbered mode” (connectionless unacknowledged service) is used to carry IP packets
 Errors are detected with a checksum
0x21 for IPv4 IP packet
Link Layer
PPP (2)
A link control protocol brings the PPP link up and
down
State machine for link control
Link Layer
PPP – Point to Point Protocol (3)
The LCP frame types.
Link Layer
69
ADSL (1)
Widely used for broadband Internet over local
loops
 ADSL runs from modem (customer) to DSLAM (ISP)
 IP packets are sent over PPP and AAL5/ATM (over)
Link Layer
ADSL (2)
PPP data is sent in AAL5 frames over ATM cells:
 ATM is a link layer that uses short, fixed-size cells (53
bytes); each cell has a virtual circuit identifier
 AAL5 is a format to send packets over ATM
 PPP frame is converted to a AAL5 frame (PPPoA)
AAL5 frame is divided into 48 byte pieces, each of
which goes into one ATM cell with 5 header bytes
Link Layer
High-Level Data Link Control
Frame format for bit-oriented protocols.
Link Layer
72
High-Level Data Link Control (2)
Control field of
(a) An information frame.
(b) A supervisory frame.
(c) An unnumbered frame.
Link Layer
73
The Data Link Layer in the
Internet
A home personal computer acting as an internet
host.
Link Layer
74
Link layer, LANs: outline
5.1 introduction, services 5.5 link virtualization:
MPLS
5.2 error detection,
correction
5.6 data center
networking
5.3 multiple access
protocols
5.7 a day in the life of a
web request
5.4 LANs




addressing, ARP
Ethernet
switches
VLANS
Link Layer 5-75
The MAC Sublayer
Responsible for deciding who
sends next on a multi-access
link
 An important part of the link
layer, especially for LANs
Application
Transport
Network
Link
Physical
MAC is in here!
CN5E by Tanenbaum & Wetherall, © Pearson EducationPrentice Hall and D. Wetherall, 2011
MAC protocols: taxonomy
three broad classes:
 channel partitioning
 divide channel into smaller “pieces” (time slots, frequency, code)
 allocate piece to node for exclusive use

random access
 channel not divided, allow collisions
 “recover” from collisions

“taking turns”
 nodes take turns, but nodes with more to send can take longer
turns
Link Layer 5-77
Channel partitioning MAC protocols: TDMA
TDMA: time division multiple access




access to channel in "rounds"
each station gets fixed length slot (length = pkt
trans time) in each round
unused slots go idle
example: 6-station LAN, 1,3,4 have pkt, slots
2,5,6 idle
6-slot
frame
6-slot
frame
1
3
4
1
3
4
Link Layer 5-78
Channel partitioning MAC protocols: FDMA
FDMA: frequency division multiple access



channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6
idle
FDM cable
frequency bands

Link Layer 5-79
Multiple access protocols


single shared broadcast channel
two or more simultaneous transmissions by nodes:
interference
 collision if node receives two or more signals at the same
time
multiple access protocol


distributed algorithm that determines how nodes share
channel, i.e., determine when node can transmit
communication about channel sharing must use channel itself!
 no out-of-band channel for coordination
Link Layer 5-80
An ideal multiple access protocol
given: broadcast channel of rate R bps
desiderata:
1. when one node wants to transmit, it can send at rate R.
2. when M nodes want to transmit, each can send at average
rate R/M
3. fully decentralized:
• no special node to coordinate transmissions
• no synchronization of clocks, slots
4. simple
Link Layer 5-81
Multiple access links, protocols
two types of “links”:
 point-to-point
 PPP for dial-up access
 point-to-point link between Ethernet switch, host

broadcast (shared wire or medium)
 old-fashioned Ethernet
 upstream HFC
 802.11 wireless LAN
shared wire (e.g.,
cabled Ethernet)
shared RF
(e.g., 802.11 WiFi)
shared RF
(satellite)
humans at a
cocktail party
(shared air, acoustical)
Link Layer 5-82
Random access protocols

when node has packet to send
 transmit at full channel data rate R.
 no a priori coordination among nodes


two or more transmitting nodes ➜ “collision”,
random access MAC protocol specifies:
 how to detect collisions
 how to recover from collisions (e.g., via delayed
retransmissions)

examples of random access MAC protocols:
 slotted ALOHA
 ALOHA
 CSMA, CSMA/CD, CSMA/CA
Link Layer 5-83
ALOHA (1)
User
A
B
C
D
E
Collision
Time
Collision
In pure ALOHA, frames are transmitted
at completely arbitrary times
Pure ALOHA (2)
Vulnerable period for the shaded frame.
November 12
CMSC417 Set 5
85
Pure (unslotted) ALOHA



unslotted Aloha: simpler, no synchronization
when frame first arrives
 transmit immediately
collision probability increases:
 frame sent at t0 collides with other frames sent in [t01,t0+1]
Link Layer 5-86
Pure ALOHA efficiency
P(success by given node) = P(node transmits) .
P(no other node transmits in [t0-1,t0] .
P(no other node transmits in [t0-1,t0]
= p . (1-p)N-1 . (1-p)N-1
= p . (1-p)2(N-1)
… choosing optimum p and then letting n
= 1/(2e) = .18
even worse than slotted Aloha!
Link Layer 5-87
Pure ALOHA (3)
Throughput versus offered traffic for ALOHA
systems.
November 12
CMSC417 Set 5
88
Slotted ALOHA
assumptions:





all frames same size
time divided into equal size
slots (time to transmit 1
frame)
nodes start to transmit
only slot beginning
nodes are synchronized
if 2 or more nodes transmit
in slot, all nodes detect
collision
operation:

when node obtains fresh
frame, transmits in next slot
 if no collision: node can send
new frame in next slot
 if collision: node retransmits
frame in each subsequent
slot with prob. p until
success
Link Layer 5-89
Slotted ALOHA
node 1
1
1
node 2
2
2
node 3
3
C
2
3
E
C
S
E
Pros:



