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

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Date:   Sat,  1 Feb 2020 07:08:43 +0100
From:   SeongJae Park <sj38.park@...il.com>
To:     Neal Cardwell <ncardwell@...gle.com>
Cc:     SeongJae Park <sj38.park@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>, sjpark@...zon.com,
        Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>, shuah@...nel.org,
        Netdev <netdev@...r.kernel.org>, linux-kselftest@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>, aams@...zon.com,
        SeongJae Park <sjpark@...zon.de>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: 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 <ncardwell@...gle.com> wrote:

> On Fri, Jan 31, 2020 at 5:18 PM SeongJae Park <sj38.park@...il.com> wrote:
> >
> > On Fri, 31 Jan 2020 17:11:35 -0500 Neal Cardwell <ncardwell@...gle.com> wrote:
> >
> > > On Fri, Jan 31, 2020 at 1:12 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
> > > >
> > > >
> > > >
> > > > On 1/31/20 7:10 AM, Neal Cardwell wrote:
> > > > > On Fri, Jan 31, 2020 at 7:25 AM <sjpark@...zon.com> wrote:
> > > > >>
> > > > >> From: SeongJae Park <sjpark@...zon.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?

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
>