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
