On 6to4 gateway and recommended MTU setting

Martin List-Petersen martin at airwire.ie
Thu Mar 11 13:39:51 CET 2010

Martin Millnert wrote:
> On Thu, 2010-03-11 at 12:05 +0000, Martin List-Petersen wrote:
>> Hi,
>> 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.
> I've missed the memo saying that I should change it, if I run the
> gateway in Linux. :) I mean, that's why I'm asking. I'm missing a
> relevant section in a RFC stating that a global relay should be
> configured according to XYZ, due to ABC. Even after reading tens of RFCs
> multiple times, it can be hard to distill that XYZ.

I guess there has been no MTU specified in any RFC, because pMTU
discovery should be taking care of it. My point really is, that you
can't go beyond 1480 because you are tunneling over an unknown network,
where you have to assume that a lot of it is no higher than 1500, if not
less. The vast majority of end-users will be sitting behind a PPPoE or
PPPoA tunnel, so they have less than 1500.

1280 has sort of been adapted for 6in4 tunneling and thus most 6to4
implementations follow that same setup.

Kind regards,
Martin List-Petersen

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