1
1
single active node can
continuously transmit at
full rate of channel
highly decentralized: only
slots in nodes need to be
in sync
simple
C
3
E
S
S
Cons:




collisions, wasting slots
idle slots
nodes may be able to
detect collision in less
than time to transmit
packet
clock synchronization
Link Layer 5-90
Slotted ALOHA: efficiency
efficiency: long-run
fraction of successful slots
(many nodes, all with many
frames to send)



suppose: N nodes with
many frames to send, each
transmits in slot with
probability p
prob that given node has
success in a slot = p(1p)N-1
prob that any node has a
success = Np(1-p)N-1


max efficiency: find p* that
maximizes
Np(1-p)N-1
for many nodes, take limit
of Np*(1-p*)N-1 as N goes
to infinity, gives:
max efficiency = 1/e = .37
at best: channel
used for useful
transmissions 37%
of time!
!
Link Layer 5-91
CSMA (carrier sense multiple access)
CSMA: listen before transmit:
if channel sensed idle: transmit entire frame
 if channel sensed busy, defer transmission

human analogy: don’t interrupt others!
Link Layer 5-92
CSMA (1)
CSMA improves on ALOHA by sensing the channel!
 User doesn’t send if it senses someone else
Variations on what to do if the channel is busy:
 1-persistent (greedy) sends as soon as idle
 Nonpersistent waits a random time then tries again
 p-persistent sends with probability p when idle
CN5E by Tanenbaum & Wetherall, © Pearson EducationPrentice Hall and D. Wetherall, 2011
Persistent and Nonpersistent
CSMA
Comparison of the channel utilization versus load for
various random access protocols.
November 12
CMSC417 Set 5
94
CSMA Collisions
Collisions can still occur:
propagation delay means
two nodes may not hear
each other’s transmission
Collision:
entire packet transmission
time wasted
October 08
CMSC417 Set 11
95
CSMA/CD (collision detection)
CSMA/CD: carrier sensing, deferral as in CSMA
 collisions detected within short time
 colliding transmissions aborted, reducing channel wastage

collision detection:
 easy in wired LANs: measure signal strengths, compare
transmitted, received signals
 difficult in wireless LANs: received signal strength
overwhelmed by local transmission strength

human analogy: the polite conversationalist
Link Layer 5-96
Ethernet CSMA/CD algorithm
1. NIC receives datagram
from network layer,
creates frame
2. If NIC senses channel
idle, starts frame
transmission. If NIC
senses channel busy,
waits until channel idle,
then transmits.
3. If NIC transmits entire
frame without detecting
another transmission,
NIC is done with frame !
4. If NIC detects another
transmission while
transmitting, aborts and
sends jam signal
5. After aborting, NIC
enters binary (exponential)
backoff:
 after mth collision, NIC
chooses K at random
from {0,1,2, …, 2m-1}.
NIC waits K·512 bit
times, returns to Step 2
 longer backoff interval
with more collisions
Link Layer 5-97
CSMA/CD efficiency


Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency =


1
1 + 5t prop /t trans
efficiency goes to 1
 as tprop goes to 0
 as ttrans goes to infinity
better performance than ALOHA: and simple, cheap,
decentralized!
Link Layer 5-98
CSMA/CD Collision Detection
October 08
CMSC417 Set 11
99
CSMA (3) – Collision Detection
CSMA/CD improvement is to detect/abort
collisions
 Reduced contention times improve performance
Collision time is
much shorter
than frame
time
CN5E by Tanenbaum & Wetherall, © Pearson EducationPrentice Hall and D. Wetherall, 2011
“Taking turns” MAC protocols
channel partitioning MAC protocols:
 share channel efficiently and fairly at high load
 inefficient at low load: delay in channel access, 1/N
bandwidth allocated even if only 1 active node!
random access MAC protocols
 efficient at low load: single node can fully utilize
channel
 high load: collision overhead
“taking turns” protocols
look for best of both worlds!
Link Layer5-101
“Taking turns” MAC protocols
polling:



master node “invites”
slave nodes to transmit
in turn
typically used with
“dumb” slave devices
concerns:
 polling overhead
 latency
 single point of
failure (master)
data
poll
master
data
slaves
Link Layer5-102
“Taking turns” MAC protocols
token passing:



control token passed
from one node to next
sequentially.
token message
concerns:
 token overhead
 latency
 single point of failure
(token)
T
(nothing
to send)
T
data
Link Layer5-103
Collision-Free Protocols (1) –
Bitmap
Collision-free protocols avoid collisions entirely
 Senders must know when it is their turn to send
The basic bit-map protocol:
 Sender set a bit in contention slot if they have data
 Senders send in turn; everyone knows who has data
CN5E by Tanenbaum & Wetherall, © Pearson EducationPrentice Hall and D. Wetherall, 2011
Collision-Free Protocols– Countdown
Binary countdown improves on the bitmap protocol
 Stations send their address in
contention slot (log N bits
instead of N bits)
 Medium ORs bits; stations
give up when they send a “0”
but see a “1”
 Station that sees its full
address is next to send
CN5E by Tanenbaum & Wetherall, ©
Limited-Contention Protocols
(1)
Idea is to divide stations into groups within which
only a very small number are likely to want to
send
 Avoids wastage due to idle periods and collisions
Already too many contenders
for a good chance of one winner
CN5E by Tanenbaum & Wetherall, © Pearson EducationPrentice Hall and D. Wetherall, 2011
Limited Contention
Adaptive Tree Walk
Tree divides stations into groups (nodes) to poll
 Depth first search under nodes with poll collisions
 Start search at lower levels if >1 station expected
Level 0
Level 1
Level 2
CN5E by Tanenbaum & Wetherall, © Pearson EducationPrentice Hall and D. Wetherall, 2011
Wavelength Division Multiple Access
Protocols
Wavelength division multiple access.
November 12
CMSC417 Set 5 108
Ethernet
•
•
•
•
•
•
•
•
•
•
Ethernet Cabling
Manchester Encoding
The Ethernet MAC Sublayer Protocol
The Binary Exponential Backoff Algorithm
Ethernet Performance
Switched Ethernet
Fast Ethernet
Gigabit Ethernet
IEEE 802.2: Logical Link Control
Retrospective on Ethernet
November 12
CMSC417 Set 5 109
Ethernet
“dominant” wired LAN technology:
 cheap $20 for NIC
 first widely used LAN technology
 simpler, cheaper than token LANs and ATM
 kept up with speed race: 10 Mbps – 10 Gbps
Metcalfe’s Ethernet sketch
Link Layer 5-110
Classic Ethernet– Physical Layer
One shared coaxial cable to which all hosts attached
 Up to 10 Mbps, with Manchester encoding
 Hosts ran the classic Ethernet protocol for access
CN5E by Tanenbaum & Wetherall, © Pearson EducationPrentice Hall and D. Wetherall, 2011
Ethernet Transceiver and
Adapter





