One can characterize distributed computing ... spectrum of activities varying in ... 1. Background

One  can  characterize  distributed  computing ... spectrum  of  activities  varying  in ... 1.  Background
1. Background
Computer
Systems
G. Bell, S. Fuller and
D. Siewiorek, Editors
Ethernet: Distributed
Packet Switching for
Local Computer
Networks
Robert M. Metcalfe and David R. Boggs
Xerox Palo Alto Research Center
Ethernet is a branching broadcast communication
system for carrying digital data packets among locally
distributed computing stations. The packet transport
mechanism provided by Ethernet has been used to build
systems which can be viewed as either local computer
networks or loosely coupled multiprocessors. An Ethernet's shared communication facility, its Ether, is a passive broadcast medium with no central control. Coordination of access to the Ether for packet broadcasts is
distributed among the contending transmitting stations
using controlled statistical arbitration. Switching of
packets to their destinations on the Ether is distributed
among the receiving stations using packet address
recognition. Design principles and implementation are
described, based on experience with an operating Ethernet of 100 nodes along a kilometer of coaxial cable. A
model for estimating performance under heavy loads
and a packet protocol for error controlled communication are included for completeness.
Key Words and Phrases: computer networks, packet
switching, multiprocessing, distributed control, distributed computing, broadcast communication, statistical arbitration
CR Categories: 3.81, 4.32, 6.35
Copyright © 1976, Association for Computing Machinery, Inc.
General permission to republish, but not for profit, all or part
of this material is granted provided that ACM's copyright notice
is given and that reference is made to the publication, to its date
of issue, and to the fact that reprinting privileges were granted
by permission of the Association for Computing Machinery.
Author's present addresses: R.M. Metcalfe, Transaction Technology, Inc., 10880 Wilshire Boulevard, Los Angeles, CA 94304; D.
Boggs, Xerox Palo Alto Research Center, 3333 Coyote Hill Road,
Palo Alto, CA 94304.
395
One can characterize distributed computing as a
spectrum of activities varying in their degree o f decentralization, with one extreme being remote computer
networking and the other extreme being multiprocessing. Remote computer networking is the loose interconnection of previously isolated, widely separated, and
rather large computing systems. Multiprocessing is the
construction of previously monolithic and serial computing systems from increasingly numerous and smaller
pieces computing in parallel. Near the middle of this
spectrum is local networking, the interconnection of
computers to gain the resource sharing of computer
networking and the parallelism of multiprocessing.
The separation between computers and the associated bit rate of their communication can be used to divide the distributed computing spectrum into broad
activities. The product of separation and bit rate, now
about 1 gigabit-meter per second (1 Gbmps), is an indication of the limit of current communication technology and can be expected to increase with time:
Activity
Separation
Bit rate
Remote networks
Local networks
Multiprocessors
> 10 km
10-.1 km
< .1 km
< .1 Mbps
.1-10 Mbps
> 10 Mbps
1.1 Remote Computer Networking
Computer networking evolved from telecommunications terminal-computer communication, where the object was to connect remote terminals to a central computing facility. As the need for computer-computer
interconnection grew, computers themselves were used
to provide communication [2, 4, 29]. Communication
using computers as packet switches [15-21, 26] and
communications among computers for resource sharing
[I0, 32] were both advanced by the development of the
Arpa Computer Network.
The Aloha Network at the University of Hawaii was
originally developed to apply packet radio techniques
for communication between a central computer and its
terminals scattered among the Hawaiian Islands [1, 2].
Many of the terminals are now minicomputers communicating among themselves using the Aloha Network's Menehune as a packet switch. The Menehune
and an Arpanet Imp are now connected, providing terminals on the Aloha Network access to computing
resources on the U.S. mainland.
Just as computer networks have grown across continents and oceans to interconnect major computing
facilities around the world, they are now growing down
corridors and between buildings to interconnect minicomputers in offices and laboratories [3, 12, 13, 14, 35].
1.2 Multiprocessing
Multiprocessing first took the form of connecting an
I/O controller to a large central computer; raM's Asp is a
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classic example [29]. Next, multiple central processors
were connected to a common memory to provide more
power for compute-bound applications [33]. For certain
of these applications, more exotic multiprocessor architectures such as Illiae IV were introduced [5].
More recently minicomputers have been connected
in multiprocessor configurations for economy, reliability, and increased system modularity [24, 36]. The
trend has been toward decentralization for reliability;
loosely coupled multiprocessor systems depend less on
shared central memory and more on thin wires for interprocess communication with increased component
isolation [18, 26]. With the continued thinning of interprocessor communication for reliability and the development of distributable applications, multiprocessing
is gradually approaching a local form of distributed
computing.
