NWG/RFC 59 Flow Control - Fixed Versus Demand Allocation
manner. IMP messages for a particular "receive" link may be coming in
to the destination HOST faster than the attached process is reading them
out of the NCP's buffers. At some point the NCP will decide that the
input queue for that link is too large in relation to the total amount
of free NCP buffer space remaining. At this time the NCP initiates
quenching by sending a "cease on link" IMP message to its IMP. This does
nothing until the next message for that link comes in to the destination
IMP. The message still gets transmitted to the receiving HOST. However,
the RFNM returned to the transmitting HOST has a special bit set. This
indicates to the originating NCP that it should stop sending over that
link. As a way of confirming the suspension, the NCP sends an SPD
"suspended" NCP control message to the receiving HOST, telling it that
it indeed has stopped transmitting. At a future time the receiving pro-
cess will have cut the input queue for the link down to reasonable size,
and the NCP tells the sending NCP to begin sending messages by issuing a
RSM
"resume" NCP control message.
The flow control argument is based on the following premises:
(1) Most network transmission falls into two categories:
Type 1 - short messages (
