[128959] in North American Network Operators' Group
Re: IPv6 PMTUD and OS-X
daemon@ATHENA.MIT.EDU (bmanning@vacation.karoshi.com)
Sat Aug 21 03:19:35 2010
Date: Sat, 21 Aug 2010 07:18:57 +0000
From: bmanning@vacation.karoshi.com
To: Jeroen Massar <jeroen@unfix.org>
In-Reply-To: <4C6EF4DF.90709@unfix.org>
Cc: nanog@nanog.org
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
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
