owens at nysernet
Apr 17, 2012, 5:05 PM
Post #4 of 5
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On Tue, Apr 17, 2012 at 05:24:35PM -0400, Ryan Rawdon wrote:
> From a host behind a 6in4 tunnel a request to www.comcast6.net hangs. It looks like a PMTU brokenness issue since the connection is established and the request is sent successfully.
If you have access to an ipfw-based machine (OSX definitely counts, other BSDs as well, but not Linux) then the definitive way to test is with scamper:
[cookiemonster:~] owens% sudo /usr/local/bin/scamper -F ipfw -I "tbit -M 1280 -u 'http://www.comcast6.g.comcast.net' 2001:558:1002:4:68:87:29:36"
tbit from 2620:f:1:1201:21b:63ff:fea4:4d92 to 2001:558:1002:4:68:87:29:36
server-mss 1440, result: pmtud-success
app: http, url: http://www.comcast6.g.comcast.net/
[ 0.050] TX SYN 64 seq = 0:0
[ 0.073] RX SYN/ACK 64 seq = 0:1
[ 0.100] TX 60 seq = 1:1
[ 0.150] TX 245 seq = 1:1(185)
[ 0.174] RX 60 seq = 1:186
[ 0.174] RX 1500 seq = 1:186(1440)
[ 0.174] RX 1500 seq = 1441:186(1440)
[ 0.200] TX PTB 1280 mtu = 1280
[ 0.223] RX 1280 seq = 1:186(1220)
[ 0.223] RX 1280 seq = 1221:186(1220)
[ 0.250] TX 60 seq = 186:1221
[ 0.273] RX 500 seq = 2441:186(440)
[ 0.273] RX 1280 seq = 2881:186(1220)
[ 0.300] TX FIN 60 seq = 186:2441
[ 0.323] RX 60 seq = 4101:187
[ 0.323] RX 1280 seq = 4101:187(1220)
[ 0.323] RX 500 seq = 5321:187(440)
[ 0.350] TX FIN 60 seq = 186:2881
[ 0.374] RX 1280 seq = 5761:187(1220)
[ 0.374] RX 1280 seq = 6981:187(1220)
[ 0.399] TX FIN 60 seq = 186:4101
[ 0.423] RX 500 seq = 8201:187(440)
[ 0.450] TX 60 seq = 187:5321
[ 0.474] RX 1280 seq = 8641:187(1220)
[ 0.474] RX 1280 seq = 9861:187(1220)
[ 0.474] RX 500 seq = 11081:187(440)
[ 0.500] TX 60 seq = 187:5761
[ 0.550] TX 60 seq = 187:6981
[ 0.573] RX 1280 seq = 11521:187(1220)
[ 0.573] RX 1280 seq = 12741:187(1220)
[ 0.574] RX 280 seq = 13961:187(220)
[ 0.600] TX 60 seq = 187:8201
[ 0.623] RX 280 seq = 14181:187(220)
[ 0.623] RX FIN 889 seq = 14401:187(829)
[ 0.650] TX 60 seq = 187:8641
[ 0.700] TX 60 seq = 187:9861
[ 0.750] TX 60 seq = 187:11081
[ 0.800] TX 60 seq = 187:11521
[ 0.850] TX 60 seq = 187:12741
[ 0.900] TX 60 seq = 187:13961
[ 0.950] TX 60 seq = 187:14181
[ 1.000] TX 60 seq = 187:14401
Here's the odd thing though - the first time I tested, I saw the response above. Successive tests have resulted in 'pmtud-toosmall', which indicates that the server didn't send any individual packets above 1280 bytes. I wonder if there are more than one server behind a balancer, and somewhat different behavior?
Bill.
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