Medium – 50 ohm
cable
Taps 2.5 m apart
Transceiver – can
send and receive
Multiple segments can
be joined by repeaters
– no more than 4
Max end-to-end
distance – 2500 m
October 08
CMSC417 Set 11 112
Ethernet Uses CSMA/CD

Carrier sense: wait for link to be idle
 Channel idle: start transmitting
 Channel busy: wait until idle

Collision detection: listen while transmitting
 No collision: transmission is complete
 Collision: abort transmission, and send jam signal

Random access: exponential back-off
 After collision, wait a random time before trying again
 After mth collision, choose K randomly from {0, …, 2m-1}
 … and wait for K*512 bit times before trying again
October 08
CMSC417 Set 11 113
Ethernet Cabling
The most common kinds of Ethernet cabling.
November 12
CMSC417 Set 5 114
Ethernet Cabling (2)
Three kinds of Ethernet cabling.
(a) 10Base5, (b) 10Base2, (c) 10Base-T.
November 12
CMSC417 Set 5 115
Ethernet Cabling (3)
Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d)
Segmented.
November 12
CMSC417 Set 5 116
Ethernet Signalling
(a) Binary encoding, (b) Manchester encoding,
(c) Differential Manchester encoding.
November 12
CMSC417 Set 5 117
Ethernet Frame Format

Preamble –
 For synchronizing
 Alternate 0 and 1




Address – 48 bit MAC
Type – Id for higher
level protocol
Length – up to 1500
bytes, with minimum
of 46 bytes, of data
802.3 – Length for
Type field.
October 08
CMSC417 Set 11 118
Ethernet frame structure
sending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
type
dest.
source
preamble address address
data
(payload)
CRC
preamble:
 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011
 used to synchronize receiver, sender clock rates
Link Layer 5-119
Ethernet frame structure (more)

addresses: 6 byte source, destination MAC addresses
 if adapter receives frame with matching destination
address, or with broadcast address (e.g. ARP packet), it
passes data in frame to network layer protocol
 otherwise, adapter discards frame


type: indicates higher layer protocol (mostly IP but
others possible, e.g., Novell IPX, AppleTalk)
CRC: cyclic redundancy check at receiver
 error detected: frame is dropped
type
dest.
source
preamble address address
data
(payload)
CRC
Link Layer5-120
Ethernet: unreliable, connectionless



connectionless: no handshaking between sending and
receiving NICs
unreliable: receiving NIC doesnt send acks or nacks
to sending NIC
 data in dropped frames recovered only if initial
sender uses higher layer rdt (e.g., TCP), otherwise
dropped data lost
Ethernet’s MAC protocol: unslotted CSMA/CD wth
binary backoff
Link Layer5-121
802.3 Ethernet standards: link & physical layers

many different Ethernet standards
 common MAC protocol and frame format
 different speeds: 2 Mbps, 10 Mbps, 100 Mbps, 1Gbps,
10G bps
 different physical layer media: fiber, cable
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T2
100BASE-FX
100BASE-T4
100BASE-SX
100BASE-BX
copper (twister
pair) physical layer
fiber physical layer
Link Layer5-122
Classic Ethernet (2) – MAC
MAC protocol is 1-persistent CSMA/CD Random
delay (backoff) after collision is computed with
BEB (Binary Exponential Backoff)
 Frame format is still used with modern Ethernet.
Ethernet
(DIX)
IEEE
802.
3
CN5E by Tanenbaum & Wetherall, © Pearson EducationPrentice Hall and D. Wetherall, 2011
Ethernet Transmitter Algorithm

P-persistent
 Transmit with prob p
when line goes idle


Ethernet uses 1persistent algorithm
On Collision 32 bit jamming
sequence
 Stops transmitting
 Runt frame – 64 synch
+ 32 bit jamming
sequence



Min frame size – 64
bytes – 46 + 14 + 4
For 2500 m line with
up to 4 repeaters max
round trip delay –
51.2µs
Exponential Backoff
 In steps of 51.2µs
 Wait k* rand (0,.. 2k-1)
October 08
CMSC417 Set 11 124
IEEE 802.2: Logical Link Control
(a) Position of LLC. (b) Protocol formats.
November 12
CMSC417 Set 5 125
Limitations on Ethernet Length
B
A

latency d
Latency depends on physical length of link
 Time to propagate a packet from one end to the other

Suppose A sends a packet at time t
 And B sees an idle line at a time just before t+d
 … so B happily starts transmitting a packet

