RFC 442 (rfc442) - Page 1 of 7
Current flow-control scheme for IMPSYS
Alternative Format: Original Text Document
Network Working Group V. Cerf
Request for Comments: 442 24 January 1973
NIC: 13774
The Current Flow-Control Scheme for IMPSYS
BB&N quarterly report #13 outlines part of the current flow control
scheme in the IMP operating system. A meeting held March 16, 1972,
at BB&N was devoted to the description of this new scheme for the
benefit of interested network participants.
This note represents my understanding of the flow control mechanism.
The essential goal is to eliminate unnecessary retransmissions when
the load is heavy, eliminate the retransmission time-out period when
the load is light, increase bandwidth, prevent re-assembly lock-up,
control traffic from HOSTS into the net more strictly than the
earlier link blocking method, and secure the rights of life, liberty,
and the pursuit of happiness for ourselves and our posterity,...oops.
Source IMP-to-Destination IMP Protocol
There are two different protocols depending on message length (i.e.
single or multi-packet). We illustrate first the single packet case.
Source Imp Destination Imp
---------- ---------------
case 1) message (1) + implicit req (1)--->
<--- RFNM (arrived ok)
[discard copy of msg]
case 2) message (1) + implicit req (1)---> no room, don't respond
<--- All (1) (room available)
message (1) --->
[discard copy of msg] <--- RFNM (arrived ok)
In the first case, a single packet message is sent to the destination
IMP. This message acts as an implicit request for single packet
buffer space. If there is room, as in case 1, the destination IMP
responds with a RFNM. The source IMP, which has retained a copy of
the message, deletes its copy and goes on.
The second case illustrates what happens when the source IMP sends a
message to a destination IMP at which there is no room for the one-
packet message. The arrival of the single packet message constitutes
a request for single packet buffer space, and is recorded as such by
the destination IMP in a first-come-first-served buffer reservation
Cerf
