[ih] Question on Flow Control

[Vint Cerf]

John,
there is another term "go-back-N" which sounds like a fixed window and is a
form of ARQ.
would the terminology uncover any earlier schemes?

v


On Mon, Dec 29, 2025 at 9:59 AM John Day via Internet-history <
internet-history at elists.isoc.org> wrote:

> 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
>
>
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