B detects a collision, and sends jamming signal
 But A doesn’t see collision till t+2d
October 08
CMSC417 Set 11 126
Limitations on Ethernet Length
B
A

latency d
A needs to wait for time 2d to detect collision
 So, A should keep transmitting during this period
 … and keep an eye out for a possible collision

Imposes restrictions on Ethernet
 Maximum length of the wire: 2500 meters
 Minimum length of the packet: 512 bits (64 bytes)
October 08
CMSC417 Set 11 127
Ethernet MAC Sublayer Protocol (2)
Collision detection can take as long as 2τ .
November 12
CMSC417 Set 5 128
Worst Case Secnario
A sends a frame at time t
Frame arrives at B at t+d
B begins transmitting at t+d and
collides with A’s Frame
B’s 32 bit frame arrives at A at t+2d
October 08
CMSC417 Set 11 129
Ethernet Performance





k stations, always ready to transmit
Assume constant retransmission probability p
Probability, A that some station acquires channel during a given slot
𝐴 = 𝑘𝑘(1 − 𝑝)𝑘−1
Optimal value of p – differentiate w.r.t. p and equate to 0
•
•






𝑝=
1
𝑘
then optimal 𝐴 = [
As k → ∞
𝑝 →
1
𝑒
𝑘−1 𝑘−1
]
𝑘
Probability that Contention Interval is exactly j slots is 𝐴(1 − 𝐴)𝑗−1
So, the mean number of slots per contention
𝑗−1
 ∑∞
=
𝑗=0 𝑗𝑗(1 − 𝐴)
Slot duration = 2𝜏
1
𝐴
Mean contention interval 𝑤 =
If frame transmission time = P
Channel efficiency =
𝑃
𝑃+2𝜏/𝐴
=
2𝜏
𝐴
…. Mean number of cont. slots = e
1
1+2𝐵𝐵𝐵/𝑐𝑐
 Where F=Frame length, B=network Bandwidth, L = cable length
November 12
CMSC417 Set 5 130
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times.
November 12
CMSC417 Set 5 131
Ethernet Repeaters


Up to 4 Repeaters
Cables
 10Base 5
• 10Mbps
• Baseband
• 500 Meters
 10Base2
 10BaseT
• Twisted Pair 100 M
 Category 5 (Cat 5)
• 10 M and 100 M
• Twisted Pair
October 08
CMSC417 Set 11 132
Hubs: Physical-Layer Repeaters

Hubs are physical-layer repeaters
 Bits coming from one link go out all other links
 At the same rate, with no frame buffering
 No CSMA/CD at hub: adapters detect
collisions
twisted pair
hub
October 08
CMSC417 Set 11 133
Interconnecting with Hubs



Backbone hub interconnects LAN segments
All packets seen everywhere, forming one large
collision domain
Can’t interconnect Ethernets of different speeds
hub
hub
hub
hub
October 08
CMSC417 Set 11 134
Switch

Link layer device
 Stores and forwards Ethernet frames
 Examines frame header and selectively
forwards frame based on MAC dest address
 When frame is to be forwarded on segment,
uses CSMA/CD to access segment

Transparent
 Hosts are unaware of presence of switches

Plug-and-play, self-learning
 Switches do not need to be configured
October 08
CMSC417 Set 11 135
Switched/Fast Ethernet (1)
 Hubs wire all lines into a single CSMA/CD domain
 Switches isolate each port to a separate domain
• Much greater throughput for multiple ports
• No need for CSMA/CD with full-duplex lines
CN5E by Tanenbaum & Wetherall, © Pearson EducationPrentice Hall and D. Wetherall, 2011
Switched/Fast Ethernet (2)
Switches can be wired to computers, hubs and
switches
 Hubs concentrate traffic from computers
 More on how to switch frames the in 4.8
Switch
Switch ports
Hu
b
Twisted pair
CN5E by Tanenbaum & Wetherall, © Pearson EducationPrentice Hall and D. Wetherall, 2011
Switch: Traffic Isolation


Switch breaks subnet into LAN segments
Switch filters packets
 Same-LAN-segment frames not usually forwarded onto other LAN
segments
 Segments become separate collision domains
switch
collision
domain
hub
October
08
collision
domain
hub
collision domain
hub
CMSC417 Set 11 138
Switched/Fast Ethernet (3)
Fast Ethernet extended Ethernet from 10 to 100
Mbps
 Twisted pair (with Cat 5) dominated the market
CN5E by Tanenbaum & Wetherall, © Pearson EducationPrentice Hall and D. Wetherall, 2011
Gigabit Ethernet (1)
A two-station Ethernet
Gigabit / 10 Gigabit Ethernet (1)
Switched Gigabit Ethernet is now the garden variety
 With full-duplex lines between computers/switches
CN5E by Tanenbaum & Wetherall, © Pearson EducationPrentice Hall and D. Wetherall, 2011
Gigabit / 10 Gigabit Ethernet (1)
 Gigabit Ethernet is commonly run over twisted pair
 10 Gigabit Ethernet is being deployed where needed
 40/100 Gigabit Ethernet is under development
CN5E by Tanenbaum & Wetherall, © Pearson EducationPrentice Hall and D. Wetherall, 2011
Benefits of Ethernet

Easy to administer and maintain
Inexpensive
Increasingly higher speed

Moved from shared media to switches


 Change everything except the frame format
 A good general lesson for evolving the Internet
October 08
CMSC417 Set 11 143
Cable access network
Internet frames,TV channels, control transmitted
downstream at different frequencies
cable headend
…
CMTS
cable modem
termination system
ISP


…
splitter
cable
modem
upstream Internet frames, TV control, transmitted
upstream at different frequencies in time slots
multiple 40Mbps downstream (broadcast) channels
 single CMTS transmits into channels
multiple 30 Mbps upstream channels
 multiple access: all users contend for certain upstream
channel time slots (others assigned)
Cable access network
cable headend
MAP frame for
Interval [t1, t2]
Downstream channel i
CMTS
Upstream channel j
t1
Minislots containing
minislots request frames
t2
Residences with cable modems
Assigned minislots containing cable modem
upstream data frames
DOCSIS: data over cable service interface spec


