*BSD News Article 74130


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From: Pedro Roque Marques <roque@di.fc.ul.pt>
Newsgroups: comp.os.linux.networking,comp.unix.bsd.netbsd.misc,comp.unix.bsd.freebsd.misc
Subject: Re: TCP latency in the presense of RFC1323 (Was: Re: TCP latency)
Date: 18 Jul 1996 18:01:58 +0100
Organization: Faculdade de Ciencias da Universidade de Lisboa
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>>>>> "Ignatios" == Ignatios Souvatzis <souva@aibn58.astro.uni-bonn.de> writes:

    Ignatios> In article <4sbrcr$rqd@enomem.lkg.dec.com>
    Ignatios> matt@3am-software.com (Matt Thomas) writes:

    Ignatios>    There are a number of problems with the way timers
    Ignatios> are implemented in TCP.  The first is granularity.  A
    Ignatios> slow/fast timeout has an inaccuarcy of up to
    Ignatios> 500ms/200ms.  That may cause inaccuaries to creep into
    Ignatios> round-trip estimates.

    Ignatios> Shouldn't this be unimportant in the presence of the
    Ignatios> RFC1323 round trip _measurement_ (instead of "educated
    Ignatios> guessing", which old BSD TCP did)?

No.
Where talking about completly different things. RFC 1323 changes the 
precision of the input to rtt (and rto) estimatives. We are talikng about
the internal software timer that has to go off when rto msecs have elapsed
since a unacked send or {random(0,200) in BSD, delay_ack_estimate in Linux}
has elapsed since the receive of a segment.

Also actually the input is only better if
a) you have broken messurements (Linux pre 2.0 had)
b) you retransmited the segment

    Ignatios> (Assuming that no other part of the networking code
    Ignatios> violates layering).

Is layering a "good thing" (TM) ? :-)


regards,
  Pedro.