[ih] Question on Flow Control

[John Day]

Yes, it appeared to come later. Or at least I thought so.

Does Go-Back-N require sequence numbers?  I guess not. Just go back N-messages.

Do we know when GBN appeared?

John

> On Dec 29, 2025, at 10:29, Vint Cerf <vint at google.com> wrote:
> 
> 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 <mailto: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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