Re: [BEHAVE] Fwd: IPv6 hosts sending <1280 byte packets

[Iljitsch van Beijnum <[e-mail]>]

On 9 feb 2010, at 19:50, Dan Wing wrote:

>> Only in the case where the IPv6 host supports an MTU larger 
>> than 1500 but the translator doesn't, there is a possible 
>> optimization by having the translator rewrite the MSS to the 
>> maximum that the translator supports so the translator 
>> wouldn't have to send too bigs and there is less risk of 
>> PMTUD black holes. However, it's not clear to me that this 
>> case is worth the trouble.

> There are long-standing desires to MTUs larger than 1500
> with IPv6, under the theory that IPv6 Did Things Correctly.

You can send large packets with IPv4, too. Having a bigger than 1500 byte MTU causes much fewer problems than having a smaller than 1500 byte MTU for a number of reasons which are explained in http://tools.ietf.org/html/draft-van-beijnum-multi-mtu-02

However, if you have a 1500+ MTU on the IPv6 side (but the translator has 1500), the MSS doesn't fix the PMTUD black holes that are caused by the fact that that the IPv6 host has a 1480 path MTU seen from the IPv4 side while it advertising a larger one in the MSS.

If we want to avoid this particular black hole the solution is to rewrite the MSS in the translator to the minimum of the IPv6 host's MSS and the translator MTU on the IPv6 side + sizeof(IPv4) - sizeof(IPv6).

I don't remember the discussion about MSS rewriting in the softwires context, just that my conclusion was that that issue doesn't apply here. In any event, MSS clamping has been a staple of cheap NAT/PPPoE boxes and I don't believe it causes any problems.