IPv6 PMTUD and OS-X

Fri Aug 20 21:34:23 UTC 2010

On 2010-08-20 23:27, Franck Martin wrote:
> I'm trying to debug a pesky PMTUD issue with IPv6 on Mac OS-X 10.6. 
> 
> It happens only from home, on wireless, when connected to a mac aiport
> that does an automatic tunnel (teredo) to IPv6 backbone.

Welcome to the great world of Teredo/6to4 where the endpoints/relays of
the tunnel are anycasted in both IPv4 and IPv6 and thus can be quite
difficult to debug, it can be done but requires quite a lot of vision in
the network on both IPv4 and which will be generally near impossible.

> There are IPv6 web site that I cannot browse until I lower the MTU to
1400.

Why don't you just do 1280 which is the default?

Do also note that you have two levels of PMTU, the IPv6 one and the IPv4
one. If you configure your MTU of the tunnel incorrectly compared to the
relay that you are using you will not see the PMTU's coming through
either or they might not accept your large packets.

Both MTUs can be broken due to folks filtering ICMP which is generally a
bad thing to do.

Greets,
 Jeroen