Re: the correct ipv6 behavior for interfaces with gif tunnel on them

gnn_at_freebsd.org
Date: 12/08/04

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    Date: Wed, 08 Dec 2004 12:49:52 -0800
To: "Konstantin KABASSANOV" <Konstantin.Kabassanov@lip6.fr>

    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
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