VPN XP woes
March 9, 2013 3:39 PM   Subscribe

I've installed Private Internet Access VPN software on a Win 7 machine and an XP machine on my home network. The VPN connected Windows 7 machine reports good speeds at www.speedtest.net and streams content like a champ. The XP machine when connected to the VPN shows terrible speeds and buffers and stutters like something not so useful. More details after the fold...

The Win 7 machine is connecting to my home network via wifi. The XP machine is hard cabled to the same wifi router.

The speedtest results for both machines when the VPN is switched off are the same. Nice and fast.

Why is my XP machine so slow when going via the VPN?
posted by merocet to Computers & Internet (8 answers total) 2 users marked this as a favorite
Are the machines significantly different, hardware-wise? It could simply be that the XP machine's CPU can't handle encrypting VPN traffic at the same rate as the Windows 7 machine.
posted by strangecargo at 4:06 PM on March 9, 2013

The xp machine is a Pentium 4 2.6Ghz with 1.5gb RAM
The Win 7 Machine is an I7 2670QM(2.20GHz) with 8 GB of RAM

If VPN encrypting takes a fair amount of processing power then I can see why the Win 7 machine would be faster.

Would the XP machine be under specd for streaming video via VPN?
posted by merocet at 4:19 PM on March 9, 2013

Look at your CPU usage on the XP machine when running a speedtest, then activate the VPN and run the same test. I would bet that you'll see that CPU usage goes up significantly.

An i7 versus a Pentium 4, despite similar frequencies, is a whole different ballgame. Multiple cores is a big difference- when you've got a machine with 4 cores, one core can deal with background OS stuff, one can deal with the VPN overhead, one can deal with the streaming and there is still one left over to do other stuff. Each process gets 2.2ghz of power. Same thing on the single core Pentium 4 machine means that (very roughly) each of those three processes only gets 0.86ghz of power. Without the VPN, the XP machine is keeping up. But with the VPN, you've pushed it over the limit.

Also, there is a lot of video decoding performance built into the newer processors that isn't built into the pentium 4.

That's my guess, anyway. There could be an issue with the network card in the XP machine needing significant CPU time to just do its job. Some network cards can do all their stuff in hardware, some use the machine's CPU to do some of the work.
posted by gjc at 5:38 PM on March 9, 2013

Win XP was in the days of spectacularly crappy networking stacks on Windows. You don't notice until you get to larger network delays (say encrypting, going through a VPN somewhere, decrypting, traveling the internets, back to the VPN, encrypted, back over the tunnel, decrypted). Mainly because of the size of the transmit buffers for TCP.

See, when you send out data you have to keep it buffered until it gets to its destination and an ACKnowledgement makes it back to your machine. When you're sending fast volumes of data and the Round Trip Time is also large, these transmit buffers fill up and you can't send any more data until you receive some replies and then can re-use those buffers that were ACKed.

Now around Vista time, Microsoft started doing better at automatically increasing the size of these buffers as needed.

What are the differences in speed with and without the VPN on the two systems? And when you hit the speedtest site I think they show you the Round Trip Time (or Ping Time or some similar measure), what are the differences in that time between machines with and without the VPN?

It could be the CPU speed, or some other problem with the XP box. It could be that you've hit the point where the delay of the VPN is too much for XP's network stack.

If it's the TCP stack you can tune it (not for the faint of heart) start here.

Really, on the best of days XP at say cable modem speeds over long distances/high delay just doesn't perform without tweaking.
posted by zengargoyle at 11:12 PM on March 9, 2013

Thanks for the answers so far. The pings and speeds are as follows;

XP (cabled to router)
Ping 155ms
Download Speed 0.70Mbps
non VPN:
Ping 85 ms
Download Speed 16.53Mbps

Win 7 (over wifi)
Ping 140ms
Download Speed 9.64Mbps
non VPN:
Ping 62 ms
Download Speed 11.24Mbps

Running CPU tests on the XP machine while running the speed test show's pretty similar results with the CPU maxing in the low 90%'s in both cases.

I ran the SG TCP Optimizer and let it put in what it considered to be the optimal setting for all the adapters on the XP machine. It raised the TCP receive window to 256960 and switched on both MTU discovery and selective ACKs.

I then reran the tests with no change at all to the download speeds.

Does that mean the CPU is still most likely the culprit even though it's not maxing out? If not, any suggestions of where else to look?

posted by merocet at 10:18 AM on March 10, 2013

Additionally, running the cpu test while streaming video via the VPN shows cpu usage running at around 50% but the video repeated stopping to buffer so that would appear to indicate that the CPU isn't the problem in this case no?
posted by merocet at 11:22 AM on March 10, 2013

Yeah, ~10Mb/s @ ~150ms isn't too far into buffer problems (I expected the VPN to push you into ~500ms or more but evidently you aren't going that far). The XP dropping from 16Mb/s to .7Mb/s is pretty bad. Maybe it just doesn't have the memory/bus-speed needed to handle a VPN at better than 1Mb/s. You could try checking the memory usage when the VPN is running.

Things it's probably not: cables/NIC - get ~16Mb/s without VPN; Raw CPU - only at ~50%.

Could be: lack of memory; XP VPN sofware is lame; Some other VPN setting on the XP box like a bad MTU setting (If you see a MTU configuration option in your VPN software and it's ~1480 - 1500, you can try lowering it to ~1400. But that's a bit of a longshot).
posted by zengargoyle at 12:34 PM on March 10, 2013

Thanks for all the answers. Food for thought but unfortunately no resolution. I think in this case I'll save time and money in the long run by gifting this old box to someone who needs it for what it can do, and getting a new media box for myself further down the line.

Again thanks for the input.
posted by merocet at 3:25 PM on March 10, 2013

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