google path mtu?

Andras Toth diosbejgli at
Sat Jan 24 11:59:43 CET 2015

Airport Express is setting the IPv6 Tunnel MTU to 1280 in all cases and
it's not configurable, as far as I'm aware.


On Fri, Jan 23, 2015 at 6:56 AM, Mikael Abrahamsson <swmike at>

> On Fri, 23 Jan 2015, Lorenzo Colitti wrote:
>  Thanks for reporting this. What was the effect? Connections just getting
>> stuck? Was there fallback to IPv4?
> I would get all the CSS, comments etc, the player would start to try to
> load the video stream, but there would be no video loaded at all (ie the
> progress bar never moved).
>  Personally I don't understand why everyone behind a manually-configured
>> tunnel doesn't set the MTU in the RA to the MTU of the tunnel... but
>> that's
>> not an excuse for things not working.
> I did that before, but I can't do it in my Airport Express. I have been
> considering changing my IPv6 tunneling to another device.
> However, I have no clue why my OSX box thought it couldn't send 1350 byte
> sized UDP packets to the Internet. Could Google have sent me PTB=1280 from
> this host, so my OSX box thinks it can't send them unfragmented?
> Anyone know how I can check in OSX for what destinations it has received
> PTB packets and what the PMTU it think it has for these destionations?
> --
> Mikael Abrahamsson    email: swmike at
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