netdev - Re: TCP connection closed without FIN or RST

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Date:   Fri, 03 Nov 2017 10:58:30 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Vitaly Davidovich <vitalyd@...il.com>
Cc:     netdev <netdev@...r.kernel.org>
Subject: Re: TCP connection closed without FIN or RST

On Fri, 2017-11-03 at 13:23 -0400, Vitaly Davidovich wrote:
> On Fri, Nov 3, 2017 at 12:05 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > On Fri, 2017-11-03 at 11:13 -0400, Vitaly Davidovich wrote:
> >> Ok, an interesting finding.  The client was originally running with
> >> SO_RCVBUF of 75K (apparently someone decided to set that for some
> >> unknown reason).  I tried the test with a 1MB recv buffer and
> >> everything works perfectly! The client responds with 0 window alerts,
> >> the server just hits the persist condition and sends keep-alive
> >> probes; the client continues answering with a 0 window up until it
> >> wakes up and starts processing data in its receive buffer.  At that
> >> point, the window opens up and the server sends more data.  Basically,
> >> things look as one would expect in this situation :).
> >>
> >> /proc/sys/net/ipv4/tcp_rmem is 131072  1048576   20971520.  The
> >> conversation flows normally, as described above, when I change the
> >> client's recv buf size to 1048576.  I also tried 131072, but that
> >> doesn't work - same retrans/no ACKs situation.
> >>
> >> I think this eliminates (right?) any middleware from the equation.
> >> Instead, perhaps it's some bad interaction between a low recv buf size
> >> and either some other TCP setting or TSO mechanics (LRO specifically).
> >> Still investigating further.
> >
> > Just in case, have you tried a more recent linux kernel ?
> I haven't but will look into that.  I was mostly hoping to see if
> anyone perhaps has seen similar symptoms/behavior and figured out what
> the root cause is - just a stab in the dark with the well-informed
> folks on this list :).  As of right now, based on the fact that a 1MB
> recv buffer works, I would surmise the issue is perhaps some poor
> interaction between a lower recv buffer size and some other tcp
> settings.  But I'm just speculating - will continue investigating, and
> I'll update this thread if I get to the bottom of it.
> >
> > I would rather not spend time on some problem that might already be
> > fixed.
> Completely understandable - I really appreciate the tips and pointers
> thus far Eric, they've been helpful in their own right.

I am interested to see if the issue with small sk_rcvbuf is still there.

We have an upcoming change to rcvbuf autotuning to not blindly give
tcp_rmem[2] to all sockets, but use a function based on RTT.

Meaning that local flows could use small sk_rcvbuf instead of inflated
ones.

And meaning that we could increase tcp_rmem[2] to better match modern
capabilities (more memory on hosts, larger BDP)