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Date: Mon, 5 Mar 2007 11:57:31 -0800 (PST)
From: Lyndon Nerenberg <lyndon@orthanc.ca>
To: Tom Judge <tom@tomjudge.com>
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Cc: freebsd-net@freebsd.org, aditya kiran <adityaa.kiran@gmail.com>
Subject: Re: PMTU Discovery support
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> As I understand it, it is not possible to detect upward changes in the path 
> MTU as there is no mechanism for a router to generate an error when a packet 
> is small enough to be accommodated by the MTU of the link to the next hop.  I 
> may be wrong though.

RFC1191 section 6.3.

http://www.ietf.org/rfc/rfc1191.txt

--lyndon