On 6to4 gateway and recommended MTU setting

Martin Millnert martin at millnert.se
Thu Mar 11 12:40:17 CET 2010

Hello list,

was discussing best 6to4-relay interface MTU size with a friend and
basically need more and wiser input.

Apparently, Free BSD has a unmodifiable, static value of 1280 for the
6in4-interface. It could be argued that this follows 4213 3.2.1
( http://tools.ietf.org/html/rfc4213#section-3.2.1 ). OSX also has a
static value of 1280, unmodifiable (I try to stay away from OSX, could
this be due to the kernel being the same?). XP reportedly also defaults
to 1280, and I see my Windows 7-box does the same for 6to4, which
appears once I deactivate native IPv6 on my LAN interface  (netsh
interface ipv6 show subinterfaces).

Linux on the other hand defaults to 1480, but it is changeable. 

A working PMTUd will resolve MTU problems, both on underlying IPv4 node
<-> 6to4-gateway, but this isn't the issue at the moment.

The issue is what MTU value a single random 6to4-gateway today should
have (purposefully not considering "the whole"), to maximize good
interoperability and minimize problems.

I'm not really concerned with the values of a end-node's 6to4 interface.

I am not satisfied by the reasoning in 4213#3.2.1 in this regard, hence
this mail. 3.2.1 to me doesn't seem to talk about a the MTU value of the
interface on a globally announced relay gateway.

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