MTU issues s0.wp.com
Matthew Luckie
mjl at caida.org
Tue Nov 6 15:46:43 UTC 2012
> 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)
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