Re: TCP connection closed without FIN or RST

On Fri, Nov 3, 2017 at 12:05 PM, Eric Dumazet [off-list ref] wrote:

On Fri, 2017-11-03 at 11:13 -0400, Vitaly Davidovich wrote:

quoted

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.