IPv6 PMTU issues to www.citrix.com

Gert Doering gert at space.net
Fri Jul 6 11:23:52 CEST 2012


someone from Citrix here, or with contacts to their networking folks?

We're getting complaints from one of our (tunneled) customers that 
IPv6 connects to 2001:4868:10c:3::15 sometimes (!) return no data
after the TCP handshake succeeds - and sometimes it works.

The client's IPv6 MTU is 1400, and forcing MSS to 1340 makes it work in 
every case, so this looks like "load balanced to different servers, some 
of them have working PMTUd, some not".

>From a different test host with an IPv6 MTU of 1492, it worked all
the time - so I checked with tcpdump, and it seems the servers always
send out with a somewhat smaller segment size anyway:

11:20:47.364247 IP6 2001:4868:10c:3::15.80 > 2001:608:2:2::250.55317: . 23121:24481(1360) ack 40 win 64410

(client asked for MSS 1440, server never sends anything bigger than 1360,
but that's still too big for some cases)

so maybe they are aware that their PTMU is not working fully reliable...

Gert Doering
        -- NetMaster
have you enabled IPv6 on something today...?

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