[46807] in North American Network Operators' Group
RE: packet reordering at exchange points
daemon@ATHENA.MIT.EDU (Jim Forster)
Thu Apr 11 02:55:27 2002
From: "Jim Forster" <forster@cisco.com>
To: "Peter Galbavy" <peter.galbavy@knowtion.net>,
"Richard A Steenbergen" <ras@e-gerbil.net>,
"E.B. Dreger" <eddy+public+spam@noc.everquick.net>
Cc: "Paul Vixie" <paul@vix.com>, <nanog@merit.edu>
Date: Wed, 10 Apr 2002 23:54:30 -0700
Message-ID: <00ae01c1e125$ba6b5380$dc9247ab@amer.cisco.com>
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Errors-To: owner-nanog-outgoing@merit.edu
> > To transfer 1Gb/s across 100ms I need to be prepared to buffer at least
> > 25MB of data. According to pricewatch, I can pick up a high density
512MB
>
> Why ?
>
> I am still waiting (after many years) for anyone to explain to me the
issue
> of buffering. It appears to be completely unneccesary in a router.
>
> Everyone seems to answer me with 'bandwidth x delay product' and similar,
> but think about IP routeing. The intermediate points are not doing any
form
> of per-packet ack etc. and so do not need to have large windows of data
etc.
>
> I can understand the need in end-points and networks (like X.25) that do
> per-hop clever things...
>
> Will someone please point me to references that actually demonstrate why
an
> IP router needs big buffers (as opposed to lots of 'downstream' ports) ?
Sure, see the original Van Jacobson-Mike Karels paper "Congestion Avoidance
and Control", at http://www-nrg.ee.lbl.gov/papers/congavoid.pdf. Briefly,
TCP end systems start pumping packets into the path until they've gotten
about RTT*BW worth of packets "in the pipe". Ideally these packets are
somewhat evenly spaced out, but in practice in various circumtances they can
get clumped together at a bottleneck link. If the bottleneck link router
can't handle the burst then some get dumped.
-- Jim
