From: Neal Cardwell <ncardwell@google.com>
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>
Subject: Re: [PATCH 2/3] tcp: Reduce SYN resend delay if a suspicous ACK is received
Date: Fri, 31 Jan 2020 10:10:03 -0500 [thread overview]
Message-ID: <CADVnQyk9xevY0kA9Sm9S9MOBNvcuiY+7YGBtGuoue+r+eizyOA@mail.gmail.com> (raw)
In-Reply-To: <20200131122421.23286-3-sjpark@amazon.com>
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
next prev parent reply other threads:[~2020-01-31 15:10 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-31 12:24 [PATCH 0/3] Fix reconnection latency caused by FIN/ACK handling race sjpark
2020-01-31 12:24 ` [PATCH 1/3] net/ipv4/inet_timewait_sock: Fix inconsistent comments sjpark
2020-01-31 14:54 ` Eric Dumazet
2020-01-31 15:09 ` sjpark
2020-01-31 12:24 ` [PATCH 2/3] tcp: Reduce SYN resend delay if a suspicous ACK is received sjpark
2020-01-31 15:01 ` Eric Dumazet
2020-01-31 16:12 ` sjpark
2020-01-31 16:55 ` Eric Dumazet
2020-01-31 17:05 ` sjpark
2020-01-31 17:08 ` Eric Dumazet
2020-01-31 15:10 ` Neal Cardwell [this message]
2020-01-31 18:12 ` Eric Dumazet
2020-01-31 22:11 ` Neal Cardwell
2020-01-31 22:17 ` SeongJae Park
2020-02-01 3:55 ` Neal Cardwell
2020-02-01 6:08 ` SeongJae Park
2020-02-01 13:30 ` Neal Cardwell
2020-01-31 22:53 ` Eric Dumazet
2020-02-03 15:40 ` David Laight
2020-02-03 15:54 ` Eric Dumazet
2020-01-31 12:24 ` [PATCH 3/3] selftests: net: Add FIN_ACK processing order related latency spike test sjpark
2020-01-31 14:56 ` Eric Dumazet
2020-01-31 15:13 ` sjpark
2020-01-31 14:00 ` [PATCH 0/3] Fix reconnection latency caused by FIN/ACK handling race David Laight
2020-01-31 15:05 ` sjpark
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