Ideas on why using WPA2 encryption speeds up many TCP connections?

I'm seeing something a bit strange and wondering if anyone had an opinion on why...

I am testing up to 200 wifi station systems, each with a TCP connection running on them (download only, from VAP to stations).

Without encryption (ie, open network), I see total throughput go from about 108Mbps down to 69Mbps as I add more stations (I add 25 at a time, so the 108Mbps is with 25 active, and 69Mbps is with 200 active).

However, if I enable encryption, the throughput is actually higher (111Mbps to 71Mbps). I'm doing encryption in software, so it adds a fair bit of CPU load in this test. The numbers bounce around since this is wifi after all, but in general encryption tends to win reliably in this test.

When testing with a single station (and 5 tcp streams with jacked up snd/rcv buffers) the open networks perform significantly better at total throughput: 263Mbps vs 246Mbps.

Maybe the extra delay for decryption increases odds that GRO will take affect for the many, slower streams (and maybe that will decrease ACK traffic?)

Any other ideas?

Thanks,
Ben

Ben Greear <[email protected]>
Candela Technologies Inc  http://www.candelatech.com

Fewer times two or more stations step on one another? The recievers will only try to transmit when they receive data. Modulo timing, if the individual downloads are a bit slower, less chance of the receivers looking to send ACKs back through at the same time? Got any low-level stats for the health and well being of the wireless network?

rick jones

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The tcp connection stats are taken after running for 60 seconds, and I take 3-sec running averages as well as 60 second averages. So, I think that it would have to be total decrease in ACKs, not just timing, to make a difference. The 3 and 60 second stats show consistently higher throughput with encryption when using 25+ stations/connections.

Also, it works out that the sending sockets all sort of send randomly as they are able, so I don't think there would be any particular ACK flood seen..

I have great quantities of low level stats, but I have not dug into them in detail just yet. In general, my RF environment in this test is fairly controlled, as I am cabling the systems using good semi-rigid SMA cables and an RF attenuator. There will be some external interference of course, as they are not in an isolation chamber.

As for the difference in 1 stations vs 25+, then it is very likely related to low level things like MPDU working better with a single station, and probably better ACK avoidance (I recall about 20kpps download, 4kpps upload in a previous test with a single station, which indicates to me we must not be acking every packet-on-the-air..somehow).

(For grins, I played with the delayed-ack-segs from an out-of-tree patch and can get TCP throughput up to 300Mbps by setting delayed ack segs to 64 in single station/5 stream, open network test).

Thanks,
Ben

Ben Greear <[email protected]>
Candela Technologies Inc  http://www.candelatech.com

I was seeing an un-expectedly bad wifi train rates, so I changed to ath9k-rate-control, rebooted, and re-ran the tests. Throughput is much improved. I really hope it wasn't just a reboot that fixed it, but too burned out to do more tests today.

I still see better TCP throughput with WPA2 when using 25-200 stations/streams.

For anyone who wants to wade through some big automated reports, see the links near the top of this page (suggestions for improving those reports are welcome):

http://www.candelatech.com/lf_wifi_examples.php

(The 600 station reports are a bit dated and were done in a fairly busy
wifi environment. We'll re-run those sometime soon-ish.)

Thanks,
Ben

Ben Greear <[email protected]>
Candela Technologies Inc  http://www.candelatech.com