netdev - bug in tcp?

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Message-Id: <E1HdZgM-0001n2-F0@highlab.com>
Date:	Mon, 16 Apr 2007 16:28:22 -0600
From:	Sebastian Kuzminsky <seb@...hlab.com>
To:	netdev@...r.kernel.org
Subject: bug in tcp?

I'm seeing some weird behavior in TCP.  The issue is perfectly
reproducible using netcat and other programs.  This is what I do:

    1.  Open a TCP connection over the loopback (over IPv4).

    2.  Send a couple of bytes of data each way.  No problems.

    3.  Wait about 120 hours with no writes on either side of the
        connection.

    4.  write() a few bytes to the server's socket.  I'd expect the data
        to go through, but it doesnt.  I see the TCP frame from the
        server to the client, but instead of an ACK, the client sends
        back a RST.  netstat shows the bytes sitting in the server's
        socket's send-buffer.

    5.  write a few bytes to the client's socket.  The server gets
        these immediately.

    6.  On the next server-to-client retransmit, the client gets the
        bytes from the server.  After this, the connection works normally.

The libpcap capture file is here (only shows steps 4-6):

    http://highlab.com/~seb/tcp-idleness-bug

The behavior is reproducible on all kernels I've tried: 2.4.32, 2.6.19.1,
and 2.6.20.4.  I dont think it's iptables-related, though I'm rerunning
the tests on a machine without iptables to be sure.  I'll have results
for you in 120 hours.  ;-)

-- 
Sebastian Kuzminsky
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