On 6to4 gateway and recommended MTU setting

[Bernhard Schmidt]

On 11.03.2010 12:40, Martin Millnert wrote:

Hi Martin,

> 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.

I suggest 1280 as well. RFC 3056 Section 4 (and most of the 6to4 proto41 
traffic I see on the network) suggests that the DF bit is not to be set. 
Which means you won't have pMTU discovery, but slow and painful 
fragmentation in transit.

Bernhard