Subject: [pmtud] Path MTU Discovery using Options

From: Michael Welzl <[email-masked]>
Date: 04 Nov 2003 09:22:43 +0100

Dear all,

I've been very busy with lecture slides lately, and it doesn't
look like this situation will change the next couple of
days - however, I managed to write a very preliminary version
of my "path mtu discovery using options" idea, and I figured
I'd make it available to you before the IETF meeting:

http://www.welzl.at/tmp/draft-welzl-mtuoption-preliminary.txt

This might be relevant for the discussion of tunneling; even
if we decide that we don't want to place this extra burden on
routers every time we set up a connection, there can be
special occasions where my option approach is useful, e.g.:

* to probe for a larger MTU while a long-lived connection is
  active

* to cope with tunneling problems (use the options just
  in between the tunnel endpoints)

Our option processing study is over, including the necessary
statistical analysis. We did two major measurements - 2001
and 2003; in 2003, we traversed some 70000 router addresses
- of course, some may belong to the same device - and
weighed the measurements based on uniqueness of routers in
a path and measurement variance ... the result shows that
option processing appears to be much less severe than expected:
maybe 1%, or up to 10% extra delay, but rarely more. We are
now working on the documentation and hope to have a technical
report available soon.

Cheers,
Michael

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