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From:   Neal Cardwell <ncardwell@google.com>
Date:   Fri, 31 Jan 2020 22:55:34 -0500
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Subject: Re: Re: [PATCH 2/3] tcp: Reduce SYN resend delay if a suspicous ACK
 is received
To:     SeongJae Park <sj38.park@gmail.com>
Cc:     Eric Dumazet <eric.dumazet@gmail.com>, sjpark@amazon.com,
        Eric Dumazet <edumazet@google.com>,
        David Miller <davem@davemloft.net>, shuah@kernel.org,
        Netdev <netdev@vger.kernel.org>, linux-kselftest@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>, aams@amazon.com,
        SeongJae Park <sjpark@amazon.de>,
        Yuchung Cheng <ycheng@google.com>
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On Fri, Jan 31, 2020 at 5:18 PM SeongJae Park <sj38.park@gmail.com> wrote:
>
> On Fri, 31 Jan 2020 17:11:35 -0500 Neal Cardwell <ncardwell@google.com> wrote:
>
> > On Fri, Jan 31, 2020 at 1:12 PM Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > >
> > >
> > >
> > > On 1/31/20 7:10 AM, Neal Cardwell wrote:
> > > > On Fri, Jan 31, 2020 at 7:25 AM <sjpark@amazon.com> wrote:
> > > >>
> > > >> From: SeongJae Park <sjpark@amazon.de>
> > > >>
> > > >> When closing a connection, the two acks that required to change closing
> > > >> socket's status to FIN_WAIT_2 and then TIME_WAIT could be processed in
> > > >> reverse order.  This is possible in RSS disabled environments such as a
> > > >> connection inside a host.
> [...]
> >
> > I looked into fixing this, but my quick reading of the Linux
> > tcp_rcv_state_process() code is that it should behave correctly and
> > that a connection in FIN_WAIT_1 that receives a FIN/ACK should move to
> > TIME_WAIT.
> >
> > SeongJae, do you happen to have a tcpdump trace of the problematic
> > sequence where the "process A" ends up in FIN_WAIT_2 when it should be
> > in TIME_WAIT?
>
> Hi Neal,
>
>
> Yes, I have.  You can get it from the previous discussion for this patchset
> (https://lore.kernel.org/bpf/20200129171403.3926-1-sjpark@amazon.com/).  As it
> also has a reproducer program and how I got the tcpdump trace, I believe you
> could get your own trace, too.  If you have any question or need help, feel
> free to let me know. :)

Great. Thank you for the pointer.

I had one quick question: in the message:
  https://lore.kernel.org/bpf/20200129171403.3926-1-sjpark@amazon.com/
... it showed a trace with the client sending a RST/ACK, but this
email thread shows a FIN/ACK. I am curious about the motivation for
the difference?

Anyway, thanks for the report, and thanks to Eric for further clarifying!

neal