Date: Wed, 08 Aug 2001 12:15:10 -0400 From: Mike Tancsa <mike@sentex.net> To: Jeroen Ruigrok/Asmodai <asmodai@wxs.nl> Cc: freebsd-net@freebsd.org Subject: Re: gif MTU of 1280 ? Message-ID: <5.1.0.14.0.20010808121005.04473600@marble.sentex.ca> In-Reply-To: <20010808181454.Q2937@daemon.ninth-circle.org> References: <5.1.0.14.0.20010808101139.0277e010@marble.sentex.ca> <5.1.0.14.0.20010808101139.0277e010@marble.sentex.ca>
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Thanks for the clarification. I just had a read of the man pages as well
and there is mention of that too. I guess the question I am left with is
that can I safely set the MTU to 1500 if I am using it to tunnel IPV4
traffic only, and in another case, IPV4 and IPSEC traffic. When using 1280
in a strict tunnel mode, I have problems with large packets from certain
sites. Broken PMTU somewhere ? Not sure, but setting the MTU to 1500
seemed to fix it.
---Mike
At 06:14 PM 8/8/01 +0200, Jeroen Ruigrok/Asmodai wrote:
>-On [20010808 16:30], Mike Tancsa (mike@sentex.net) wrote:
> >
> >Just wondering, is there a reason why the MTU of the gif interface defaults
> >to 1280 ? Why not 1500 ?
>
>Per RFC2460:
>
>"IPv6 requires that every link in the internet have an MTU of 1280
>octets or greater. On any link that cannot convey a 1280-octet packet
>in one piece, link-specific fragmentation and reassembly must be
>provided at a layer below IPv6.
>
>Links that have a configurable MTU (for example, PPP links [RFC1661])
>must be configured to have an MTU of at least 1280 octets; it is
>recommended that they be configured with an MTU of 1500 octets or
>greater, to accommodate possible encapsulations (i.e., tunneling)
>without incurring IPv6-layer fragmentation."
>
>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?
>
>--
>Jeroen Ruigrok van der Werven/Asmodai asmodai@[wxs.nl|freebsd.org|xmach.org]
>Documentation nutter/C-rated Coder, finger asmodai@ninth-circle.dnsalias.net
>http://www.freebsd.org/doc/en_US.ISO8859-1/books/developers-handbook/
>Light-in-Darkness, lift me up from here...
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