IPv6 PMTU issues to www.citrix.com

[Bill Owens]

On Fri, Jul 06, 2012 at 11:23:52AM +0200, Gert Doering wrote:
> 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".

Based on the name, I think you're right about the load balancer:
www.citrix.com.     28800   IN  CNAME   www.gslb.citrix.com.

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

They block PMTUD and they're trying to clamp the MSS to avoid MTU problems, but they aren't trying hard enough:

[cookiemonster:~/Projects/IPv6] owens% sudo /usr/local/bin/scamper -F ipfw -I "tbit -M 1280 -u 'http://www.citrix.com/lang/English/home.asp' 2001:4868:10c:3::15"
tbit from 2620:f:1:1201:21b:63ff:fea4:4d92 to 2001:4868:10c:3::15
 server-mss 1360, result: pmtud-fail
 app: http, url: http://www.citrix.com/lang/English/home.asp
 [  0.049] TX SYN             64  seq = 0:0             
 [  0.105] RX SYN/ACK         64  seq = 0:1             
 [  0.105] TX                 60  seq = 1:1             
 [  0.149] TX                254  seq = 1:1(194)        
 [  0.209] RX               1420  seq = 1:195(1360)     
 [  0.209] TX PTB           1280  mtu = 1280
 [  0.209] RX               1420  seq = 1361:195(1360)  
 [  0.209] RX               1420  seq = 2721:195(1360)  
 [  0.209] RX               1420  seq = 4081:195(1360)  
 [  0.209] RX               1420  seq = 5441:195(1360)  
 [  0.209] RX               1420  seq = 6801:195(1360)  
 [  0.773] RX               1420  seq = 1:195(1360)     
 [  0.773] TX PTB           1280  mtu = 1280
 [  2.210] RX               1420  seq = 1:195(1360)     
 [  2.210] TX PTB           1280  mtu = 1280
 [  5.038] RX               1420  seq = 1:195(1360)     
 [  5.038] TX PTB           1280  mtu = 1280
 [ 10.616] RX               1420  seq = 1:195(1360)     

MSS of 1220 would work, 1420 won't.

And no, I don't have any contacts there. . .

Bill.