RFC 442 (rfc442) - Page 2 of 7
Current flow-control scheme for IMPSYS
Alternative Format: Original Text Document
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?----->
Cerf