IPv6 PMTUD and OS-X

Sat Aug 21 07:18:57 UTC 2010

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.


--bill