Geoff Huston gih at
Mon Feb 2 22:19:08 CET 2009

1480 still appears to be a little too high if you want to be  
conservative. A teredo packet requires a 40 octet header, not 20  
becuase of the UDP packet.

Google offer an initial TCP mss of 1212 (which corresponds to an IPv6  
packet of 1272 octects - I'm not sure why they left 'headroom' of 8  
octets here - maybe some TCP option? Or allowing for an MPLS header?  
Or ...?

I've seen 1300 octets and 1400 octets being used.

but I'd offer the view that 1480 is pushing it close.

But perhaps there is another way of looking at this. Ricardo, why do  
you want to keep the MTU as large as you can? What issues do you think  
you will encounter for your web pages with a MTU of, say, 1300 octets,  
as compared to using 1480 octets?


On 03/02/2009, at 8:06 AM, Ricardo Patara wrote:

> Hello Erik,
> Actually I tried to send an email to point of contact associated  
> with the IP block :)
> Then I search for some google folks email address with no success.  
> Thanks for your promptly answer and attention on this.
> In our case, in order to test if the MTU was the problem, we lowered  
> it to 1480. But maybe it should be something around the number you  
> mentioned.
> Just for curiosity, did google tried to contact upstream providers  
> or peering to find out if there was any icmp6 filtering in some of  
> the paths?
> thanks again.
> --
>  Ricardo Patara
> Em 02/02/2009, às 16:59, Erik Kline escreveu:
>> You tried to contact us?  I must have missed that email, sorry.
>> We set the MTU to 1280 near the server.  We've not had any reported
>> MTU problems (knock on wood), to the best of my knowledge.
>> FWIW:
>> 2009/2/2 Ricardo Patara <patara at>:
>>> Hi,
>>> Just wondering if some of you are using this technique to avoid  
>>> problems
>>> from someone else accessing your website, for instance.
>>> We have our site with ipv6 for a while now, and have received  
>>> complaints and
>>> we had opportunity to face the same problem several times.
>>> This happens when trying to access the site via a tunnel  
>>> connection. The
>>> browser simply gives a timeout.
>>> We think this is related to pmtu and sites filtering icmp6.
>>> One of the solutions was to lower down the MTU at the site server.
>>> The question is, if this is the solution you did in case you also  
>>> faced this
>>> problem.
>>> I tried to contact someone from google, but with no success.
>>> I mention google as we always try when having this  
>>> type of
>>> problem just to make sure there is nothing "completely" wrong the  
>>> network
>>> setup.
>>> thanks
>>> --
>>> Ricardo Patara

More information about the ipv6-ops mailing list