1.3 Local Computer Networking
Ethernet shares many objectives with other local
networks such as Mitre's Mitrix, Bell Telephone Laboratory's Spider, and U.C. Irvine's Distributed Computing
System (DCS) [12, 13, 14, 35]. Prototypes of all four
local networking schemes operate at bit rates between
one and three megabits per second. Mitrix and Spider
have a central minicomputer for switching and bandwidth allocation, while DCS and Ethernet use distributed control. Spider and DCS use a ring communication
path, Mitrix uses off-the-shelf CATV technology to
implement two one-way busses, and our experimental
Ethernet uses a branching two-way passive bus. Differences among these systems are due to differences among
their intended applications, differences among the cost
constraints under which trade-offs were made, and
differences of opinion among researchers.
Before going into a detailed description of Ethernet,
we offer the following overview (see Figure 1).
ing elements. To handle the demands of growth, an
Ethernet can be extended using packet repeaters for
signal regeneration, packet filters for traffic localization,
and packet gateways for internetwork address extension.
Control is completely distributed among stations,
with packet transmissions coordinated through statistical arbitration. Transmissions initiated by a station defer to any which may already be in progress. Once
started, if interference with other packets is detected, a
transmission is aborted and rescheduled by its source
station. After a certain period of interference-free transmission, a packet is heard by all stations and will run to
completion without interference. Ethernet controllers
in colliding stations each generate random retransmission intervals to avoid repeated collisions. The mean of
a packet's retransmission intervals is adjusted as a function of collision history to keep Ether utilization near
the optimum with changing network load.
Even when transmitted without source-detected interference, a packet may still not reach its destination
without error; thus, packets are delivered only with high
probability. Stations requiring a residual error rate
lower than that provided by the bare Ethernet packet
transport mechanism must follow mutually agreed upon
packet protocols.
3. Design Principles
Ethernet is a system for local communication among
computing stations. Our experimental Ethernet uses
tapped coaxial cables to carry variable length digital
data packets among, for example, personal minicomputers, printing facilities, large file storage devices,
magnetic tape backup stations, larger central computers,
and longer-haul communication equipment.
The shared communication facility, a branching
Ether, is passive. A station's Ethernet interface connects bit-serially through an interface cable to a transceiver which in turn taps into the passing Ether. A
packet is broadcast onto the Ether, is heard by all stations, and is copied from the Ether by destinations
which select it according to the packet's leading address
bits. This is broadcast packet switching and should be
distinguished from store-and-forward packet switching,
in which routing is performed by intermediate process-
Our object is to design a communication system
which can grow smoothly to accommodate several
buildings full of personal computers and the facilities
needed for their support.
Like the computing stations to be connected, the
communication system must be inexpensive. We choose
to distribute control of the communications facility
among the communicating computers to eliminate the
reliability problems of an active central controller, to
avoid creating a bottleneck in a system rich in parallelism, and to reduce the fixed costs which make small systems uneconomical.
Ethernet design started with the basic idea of packet
collision and retransmission developed in the Aloha
Network [1 ]. We expected that, like the Aloha Network,
Ethernets would carry bursty traffic so that conventional synchronous time-division multiplexing (STDM)
would be inefficient [1, 2, 21, 26]. We saw promise in the
Aloha approach to distributed control of radio channel
multiplexing and hoped that it could be applied effectively with media suited to local computer communication. With several innovations of our own, the promise
is realized.
Ethernet is named for the historical luminiferous
ether through which electromagnetic radiations were
once alleged to propagate. Like an Aloha radio transmitter, an Ethernet transmitter broadcasts completelyaddressed transmitter-synchronous bit sequences called
packets onto the Ether and hopes that they are heard by
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Fig. 1. A two-segment Ethernet.
an Ethernet taps into the Ether at the nearest convenient
point.
Looking at the relationship of interconnection and
control, we see that Ethernet is the dual of a star network. Rather than distributed interconnection through
many separate links and central control in a switching
node, as in a star network, the Ethernet has central interconnection through the Ether and distributed control
among its stations.
Unlike an Aloha Network, which is a star network
with an outgoing broadcast channel and an incoming
multi-access channel, an Ethernet supports many-tomany communication with a single broadcast multiaccess channel.
TERMINATOR
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INTERFACE
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the intended receivers. The Ether is a logically passive
medium for the propagation of digital signals and can
be constructed using.any number of media including
coaxial cables, twisted pairs, and optical fibers.
3.1 Topology
We cannot afford the redundant connections and
dynamic routing of store-and-forward packet switching
to assure reliable communication, so we choose to
achieve reliability through simplicity. We choose to
make the shared communication facility passive so that
the failure of an active element will tend to affect the
communications of only a single station. The layout and
changing needs of office and laboratory buildings leads
us to pick a network topology with the potential for
convenient incremental extention and reconfiguration
with minimal service disruption.
