RFC 442 (rfc442) - Page 2 of 7

Current flow-control scheme for IMPSYS

RFC 442        The Current Flow-Control Scheme for IMPSYS   January 1973


   request queue.  When space is available, the destination IMP will
   transmit an ALL (1) to the requesting source IMP which can then send
   the single packet message again, this time knowing that space has
   been reserved at the destination.

   For multi-packet messages, the procedure is somewhat different.  When
   a message enters an IMP from a HOST, and the "last bit" flag is not
   set when the number of bits in a maximum length single packet have
   arrived, the IMP halts the HOST->IMP transmission line while it
   determines whether space has been reserved at the dest. IMP.  If
   space (8 packets worth) has been reserved, the HOST->IMP line is re-
   opened, and the message is sent out normally.  If space has not been
   reserved, the HOST->IMP line is kept closed while the source IMP
   makes a request for multi-packet buffer storage at the destination
   IMP.  When 8 buffers are available, the destination IMP responds with
   an ALL (8).  The source IMP then transmits the message, and waits for
   a combination RFNM and ALL (8) from the destination IMP.  The
   destination IMP will delay its RFNM, if necessary, until it has
   another 8 buffers available for the next multipacket message.

   This sequence is illustrated below:

            Source IMP                   Destination IMP
            ----------                   ---------------

H-> I line
----------> First packet of multipacket
            arrives. Halt H->I line and
            send REQ (8)  -------------->
            start 30 sec. Time-out

            If time-out, resend
            REQ (8) and restart -------->
            time-out.
                                <--------ALL (8) when available. Start
                                         long term (2 min.) time-out.
                                         On time-out, reset all
                                         outstanding reservations.

            Send the message:
                        |   ----------->
            Start 30 sec. time-out
            for INComplete transmission.
            If time-out, send INC?----->







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