On 6to4 gateway and recommended MTU setting

Martin List-Petersen martin at airwire.ie
Thu Mar 11 13:05:13 CET 2010


Your underlying network is the factor, that limits you and that's the
IPv4 network. Remember, 6to4 is tunneling, just like 6in4.

So your MTU has to be smaller than 1500. EVERYONE is running 1280, why
do you want to run something else and break things even further.

Beyond that, fine if you run a gateway, but I'd focus my efforts to get
people to go native and not use 6to4 or try to change 6to4. 6to4 is
defined, it (sort of) works as it is and it will die eventually.

Kind regards,
Martin List-Petersen

Martin Millnert wrote:
> 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.
> Cheers,
> Martin

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