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From:   Neal Cardwell <ncardwell@google.com>
Date:   Fri, 31 Jan 2020 10:10:03 -0500
Message-ID: <CADVnQyk9xevY0kA9Sm9S9MOBNvcuiY+7YGBtGuoue+r+eizyOA@mail.gmail.com>
Subject: Re: [PATCH 2/3] tcp: Reduce SYN resend delay if a suspicous ACK is received
To:     sjpark@amazon.com
Cc:     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>, sj38.park@gmail.com,
        aams@amazon.com, SeongJae Park <sjpark@amazon.de>,
        Yuchung Cheng <ycheng@google.com>
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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.
>
> For example, expected state transitions and required packets for the
> disconnection will be similar to below flow.
>
>          00 (Process A)                         (Process B)
>          01 ESTABLISHED                         ESTABLISHED
>          02 close()
>          03 FIN_WAIT_1
>          04             ---FIN-->
>          05                                     CLOSE_WAIT
>          06             <--ACK---
>          07 FIN_WAIT_2
>          08             <--FIN/ACK---
>          09 TIME_WAIT
>          10             ---ACK-->
>          11                                     LAST_ACK
>          12 CLOSED                              CLOSED

AFAICT this sequence is not quite what would happen, and that it would
be different starting in line 8, and would unfold as follows:

          08                                     close()
          09                                     LAST_ACK
          10             <--FIN/ACK---
          11 TIME_WAIT
          12             ---ACK-->
          13 CLOSED                              CLOSED


> The acks in lines 6 and 8 are the acks.  If the line 8 packet is
> processed before the line 6 packet, it will be just ignored as it is not
> a expected packet,

AFAICT that is where the bug starts.

AFAICT, from first principles, when process A receives the FIN/ACK it
should move to TIME_WAIT even if it has not received a preceding ACK.
That's because ACKs are cumulative. So receiving a later cumulative
ACK conveys all the information in the previous ACKs.

Also, consider the de facto standard state transition diagram from
"TCP/IP Illustrated, Volume 2: The Implementation", by Wright and
Stevens, e.g.:

  https://courses.cs.washington.edu/courses/cse461/19sp/lectures/TCPIP_State_Transition_Diagram.pdf

This first-principles analysis agrees with the Wright/Stevens diagram,
which says that a connection in FIN_WAIT_1 that receives a FIN/ACK
should move to TIME_WAIT.

This seems like a faster and more robust solution than installing
special timers.

Thoughts?

neal