RFC 632 (rfc632) - Page 2 of 6

Throughput degradations for single packet messages

RFC 632    Throughput Degradations for Single Packet Messages   May 1974


is then sent back to the source IMP only after the RFNM for the previous
message has been processed.  We therefore may have the following
sequence of events:

     1  MSG(i) sent from SOURCE-IMP (message i is sent from the source
        IMP to the destination IMP).

     2  MSG(i+1) sent from SOURCE-IMP.

     3  MSG(i+1) arrives at DEST-IMP (due to an alternate path or a line
        error, message (i+1) arrives at the destination IMP out of
        order; it is treated as a request for one reassembly buffer
        allocation and then discarded).

     4  MSG(i) arrives at DEST-IMP (message i arrives at the destination
        IMP; it is put on the proper HOST output queue).

     5  RFNM(i) sent from DEST-IMP (after message i has been accepted by
        the destination HOST the RFNM is sent to the source IMP).

     6  ALL(i+1) sent from DEST-IMP (only after the RFNM for message i
        has been processed can the ALLOCATE for message i + 1 be sent).

     7  RFNM(i) arrives at SOURCE-IMP.

     8  ALL(i+1) arrives at SOURCE-IMP.

     9  MSG(i+1) arrives at DEST-IMP (now message i+1 is put on the
        proper HOST output queue.)

    10  RFNM(i+1) sent form DEST-IMP.

    11  RFNM(i+1) arrives at SOURCE-IMP.

Note that the round-trip time for message i+1 is the time interval
between event 2 and event 11.  Therefore the round-trip time for message
i+1 is more than twice as large as it would have been if it had arrived
in order, other conditions being unchanged.  Therefore a line error will
in many cases not only delay the message in error but also the next
single-packet message if this message follows the preceding message
within 125 msec, the error retransmission timeout interval.  Also, a
faster, alternate path to the destination IMP can actually slow down the
transmission since it causes messages to arrive there out of order.








Opderbeck