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From: seasterb@CS.UCL.AC.UK (Steve Easterbrook)
Newsgroups: comp.protocols.tcp-ip
Subject: tcp smooth rtt function
Message-ID: <8704300123.AA27080@ucbvax.Berkeley.EDU>
Date: Wed, 29-Apr-87 22:07:26 EDT
Article-I.D.: ucbvax.8704300123.AA27080
Posted: Wed Apr 29 22:07:26 1987
Date-Received: Sat, 2-May-87 05:01:04 EDT
Sender: daemon@ucbvax.BERKELEY.EDU
Distribution: world
Organization: The ARPA Internet
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Whilst building a tool to monitor tcp's behaviour under various conditions
I came across an interesting feature of the smooth round trip time function.
As expected, this hovers around a value, responding to such things as
rtx time-outs, etc. However, when running tcp across a local net, (i.e. with
very small round trip time), I noticed a series of apparently random 'kicks'
where the SRTT suddenly shoots up to a relatively enormous value, coming
down only slightly less quick. After some careful analysis, I discovered
the cause. On the test I was doing, the round trip time was small enough
for the SRTT to reach zero occasionally. Examination of the tcp code reveals
that when modifying the SRTT, a test is made to see if it is zero, and if
so the usual RSRE smoothing function isn't used. 

The question is this: Can anyone give me some pointer as to what the
philosophy behind this is, and maybe some reference as well. I gather
its main purpose is to prevent the SRTT from staying at zero, as this
will cause tcp to be a bit keen on the retransmissions, but it seems
to me that substitute value is a little on the overkill side.

Ta muchly,
Steve