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Message-ID: <1307714177.4044.4.camel@edumazet-laptop>
Date: Fri, 10 Jun 2011 15:56:17 +0200
From: Eric Dumazet <eric.dumazet@...il.com>
To: Charles Bearden <Charles.F.Bearden@....tmc.edu>
Cc: netdev@...r.kernel.org
Subject: Re: TCP keepalives ignored by kernel when the contain timestamps
Le jeudi 09 juin 2011 à 10:26 -0500, Charles Bearden a écrit :
> I have come across a case that looks like it might be a kernel bug. It appears
> that tcp keepalives sent by a remote system are ignored when they contain tcp
> timestamps, but are ACKed when they don't. When they are ignored, the remote
> system resets the connection after a number of retries.
>
> I have replicated this problem on both Ubuntu 10.04 with a 2.6.32-32-server
> kernel (x86_64) and CentOS 5.6 with a 2.6.18-238.12.1.el5 kernel. I'm sorry that
> I haven't had a chance to try to replicate the bug with a newer kernel, though a
> co-worker has looked through changelogs for more recent kernels and didn't find
> anything that looked relevant.
>
> From either of these hosts I run an application that connects to a remote host
> for 2-3 minutes, and that for most of that time sends no application data back
> and forth. After 30 seconds of no data from the Linux host, the remote host
> sends a garden variety keepalive. When the remote host includes tcp timestamps
> in the keepalives, they are ignored by the Linux host, and the remote host
> resets the connection after 10 unACKed keepalives. When timestamps are absent
> from the keepalives, the Linux host ACKs each one, and all is copacetic.
>
> Text output of a tcpdump trace of a connection that fails:
> http://pastebin.com/v6CpteJ9
>
> Text output of a tcpdump trace of a connection that succeeds:
> http://pastebin.com/KVLb3Mzh
>
> More details, in case you think they are relevant:
>
> My application creates a JDBC connection to a remote MS SQL Server and
> executes a statement that does not return a result set, and so it doesn't
> need to pass application data back and forth while it executes. The
> statement takes 2 or 3 minutes to complete. I connect to two different
> remote hosts: a Win2003 machine, and a Win2008R2 machine. The Win2003
> machine doesn't put timestamps in its keep-alives, so the application
> completes successfully when connecting to that host. If tcp timestamps
> are enabled on the Linux host, the Win2008 host includes them in its
> keepalives, and they are unACKed, so the connection is reset; if they
> are disabled on the Linux host, the Win2008 host doesn't include them in
> the keepalives, and the application completes successfully. I use (as
> you might expect) sysctl to disable tcp timestamps on the Linux hosts.
>
> I have dumps for all permutations of CentOS/Ubuntu, Win200[38], and +/-
> timestamps on the Linux side, and I will post them if the developers think that
> they would be useful.
Hi Charles
I could not reproduce the problem here, even using a quite old kernel as
receiver (2.6.9)
15:54:33.566192 IP 192.168.20.108.55926 > 192.168.20.124.777: SWE
479814493:479814493(0) win 14600 <mss 1460,sackOK,timestamp 151666
0,nop,wscale 7>
15:54:33.566265 IP 192.168.20.124.777 > 192.168.20.108.55926: S
3714869381:3714869381(0) ack 479814494 win 5792 <mss
1460,sackOK,timestamp 54553041 151666,nop,wscale 2>
15:54:33.566274 IP 192.168.20.108.55926 > 192.168.20.124.777: . ack 1
win 115 <nop,nop,timestamp 151666 54553041>
15:54:33.566281 IP 192.168.20.108.55926 > 192.168.20.124.777: P 1:5(4)
ack 1 win 115 <nop,nop,timestamp 151666 54553041>
15:54:33.566351 IP 192.168.20.124.777 > 192.168.20.108.55926: . ack 5
win 1448 <nop,nop,timestamp 54553041 151666>
15:54:33.566375 IP 192.168.20.124.777 > 192.168.20.108.55926: P 1:5(4)
ack 5 win 1448 <nop,nop,timestamp 54553041 151666>
15:54:33.566380 IP 192.168.20.108.55926 > 192.168.20.124.777: . ack 5
win 115 <nop,nop,timestamp 151666 54553041>
15:54:43.577945 IP 192.168.20.108.55926 > 192.168.20.124.777: . 4:5(1)
ack 5 win 115 <nop,nop,timestamp 152668 54553041>
15:54:43.578012 IP 192.168.20.124.777 > 192.168.20.108.55926: . ack 5
win 1448 <nop,nop,timestamp 54563053 152668,nop,nop,sack sack 1 {4:5} >
15:54:53.597946 IP 192.168.20.108.55926 > 192.168.20.124.777: . 4:5(1)
ack 5 win 115 <nop,nop,timestamp 153670 54563053>
15:54:53.598012 IP 192.168.20.124.777 > 192.168.20.108.55926: . ack 5
win 1448 <nop,nop,timestamp 54573073 153670,nop,nop,sack sack 1 {4:5} >
Are you sure frame tcp checksums are OK when the 'faulty' linux receive
them ? (tcpdump -v)
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