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TCP performance vs delay!!

Posted: Mon Jul 08, 2019 1:22 pm
by roymickson
I have got some kind of confuse in TCP performance depending on delay.

My lab schema:

CPE --- (p1) Router (p2) --- Server
The CPE connects to the Server by OpenVPN using TCP. I know about TCP-over-TCP but TCP proto is mandatory! Next I do series of network benchmarks through the OpenVPN tunnel.

WITHOUT artificial delay.
WITH artificial delay of 30 ms (it's RTT) made on a Router's p1 and p2 interfaces.
WITH artificial delay of 60 ms.
The result of series:
"to Server": 40 mbps, "from Server": 50 mbps
"to Server": 34 mbps, "from Server": 38 mbps
"to Server": 20 mbps, "from Server": 10 mbps
Rate limit in test 1 due to CPU on CPE. It's normal.
But why third test with delay 60ms reduces performance "from Server" by 5 times while "to Server" only by 2 times?

P.S. CPE is an OpenWRT on a Banana PI R2 board. Server is a Ubuntu server, Router is a Ubuntu server too. All devices connect via Ethernet. CPE intefrace is 100M. Others — 1G.

TCP congestion control for both (CPE and Server) is BBR: