netdev - Re: TCP connection will hang in FIN_WAIT1 after closing if zero window is advertised

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Date:	Tue, 16 Sep 2014 12:31:42 -0400
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Yuchung Cheng <ycheng@...gle.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Andrey Dmitrov <andrey.dmitrov@...etlabs.ru>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	netdev <netdev@...r.kernel.org>,
	"Alexandra N. Kossovsky" <Alexandra.Kossovsky@...etlabs.ru>,
	Konstantin Ushakov <kostik@...etlabs.ru>
Subject: Re: TCP connection will hang in FIN_WAIT1 after closing if zero
 window is advertised

On Tue, Sep 16, 2014 at 11:11 AM, Yuchung Cheng <ycheng@...gle.com> wrote:
> On Tue, Sep 16, 2014 at 6:09 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> Normally SO_LINGER could be used, or TCP_USER_TIMEOUT. This requires a
>> system call before doing the close().
>>
>> 1) TCP_USER_TIMEOUT would be the fit for this, but its current
>> implementation do not take care of the probes sent, even in FIN_WAIT
>> state when in this zero window mode. A patch would be needed.
> Yes that's what I meant. I am proposing we should patch
> TCP_USER_TIMEOUT to do this.

We should probably be careful here. It would be a non-trivial change
in semantics to have TCP_USER_TIMEOUT solve this issue.

TCP_USER_TIMEOUT, in both the man page, the RFC, and the existing
code, is about a user-specific limit on the maximum amount of time the
TCP stack will attempt to transmit a single packet. (For example, the
man page: "specifies the maximum amount of time in milliseconds that
transmitted data may remain unacknowledged before TCP will forcibly
close the corresponding connection"). Any existing apps that are
setting TCP_USER_TIMEOUT are probably setting it to something in the
range of seconds to a few minutes, and may reasonably expect their
orphaned connections to last minutes to hours, as long as they are
making progress (each of the packets is ACKed in the
seconds-to-minutes range).

By contrast, AFAICT what we are talking about here for these
ZWP-forever/tarpit scenarios is to be able to cap the maximum overall
lifetime of an orphan connection. That's a different parameter, and
folks might want to set it in the minutes-to-hours range.

neal
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