RFC 203 (rfc203) - Page 2 of 4

Achieving reliable communication

RFC 203             ACHIEVING RELIABLE COMMUNICATION      10 August 1971


   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:



Kalin