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

On Wed, Aug 23, 2017 at 1:08 AM, Akshat Kakkar [off-list ref] wrote:

On Tue, Aug 22, 2017 at 5:58 PM, Neal Cardwell [off-list ref] wrote:

quoted

On Tue, Aug 22, 2017 at 1:42 AM, Akshat Kakkar [off-list ref] wrote:

quoted

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.

...

quoted

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