google path mtu?

Mikael Abrahamsson swmike at
Fri Jan 23 08:13:55 CET 2015

On Fri, 23 Jan 2015, Tore Anderson wrote:

> I highly doubt that Google would be sending you PTBs. 10 SEK says it's
> your tunnel ingress router (i.e., your Airport Express)...

I just took for granted that this would be the case because I've been told 
that google would be capping their communication at 1280. That's what I 
get for taking things for granted.

> But this could be found out, run "tcpdump -nvi eth0 'icmp6 and ip6[40]
> == 2'" in a terminal while reproducing the problem and see what's the
> source address of the PTBs?

08:10:06.394230 IP6 (hlim 64, next-header ICMPv6 (58) payload length: 1240) 2001:470:X:X::1 > 2001:470:28:35e:650d:12ec:4019:6a38: [icmp6 sum ok] ICMP6, packet too big, mtu 1280

You're correct. Wonder why I never noticed this before.

So I guess this explains why I sometimes saw SYN packets coming from my 
machine with MSS 1440 and sometimes with 1220, the 1440 was for 
destinations by which there was no PTB seen (yet), and 1220 for the ones 
where PTB had been seen.

Ok, thanks for helping clearing that up!

Mikael Abrahamsson    email: swmike at

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