Re: IPv6 PMTUD and OS-X

[Jeroen Massar <jeroen () unfix org>]

On 2010-08-21 09:18, bmanning () vacation karoshi com wrote:
> On Fri, Aug 20, 2010 at 11:34:23PM +0200, Jeroen Massar wrote:
> > 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
>
>
>       or - if you are tunneled more than once, you might be ultra conservative
>       and drop your MTU to 1220 - that should weed out the edge cases where even 1280
>       is too large.

1220? I am pretty sure the minimal IPv6 MTU is 1280 and that below it
fragmentation should be handled by the medium that transports packets
smaller than that.... Can you enlighten me Bill? :)

Greets,
 Jeroen