netdev - Re: TCP connection closed without FIN or RST

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Date:   Fri, 3 Nov 2017 13:23:55 -0400
From:   Vitaly Davidovich <vitalyd@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     netdev <netdev@...r.kernel.org>
Subject: Re: TCP connection closed without FIN or RST

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.
>
>
>