option 212 for 6RD

[Sam Wilson]

On 17 Jan 2013, at 11:46, Lorenzo Colitti wrote:

> On Thu, Jan 17, 2013 at 7:01 PM, Sam Wilson <Sam.Wilson at ed.ac.uk>  
> wrote:
>>> No, there's another way to get around it: lower the LAN MTU so  
>>> the host lowers the MSS itself.
>>> [snip...]
>>
>> The solution is to mandate the same MTU on every link.  I'm not  
>> serious about that but it looks like the obvious direction that  
>> one side of this argument is going.
>
> Not saying this is a good outcome, bear in mind that's effectively  
> what we do in IPv4. In IPv4, PMTUD is basically useless and the  
> only tools we have are MSS rewriting and "sending smaller packets".  
> Pick some well-known content providers and you might see they don't  
> send you 1500-byte packets in IPv4 either.
>
> Hopefully we can do better than that in IPv6. Who knows.

In principle PMTUD is the more technically elegant way - finding out  
what's out there and adapting to it.  Mandating a fixed MTU is  
actually not simple since I guess you'd have to allow for tunnelling,  
extension headers and so on that might be added along the path and  
which would rob space from the data field.  You'd have to require a  
physical MTU larger than the logical MTU with enough headroom to  
allow for extra overhead, and *still* have a method of dealing with  
any overflows en route caused by, say, multiple layers of tunnelling.

It all gets a bit awkward.

Sam

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