The topology of the Ethernet is that of an unrooted
tree. It is a tree so that the Ether can branch at the entrance to a building's corridor, yet avoid multipath interference. There must be only one path through the
Ether between any source and destination; if more than
one path were to exist, a transmission would interfere
with itself, repeatedly arriving at its intended destination having travelled by paths of different length. The
Ether is unrooted because it can be extended from any of
its points in any direction. Any station wishing to join
397
3.2 Control
Sharing of the Ether is controlled in such a way that
it is not only possible but probable that two or more stations will attempt to transmit a packet at roughly the
same time. Packets which overlap in time on the Ether
are said to collide; they interfere so as to be unrecognizable by a receiver. A station recovers from a detected
collision by abandoning the attempt and retransmitting
the packet after some dynamically chosen random time
period. Arbitration of conflicting transmission demands
is both distributed and statistical.
When the Ether is largely unused, a station transmits
its packets at will, the packets are received without error,
and all is well. As more stations begin to transmit, the
rate of packet interference increases. Ethernet controllers
in each station are built to adjust the mean retransmission
interval in proportion to the frequency of collisons;
sharing of the Ether among competing station-station
transmissions is thereby kept near the optimum [20, 21].
A degree of cooperation among the stations is required to share the Ether equitably. In demanding applications certain stations might usefully take transmission priority through some systematic violation of
equity rules. A station could usurp the Ether by not adjusting its retransmission interval with increasing traffic
or by sending very large packets. Both practices are now
prohibited by low-level software in each station.
3.3 Addressing
Each packet has a source and destination, both of
which are identified in the packet's header. A packet
placed on the Ether eventually propagates to all stations. Any station can copy a packet from the Ether into
its local memory, but normally only an active destination station matching its address in the packet's header
will do so as the packet passes. By convention, a zero
destination address is a wildcard and matches all addresses; a packet with a destination of zero is called a
broadcast packet.
3.4 Reliability
An Ethernet is probabilistic. Packets may be lost due
to interference with other packets, impulse noise on the
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Ether, an inactive receiver at a packet's intended destination, or purposeful discard. Protocols used to communicate through an Ethernet must assume that packets
will be received correctly at intended destinations only
with high probability.
An Ethernet gives its best efforts to transmit packets
successfully, but it is the responsibility of processes in the
source and destination stations to take the precautions
necessary to assure reliable communication of the quality
they themselves desire [18, 21]. Recognizing the costliness and dangers of promising "error-free" communication, we refrain from guaranteeing reliable delivery
of any single packet to get both economy of transmission and high reliability averaged over many packets
[21 ]. Removing the responsibility for reliable communication from the packet transport mechanism allows us to
tailor reliability to the application and to place error recovery where it will do the most good. This policy becomes more important as Ethernets are interconnected
in a hierarchy of networks through which packets must
travel farther and suffer greater risks.
3.5 Mechanisms
A station connects to the Ether with a tap and a
transceiver. A tap is a device for physically connecting to
the Ether while disturbing its transmission characteristics as little as possible. The design of the transceiver
must be an exercise in paranoia. Precautions must be
taken to insure that likely failures in the transceiver or
station do not result in pollution of the Ether. In particular, removing power from the transceiver should
cause it to disconnect from the Ether.
Five mechanisms are provided in our experimental
Ethernet for reducing the probability and cost of losing
a packet. These are (1) carrier detection, (2) interference
detection, (3) packet error detection, (4) truncated
packet filtering, and (5) collision consensus enforcement.
3.5.1 Carrier detection. As a packer's bits are placed
on the Ether by a station, they are phase encoded (like
bits on a magnetic tape), which guarantees that there is
at least one transition on the Ether during each bit time.
The passing of a packet on the Ether can therefore be detected by listening for its transitions. To use a radio
analogy, we speak of the presence of carrier as a packet
passes a transceiver. Because a station can sense the carrier of a passing packet, it can delay sending one of its
own until the detected packet passes safely. The Aloha
Network does not have carrier detection and consequently suffers a substantially higher collision rate.
Without carrier detection, efficient use of the Ether
would decrease with increasing packet length. In Section
6 below, we show that with carrier detection, Ether
efficiency increases with increasing packet length.
With carrier detection we are able to implement
deference: no station will start transmitting while hearing
carrier. With deference comes acquisition: once a packet
transmission has been in progress for an Ether end-to398
end propagation time, all stations are hearing carrier
and are deferring; the Ether has been acquired and the
transmission will complete without an interfering collision.
