Network Working Group R. Kalin
Request for Comments: 203 MIT Lincoln Lab
NIC: 7168 10 August 1971
Achieving Reliable Communication
'This material has not been reviewed for public release and is
intended only for use with the ARPA network. It should not be quoted
or cited in any publication not related to the ARPA network.'
ABSTRACT
A non-standard protocol, suitable for either second or third level
use, is proposed with the intent of providing error resistant and
highly reliable communication channels. Errors introdUCed by message
garbling, message loss, and message pickup are considered. Measures
for increasing throughput are also discussed.
AIMS AND LIMITATIONS
It is not our intent to propose the design of a perfect communication
channel, rather it is our contention that in the real world there can
be no perfect channels and that no amount of protocol can insure the
error free transfer of information. Our goal is to eXPlicate the
various types of errors that are possible and to provide for each
techniques of detection and recovery that, at a cost, can be made
arbitrarily good. In this way the mean time between undetected
errors can be made as large as necessary.
ERROR TYPES AND DETECTION
Over a message switching facility, such as the ARPA network, all
transmission errors can be divided into two classes -- those that
result in the loss of an expected message, and those that result in
the picking up of an unexpected message. A single bit inversion can
cause errors of both types. Error detection can therefore be divided
into two components -- one which attempts to determine if the message
just received is appropriate at that time, and another which attempts
to determine if a message has been lost.
The detection of garbled input messages has been adequately covered
by classical coding ( elsewhere, mistakenly termed 'communication' )
theory. Internal message consistency can be determined through the
use parity bits, checksum fields, or any of the various coding
techniques available for adding some measure of redundancy. With
relative simplicity, the likelyhood of an undetected error of this
type can be made small enough so as to become inconsequential.
Because it is adequately covered elsewhere, no further discussion
shall be given here.
The detection of a message's external consistency, whether or not it
can possibly follow the message that arrived just before it, can also
be straight forward. Sequence numbers, if used, can be easily
checked. A modulo N sequence field will allow detection of up to N-1
successive message losses. If several concurrent links are in use
then sequencing can be maintained for each link. Multi-link single
sequence schemes are more complicated and, although used between IMPs
for transmission of message packets, they shall be ignored here.
The detection by a receiving host of a lost message can not be
determined directly, but rather must be inferred from other
observations. Any automatic correction scheme must be prepared to
handle the possibility of faulty inference. Message loss would
normally be inferred upon the arrival of a message that should follow
the one expected. It might also be inferred by the fact that the
message expected is long overdue.
ERROR CORRECTION
If a BCH or other error correcting code is used for transmission,
errors detected in a message's internal consistency can sometimes be
corrected by the receiving host. In the event that this is not
possible, the content of the message is of little use because it can
not be relied upon. The only reasonable solution is that of
discarding the message and relying upon the recovery procedures
implemented for lost messages.
Errors of external consistency can also be treated in the same way.
The message can be thrown away and the techniques for recovering lost
messages relied upon. Over a critical channel, a slightly fancier
technique can at times save some retransmissions. If message N is
expected, but message N+1 arrives, there is no need to throw away
message N+1 and then recover two messages, it could be saved, and
only message N retransmitted.
On noisy channels the technique of saving out of sequence messages
can be used to some advantage, especially if recovering from a lost
message requires several messages of overhead. On the ARPA network,
the measured error rate is so low that its advantages are outweighed
by the increase in resident coding.
RECOVERING LOST MESSAGES
The simplest technique I know of for recovering lost can be defined
by the following rules:
1) All undiscarded messages have reply messages.
2) Messages with coding errors that can not be corrected are
discarded.
3) The receiver can determine if a message is in sequence.
4) Messages received that are out of sequence are discarded.
5) If no reply message is received in N time units since the last
transmission, the last message sent is retransmitted ( space need
not be isochronic ).
6) A new message is not sent until the reply for the last one has
been received.
The above protocol, if run, is highly effective for continuous
communication. Since by rule 6) only one message can be in transit
at a time, the necessary sequencing information can be contained in a
single bit. Unmodified, it is not suitable for finite communication,
since rules 1) and 5) guarantee that there will be no 'last message'.
The protocol also does not make very effective use of a pipelined
channel, since there is only one message being sent at a time.
Channel throughput can be increased by several techniques, the first
of which would be to disassemble the data stream into several ( eg.
four or eight ) streams, transmit each using the above protocol, and
then reassemble the streams at the far end. Another technique is to
modify rules 5) and 6).
5a) If no reply has been received to message M in N time units
since the last transmission, then messages M, M+1,... are
retransmitted.
6a) There must be no more than L outstanding unreplied messages.
With L equal to one, this protocol degenerates into the first
protocol. Increasing L increases throughput until the gain is
outweighed by the time spent in error recovery. The larger L, the
costlier error recovery. The value of N should be adjusted so that
the reply time for a message is usually less than N plus the time to
send L-1 messages. Increasing N too much will have the effect of
lowering the response time to errors. Decreasing N increases the
probability initiating unnecessary retransmissions.
A CRITICAL RACE
The above protocols leave unresolved the the particulars of starting
and stopping a finite transmission. In opening a communication
channel, what is the sequence number of the first message sent? What
will be the first sequence number of the first message sent? What
will be the first sequence number of the first reply received? At
the end of transmission, how does one signal the 'last message'? The
following two rules are introduced:
7) If the same message has been received K times ( eg. 50 ), then it
should be accepted as being 'in sequence'. The expected
sequencing should be adjusted accordingly. K identical reply
messages are then sent.
8) If no reply has been received in J seconds, then the
retransmission of the last unreplied message should cease.
With these additional rules a finite transmission is started by
repeatedly transmitting the first message until K identical reply
messages are received. Sequencing is adjusted accordingly and then
subsequent messages can be sent. A conversation is broken by
quitting transmission after the reply to the last message you care
about has been received. Eventually the other end will stop
resending the reply. To avoid ambiguity, the variable J should be
less than N times K. Problems will arise if the network crashes for
J seconds, for there is a race condition over whether or not the lack
of a reply is the result of a channel failure or the end of a
conversation.
[ This RFCwas put into machine readable form for entry ]
[ into the online RFCarchives by Ryan Kato 6/01]
