MTU/MSS testing IPv6

[Mikael Abrahamsson]

Hi,

I've run into a scenario where a website doesn't seem to be listening to 
PTB. I can reach them just fine from an MTU1500 clean IPv6 connection, but 
if I reach from a MTU1500<->MTU1480<->MTU1500 connection, it doesn't work. 
I don't get the big packets after SYN handshake.

I've been considering asking iis.se (the .SE ccTLD registry) who are 
already running multiple testing tools for web sites and domain name 
owners) to include these kinds of testing, and perhaps develop more of 
them.

So I'd like to gather some information and feedback here.

1. Are there are already FOSS tools out there that could be used for this, 
or would be good to enhance to include capability for this kind of 
testing. I don't want to waste work, and if I can enhance FOSS tools 
already existing and also solve my problem, that's a double win.

2. Test cases? From my testing, I've seen two different behavior just in 
the last two days:

Site A as described in top paragraph, probably doesn't listen to PTB. Can 
be either because they drop PTBs, or traffic traverses a load 
balancer/IPv4v6proxy that doesn't correctly handle PTB.

Site B which sends all data packets as fragments. This is most likely 
because they have some kind of AFTR where the IPv4 side has MTU1500 and 
the IPv6 side has MTU1320 or something like that.

Neither of this is of course optimal, and I'd like to be able to test for 
these and tell the site owner that their solution either is broken or 
suboptimal (the fragment case isn't strictly broken, it's just not a good 
way to do things).

Opinions? Thoughts?

-- 
Mikael Abrahamsson    email: swmike at swm.pp.se