FDM over upstream, downstream frequency channels
TDM upstream: some slots assigned, some have contention
 downstream MAP frame: assigns upstream slots
 request for upstream slots (and data) transmitted
random access (binary backoff) in selected slots
Link Layer5-145
Summary of MAC protocols

channel partitioning, by time, frequency or code
 Time Division, Frequency Division


random access (dynamic),
 ALOHA, S-ALOHA, CSMA, CSMA/CD
 carrier sensing: easy in some technologies (wire), hard
in others (wireless)
 CSMA/CD used in Ethernet
 CSMA/CA used in 802.11
taking turns
 polling from central site, token passing
 bluetooth, FDDI, token ring
Link Layer5-146
Link layer, LANs: outline
5.1 introduction, services 5.5 link virtualization:
MPLS
5.2 error detection,
correction
5.6 data center
networking
5.3 multiple access
protocols
5.7 a day in the life of a
web request
5.4 LANs




addressing, ARP
Ethernet
switches
VLANS
Link Layer5-147
MAC addresses and ARP

32-bit IP address:
 network-layer address for interface
 used for layer 3 (network layer) forwarding

MAC (or LAN or physical or Ethernet) address:
 function: used ‘locally” to get frame from one interface to
another physically-connected interface (same network, in IPaddressing sense)
 48 bit MAC address (for most LANs) burned in NIC
ROM, also sometimes software settable
 e.g.: 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation
(each “number” represents 4 bits)
Link Layer5-148
LAN addresses and ARP
each adapter on LAN has unique LAN address
1A-2F-BB-76-09-AD
LAN
(wired or
wireless)
adapter
71-65-F7-2B-08-53
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
Link Layer5-149
LAN addresses (more)



MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space
(to assure uniqueness)
analogy:
 MAC address: like Social Security Number
 IP address: like postal address

MAC flat address ➜ portability
 can move LAN card from one LAN to another

IP hierarchical address not portable
 address depends on IP subnet to which node is
attached
Link Layer5-150
ARP: address resolution protocol
Question: how to determine
interface’s MAC address,
knowing its IP address?
137.196.7.78
1A-2F-BB-76-09-AD
137.196.7.23
137.196.7.14
LAN
71-65-F7-2B-08-53
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
ARP table: each IP node (host,
router) on LAN has table
 IP/MAC address
mappings for some LAN
nodes:
< IP address; MAC address; TTL>
 TTL (Time To Live):
time after which address
mapping will be
forgotten (typically 20
min)
137.196.7.88
Link Layer5-151
ARP protocol: same LAN

A wants to send datagram
to B
 B’s MAC address not in
A’s ARP table.

A broadcasts ARP query
packet, containing B's IP
address
 dest MAC address = FF-FFFF-FF-FF-FF
 all nodes on LAN receive
ARP query


B receives ARP packet,
replies to A with its (B's)
MAC address
A caches (saves) IP-toMAC address pair in its
ARP table until
information becomes old
(times out)
 soft state: information that
times out (goes away)
unless refreshed

ARP is “plug-and-play”:
 nodes create their ARP
tables without intervention
from net administrator
 frame sent to A’s MAC
address (unicast)
Link Layer5-152
Addressing: routing to another LAN
walkthrough: send datagram from A to B via R
 focus on addressing – at IP (datagram) and MAC layer (frame)
 assume A knows B’s IP address
 assume A knows IP address of first hop router, R (how?)
 assume A knows R’s MAC address (how?)
A
R
111.111.111.111
74-29-9C-E8-FF-55
B
222.222.222.222
49-BD-D2-C7-56-2A
222.222.222.220
1A-23-F9-CD-06-9B
111.111.111.112
CC-49-DE-D0-AB-7D
111.111.111.110
E6-E9-00-17-BB-4B
222.222.222.221
88-B2-2F-54-1A-0F
Link Layer5-153
Addressing: routing to another LAN
A creates IP datagram with IP source A, destination B
A creates link-layer frame with R's MAC address as dest, frame
contains A-to-B IP datagram


MAC src: 74-29-9C-E8-FF-55
MAC dest: E6-E9-00-17-BB-4B
IP src: 111.111.111.111
IP dest: 222.222.222.222
IP
Eth
Phy
A
R
111.111.111.111
74-29-9C-E8-FF-55
B
222.222.222.222
49-BD-D2-C7-56-2A
222.222.222.220
1A-23-F9-CD-06-9B
111.111.111.112
CC-49-DE-D0-AB-7D
111.111.111.110
E6-E9-00-17-BB-4B
222.222.222.221
88-B2-2F-54-1A-0F
Link Layer5-154
Addressing: routing to another LAN
frame sent from A to R
frame received at R, datagram removed, passed up to IP


MAC src: 74-29-9C-E8-FF-55
MAC dest: E6-E9-00-17-BB-4B
IP src: 111.111.111.111
IP dest: 222.222.222.222
IP src: 111.111.111.111
IP dest: 222.222.222.222
IP
Eth
Phy
A
IP
Eth
Phy
R
111.111.111.111
74-29-9C-E8-FF-55
B
222.222.222.222
49-BD-D2-C7-56-2A
222.222.222.220
1A-23-F9-CD-06-9B
111.111.111.112
CC-49-DE-D0-AB-7D
111.111.111.110
E6-E9-00-17-BB-4B
222.222.222.221
88-B2-2F-54-1A-0F
Link Layer5-155
Addressing: routing to another LAN


