google path mtu?

Mikael Abrahamsson swmike at
Wed Jan 21 15:02:23 CET 2015

On Wed, 21 Jan 2015, Ignatios Souvatzis wrote:

> Hi,
> On Tue, Jan 20, 2015 at 03:40:23PM +0100, Marco d'Itri wrote:
>> On Jan 20, Mikael Abrahamsson <swmike at> wrote:
>>> I turned off my IPv6 tunnel yesterday because family was complaining
>>> about Youtube not working. I haven't enabled it again. Other people who had
>>> problems, are things working again now?
>> Yes for me (2a00:1450:400c:c04::93).
> For me (2001:638:403::), Google servers worked again yesterday,
> but this was apparently implemented by our address ranges being
> put onto Google's AAAA blacklist.
> Today it seems to work with AAAA again.

I switched my HE tunnel back on again today and everything seems to work 
fine. As a data point, my "fullsize" packets from Youtube are IPv6 packets 
with length 1408 (as reported by tcpdump in OSX).

Flags [S.], seq 825239677, ack 2182453102, win 28160, options [mss 1420,sackOK,TS val 1317228847 ecr 493983801,nop,wscale 7], length 0

The weird thing is that when I checked it again later, now both my machine 
and googles machine are saying MSS 1220 and I'm seeing packets that are 
1208. Some examples:

13:48:01.810082 IP6 2001:470:XXX.63869 > 2001:7f8:49:1::c.443: Flags [SEW], seq 4278620568, win 65535, options [mss 1220,nop,wscale 5,nop,nop,TS val 494398939 ecr 0,sackOK,eol], length 0
13:48:01.888071 IP6 2001:7f8:49:1::c.443 > 2001:470:XXXX.63869: Flags [S.], seq 1292334177, ack 4278620569, win 28160, options [mss 1420,sackOK,TS val 1317649207 ecr 494398939,nop,wscale 7], length 0

Then I get:

13:50:28.876356 IP6 2001:470:XX.63894 > 2001:7f8:49:1::c.443: Flags [SEW], seq 2121307480, win 65535, options [mss 1220,nop,wscale 5,nop,nop,TS val 494544092 ecr 0,sackOK,eol], length 0
13:50:28.970365 IP6 2001:7f8:49:1::c.443 > 2001:470:X.63894: Flags [S.], seq 2118432000, ack 2121307481, win 28560, options [mss 1440,sackOK,TS val 1317796289 ecr 494544092,nop,wscale 7], length 0

Notice the difference in MSS between those two machines that claim to have 
the same IPv6 address?

$ dig +short AAAA

Mikael Abrahamsson    email: swmike at

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