avoiding multicast fragmentation over tunnels

I am wondering if there is any help when it comes to multicast streaming over tunnels. How to avoid fragmentation of these packets. I am not talking about small packets like IGP control plane messages. It's about bigger packets that can get fragmented (like video streams, for instance). PMTUD and TCP MSS cannot help here because multicast is generally speaking always UDP or RTP.

The only thing that pops into my mind is to increase allowed frame sizes on the tunnel path, on the routers. Is there some way to do this, similar to using jumbo frames on switches? For instance, if I am using GRE tunnels I would need to boost MTU with 24 bytes on every interface on the path, but if I am using ethernet network I would need to tell that network to allow jumbo frames in the first place.

Anybody has experience on this, or ideas?

Comments

  • Best thing to configure the max MTU at the source, the multicast sender.

    Sent from my iPhone

    On Apr 23, 2015, at 14:38, bokus <[email protected]> wrote:

    I am wondering if there is any help when it comes to multicast streaming over tunnels. How to avoid fragmentation of these packets. I am not talking about small packets like IGP control plane messages. It's about bigger packets that can get fragmented (like video streams, for instance). PMTUD and TCP MSS cannot help here because multicast is generally speaking always UDP or RTP.

    The only thing that pops into my mind is to increase allowed frame sizes on the tunnel path, on the routers. Is there some way to do this, similar to using jumbo frames on switches? For instance, if I am using GRE tunnels I would need to boost MTU with 24 bytes on every interface on the path, but if I am using ethernet network I would need to tell that network to allow jumbo frames in the first place.

    Anybody has experience on this, or ideas?




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  • My first reaction was "oh yeah nice idea, but I never saw MTU in any menu on the video encoder products". No way. Wishful thinking.

    On the second thought, you got it right, thanks a lot. There is a way to do it, but indirect. There is always an option to choose how many MPEG packets to put inside one IP frame, and that does the job. Spot on. I completely forgot about it, it's never used actually - maybe when you have excessive tunelling...

    I tried it now, checked with wireshark, calculated the headers and all, it really fits in nicely and does the job.

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