R forwards datagram with IP source A, destination B
R creates link-layer frame with B's MAC address as dest, frame
contains A-to-B IP datagram
MAC src: 1A-23-F9-CD-06-9B
MAC dest: 49-BD-D2-C7-56-2A
IP src: 111.111.111.111
IP dest: 222.222.222.222
IP
Eth
Phy
A
R
111.111.111.111
74-29-9C-E8-FF-55
IP
Eth
Phy
B
222.222.222.222
49-BD-D2-C7-56-2A
222.222.222.220
1A-23-F9-CD-06-9B
111.111.111.112
CC-49-DE-D0-AB-7D
111.111.111.110
E6-E9-00-17-BB-4B
222.222.222.221
88-B2-2F-54-1A-0F
Link Layer5-156
Addressing: routing to another LAN


R forwards datagram with IP source A, destination B
R creates link-layer frame with B's MAC address as dest, frame
contains A-to-B IP datagram
MAC src: 1A-23-F9-CD-06-9B
MAC dest: 49-BD-D2-C7-56-2A
IP src: 111.111.111.111
IP dest: 222.222.222.222
IP
Eth
Phy
A
R
111.111.111.111
74-29-9C-E8-FF-55
IP
Eth
Phy
B
222.222.222.222
49-BD-D2-C7-56-2A
222.222.222.220
1A-23-F9-CD-06-9B
111.111.111.112
CC-49-DE-D0-AB-7D
111.111.111.110
E6-E9-00-17-BB-4B
222.222.222.221
88-B2-2F-54-1A-0F
Link Layer5-157
Addressing: routing to another LAN


R forwards datagram with IP source A, destination B
R creates link-layer frame with B's MAC address as dest, frame
contains A-to-B IP datagram
MAC src: 1A-23-F9-CD-06-9B
MAC dest: 49-BD-D2-C7-56-2A
IP src: 111.111.111.111
IP dest: 222.222.222.222
IP
Eth
Phy
A
R
111.111.111.111
74-29-9C-E8-FF-55
B
222.222.222.222
49-BD-D2-C7-56-2A
222.222.222.220
1A-23-F9-CD-06-9B
111.111.111.112
CC-49-DE-D0-AB-7D
111.111.111.110
E6-E9-00-17-BB-4B
222.222.222.221
88-B2-2F-54-1A-0F
Link Layer5-158
Link layer, LANs: outline
5.1 introduction, services 5.5 link virtualization:
MPLS
5.2 error detection,
correction
5.6 data center
networking
5.3 multiple access
protocols
5.7 a day in the life of a
web request
5.4 LANs




addressing, ARP
Ethernet
switches
VLANS
Link Layer5-159
Ethernet switch



link-layer device: takes an active role
 store, forward Ethernet frames
 examine incoming frame’s MAC address,
selectively forward frame to one-or-more
outgoing links when frame is to be forwarded on
segment, uses CSMA/CD to access segment
transparent
 hosts are unaware of presence of switches
plug-and-play, self-learning
 switches do not need to be configured
Link Layer5-160
Switch: multiple simultaneous transmissions




hosts have dedicated, direct
connection to switch
switches buffer packets
Ethernet protocol used on each
incoming link, but no collisions;
full duplex
 each link is its own collision
domain
switching: A-to-A’ and B-to-B’
can transmit simultaneously,
without collisions
A
B
C’
6
1
2
4
5
3
C
B’
A’
switch with six interfaces
(1,2,3,4,5,6)
Link Layer5-161
Switch forwarding table
Q: how does switch know A’
reachable via interface 4, B’
reachable via interface 5?
 A: each switch has a switch
table, each entry:
 (MAC address of host, interface to
reach host, time stamp)
 looks like a routing table!
A
B
C’
6
1
2
4
5
3
C
B’
A’
Q: how are entries created,
maintained in switch table?
switch with six interfaces
(1,2,3,4,5,6)
 something like a routing protocol?
Link Layer5-162
Switch: self-learning

switch learns which hosts
can be reached through
which interfaces
 when frame received,
switch “learns”
location of sender:
incoming LAN segment
 records sender/location
pair in switch table
Source: A
Dest: A’
A
A A’
B
C’
6
1
2
4
5
3
C
B’
A’
MAC addr interface
A
1
TTL
60
Switch table
(initially empty)
Link Layer5-163
Switch: frame filtering/forwarding
when frame received at switch:
1. record incoming link, MAC address of sending host
2. index switch table using MAC destination address
3. if entry found for destination
then {
if destination on segment from which frame arrived
then drop frame
else forward frame on interface indicated by entry
}
else flood /* forward on all interfaces except arriving
interface */
Link Layer5-164
Self-learning, forwarding: example


frame destination, A’,
locaton unknown: flood
destination A location
known: selectively send
on just one link
Source: A
Dest: A’
A
A A’
B
C’
6
1
2
A A’
4
5
3
C
B’
A’ A
A’
MAC addr interface
A
A’
1
4
TTL
60
60
switch table
(initially empty)
Link Layer5-165
Interconnecting switches

switches can be connected together
S4
S1
S3
S2
A
B
C
F
D
E
I
G
H
Q: sending from A to G - how does S1 know to
forward frame destined to F via S4 and S3?
 A: self learning! (works exactly the same as in
single-switch case!)
Link Layer5-166
Self-learning multi-switch example
Suppose C sends frame to I, I responds to C
S4
S1
S3
S2
A
B
C
F
D
E

I
G
H
Q: show switch tables and packet forwarding in S1, S2, S3, S4
Link Layer5-167
Institutional network
mail server
to external
network
router
web server
IP subnet
Link Layer5-168
Switches vs. routers
both are store-and-forward:
 routers: network-layer
devices (examine networklayer headers)
 switches: link-layer devices
(examine link-layer
headers)
both have forwarding tables:
 routers: compute tables
using routing algorithms, IP
addresses
 switches: learn forwarding
table using flooding,
learning, MAC addresses
datagram
frame
application
transport
network
link
physical
frame
link
physical
switch
network datagram
link
frame
physical
application
transport
network
link
physical
Link Layer5-169
VLANs: motivation
consider:


Computer
Science
Electrical
Engineering
Computer
Engineering
CS user moves office to
EE, but wants connect to
CS switch?
single broadcast domain:
 all layer-2 broadcast
traffic (ARP, DHCP,
unknown location of
destination MAC
address) must cross
entire LAN
 security/privacy,
efficiency issues
Link Layer5-170
VLANs
Virtual Local
Area Network
switch(es) supporting
VLAN capabilities can
be configured to
define multiple virtual
LANS over single
physical LAN
infrastructure.
port-based VLAN: switch ports
grouped (by switch management
software) so that single physical
switch ……
1
7
9
15
2
8
10
16
…
…
Electrical Engineering
(VLAN ports 1-8)
Computer Science
(VLAN ports 9-15)
… operates as multiple virtual switches
1
7
9
15
2
8
10
16
…
Electrical Engineering
(VLAN ports 1-8)
…
Computer Science
(VLAN ports 9-16)
Link Layer5-171
Port-based VLAN

router
traffic isolation: frames to/from
ports 1-8 can only reach ports
1-8
 can also define VLAN based on
MAC addresses of endpoints,
rather than switch port


dynamic membership: ports
can be dynamically assigned
among VLANs
1
7
9
15
2
8
10
16
…
Electrical Engineering
(VLAN ports 1-8)
…
Computer Science
(VLAN ports 9-15)
forwarding between VLANS: done via
routing (just as with separate
switches)
 in practice vendors sell combined
switches plus routers
Link Layer5-172
VLANS spanning multiple switches
1
7
9
15
1
3
5
7
2
8
10
16
2
4
6
8
…
Electrical Engineering
(VLAN ports 1-8)

…
Computer Science
(VLAN ports 9-15)
Ports 2,3,5 belong to EE VLAN
Ports 4,6,7,8 belong to CS VLAN
trunk port: carries frames between VLANS defined over
multiple physical switches
 frames forwarded within VLAN between switches can’t be vanilla
802.1 frames (must carry VLAN ID info)
 802.1q protocol adds/removed additional header fields for frames
forwarded between trunk ports
Link Layer5-173
802.1Q VLAN frame format
type
preamble
dest.
address
source
address
data (payload)
CRC
802.1 frame
type
preamble
dest.
address
source
address
data (payload)
2-byte Tag Protocol Identifier
(value: 81-00)
CRC
802.1Q frame
Recomputed
CRC
Tag Control Information (12 bit VLAN ID field,
3 bit priority field like IP TOS)
Link Layer5-174
Link layer, LANs: outline
5.1 introduction, services 5.5 link virtualization:
MPLS
5.2 error detection,
correction
5.6 data center
networking
5.3 multiple access
protocols
5.7 a day in the life of a
web request
5.4 LANs




addressing, ARP
Ethernet
switches
VLANS
Link Layer5-175
Multiprotocol label switching (MPLS)

initial goal: high-speed IP forwarding using fixed
length label (instead of IP address)
 fast lookup using fixed length identifier (rather than
shortest prefix matching)
 borrowing ideas from Virtual Circuit (VC) approach
 but IP datagram still keeps IP address!
PPP or Ethernet
header
MPLS header
label
20
IP header
remainder of link-layer frame
Exp S TTL
3
1
5
Link Layer5-176
MPLS capable routers


a.k.a. label-switched router
forward packets to outgoing interface based only on
label value (don’t inspect IP address)
 MPLS forwarding table distinct from IP forwarding tables

flexibility: MPLS forwarding decisions can differ from
those of IP
 use destination and source addresses to route flows to
same destination differently (traffic engineering)
 re-route flows quickly if link fails: pre-computed backup
paths (useful for VoIP)
Link Layer5-177
MPLS versus IP paths
R6
D
R4
R3
R5
A
R2

IP routing: path to destination determined
by destination address alone
IP router
Link Layer5-178
MPLS versus IP paths
entry router (R4) can use different MPLS
routes to A based, e.g., on source address
R6
D
R4
R3
R5
A
R2


IP routing: path to destination determined
by destination address alone
IP-only
router
MPLS routing: path to destination can be
based on source and dest. address
MPLS and
IP router
 fast reroute: precompute backup routes in
case of link failure
Link Layer5-179
MPLS signaling

modify OSPF, IS-IS link-state flooding protocols to
carry info used by MPLS routing,
 e.g., link bandwidth, amount of “reserved” link bandwidth

entry MPLS router uses RSVP-TE signaling protocol to set
up MPLS forwarding at downstream routers
RSVP-TE
R6
D
R4
R5
modified
link state
flooding
A
Link Layer5-180
MPLS forwarding tables
in
label
out
label dest
10
12
8
out
interface
A
D
A
0
0
1
in
label
out
label dest
out
interface
10
6
A
1
12
9
D
0
R6
0
0
D
1
1
R3
R4
R5
0
0
R2
in
label
8
out
label dest
6
A
out
interface
in
label
6
outR1
label dest
-
A
A
out
interface
0
0
Link Layer5-181
Link layer, LANs: outline
5.1 introduction, services 5.5 link virtualization:
MPLS
5.2 error detection,
correction
5.6 data center
networking
5.3 multiple access
protocols
5.7 a day in the life of a
web request
5.4 LANs




addressing, ARP
Ethernet
switches
VLANS
Link Layer5-182
Data center networks

10’s to 100’s of thousands of hosts, often closely
coupled, in close proximity:
 e-business (e.g. Amazon)
 content-servers (e.g., YouTube, Akamai, Apple, Microsoft)
 search engines, data mining (e.g., Google)

