On 6to4 gateway and recommended MTU setting

Bernhard Schmidt berni at birkenwald.de
Thu Mar 11 13:12:58 CET 2010

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.


More information about the ipv6-ops mailing list