Date: Thu, 29 Mar 2012 16:29:51 +0300 From: Sergey Smitienko <hunter@comsys.com.ua> To: freebsd-net@freebsd.org Subject: FreeBSD 9.0 generates incorrect SEC/ACK numbers under load Message-ID: <4F7463CF.8010206@comsys.com.ua>
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Hello.
I've run into a problem with a web server runing FreeBSD 9.0/amd64. What
I believe is happening, is what server loses track of correct SEQ/ACK
numbers
on some connections. Here is an example:
15:20:00.347514 IP (tos 0x68, ttl 123, id 1181, offset 0, flags [DF],
proto TCP (6), length 52)
93.72.14.220.49239 > 193.178.147.113.80: Flags [S], cksum 0x6995
(correct), seq 3881466934, win 8192, options [mss 1460,nop,wscale
2,nop,nop,sackOK], length 0
15:20:00.347526 IP (tos 0x10, ttl 254, id 28065, offset 0, flags [DF],
proto TCP (6), length 44)
193.178.147.113.80 > 93.72.14.220.49239: Flags [S.], cksum 0x79fa
(correct), seq 2151790680, ack 3881466935, win 0, options [mss 1460],
length 0
15:20:00.361812 IP (tos 0x68, ttl 123, id 1183, offset 0, flags [DF],
proto TCP (6), length 40)
93.72.14.220.49239 > 193.178.147.113.80: Flags [.], cksum 0x96c6
(correct), seq 3881466935, ack 2151790681, win 64240, length 0
15:20:00.361869 IP (tos 0x10, ttl 254, id 31305, offset 0, flags [DF],
proto TCP (6), length 40)
193.178.147.113.80 > 93.72.14.220.49239: Flags [.], cksum 0x71b7
(correct), seq 2151790681, ack 3881466935, win 8192, length 0
Client sends "GET" request
15:20:48.236181 IP (tos 0x68, ttl 123, id 1353, offset 0, flags [DF],
proto TCP (6), length 626)
93.72.14.220.49239 > 193.178.147.113.80: Flags [P.], cksum 0x7fc9
(correct), seq 3881466935:3881467521, ack 2151790681, win 64240, length 586
and then the "ping-pong" starts:
15:20:48.236198 IP (tos 0x0, ttl 254, id 63530, offset 0, flags [DF],
proto TCP (6), length 40)
193.178.147.113.80 > 93.72.14.220.49239: Flags [.], cksum 0x8a97
(correct), seq 2991748588, ack 1985077892, win 8760, length 0
15:20:48.255998 IP (tos 0x68, ttl 123, id 1357, offset 0, flags [DF],
proto TCP (6), length 40)
93.72.14.220.49239 > 193.178.147.113.80: Flags [.], cksum 0x947c
(correct), seq 3881467521, ack 2151790681, win 64240, length 0
15:20:48.256015 IP (tos 0x0, ttl 254, id 53518, offset 0, flags [DF],
proto TCP (6), length 40)
193.178.147.113.80 > 93.72.14.220.49239: Flags [.], cksum 0x8a97
(correct), seq 2991748588, ack 1985077892, win 8760, length 0
15:20:48.276084 IP (tos 0x68, ttl 123, id 1360, offset 0, flags [DF],
proto TCP (6), length 40)
93.72.14.220.49239 > 193.178.147.113.80: Flags [.], cksum 0x947c
(correct), seq 3881467521, ack 2151790681, win 64240, length 0
15:20:48.276099 IP (tos 0x0, ttl 254, id 42983, offset 0, flags [DF],
proto TCP (6), length 40)
193.178.147.113.80 > 93.72.14.220.49239: Flags [.], cksum 0x8a97
(correct), seq 2991748588, ack 1985077892, win 8760, length 0
15:20:48.290914 IP (tos 0x68, ttl 123, id 1361, offset 0, flags [DF],
proto TCP (6), length 40)
93.72.14.220.49239 > 193.178.147.113.80: Flags [.], cksum 0x947c
(correct), seq 3881467521, ack 2151790681, win 64240, length 0
This happens on about 0.01% of connections. This tcpdump is recorded on
the 193.178.147.113, before traffic hits the wire.
So it's not a NIC fault. Server is running nginx and serving static
content 200-500 request per second.
Any ideas ?
--
Sergey Smitienko
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