Jeroen Massar jeroen at unfix.org
Sat Aug 21 09:42:04 UTC 2010

On 2010-08-21 09:18, bmanning at 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? :)


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