Iljitsch van Beijnum iljitsch at muada.com
Fri Aug 25 15:19:31 CEST 2006

On 25-aug-2006, at 14:52, Nick Hilliard wrote:

>> PMTUD problems aren't caused by any particular MTU, but by the source
>> host not seeing "packet too big" messages. If you remove this router
>> and use one that allows for a bigger MTU the problem will simply
>> resurface once packets hit another link with a smaller MTU.

> This is true, but it's also true that having links with MTU < 1500  
> tends
> to cause trouble.  Of course, in a perfect world, you could run at any
> MTU you cared for, but we don't live in that world.

I would say that's true for IPv4, where having links with an MTU  
smaller than 1500 bytes causes so much trouble that you can't  
realistically do anything other than ignore the DF bit or have a 1500  
byte or larger MTU. But in IPv6, you can't clear the DF bit and there  
is still a lot tunneling going on, so both of those aren't options,  
and we have a chance to get it right so we should really kick people  
in the sensitive parts until they get their PMTUD working. It's not  
like getting PMTUD to work is so incredibly hard...

There is one other option that may work to some degree in IPv6:  
rewriting the TCP MSS option. But I'm not aware of any gear that  
supports this for IPv6.

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