netdev - Re: [Bugme-new] [Bug 15507] New: kernel misses 3rd part of tcp handshake (ACK), stays in SYN

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Message-ID: <1268953951.2894.194.camel@edumazet-laptop>
Date:	Fri, 19 Mar 2010 00:12:31 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	netdev@...r.kernel.org, bugzilla-daemon@...zilla.kernel.org,
	bugme-daemon@...zilla.kernel.org, roysjosh@...il.com
Subject: Re: [Bugme-new] [Bug 15507] New: kernel misses 3rd part of tcp
 handshake (ACK), stays in SYN_RECV state

Le jeudi 18 mars 2010 à 16:01 -0700, Andrew Morton a écrit :
> (switched to email.  Please respond via emailed reply-to-all, not via the
> bugzilla web interface).
> 
> On Wed, 10 Mar 2010 14:50:51 GMT
> bugzilla-daemon@...zilla.kernel.org wrote:
> 
> > http://bugzilla.kernel.org/show_bug.cgi?id=15507
> > 
> >            Summary: kernel misses 3rd part of tcp handshake (ACK), stays
> >                     in SYN_RECV state
> >            Product: Networking
> >            Version: 2.5
> >     Kernel Version: 2.6.32.9-67.fc12
> >           Platform: All
> >         OS/Version: Linux
> >               Tree: Fedora
> >             Status: NEW
> >           Severity: normal
> >           Priority: P1
> >          Component: IPV4
> >         AssignedTo: shemminger@...ux-foundation.org
> >         ReportedBy: roysjosh@...il.com
> >         Regression: Yes
> > 
> > 
> > Created an attachment (id=25450)
> >  --> (http://bugzilla.kernel.org/attachment.cgi?id=25450)
> > tcpdump from virtual guest over ::1 showing dup synacks
> > 
> > Setup:
> > 64-bit dual-core, latest F12.  (I've experienced this on .31 and .32 kernels.)
> > 
> > I have also experienced this on 2 other machines, all 64-bit.  One was a
> > single-core virtual guest.
> > 
> > 1. Install httpd.
> > 2. Start it.  Don't bother configuring anything unless you need to.
> > 3. tcpdump .....
> > 4. connect from localhost or (possibly) a nearby host.  I'm not sure if this is
> > timing related yet.
> > 5. note duplicate SYN-ACKs sent out at the exponential backoff.  Some setups
> > exhibit this more/worse than others, and will actually time out the connection.
> >  (Others will only send one or two duplicate SYN-ACKs, and then enter the
> > ESTABLISHED state.)  However, if any data is sent the kernel enters
> > ESTABLISHED.
> > 
> > Note that this isn't ipv4 specific.  I have seen it under ipv4 and ipv6 (via
> > loopback).
> > 
> 
> --

I would say this is expected if httpd server set DEFER_ACCEPT socket
option.



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