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Thread: Poor performance with iperf3 and OpenVPN under Windows 10

  1. #1

    Poor performance with iperf3 and OpenVPN under Windows 10

    Hi to everyone.

    As the title says, i'm doing some tests with OpenVPN and a VPS (Ubuntu Linux version 16 LTS) with 1gigabit port (both directions). My ISP connection has around 160Mbit in down and 20Mbit in up.

    If i launch a speedtest while the VPN is active from my client (Windows 10 1803 build 17133.1) i have very poor performance: at the beginning of my tests i was reaching a super slow speed: about 20mbit in down. After some research and tweaking with openvpn configurations i managed to reach about 40 mbit in down which is still not enough. There is absolutely something wrong so i continued with experiments: i decided to give a try to the connection between me and the vps without going through the vpn. In this scenario the result with iperf3 is super clear: the connection between me and the vps is capped at around 50mbit. After some other tests and research, i got a doubt and i decided to try everything under a linux distro. So i ran an ubuntu live session on the same pc client i was using before and, big surprise, everything work as expected: i can reach max speed (around 160Mbit) in both iperf3 and openvpn. So, finally, i realized that the problem is something under Windows 10 indeed. I already tried with different settings of Autotuning level under the tcp/ip settings with no luck: the max speed i can reach is always 50mbit. The strange thing is that if i try to download something else from the vps (with chrome for example with http) i can get full speed instead: the only problems seems to be with openvpn and iperf and seem to be connected. Any clue about this? Thank you!

  2. #2
    Administrator Philip's Avatar
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    Try to set your MTU down to ~1200 bytes and see if that helps?

    I think the correct way is to first set in in Windows (you can use the TCP Optimizer), then also add the following to the vpn.conf on both ends (increases send/receive buffers as well):

    fragment 1200
    mssfix 1200
    sndbuf 2000000
    rcvbuf 2000000
    txqueuelen 4000

    You should also try UDP vs. TCP...
    Someone reported this MS security update also caused issues with openvpn --> KB 4074588 (It has something to do with driver signing for USB and TAP adapters). Uninstalling/reinstalling the TAP adapter may help with this also, not sure.

    Also, you may want to try an older TAP adapter driver, there is some issue with the latest driver I believe.

  3. #3
    Thank you for reply. I will try with those settings and report it back. And what about the iperf3 test? It's also limited to 50mbit.


    I've tried your config but i get even worst performance.

    The option

    txqueuelen 4000

    makes the openvpn client crash on start. I already tried tcp and udp but still nothing.
    Last edited by gabemorgath; 04-23-18 at 12:57 PM.

  4. #4
    Administrator Philip's Avatar
    Join Date
    May 1999
    Jacksonville, Florida, United States
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    Not sure about the Iperf3 limitation, could also be driver related/Windows build related (you may have to pass more parameters to it specifying larger TCP Window, etc.).

    Have you tried the TCP Optimizer?

  5. #5
    Yeah, already tried tcpoptimizer but without luck: always same results.

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