Re: Re: Re: [PATCH 2/3] tcp: Reduce SYN resend delay if a suspicous ACK is received

On Fri, 31 Jan 2020 22:55:34 -0500 Neal Cardwell [off-list ref] wrote:

On Fri, Jan 31, 2020 at 5:18 PM SeongJae Park [off-list ref] wrote:

quoted

On Fri, 31 Jan 2020 17:11:35 -0500 Neal Cardwell [off-list ref] wrote:

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On Fri, Jan 31, 2020 at 1:12 PM Eric Dumazet [off-list ref] wrote:

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On 1/31/20 7:10 AM, Neal Cardwell wrote:

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On Fri, Jan 31, 2020 at 7:25 AM [off-list ref] wrote:

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From: SeongJae Park <redacted>

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.

[...]

quoted

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/ (local)).  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/ (local)
... 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?

RST/ACK is traced if LINGER socket option is applied in the reproduce program,
and FIN/ACK is traced if it is not applied.  LINGER applied version shows the
spikes more frequently, but the main problem logic has no difference.  I
confirmed this by testing both of the two versions.

In the previous discussion, I showed the LINGER applied trace.  However, as
many other documents are using FIN/ACK, I changed the trace to FIN/ACK version
in this patchset for better understanding.  I will comment that it doesn't
matter whether it is FIN/ACK or RST/ACK in the next spin.


Thanks,
SeongJae Park

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

neal