Some very nice broken IPv6 networks at Google and Akamai

Pim van Pelt pim at
Tue Nov 11 20:32:22 CET 2014


2014-11-11 11:13 GMT-08:00 Jeroen Massar <jeroen at>:
> As stated, the MSS clamping is just hiding the real problems. It does
> not properly resolve anything.
You are simply wrong about this statement. MSS clamping effectively
resolves issues with PMTUD by reducing its necessity in the first
place. I think I'm the ninth person to point that out in this thread?

The reason why operators resort to MSS clamping, is because they then
take end to end delivery reliability into their own hands, and have
more control over the flow of their data onto the internet. The "real
problems" you bring up, are almost impossible to address without
explicit cooperation from all affected parties - this is a method that
does not scale, and is not considered a winning strategy by operators
who wish to actually see their packets reach the intended recipient.

> If I had not commented about this problem,
> it would never have come to light... maybe in several years when nothing
> could have been done anymore. But today, we still can fix things.
While it's great that you noticed it, I think it's mendacious to claim
that if you had not commented on the problem, it would not have come
to light. Also, I'd like you to keep in mind that there can be some
significant wall clock time between noticing a problem and completing
its resolution, in a network or server deployment of sufficient size
this can take a while. So saying "they just did not notice it this
time around and thus it took a while for them to wake up (timezones :)
figure out what it is and fix the issue" carries little merit if you
don't actually know what happened, how it got noticed, or how it got

> Noting problems and properly fixing them are important.
but but but ... the problem *was* fixed, and whether you like it or
not, it was fixed by restoring the intended behavior of MSS clamping
in the affected Google servers after they had a regression.

It's fine if you want to practice pedantry, and I applaud your
persistence. But you must understand that ipv6-ops being network
engineers by trade will in general resort to "doing that, which
actually gets the job done".

Pim van Pelt <pim at>

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