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Bolt Beranek and Newman
Inc.
The Modem-to-IMP routine (M-I) handles inputs from the
modems. This routine consists of several identical routines,
each devoted to a modem channel. Such duplication is useful
to obtain higher speed. This routine first sets up a new input
buffer, normally obtained from the free list. If a buffer
cannot be obtained, the received buffer is not acknowledged
and is reused immediately. The discarded packet will be
retransmitted by the distant IMP. The routine processes
returning acknowledgments for previously transmitted packets
and either releases the packets to the free list or signals their
subsequent release to the IMP--to-Modem routine. The M-I routine
then places the buffer on the end of the task queue and triggers
the programmable task interrupt.
The TASK routine uses the header information to direct packets
to their proper destination. The routine is driven by the task
interrupt, which is set whenever a packet is put on the task
queue. The routine routes packets from the task queue onto an
output modem or Host queue determined from the routing algorithm.
If the packet ls for non-local delivery, the routine determines
whether sufficient store and forward buffer space is available.
If not, buffers from modem lines are flushed and no subsequent
acknowledgment is returned by the IMP-to-Modem routine. Normally,
an acknowledgment is returned in the next outgoing packet over that
modem line. Packets from Hosts which cannot get store and forward
space are removed from the queue and replaced at a later time by
the H-I routine.
If a packet from a modem line is addressed for local delivery,
its message number is checked to see whether a duplicate packet
has been received. As mentioned previously, each IMP maintains
a window of contiguous numbers which it will accept from a source
IMP. Packets with out-of-range numbers are considered duplicate
3-10 2/73
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