netdev - Re: Per-connection tcp_retries2 and RFC 1122 compliance

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Date:	Tue, 03 Feb 2015 13:11:46 -0500
From:	John Eckersberg <jeckersb@...hat.com>
To:	Neal Cardwell <ncardwell@...gle.com>
Cc:	Netdev <netdev@...r.kernel.org>
Subject: Re: Per-connection tcp_retries2 and RFC 1122 compliance

Neal Cardwell <ncardwell@...gle.com> writes:
> I believe the functionality you are looking  for is the
> TCP_USER_TIMEOUT socket option:

I had tried that previously, and it did not help my case.  The reason
why is that I was using a downstream kernel (Fedora 21, 3.17.8 in this
case) and it was missing this commit that went into 3.18:

commit b248230c34970a6c1c17c591d63b464e8d2cfc33
Author: Yuchung Cheng <ycheng@...gle.com>
Date:   Mon Sep 29 13:20:38 2014 -0700

    tcp: abort orphan sockets stalling on zero window probes
    
    Currently we have two different policies for orphan sockets
    that repeatedly stall on zero window ACKs. If a socket gets
    a zero window ACK when it is transmitting data, the RTO is
    used to probe the window. The socket is aborted after roughly
    tcp_orphan_retries() retries (as in tcp_write_timeout()).
    
    But if the socket was idle when it received the zero window ACK,
    and later wants to send more data, we use the probe timer to
    probe the window. If the receiver always returns zero window ACKs,
    icsk_probes keeps getting reset in tcp_ack() and the orphan socket
    can stall forever until the system reaches the orphan limit (as
    commented in tcp_probe_timer()). This opens up a simple attack
    to create lots of hanging orphan sockets to burn the memory
    and the CPU, as demonstrated in the recent netdev post "TCP
    connection will hang in FIN_WAIT1 after closing if zero window is
    advertised." http://www.spinics.net/lists/netdev/msg296539.html
    
    This patch follows the design in RTO-based probe: we abort an orphan
    socket stalling on zero window when the probe timer reaches both
    the maximum backoff and the maximum RTO. For example, an 100ms RTT
    connection will timeout after roughly 153 seconds (0.3 + 0.6 +
    .... + 76.8) if the receiver keeps the window shut. If the orphan
    socket passes this check, but the system already has too many orphans
    (as in tcp_out_of_resources()), we still abort it but we'll also
    send an RST packet as the connection may still be active.
    
    In addition, we change TCP_USER_TIMEOUT to cover (life or dead)
    sockets stalled on zero-window probes. This changes the semantics
    of TCP_USER_TIMEOUT slightly because it previously only applies
    when the socket has pending transmission.

The key part being that last paragraph about stalled zero-window
probes.  Here's the specific use case where I'm hitting this:

(1) Establish a TCP connection bound to a given IP address
(2) Remove IP address from host
(3) Write to socket

This gets kicked back by the IP layer as non-routable, which triggers
the same behavior as the zero-window probes.

The good news is, I confirmed this is working as expected when I tested
on 3.19.0-rc7.

Thanks for the pointer, I'll go take my harassment to the relevant
downstream folks.
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