Summary

  • RFC 1008 described optional flow-control confirmation for ISO transport: an AK message that reopens a closed credit window may be repeated quickly if its arrival is uncertain. The data sender may answer with an AK carrying an FCC parameter.
  • Wayne McCoy's June 1987 guide said there was no reported operating experience showing whether this procedure cut resynchronization delay enough to justify it. If confirmation never arrives, the fast repeats stop after a configured maximum and the ordinary window timer takes over.

Picture two transport peers after the receiver has run out of room. It advertises no credit, and the sender stops. Later the receiver has buffer space again. It sends an acknowledgment carrying the new credit. That message disappears. The sender is behaving perfectly: its last instruction was to wait. The receiver is also behaving perfectly: it has made room. The connection has become a two-person comedy in which each thinks the other holds the next line.

Wayne McCoy examined this case in the June 1987 RFC 1008 implementation guide for the ISO transport protocol. The guide treated transport as an engineering job full of buffers, timers and state, rather than as a tidy stack of boxes. Its flow-control section distinguished routine data acknowledgment from messages that carried a more consequential fact: the receiver had changed how much the sender was allowed to put in flight. An AK TPDU opening a closed window, or one sent after certain credit reductions, could be critical even when no data byte needed praise.

Losing such a message did not erase the receiver's free buffer. It erased the sender's knowledge of it. The ordinary window timer could eventually bring the two sides back into agreement, but McCoy called that timer relatively long. Waiting for it meant idle capacity and a delayed conversation. The proposed FCC procedure tried to shorten the gap. The receiver would retransmit its critical AK at brief intervals, asking for confirmation of the flow-control state. If the data sender returned an AK carrying the FCC parameter, the receiver could stop the fast repeats. It was a receipt for permission, not a receipt for a parcel.

The procedure had brakes. If no FCC answer arrived, the receiver would stop after a maximum number of retransmissions and leave the window timer in charge. FCC was optional, and the data sender was not required to respond. McCoy suggested using the same timer for fast repeats while the ordinary window timer was disabled, avoiding another running clock; the formal description showed separate timers to make the logic easier to read. His later timer discussion put the fast interval near Erl + Elr, the two directions of network delay. The clock was a design choice, not magic dust sprinkled on a lost packet.

Should an implementer add this ritual? The guide offered questions rather than a victory lap: How often does credit shrink and reopen? How likely is a TPDU to be lost? How long is the window timer? How large is the window? What acknowledgment policy is already in use? More control messages cost processing and bandwidth. A long stall costs useful transmission time. McCoy explicitly said no experience with FCC and fast acknowledgment retransmission had been reported, so the benefit could not yet be measured from deployment.

That restraint makes the episode interesting. Network flow control is often narrated as a simple traffic light: green means go, red means stop. RFC 1008 dealt with the awkward moment when the light turns green but the driver never sees it. The sender has no reason to move; the receiver has no reason to remain silent. An extra acknowledgment can tell each what the other knows, at the price of more messages and another timer path.

The lesson is not that every acknowledgment deserves a thank-you note. It is that a control message can carry operational authority out of proportion to its size. The lost AK left real buffer space unused because the permission to use it had failed to arrive. In McCoy's proposal, the receiver could knock again and ask to hear the sender say, “I saw the door open.”