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To: Steve Watt <steve@Watt.COM>
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Cc: hackers@freebsd.org
Subject: Re: Interesting TCP issue
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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@.:......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.