With carrier detection, collisions should occur only
when two or more stations find the Ether silent and begin transmitting simultaneously: within an Ether end-toend propagation time. This will almost always happen
immediately after a packet transmission during which
two or more stations were deferring. Because stations do
not now randomize after deferring, when the transmission terminates, the waiting stations pile on together,
collide, randomize, and retransmit.
3.5.2 Interference detection. Each transceiver has an
interference detector. Interference is indicated when the
transceiver notices a difference between the value of the
bit it is receiving from the Ether and the value of the bit
it is attempting to transmit.
Interference detection has three advantages. First, a
station detecting a collision knows that its packet has
been damaged. The packet can be scheduled for retransmission immediately, avoiding a long acknowledgment timeout. Second, interference periods on the Ether
are limited to a maximum of one round trip time. Colliding packets in the Aloha Network run to completion,
but the truncated packets resulting from Ethernet collisions waste only a small fraction of a packet time on the
Ether. Third, the frequency of detected interference is
used to estimate Ether traffic for adjusting retransmission intervals and optimizing channel efficiency.
3.5.3 Packet error detection. As a packet is placed
on the Ether, a checksum is computed and appended.
As the packet is read from the Ether, the checksum is
recomputed. Packets which do not carry a consistent
checksum are discarded. In this way transmission errors,
impulse noise errors, and errors due to undetected interference are caught at a paeket's destination.
3.5.4 Truncated packet filtering. Interference detection and deference cause most collisions to result in
truncated packets of only a few bits; colliding stations
detect interference and abort transmission within an
Ether round trip time. To reduce the processing load
that the rejection of such obviously damaged packets
would place on listening station software, truncated
packets are filtered out in hardware.
3.5.5 Collision consensus enforcement. When a station determines that its transmission is experiencing interference, it momentarily jams the Ether to insure that
all other participants in the collision will detect interference and, because of deference, will be forced to abort.
Without this collision consensus enforcement mechanism,
it is possible that the transmitting station which would
otherwise be the last to detect a collision might not do
so as the other interfering transmissions successively
abort and stop interfering. Although the packet may
look good to that last transmitter, different path lengths
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between the colliding transmitters and the intended receiver will cause the packet to arrive damaged.
4. Implementation
Our choices of 1 kilometer, 3 megabits per second,
and 256 stations for the parameters of an experimental
Ethernet were based on characteristics of the locally
distributed computer communication environment and
our assessments of what would be marginally achievable; they were certainly not hard restrictions essential
to the Ethernet concept.
We expect that a reasonable maximum network size
would be on the order of 1 kilometer of cable. We used
this working number to choose among Ethers of varying
signal attenuation and to design transceivers with appropriate power and sensitivity.
The dominant station on our experimental Ethernet
is a minicomputer for which 3 megabits per second is a
convenient data transfer rate. By keeping the peak rate
well below that of the computer's path to main memory,
we reduce the need for expensive special-purpose packet
buffering in our Ethernet interfaces. By keeping the peak
rate as high as is convenient, we provide for larger numbers of stations and more ambitious multiprocessing
communications applications.
To expedite low-level packet handling among 256
stations, we allocate the first 8-bit byte of the packet to
be the destination address field and the second byte to be
the source address field (see Figure 2). 256 is a number
small enough to allow each station to get an adequate
share of the available bandwidth and approaches the
limit of what we can achieve with current techniques for
tapping cables. 256 is only a convenient number for the
lowest level of protocol; higher levels can accomodate
extended address spaces with additional fields inside the
packet and software to interpret them.
Our experimental Ethernet implementation has four
major parts: the Ether, transceivers, interfaces, and controllers (see Figure 1).
4.1 Ether
We chose to implement our experimental Ether using
low-loss coaxial cable with off-the-shelf CATV taps and
connectors. It is possible to mix Ethers on a single
Ethernet; we use a smaller-diameter coax for convenient
connection within station dusters and a larger-diameter
coax for low-loss runs between clusters. The cost of
coaxial cable Ether is insignificant relative to the cost of
the distributed computing systems supported by
Ethernet.
4.2 Transceivers
Our experimental transceivers can drive a kilometer
of coaxial cable Ether tapped by 256 stations transmitting at 3 megabits per second. The transceivers can
endure (i.e. work after) sustained direct shorting, im399
proper termination of the Ether, and simultaneous
drive by all 256 stations; they can tolerate (i.e. work
during) ground differentials and everyday electrical
noise, from typewriters or electric drills, encountered
when stations are separated by as much as a kilometer.
An Ethernet transceiver attaches directly to the
Ether which passes by in the ceiling or under the floor.
It is powered and controlled through five twisted pairs
in an interface cable carrying transmit data, receive
data, interference detect, and power supply voltages.
