google path mtu?

Brian E Carpenter brian.e.carpenter at
Mon Jan 19 23:25:37 CET 2015

On 20/01/2015 01:52, Artem Viklenko wrote:
> 19.01.2015 13:12, Mikael Abrahamsson написав(ла):
>> On Mon, 19 Jan 2015, Ignatios Souvatzis wrote:
>>> I had some problems at home, which is not tunneled, but a
>>> less-than-1500-octet-mtu PPPoE DSL line, so - thinking it's tunnels
>>> only ignores an increasing number of native private users.
>> Well, PPPoE is a tunnel, but I guess that's splitting hairs.
>> But regardless, I had trouble this morning with Youtube that might be
>> related to the same issue.
> One possible solution to force TCP MSS. For example on my FreeBSD
> router at home i used PF's scrub rule for ipv6 tunnel to set
> mss to 1220 bytes.
> If you use PPPoE as underlying technology, decrease this vale little
> bit more - 1212 or less.

netsh int ipv6 set int "12" mtu=1280
seems to help (or its equivalent for your preferred host o/s).
However, I've been seeing lots of problems with Cloudflare-mediated
sites recently and that does seem to be due to faulty MSS negotiation
at their end rather than plain PMTUD fails. Clues welcome...


More information about the ipv6-ops mailing list