Interesting TCP issue

[Steve Watt]

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.

13:20:13.610975 IP (tos 0x20, ttl  58, id 12897, offset 0, flags [DF], proto: TCP (6), length: 74) share.skype.net.50406 > wattres.watt.com.smtp: P, cksum 0x1d67 (correct), 0:22(22) ack 1 win 46 <nop,nop,timestamp 2511915672 520058954>
13:20:13.611036 IP (tos 0x20, ttl  64, id 39789, offset 0, flags [DF], proto: TCP (6), length: 40) wattres.watt.com.smtp > share.skype.net.50406: R, cksum 0x5b51 (correct), 1248591103:1248591103(0) win 0
        0x0000:  4520 0028 9b6d 4000 4006 571e 425d 8582  E..(.m at .@.W.B]..
        0x0010:  d5f4 aa50 0019 c4e6 4a6b fcff 0000 0000  ...P....Jk......
        0x0020:  5004 0000 5b51 0000                      P...[Q..

The packets shown here are the complete capture.

I can hand-type stuff to the skype server and it appears to get
through just fine, but when it connects to my box, no segments make
their way back to skype.  I can see them sitting in the Send-Q:
# netstat -f inet
Active Internet connections
Proto Recv-Q Send-Q  Local Address          Foreign Address        (state)
[ ... ]
tcp4       0    213  wattres.smtp           share.skype.net.50406  ESTABLISHED
[ ... ]

Very strange.  Any opinions on possible workarounds, since I'm not
expecting Skype to fix their (broken) system?

-- 
Steve Watt KD6GGD  PP-ASEL-IA          ICBM: 121W 56' 57.5" / 37N 20' 15.3"
 Internet: steve @ Watt.COM                      Whois: SW32-ARIN
   Free time?  There's no such thing.  It just comes in varying prices...