MTU & MSS Clamping



  • First post so I'm not sure if this is the right spot for this.

    We're live streaming video through our OpenVPN setup at various hospital locations around the country. The big variable is the network connection on location. I've started to experiment with various MTU and MSS restrictions and I'm starting to see improved consistency with the network streams.

    As I've done some reading online, I came across this page: http://lartc.org/howto/lartc.cookbook.mtu-mss.html

    At the bottom, it made this suggestion:

    "This calculates the proper MSS for your link. If you are feeling brave, or think that you know best, you can also do something like this:

    iptables -A FORWARD -p tcp –tcp-flags SYN,RST SYN -j TCPMSS --set-mss 128

    This sets the MSS of passing SYN packets to 128. Use this if you have VoIP with tiny packets, and huge http packets which are causing chopping in your voice calls."


    That last part was very interesting to me because when I monitor the packets that is the exact thing that I'm seeing. However, I have found that the MSS Clamping and MTU cannot be set to a lower number than 576 or thereabouts.

    Does anyone know of a work-around to get that number lower? That is my only question. If that answer is No, that is totally fine. But as of right now, just looking for a Y/N answer.

    Thanks everyone.


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