IPv6 PMTUD and OS-X

Vyto Grigaliunas vyto at fnal.gov
Tue Aug 24 13:15:46 CDT 2010


>> 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? :)

Please correct me if I'm wrong, but last I heard IPv6 routers do not do
fragmentation...it's up to the IPv6 end hosts to properly determine the path
MTU.

Thanks...

Vyto




Date: Sat, 21 Aug 2010 11:42:04 +0200
From: Jeroen Massar <jeroen at unfix.org>
Subject: Re: IPv6 PMTUD and OS-X
To: bmanning at vacation.karoshi.com
Cc: nanog at nanog.org
Message-ID: <4C6F9F6C.80609 at unfix.org>
Content-Type: text/plain; charset=UTF-8

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? :)

Greets,
 Jeroen





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