When unpowered, the transceiver disconnects itself
electrically from the Ether. Here is where our fight for
reliability is won or lost; a broken transceiver can, but
should not, bring down an entire Ethernet. A watchdog
timer circuit in each transceiver attempts to prevent
pollution of the Ether by shutting down the output stage
if it acts suspiciously. For transceiver simplicity we use
the Ether's base frequency band, but an Ethernet could
be built to use any suitably sized band of a frequency division multiplexed Ether.
Even though our experimental transceivers are very
simple and can tolerate only limited signal attenuation,
they have proven quite adequate and reliable. A more
sophisticated transceiver design might permit passive
branching of the Ether and wider station separation.
4.3 Interface
An Ethernet interface serializes and deserializes the
parallel data used by its station. There are a number of
different stations on our Ethernet; an interface must be
built for each kind.
Each interface is equipped with the hardware necessary to compute a 16-bit cyclic redundancy checksum
(CRC) on serial data as it is transmitted and received.
This checksum protects only against errors in the Ether
and specifically not against errors in the parallel portions of the interface hardware or station. Higher-level
software checksums are recommended for applications
in which a higher degree of reliability is required.
A transmitting interface uses a packet buffer address
and word count to serialize and phase encode a variable
number of 16-bit words which are taken from the station's memory and passed to the transceiver, preceded
by a start bit (called SYNC in Figure 2) and followed by
the CRC. A receiving interface uses the appearance of
carrier to detect the start of a packet and uses the SYNC
bit to acquire bit phase. As long as carrier stays on, the
interface decodes and deserializes the incoming bit
stream depositing 16-bit words in a packet buffer in the
station's main memory. When carrier goes away, the
interface checks that an integral number of 16-bit words
has been received and that the CRC is correct. The last
word received is assumed to be the CRC and is not copied
into the packet buffer.
These interfaces ordinarily include hardware for
accepting only those packets with appropriate addresses
in their headers. Hardware address filtering helps a station avoid burdensome software packet processing when
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Fig. 2. Ethernetpacket layout.
ACCESSIBLETO SOFI'WARE
Y DEST SOURCE
ADDRESS
Ii
-
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DATA
8 BITS
CIIECKSUM
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8 BITS
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16 BITS
the Ether is very busy carrying traffic intended for other
stations.
4.4 Controller
An Ethernet controller is the station-specific lowlevel firmware or software for getting packets onto and
out of the Ether. When a source-detected collision occurs, it is the source controller's responsibility to generate a new random retransmission interval based on the
updated collision count. We have studied a number of algorithms for controlling retransmission rates in stations
to maintain Ether efficiency [20, 22]. The most practical
of these algorithms estimate traffic load using recent
collision history.
Retransmission intervals are multiples of a slot, the
maximum time between starting a transmission and detecting a collision, one end-to-end round trip delay. An
Ethernet controller begins transmission of each new
packet with a mean retransmission interval of one slot.
Each time a transmission attempt ends in collision, the
controller delays for an interval of random length with a
mean twice that of the previous interval, defers to any
passing packet, and then attempts retransmission. This
heuristic approximates an algorithm we have called
Binary Exponential Backoff (see Figure 3) [22].
When the network is unloaded and collisions are
rare, the mean seldom departs from one and retransmissions are prompt. As the traffic load increases, more
collisions are experienced, a backlog of packets builds
up in the stations, retransmission intervals increase, and
retransmission traffic backs off to sustain channel
efficiency.
the Ether and extending its signal cover, there is a tradeoff between using sophisticated transceivers and using
repeaters. With increased power and sensitivity, transceivers become more expensive and less reliable. The
introduction of repeaters into an Ethernet makes the
centrally interconnecting Ether active. The failure of a
transceiver will sever the communications of its owner;
the failure of a repeater partitions the Ether severing
many communications.
5.2 Traffic Cover
One can expand an Ethernet just so far by adding
Ether and packet repeaters. At some point the Ether will
be so busy that additional stations will just divide more
finely the already inadequate bandwidth. The traffic
cover can be extended with an unbuffered traffic-filtering
repeater or packet filter, which passes packets from one
Ether segment to another only if the destination station
is located on the new segment. A packet filter also extends the signal cover.
5.3 Address Cover
One can expand an Ethernet just so far by adding
Ether, repeaters, and traffic filters. At some point there
will be too many stations to be addressed with the Ethernet's 8-bit addresses. The address cover can be extended
with packet gateways and the software addressing conventions they implement [7]. Addresses can be expanded
in two directions: down into the station by adding fields
to identify destination ports or processes within a station, and up into the internetwork by adding fields to
identify destination stations on remote networks.