challenges:
 multiple applications, each
serving massive numbers of
clients
 managing/balancing load,
avoiding processing,
networking, data bottlenecks
Inside a 40-ft Microsoft container,
Chicago data center
Link Layer5-183
Data center networks
load balancer: application-layer routing
 receives external client requests
 directs workload within data center
 returns results to external client (hiding data
center internals from client)
Internet
Border router
Load
balancer
Access router
Tier-1 switches
B
A
Load
balancer
C
Tier-2 switches
TOR switches
Server racks
1
2
3
4
5
6
7
8
Link Layer5-184
Data center networks

rich interconnection among switches, racks:
 increased throughput between racks (multiple routing
paths possible)
 increased reliability via redundancy
Tier-1 switches
Tier-2 switches
TOR switches
Server racks
1
2
3
4
5
6
7
8
Link layer, LANs: outline
5.1 introduction, services 5.5 link virtualization:
MPLS
5.2 error detection,
correction
5.6 data center
networking
5.3 multiple access
protocols
5.7 a day in the life of a
web request
5.4 LANs




addressing, ARP
Ethernet
switches
VLANS
Link Layer5-186
Synthesis: a day in the life of a web request

journey down protocol stack complete!
 application, transport, network, link

putting-it-all-together: synthesis!
 goal: identify, review, understand protocols (at all
layers) involved in seemingly simple scenario:
requesting www page
 scenario: student attaches laptop to campus network,
requests/receives www.google.com
Link Layer5-187
A day in the life: scenario
DNS server
browser
Comcast network
68.80.0.0/13
school network
68.80.2.0/24
web page
web server
64.233.169.105
Google’s network
64.233.160.0/19
Link Layer5-188
A day in the life… connecting to the Internet
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP

connecting laptop needs to
get its own IP address, addr
of first-hop router, addr of
DNS server: use DHCP
DHCP

DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
router
(runs DHCP)


DHCP request encapsulated
in UDP, encapsulated in IP,
encapsulated in 802.3
Ethernet
Ethernet frame broadcast
(dest: FFFFFFFFFFFF) on LAN,
received at router running
DHCP server
Ethernet demuxed to IP
demuxed, UDP demuxed to
DHCP
Link Layer5-189
A day in the life… connecting to the Internet
DHCP
UDP
IP
Eth
Phy
DHCP
DHCP
DHCP
DHCP


DHCP
DHCP
DHCP
DHCP
DHCP
DHCP
UDP
IP
Eth
Phy
router
(runs DHCP)

DHCP server formulates
DHCP ACK containing
client’s IP address, IP
address of first-hop router
for client, name & IP
address of DNS server
encapsulation at DHCP
server, frame forwarded
(switch learning) through
LAN, demultiplexing at
client
DHCP client receives
DHCP ACK reply
Client now has IP address, knows name & addr of DNS
server, IP address of its first-hop router
Link Layer5-190
A day in the life… ARP (before DNS, before HTTP)
DNS
DNS
DNS
ARP query

DNS
UDP
IP
ARP
Eth
Phy

ARP
ARP reply
Eth
Phy
router
(runs DHCP)


before sending HTTP request, need
IP address of www.google.com:
DNS
DNS query created, encapsulated in
UDP, encapsulated in IP,
encapsulated in Eth. To send frame
to router, need MAC address of
router interface: ARP
ARP query broadcast, received by
router, which replies with ARP
reply giving MAC address of
router interface
client now knows MAC address
of first hop router, so can now
send frame containing DNS
query
Link Layer5-191
A day in the life… using DNS
DNS
DNS
DNS
DNS
DNS
DNS
DNS
UDP
IP
Eth
Phy
DNS
DNS
DNS
UDP
IP
Eth
Phy
DNS server
DNS
Comcast network
68.80.0.0/13
router
(runs DHCP)

IP datagram containing DNS
query forwarded via LAN
switch from client to 1st hop
router

IP datagram forwarded from
campus network into comcast
network, routed (tables created
by RIP, OSPF, IS-IS and/or BGP
routing protocols) to DNS server

demux’ed to DNS server
DNS server replies to client
with IP address of
www.google.com
Link Layer5-192

A day in the life…TCP connection carrying HTTP
HTTP
HTTP
TCP
IP
Eth
Phy
SYNACK
SYN
SYNACK
SYN
SYNACK
SYN

router
(runs DHCP)
SYNACK
SYN
SYNACK
SYN
SYNACK
SYN
TCP
IP
Eth
Phy
web server
64.233.169.105



to send HTTP request,
client first opens TCP socket
to web server
TCP SYN segment (step 1 in 3way handshake) inter-domain
routed to web server
web server responds with TCP
SYNACK (step 2 in 3-way
handshake)
TCP connection established!
Link Layer5-193
A day in the life… HTTP request/reply
HTTP
HTTP
HTTP
TCP
IP
Eth
Phy
HTTP
HTTP
HTTP
HTTP
HTTP
HTTP

web page finally (!!!) displayed

HTTP
HTTP
HTTP
HTTP
HTTP
TCP
IP
Eth
Phy
web server
64.233.169.105
router
(runs DHCP)



HTTP request sent into TCP
socket
IP datagram containing HTTP
request routed to
www.google.com
web server responds with
HTTP reply (containing web
page)
IP datagram containing HTTP
reply routed back to client
Link Layer5-194
Chapter 5: Summary

principles behind data link layer services:
 error detection, correction
 sharing a broadcast channel: multiple access
 link layer addressing

instantiation and implementation of various link
layer technologies
 Ethernet
 switched LANS, VLANs
 virtualized networks as a link layer: MPLS

synthesis: a day in the life of a web request
Link Layer5-195
Chapter 5: let’s take a breath



journey down protocol stack complete (except
PHY)
solid understanding of networking principles,
practice
….. could stop here …. but lots of interesting
topics!




wireless
multimedia
security
network management
Link Layer5-196
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