Interesting TCP issue
Julian Elischer
julian at elischer.org
Tue Jan 2 00:07:17 PST 2007
Steve Watt wrote:
> One of my users is having trouble receiving mail from Skype. So,
> after some sniffing, I discovered this:
>
> # tcpdump -vv -s 1500 -i dc0 -X net 213.244.128.0/18
> tcpdump: lestening on dc0, link-type EN10MB (Ethernet), capture size 1500 bytes
> 13:18:13.607493 IP (tos 0x20, ttl 58, id 12896, offset 0, flags [DF], proto: TCP (6), length: 74) share.skype.net.50406 > wattres.watt.com.smtp: P, cksum 0x9297 (correct), 4072464914:4072464936(22) ack 1248591103 win 46 <nop,nop,timestamp 2511885672 520058954>
> 0x0000: 4520 004a 3260 4000 3a06 c609 d5f4 aa50 E..J2`@.:......P
> 0x0010: 425d 8582 c4e6 0019 f2bc e212 4a6b fcff B]..........Jk..
> 0x0020: 8018 002e 9297 0000 0101 080a 95b8 5568 ..............Uh
> 0x0030: 1eff 784a 4548 4c4f 2073 6861 7265 2e73 ..xJEHLO.share.s
> 0x0040: 6b79 7065 2e6e 6574 0d0a kype.net..
> 0x0000: 4520 004a 3261 4000 3a06 c608 d5f4 aa50 E..J2a at .:......P
> 0x0010: 425d 8582 c4e6 0019 f2bc e212 4a6b fcff B]..........Jk..
> 0x0020: 8018 002e 1d67 0000 0101 080a 95b8 ca98 .....g..........
> 0x0030: 1eff 784a 4548 4c4f 2073 6861 7265 2e73 ..xJEHLO.share.s
> 0x0040: 6b79 7065 2e6e 6574 0d0a kype.net..
>
> And no responses from my system.
>
> Interesting. I presume it has something to do with the
> idiotically small window the remote server is advertising. So I
> set net.inet.tcp.minmss down to 46, and that resulted in a RST
> being spit back to skype's server when its retransmit happened.
> [...]
turn off window scaling (I forget the sysctl) and see if that helps
It's broken in some versions of freeBSD at least.
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