A gateway also extends the traffic and signal covers.
There can be only one repeater or packet filter connecting two Ether segments; a packet repeated onto a
segment by multiple repeaters would interfere with itself.
However, there is no limit to the number of gateways
connecting two segments; a gateway only repeats packets
addressed to itself as an intermediary. Failure of the
single repeater connecting two segments partitions the
network; failure of a gateway need not partition the net
it there are paths through other gateways between the
segments.
5. Growth
5.1 Signal Cover
6. Performance
One can expand an Ethernet just so far by adding
transceivers and Ether. At some point, the transceivers
and Ether will be unable to carry the required signals.
The signal cover can be extended with a simple unbuffered packet repeater. In our experimental Ethernet,
where because of transceiver simplicity the Ether cannot
be branched passively, a simple repeater may join any
number of Ether segments to enrich the topology while
extending the signal cover.
We operate an experimental two-segment packet repeater, but hope to avoid relying on them. In branching
We present here a simple set of formulas with which
to characterize the performance expected of an Ethernet
when it is heavily loaded. More elaborate analyses and
several detailed simulations have been done, but the
following simple model has proven very useful in
understanding the Ethernet's distributed contention
scheme, even when it is loaded beyond expectations
[1, 20, 21, 22, 23, 27].
We develop a simple model of the performance of a
loaded Ethernet by examining alternating Ether time
periods. The first, called a transmission interval, is that
40O
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during which the Ether has been acquired for a successful packet transmission. The second, called a contention
interval, is that composed of the retransmission slots of
Section 4.4, during which stations attempt to acquire
control of the Ether. Because the model's Ethernets are
loaded and because stations defer to passing packets before starting transmission, the slots are synchronized
by the tail of the preceding acquisition interval. A slot
will be empty when no station chooses to attempt transmission in it and it will contain a collision if more than
one station attempts to transmit. When a slot contains
only one attempted transmission, then the Ether has
been acquired for the duration of a packet, the contention interval ends, and a transmission interval begins.
Let P be the number of bits in an Ethernet packet.
Let C be the peak capacity in bits per second, carried on
the Ether. Let T be the time in seconds of a slot, the
number of seconds it takes to detect a collision after
starting a transmission. Let us assume that there are Q
stations continuously queued to transmit a packet;
either the acquiring station has a new packet immediately after a successful acquisition or another station
comes ready. Note that Q also happens to give the total
offered load on the network which for this analysis is always 1 or greater. We assume that a queued station attempts to transmit in the current slot with probability
1/Q, or delays with probability 1 - ( l / Q ) ; this is known
NEW
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LOADESTIMATE
OVERFLOWED?
YES
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6.1 Acquisition Probability
We now compute A, the probability that exactly one
station attempts a transmission in a slot and therefore
acquires the Ether. A is Q.(1/Q).((1 - ( l / Q ) ) * *
(Q - 1)); there are Q ways in which one station can
choose to transmit (with probability ( l / Q ) ) while Q - 1
stations choose to wait (with probability 1 -- ( l / Q ) ) .
Simplifying,
A = (1-(l/Q))
(Q-l).
6.2 Waiting Time
We now compute IV, the mean number of slots of
waiting in a contention interval before a successful acquisition of the Ether by a station's transmission. The
probability of waiting no time at all is just A, the probability that one and only one station chooses to transmit in the first slot following a transmission. The probability of waiting 1 slot is A,(1 - A); the probability of
waiting i slots is A , ( ( l -- A)**i). The mean of this geometric distribution is
w
=
(l
-
A)/A.
6.3 Efficiency
We now compute E, that fraction of time the Ether
is carrying good packets, the efficiency. The Ether's time
is divided between transmission intervals and contention intervals. A packet transmission takes P/C seconds.
The mean time to acquisition is W,T. Therefore, by our
simple model,
Fig. 3. Collision control algorithm.
ZEROLOAD
ESTIMATEFOR
PACKET
to be the optimum statistical decision rule, approximated in Ethernet stations by means of our load-estimating retransmission control algorithms [20, 21 ].
E = (P/C)/((P/C) q- (W,T)).
I
ERROR
I
Table I presents representative performance figures
(i.e. E) for our experimental Ethernet with the indicated
packet sizes and number of continuously queued stations.
The efficiency figures given do not account for inevitable
reductions due to headers and control packets nor for
losses due to imprecise control of the retransmission
parameter 1/Q; the former is straightforwardly protocol-dependent and the latter requires analysis beyond
the scope of this paper. Again, we feel that all of the
Ethernets in the table are overloaded; normally loaded
Ethernets will usually have a Q much less than 1 and
exhibit behavior not covered by this model.
