[ih] Question on Flow Control

[John Day]

As we all know, there are two forms of sliding window flow control:
1) the static window, where an Ack causes the window to be moved and requires two extra commands, and
2) the dynamic window, with a credit field that is added to the Ack value to find the Right Window Edge (RWE) and doesn’t need the two extra commands.

Static window is found in datacomm protocols such as SDLC, HDLC, and variations on them.  (HDLC was created from SDLC.)
Dynamic window is found in most Transport protocols such as TCP, TS, TP4, SCTP, QUIC, etc.

Most textbooks present these as a progression from a simple stop-and-wait protocol with increasing complexity through static window to dynamic window as if this was the order of development. That dynamic winsdow was an enhancement of static window.

However, that does not seem to be borne out by the historical record, but it isn’t clear. This is what I have been able to determine:
1) Static window is in SDLC, an integral part of IBM’s SNA.

2) SNA was released in 1974 with SDLC.

3) I have not been able to find anyone (or any paper) who knows about the development inside IBM of SDLC, nor was SDLC based on precursors either inside or outside IBM.

4) I have not been able to find anything about a sliding window flow control protocol prior to 72 or 73.

5) Dynamic window first appeared in CYCLADES TS in late 1972 or early 73 and was incorporated into the early drafts of Sept 73.

That doesn’t give much time for overlap and IBM in this period kept their cards pretty close to their chest.

So the questions are:

1) Did fixed window originate at IBM with SDLC?

2) Was there an earlier fixed window precursor to SDLC that was inside or outside IBM?

3)  Was there an external fixed window precursor that was the example for both?

4)  Is this a case of independent invention?  Were static and dynamic window flow control invented independently?

It isn’t an earth-shattering question, but it is curious and does seem to run counter to the typical exposition in textbooks.

Take care,
John Day