From owner-freebsd-net@FreeBSD.ORG  Wed Dec  8 20:50:19 2004
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Date: Wed, 08 Dec 2004 12:49:52 -0800
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From: gnn@freebsd.org
To: "Konstantin KABASSANOV" <Konstantin.Kabassanov@lip6.fr>
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References: <003101c4dd20$8ab9f070$8748e384@ipv6.lip6.fr>
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cc: net@freebsd.org
Subject: 
	Re: the correct ipv6 behavior for interfaces with gif tunnel on them
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At Wed, 8 Dec 2004 13:22:10 +0100,
Konstantin KABASSANOV wrote:
> 
> [1  <text/plain; us-ascii (7bit)>]
> Hello,
> 
> I have a freebsd box with a gif tunnel configured. I observed that even if
> ifconfig displays MTU 1500 for both gif and physical interface, 1300 bytes
> ipv6 packets transiting on the gif interface are systematically fragmented
> while transiting on the lower physical interface to 1280 bytes. Is it the
> normal behavior?

The default MTU of IPv6 is 1280 bytes.  From RFC 2460:

 It is strongly recommended that IPv6 nodes implement Path MTU
   Discovery [RFC-1981], in order to discover and take advantage of path
   MTUs greater than 1280 octets.  However, a minimal IPv6
   implementation (e.g., in a boot ROM) may simply restrict itself to
   sending packets no larger than 1280 octets, and omit implementation
   of Path MTU Discovery.

If the connection over the tunnel is not use Path MTU Discovery then
this number may not change.

Later,
George