RFC 203 (rfc203) - Page 1 of 4

Achieving reliable communication

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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.



Kalin
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