netdev - Re: Something hitting my total number of connections to the server

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Message-ID: <CADVnQy=33_0H8g7JaLtGaVpf0_hXrcgJpNiBeJEN_SvpGPT9CQ@mail.gmail.com>
Date:   Wed, 23 Aug 2017 09:35:59 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Akshat Kakkar <akshat.1984@...il.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        David Laight <David.Laight@...lab.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: Something hitting my total number of connections to the server

On Wed, Aug 23, 2017 at 1:08 AM, Akshat Kakkar <akshat.1984@...il.com> wrote:
>
> On Tue, Aug 22, 2017 at 5:58 PM, Neal Cardwell <ncardwell@...gle.com> wrote:
> > On Tue, Aug 22, 2017 at 1:42 AM, Akshat Kakkar <akshat.1984@...il.com> wrote:
> >> There are multiple hosts/clients. All are mainly windows based.
> >>
> >> Timestamp is not used as my clients mainly are windows based and in
> >> that it tcp timestamp is by defauly disabled.
> > ...
> >> net.ipv4.tcp_tw_reuse=1
> >> net.ipv4.tcp_tw_recycle=1
> >
> > I suspect the problem is there. The net.ipv4.tcp_tw_recycle setting
> > should be 0. Running with the value 1 is known to cause buggy behavior
> > related to TCP timestamps, and that feature has been removed in kernel
> > v4.12.
> >
> > Can you please re-run your tests with net.ipv4.tcp_tw_recycle=0 or a
> > newer kernel?
> >
> > neal
>
> Thanks for your reply.
>
> I understand that.
>
> But my point is, though tcp timestamp is enabled on the server, but as
> client is not using it ... so how come this _bug_ (if any) is
> triggered in first place.

You mention "clients mainly are windows based". if they are only
"mainly" Windows-based, and some are of other OSes that do use TCP
timestamps, and the remote address is the same for TCP-timestamp-using
and non-TCP-timestamp-using clients, then running with timestamps
enabled on the server could tickle the bugs in pre-4.12 kernels that
save info from TCP-timestamp-using connections and erroneously try to
use that info to validate non-TCP-timestamp-using connections.

But the main point is that the configuration you cited
(net.ipv4.tcp_tw_recycle=1) is an unsupported configuration with known
bugs. The best resolution would be to just run with
net.ipv4.tcp_tw_recycle=0. It's not worth digging any further unless
you run with net.ipv4.tcp_tw_recycle=0 or a kernel that is v4.12 or
later and still have problems.

Hope that helps,
neal