RFC516

Network Working Group Jonathan B. Postel RFC # 516 UCLA-NMC NIC # 16683 May 18, 1973

                     LOST MESSAGE DETECTION

I have three suggestions for detecting the loss of messages by the communications subsystem. The first of these is perhaps the more powerful and simpler to implement since it uses no new concepts and has the power to retransmit the message detected as lost.

The first scheme:

If upon sending a message the host saved a copy of that message and
waited until either:

    a RFNM was returned, in which case everything is ok and the next
    message is processed.

    a INCOMPLETE TRANSMISSION is returned, in which case the copy of
    the message is retransmitted (this could be a loop so put a
    finite upper bound on the number of times to retransmit the same
    message).

    a DESTINATION DEAD is returned, in which case mark the
    destination down and require the exchange of reset commands
    before further communication is allowed.

    something else is received indicating an error in the network or
    local IMP, in which case at least log the error, and probably
    close the conversation.

Following the above procedures either on a per host basis or a per
link basis should prevent a lost message problem from
developing.

The second scheme:

If on a per host basis, message numbers are included in the host to
host header of messages,, and messages are delivered in order (this
is currently the case in the network, except for priority messages
so this proposal requires that each host either send everything as
priority or nothing as priority) then each receiving host can detect
a missing message by comparing the message number of the received
message with the previously received message.

    On exchanging resets the sequence numbers between that pair of

    hosts is set to zero.

    Each time a message is sent the current send message number is
    entered into a field in the message header, and the current send
    message number is incremented (modulo N, say N=256)

    Each time a message is received the message number from the
    message is compared to the current receive message number and:

        if the received message is the expected one then the message
        is acceptable and current receive message number is
        incremented (modulo N);

        if the received message is not the expected one then a
        message has been lost.

What to do when a missing message is detected, not clear, but at
least can be logged and reported to the network control center.  A
missing message may not be fatal to an interactive conversation, but
it is critical in a file transfer, thus I suggest that missing
messages which are not recovered be cause to close the conversation.

The third scheme:

Host to host acknowledgements could be required.  Such an
acknowledgement scheme could be implemented similarly to the IMP to
IMP scheme.  This is a serious change to the current protocols so I
will not elaborate on it here, feeling that deeper study will be
necessary to fully specify a reasonable host to host acknowledgement
strategy.

Of these three suggestions the first is the most immediately practical and implementable; in fact several hosts all ready do this. These schemes also are non-conflicting, they could be implemented and used simultaneously.

   [ This RFC was put into machine readable form for entry ]
   [ into the online RFC archives by Alex McKenzie with    ]
   [ support from GTE, formerly BBN Corp.             9/99 ]