For our calculations we use a C of 3 megabits per
second and a T of 16 microseconds. The slot duration T
must be long enough to allow a collision to be detected
or at least twice the Ether's round trip time. We limit in
software the maximum length of our packets to be near
4000 bits to keep the latency of network access down and
to permit efficient use of station packet buffer storage.
For packets whose size is above 4000 bits, the efficiency of our experimental Ethernet stays well above 95
Communications
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the ACM
July 1976
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percent. For packets with a size approximating that of a
slot, Ethernet efficiency approaches l/e, the asymptotic
efficiency of a slotted Aloha network [27].
7. Protocol
There is more to the construction of a viable packet
communication system than simply providing the
mechanisms for packet transport. Methods for error
correction, flow control, process naming, security, and
accounting must also be provided through higher-level
protocols implemented on top o f the Ether control protocol described in Sections 3 and 4 above. [7, 10, 12, 21,
28, 34]. Ether control includes packet framing, error detection, addressing and multi-access control; like other
line control procedures, Ethernet is used to support
numerous network and multiprocessor architectures
[30, 31].
Here is a brief description of one simple error-controlling packet protocol. The E F T P (Ethernet File
Transfer Protocol) is of interest both because it is relatively easy to understand and implement correctly and
because it has dutifully carried many valuable files during the development of more general and efficient
protocols.
7.1. General Terminology
In discussing packet protocols, we use the following
generally useful terminology. A packet is said to have a
source and a destination. A flow of data is said to have a
sender and a receiver, recognizing that to support a flow
of data some packets (typically acknowledgments) will
be sourced at the receiver and destined for the sender.
A connection is said to have a listener and an initiator
and a service is said to have a server and a user. It is very
useful to treat these as orthogonal descriptors of the
participants in a communication. Of course, a server is
usually a listener and the source of data-bearing packets
is usually the sender.
7.2 E F T P
The first 16 bits of all Ethernet packets contain its
interface-interpretable destination and source station
addresses, a byte each, in that order (see Figure 2). By
software convention, the second 16 bits of all Ethernet
packets contain the packet type. Different protocols
use disjoint sets of packet types. The E F T P uses 5
packet types: data, ack, abort, end, and endreply. Following the 16-bit type word of an E F T P packet are 16
bits of sequence number, 16 bits of length, optionally
some 16-bit data words, and finally a 16-bit software
checksum word (see Figure 4). The Ethernet's hardware checksum is present only on the Ether and is not
counted at this level of protocol.
It should be obvious that little care has been taken
to cram certain fields into just the right number of bits.
The emphasis here is on simplicity and ease of pro402
gramming. Despite this disclaimer, we do feel that it is
more advisable to err on the side of spacious fields; try
as you may, one field or another will always turn out to
be too small.
The software checksum word is used to lower the
probability of an undetected error. It serves not only as
a backup for the experimental Ethernet's serial hardware 16-bit cyclic redundancy checksum (in Figure 2),
but also for protection against failures in parallel data
paths within stations which are not checked by the CRC.
The checksum used by the E F T P is a l's complement
add and cycle over the entire packet, including header
and content data. The checksum can be ignored at the
user's peril at either end; the sender may put all l's (an
impossible value) into the checksum word to indicate
to the receiver that no checksum was computed.
7.2.1 Data transfer. The 16-bit words of a file are
carried from sending station to receiving station in data
packets consecutively numbered from 0. Each data
packet is retransmitted periodically by the sender until
an ack packet with a matching sequence number is returned from the receiver. The receiver ignores all damaged packets, packets from a station other than the
sender, and packets whose sequence number does not
match either the expected one o r the one preceding.
When a packet has the expected sequence number, the
packet is acked, its data is accepted as part of the file,
and the sequence number is incremented. When a packet
arrives with a sequence number one less than that expected, it is acknowledged and discarded; the presumptiort is that its ack was lost and needs retransmission
[21].
7.2.2 End. When all the data has been transmitted,
an end packet is sent with the next consecutive sequence
number and than the sender waits for a matching endreply. Having accepted an end packet in sequence, the
data receiver responds with a matching endreply and
then dallys for some reasonably long period of time (10
seconds). U p o n getting the endreply, the sending station
transmits an echoing endreply and is free to go off with
the assurance that the file has been transferred successfully. The dallying receiver then gets the echoed endreply and it too goes off assured.
Table I. Ethernet Efficiency.
