Thoughts on increasing MTUs on the internet

Thu Apr 12 19:28:09 UTC 2007

mark does not have posting privs and has asked me to post the
following for him:

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To: Gian Constantine <constantinegi at corp.earthlink.net>
From: Mark Allman <mallman at icir.org>
cc: NANOG list <nanog at merit.edu>
Subject: Re: Thoughts on increasing MTUs on the internet 
Date: Thu, 12 Apr 2007 11:47:35 -0400

Folks-

> I agree. The throughput gains are small. You're talking about a
> difference between a 4% header overhead versus a 1% header overhead
> (for TCP).

This does not begin to reflect the gain.  Check out the model of TCP
performance given in:

  M. Mathis, J. Semke, J. Mahdavi, T. Ott, "The Macroscopic Behavior of
  the TCP Congestion Avoidance Algorithm", Computer Communication Review,
  volume 27, number3, July 1997.
  (number 35 at http://www.psc.edu/~mathis/papers/index.html)

The key point is that performance is directly proportional to packet
size.  So, an increase in the packet size is much more than a simple
lowering of the overhead.

In addition, the newly published RFC 4821 offers a different way to do
PMTUD without relying on ICMP feedback (essentially by trying different
packet sizes and trying to infer things from whether they get dropped).

A good general reference to the subject of bigger MTUs is Matt Mathis'
page on the subject:

  http://www.psc.edu/~mathis/MTU/

allman

-- 
Mark Allman -- ICIR/ICSI -- http://www.icir.org/mallman/