[ih] TCP RTT Estimator

[Craig Partridge]

Yes, informally known as the RSRE algorithm.

The other detail is the value for Alpha was chosen to be fast on computers
of the time and so is a fraction of 1/8.

Craig

On Tue, Mar 11, 2025 at 8:05 AM David Finnigan via Internet-history <
internet-history at elists.isoc.org> wrote:

> About 2 weeks ago I finally began writing the Round Trip Time (RTT)
> estimator for my TCP on the Mac. I had previously read many many
> documents which described the small, evolutionary changes in this
> important function of TCP:
>
> SRTT = ( ALPHA * SRTT ) + ( (1-ALPHA) * RTT )
>
> I was interested in knowing the reason behind why this particular
> algorithm was selected. I found a reference to IEN 177, "Comments on
> Action Items from the January [1981] Meeting" which stated:
>
> "The algorithm described by RSRE at the October 80 meeting should be
> implemented.  It will be included in the next edition of the TCP
> specification.
>
> The current best procedure for retransmission timeout is to measure the
> time elapsed between sending a data octet with a particular sequence
> number and receiving an ack that covers that sequence number (thus one
> does not have to match sends and acks one for one)."
>
> I continued looking back in older IEN documents and found in IEN 160, 7
> November 1980, it was reported that
>
> "Brian Davies discussed some suggestions for performance improvements
> based on the experience at RSRE.
>
> The use of an adaptive retransmission timeout seems to be very helpful.
> RSRE has experimented with one based on the following:
>
> 1.  For each segment record the sequence number and time sent.
>
> 2.  For each acknowledgment determine the round trip time (RTT) of the
> sequence number thereby acknowledged.
>
> 3.  Compute an Integrated Ack Time (IAT) as follows:
>
>         IAT = ( ALPHA * IAT ) + RTT
>
> 4.  Compute a Retransmission Time Estimate (RTE) as follows:
>
>         RTE = Min [ BOUND, ( BETA * IAT ) ]
>
> Where BOUND is an upper bound on the retransmission time and BETA is an
> adjustment to the IAT to account for variation in the delay.
>
> RSRE currently uses  ALPHA = 31/32 and BETA = 1.33.
>
> [Dave Clark noted that MIT-MULTICS uses the same algorithm but with
> ALPHA = 4/5 and BETA = 1.5.]"
>
>
>
> Going still further back to IEN 134 of 29 February 1980, it was reported
> that
> "Brian discussed some measurements of TCP conducted by RSRE to various
> other places in the internet.  The performance is regular for round
> trips from RSRE to various points at UCL, and is consistent with the
> physical facilities.  Once the round trip path includes the SATNET,
> however, the performance becomes irregular, with a few messages subject
> to very high delay.  Also some unnecessary retransmissions are detected
> in the tests form RSRE to ISIE and back, these may be due to a too low
> retransmission threshold."
>
> And the topic is discussed in IEN 121, 25 October 1979.
>
> It looks like staff at RSRE (Royal Signals and Radar Establishment) took
> the lead in experimenting with formulae and methods for dynamic
> estimation of round trip times in TCP. Does anyone here have any further
> insight or recollection into these experiments for estimating RTT, and
> the development of the RTT formula?
>
> -David Finnigan
> --
> Internet-history mailing list
> Internet-history at elists.isoc.org
> https://elists.isoc.org/mailman/listinfo/internet-history
>


-- 
*****
Craig Partridge's email account for professional society activities and
mailing lists.