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To: Jeroen Ruigrok/Asmodai <asmodai@wxs.nl>
Cc: Mike Tancsa <mike@sentex.net>, freebsd-net@freebsd.org
In-reply-to: asmodai's message of Wed, 08 Aug 2001 18:14:54 +0200.
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Subject: Re: gif MTU of 1280 ? 
From: Jun-ichiro itojun Hagino <itojun@iijlab.net>
Date: Thu, 09 Aug 2001 04:43:25 +0900
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>Actually I am wondering about it now myself.  X.25 is one of the few
>link layer protocols left which has a MTU < 1500 (aside from 802.3's
>1492).
>
>Maybe some IPv6 guru is able to shed some light?

	we picked 1280 because it is the IPv6 minimum link MTU, and
	would liked to torture-test IPv6 stack with gif (configured as
	IPv6-over-IPv4 tunnel).  for other usage, you may want to SIOCSIFMTU,
	or whatever.

	at this moment gif(4) does not keep track of the path MTU discovered
	for the outer header.  this is because, IP protocol (at least both
	v4/v6) does not cope well with L2 that changes MTU too frequently.
	(therefore, gif MTU should better be statically defined)

	as others noted, if you set gif MTU to 1500, then you will see a lot
	of fragmented packet exchanges (= poor performance).  if you use
	IPv4 as the outer header, MTU <= 1480 avoids fragmentation and
	more performant.  as 1280 <= 1480, we avoid fragmentation with the
	current default configuration.

itojun

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