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MTU issues s0.wp.com



> Since about a week or so it's become impossible to reach wp.com content 
> over IPv6.
>
> IPv4 content does work fine, using the IPv6 literal returns a 404 which 
> is small enough to fit in a smaller 1480 byte MTU.
>
> I have another test site that has a clean 1500 byte mtu and I can fetch 
> the s0.wp.com page from there.
>
> It looks like tunneled IPv6 users might be in hurt here.

Yes, a middlebox is discarding PTB messages before they reach the webserver:

$ scamper -F ipfw -I "tbit -u 'http://s0.wp.com/' 2606:2800:220:1c85:99b:1b56:207e:ea5"
tbit from 2001:48d0:101:501:d267:e5ff:fe14:a701 to 2606:2800:220:1c85:99b:1b56:207e:ea5
 server-mss 1440, result: pmtud-fail
 app: http, url: http://s0.wp.com/
 [  0.048] TX SYN             64  seq = 0:0             
 [  0.061] RX SYN/ACK         64  seq = 0:1             
 [  0.099] TX                 60  seq = 1:1             
 [  0.148] TX                228  seq = 1:1(168)        
 [  0.161] RX                 60  seq = 1:169           
 [  0.162] RX               1500  seq = 1:169(1440)     
 [  0.162] RX               1500  seq = 1441:169(1440)  
 [  0.162] RX               1500  seq = 2881:169(1440)  
 [  0.162] RX               1500  seq = 4321:169(1440)  
 [  0.162] RX               1500  seq = 5761:169(1440)  
 [  0.162] RX                914  seq = 7201:169(854)   
 [  0.198] TX PTB           1280  mtu = 1280
 [  0.248] TX                 60  seq = 169:1           
 [  3.173] RX               1500  seq = 1:169(1440)     
 [  3.173] TX PTB           1280  mtu = 1280
 [  6.044] RX FIN             60  seq = 8055:169        
 [  6.044] TX                 60  seq = 169:1           
 [  9.183] RX               1500  seq = 1:169(1440)     
 [  9.183] TX PTB           1280  mtu = 1280
 [ 21.204] RX               1500  seq = 1:169(1440)     
 [ 21.204] TX PTB           1280  mtu = 1280
 [ 45.254] RX               1500  seq = 1:169(1440)