Network Working Group                                     Wayne Hathaway
RFC # 512                                                        AMES-67
NIC # 16443                                                  25 May 1973


                    MORE ON LOST MESSAGE DETECTION

I would like to second Edwin Meyer's (RFC #492) strong opposition to the
proposals made in RFC #467 concerning solutions to the "lost allocate"
and "half-closed" phenomena.  In particular I support all of his
principles concerning the "half-closed" phenomenon.  I also agree that
the proposed "lost allocate" solution tends to mask the real problem of
lost messages.  I would, however, like to propose the following
alternative scheme for recognizing lost messages.

I propose that one of the two unused eight-bit bytes in the level 2
message leader be designated the "Sequence Control Byte" (SCB).  This
SCB would be essentially a modulo 255 message count.  Upon receipt of a
message, the receiving NCP would compare the SCB in the previous the
message with the expected SCB as computed from the SCB in the previous
message on the same link.  A discrepancy indicates a lost message, which
could then be reported immediately via an appropriate ERR message.  This
ERR message (to be defined) would contain both received and expected
SCB's, allowing possible recovery of the lost message (if sufficient
space were available in the sending host to save the last several
messages for each link).  At any rate, the lost message would be
recognized immediately, whether it was an ALL (or any control message)
or a data message.  The message with the unexpected SCB should be
processed normally, with the SCB for the next message computed from it.

For compatibility, the SCB would be defined such that an SCB of zero
indicates that no checking is to be done.  The SCB following 255 would
thus be 1.  This would mean that current NCP's would not have to be
changed unless actual checking were desired (since the level 2 protocol
specifies that these two unused bytes must be zero.)  This special
definition of zero SCB would also allow RST's and ERR's to bypass
checking, which would be useful in avoiding possible loops.

This proposed scheme is similar to the second scheme suggested by Jon
Postel (RFC #516) except that it is on a per-link basis rather than a
per-host basis.  This is significant, however, as it removes the
requirement that all messages from one host to another arrive in the
order sent (which cannot be guaranteed).  It also provides for
compatibility with existing NCP's.  Jon's first proposal (save all
messages until RFNM received) is weak in two areas: first, it is
possible that the receiving IMP has sent a RFNM for a message that in
fact never gets to its host, and second, it requires (at least for
swapped systems such as ours) either that messages be saved in resident



Hathaway                                                        [Page 1]

RFC 512              MORE ON LOST MESSAGE DETECTION             May 1973


storage (expensive) or that RFNM's be handled by a swapped process (also
expensive).  The third proposal (that of a host-to-host acknowledgment
scheme) is perhaps the best, but as that requires quite major changes to
the level 2 protocol, an interim solution such as that proposed here
seems of value.










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Hathaway                                                        [Page 2]