[ih] TCP RTT Estimator

[Barbara Denny]

 (Having trouble getting this posted so shortening the message. Hope I don't create duplicates.)

While we are on the topic..
I have always been curious how packet radio may,  or may not, have impacted the calculations.  If anyone finds any info while looking into this,  I would appreciate it if you passed on where to find this discussion.
barbara
    On Tuesday, March 11, 2025 at 11:23:21 AM PDT, vinton cerf via Internet-history <internet-history at elists.isoc.org> wrote:  
 
 one of the major participants in the Internet work in the UK in early days
was John Laws at RSRE.
I have cc'd him on this note.

John, any comments on RSRE development of RTT computations?

vint


On Tue, Mar 11, 2025 at 2:07 PM Michael Greenwald via Internet-history <
internet-history at elists.isoc.org> wrote:

> If I am remembering correctly (I'm on the move, and don't have access to
> anything other than email to checkup anything at the moment), the RTT
> estimator is not exactly the standard algorithm for computing moving
> average. I think it separately estimated RTT average and RTT variance,
> and added the 2 together (which meant the estimate actually increased,
> briefly, if the RTT was dropping rapidly.)
> Also, wasn't there some variation (Craig and MRose? or Van? or Lixia?)
> to eliminate individual round-trips that may have been computed by a
> duplicate or missing packet?
>
> On 3/11/25 10:51 AM, John Day via Internet-history wrote:
> > This algorithm and variations of it are used throughout science in many
> different places, sometimes called convolution to compute a moving average.
> >
> > It is far from unique to TCP or even networking.
> >
> > Take care,
> > John
> >
> >> On Mar 11, 2025, at 12:01, Craig Partridge via Internet-history <
> internet-history at elists.isoc.org> wrote:
> >>
> >> 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
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