Q
1
2
3
4
5
10
32
64
128
256
P = 4096
P = 1024
P = 512
P = 48
1.0000
0.9884
0.9857
0.9842
0.9834
0.9818
0.98~
0.9805
0.9804
0.9803
1.0000
0.9552
0.9447
0.9396
0.9367
0.9310
0.9272
0.9263
0.9259
0.92~
1.0000
0.9143
0.8951
0.88~
0.8810
0.8709
0.8642
0.8627
0.8620
0.8616
1.0000
0.5000
Communications
of
the ACM
July 1976
Volume 19
Number 7
0,4444
0.4219
0.4096
0.3874
0.3737
0.3708
0,3~3
0.3~6
Fig. 4. EFFP packet layout.
Destination
words belonging to unnamed files and thus the E F T P is
either terribly restrictive or demands some nested file
transfer formats internal to its data words. And fifth,
functional generality is lost because the receiver is also
the listener and server.
Source
Packet Type
Sequence Number
Length (in words)
Data (words)
8. Conclusion
I
Data
Packet
Only
Software Checksum
-
-
1 16-bit Word
The comparatively complex end-dally sequence is
intended to make it practically certain that the sender
and receiver of a file will agree on whether the file has
been transmitted correctly. If the end packet is lost, the
data sender simply retransmits it as it would any packet
with an overdue acknowledgement. If the endreply from
the data receiver is lost, the data sender will time out in
the same way and retransmit the end packet which will
in turn be acknowledged by the dallying receiver. If the
echoed endreply is lost, the dallying receiver will be inconvenienced having to wait for it, but when it has
timed out, the receiver can nevertheless be assured of
successful transfer of the file because the end packet has
been received.
At any time during all of this, either side is free to
decide communication has failed and just give up, it is
considered polite to send an abort packet to end the
communication promptly in the event of, say, a userinitiated abort or a file system error.
7.2.3 E F T P shortcomings. The E F T P has been very
useful, but its shortcomings are many. First, the protocol provides only for file transfer from station to station
in a single network and specifically not from process to
process within stations either on the same network or
through a gateway. Second, process rendezvous is degenerate in that there are no mechanisms for finding
processes by name or for convenient handling of multiple users by a single server. Third, there is no real flow
control. If data arrives at a receiver unable to accept it
into its buffers, the data can simply be thrown away with
complete assurance that it will be retransmitted eventually. There is no way for a receiver to quench the flow of
such wasted transmissions or to expedite retransmission.
Fourth, data is transmitted in integral numbers of 16-bit
403
Our experience with an operating Ethernet leads us
to conclude that our emphasis on distributed control
was well placed. By keeping the shared components of
the communication system to a minimum and passive,
we have achieved a very high level of reliability. Installation and maintenance of our experimental Ethernet has
been more than satisfactory. The flexibility of station
interconnection provided by broadcast packet switching
has encouraged the development of numerous computer
networking and multiprocessing applications.
Acknowledgments. Our colleagues at the Xerox Palo
Alto Research Center, especially Tat C. Lam, Butler W.
Lampson, John F. Shoch, and Charles P. Thacker, have
contributed in many ways to the evolution of Ethernet
ideas and to the construction of the experimental system without which such ideas would be just so much
speculation.
Received May 1975; revised December 1975
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15. Heart, F.E., Kahn, R.E., Omstein, S.M., Crowther, W.R., and
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Michel, A., and Heart, F.E. Pluribus--a reliable multiprocessor.
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404
Management
Applications
H. M o r g a n
Editor
Synthesis of
Decision Rules
Cheng-Wen Cheng and Jonas Rabin
Western Electric
Decision tables can be used as an effective tool
during an interview to record the logic of processes to be
automated. The result of such an interview is not a
structure of complete decision tables but rather sets of
decision rules. The purpose of this paper is to provide a
procedure for synthesizing the decision rules and thus
provide an aid in developing a structure of complete
decision tables.
Key Words and Phrases: decision rules, decision
tables, logical tables, logical design, system design,
specification language
CR Categories: 3.50, 4.33
1. Introduction
One of the most i m p o r t a n t stages in the systems
development process is fact finding. F a c t finding is an
investigation and recording o f current procedures and
future requirements. T h e m o s t effective m e t h o d o f fact
finding is interviewing.
Decision tables are an effective tool for recording
the logic during the interview process, as discussed b y
L o n d o n [1]. The p r i m a r y objective o f the interview is
Copyright © 1976, Association for Computing Machinery, Inc.
General permission to republish, but not for profit, all or part
of this material is granted provided that ACM's copyright notice
is given and that reference is made to the publication, to its date
of issue, and to the fact that reprinting privileges were granted
by permission of the Association for Computing Machinery.
Authors' present addresses: C. W. Cheng, Western Electric, Engineering Research Center, P.O. Box 900, Princeton, NJ 08540.
J. Rabin, Bell Laboratories, Napierville, IL 08540.
Communications
of
the ACM
July 1976
Volume 19
Number 7
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