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Thread: Download Dips

  1. #1

    Download Dips

    Hi Philip,

    I need your opinion on this:

    I'm using Vypr VPN. When downloading large files I get dips to almost 0 kbs for less than a second for every couple of minutes. Is this a form of bandwidth throttling from my ISP or just buffer issues? If it's the latter any work around?

    My specs:
    1. Windows 10 Pro x64
    2. Intel NIC
    3. Optimized by TCP Optimizer
    4. NIC is optimized based on the recommendation of this iste


  2. #2
    Administrator Philip's Avatar
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    Seems like an issue with your line or VPN...
    I would first test to see whether it happens without the VPN?
    Also, is it always the case, or intermittent/certain time of day, etc. (could be some type of EMI interference). Wired Ethernet or Wi-FI?

    If you still can't determine the cause and it is happening even without the VPN, check connectivity while this happens, i.e. what about pings, do they start timing out too?

  3. #3
    1. Happens only with VPN
    2. Happens all day as long as I'm connected to VPN
    3. Not EMI. I'm using Fiber 50/50
    4. I'm using Ethernet
    5. No problem with pings

    If indeed this is VPN issue, can you recommend a work around? Did you have any experience with this type of case?

  4. #4
    Administrator Philip's Avatar
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    Ok, it seems to be an issue with the VPN... There are a couple of things to look at on your end:

    1. Where is the VPN client, your router or your client machine? You should try to lower the MTU to 1200 to see if that solves the issue.
    2. If lowering the MTU does not work, try a different type of VPN connection, i.e. PPtP, TCP/UDP.

    Please let us know if this solves it for you.

  5. #5
    1. VPN client is in machine
    2. Present MTU is 1500. MSS is at 1250. This is the default.
    3. It only offers UDP.

    Where shall I change the MTU? OS? VPN client? or Router?

  6. #6
    Tried changing tun-mtu to 1200 in the VPN Client. Throughput is a lot better. However, there are still some dips.

    Is there an objective way of determining tun-mtu? Maybe if we could get the magic number we could completely eliminate the dips.

    The hardcoded mss-fix in the app is 1250. Isn't tun-mtu should be bigger?

  7. #7
    Administrator Philip's Avatar
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    Yes, MTU is always bigger than MSS. Your MSS will get limited by the MTU I imagine, so the hardcoded MSS in the app is getting overridden imho.

    MTU is essentially the packet size, and MSS is the packet minus all headers. For typical TCP/IP headers are at least 40 bytes:
    20-byte TCP header (or 8-byte UDP header)
    20-byte IPv4 header (or 40-byte for IPv6)
    1500 MTU minus 40-byte TCP/IP headers = 1460 MSS.

    For VPN tunnels headers get much larger than 40 bytes, however header size varies depending on the protocol, options etc.
    For example, IPSec AES-256 with SHA1 produces a typical overhead of up to 72 bytes.
    So, 1500 MTU - 72 bytes of headers = 1427 bytes MTU max. It is actually better at 1418 bytes. Anything over that will have to get split/fragmented on either end before being transmitted adding to latency and overhead. The actual header calculation can get a bit confusing, and the headers vary here is a sample link showing up to 101-bytes of headers for IPSec: https://hamwan.org/Standards/Network...ing/IPsec.html

    It is common to get disconnects and throughput issues if your MTU is larger than what is allowed without fragmentation through the VPN tunnel, that's why setting the MTU to a lower value often helps. It is hard to know the exact header size for your variety without a lot of technical info on the underlying tunnel. That said, overestimating the headers helps ensure your packets don't have to be fragmented before being transferred, that is the most important point.


    If not the MTU size, then you have to worry about:
    * Keep Alive time (usually 1-2 minutes)
    * SA Lifetime (usually set to a much larger value, like 24 hours)

    These two can also cause temporary disconnects. They can sometimes be fixed by either
    1. Disabling the isakmp keepalive on the tunnel interface, or
    2. Using group policy: configuring the vpn-idle-timeout and vpn-session-timeout
    Setting them may not work, however, since they are also set on the other end, there are global settings beyond your control.
    Linux is user friendly, it's just picky about its friends...
    Disclaimer: Please use caution when opening messages, my grasp on reality may have shaken loose during transmission (going on rusty memory circuits).
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  8. #8
    Are there softwares that can help us determining the packet overhead? How about wireshark?

  9. #9
    Administrator Philip's Avatar
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    Using 1200 bytes , even 1300 should be small enough for most scenarios. If using small MTUs doesn't make a difference I'd go back to ~1400 bytes and look into other possible causes.


    Wireshark can show you frame/header sizes, even though the output may look a bit cryptic.

  10. #10
    Thanks Philip for your thoughts! It is really appreciated

  11. #11
    Administrator Philip's Avatar
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    np, just trying to help

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