[ih] Question on Flow Control

[touch at strayalpha.com]

On Dec 29, 2025, at 9:57 AM, Craig Partridge via Internet-history <internet-history at elists.isoc.org> wrote:
> 
> On Mon, Dec 29, 2025 at 12:07 PM John Day <jeanjour at comcast.net> wrote:
> 
>> 
>> As for TCP initially using Selective-repeat or SACK, do you remember what
>> the TCP retransmission time out was at that time? It makes a difference.
>> The nominal value in the textbooks is RTT + 4D, where D is the mean
>> variation. There is an RFC that says if 4D < 1 sec, set it to 1 sec. which
>> seems high, but that is what it says.
>> 
>> Take care,
>> John
>> 
> 
> Serious study of what the RTO should be didn't happen until the late
> 1980s.  Before that, it was rather ad hoc.
> 
> RFC 793 says min(upper bound,  beta * min(lower bound, SRTT)). where SRTT
> was an incremental moving average, SRTT = (alpha * SRTT) +
> (1-alpha)(measured RTT).  But this leaves open all sorts of questions such
> as: what should alpha and beta be (RFC 793 suggests alpha of .8 or so and
> beta of 1.3 to 2), and do you measure an RTT once per window (BSD's
> approach) or once per segment (I think TENEX's approach).  Not to
> mention the retransmission ambiguity problem, which Lixia Z. and Raj Jain
> discovered in 1985-6.  (If you are wondering why we didn't use variance --
> it required a square root which was strictly a no-no in kernels of that
> era;  Van J. solved part of this issue by finding a variance calculation
> that could be done without a square root).

FWIW, I raised the point that the time window had a moving average but not a moving variance to Van at a DARPA PI meeting. I don’t know why everyone was stuck on square roots as a problem; it’s just as easy to just compare the squares instead of trying to take square roots.

The variance part came from some work I did at Cornell with Ken Birman as a grad student working on heartbeat trace analysis, as described in the first paragraph here: https://www.strayalpha.com/pubs/ecg1987.pdf I have an online version of the detail of that all, describing how the combination of avg and variance automatically adjusted for noise, which I can upload and link if there’s interest.

Joe

—
Dr. Joe Touch, temporal epistemologist
www.strayalpha.com