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I have an application that is sending some UDP packets using multicast. I looked at the network traffic and there seems to be a lot of ancillary packets related to using multicast. I don't totally understand it, but does multicast by nature result in MORE network traffic. If so how can I throttle this down?


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3 Answers

Other than the Multicast group join/remove messages, there are no ancillary messages created from you sending multicast data. However, NIC's, routers, switches, printers, etc. all usually send some kind of multicast traffic, which is probably what you are seeing if you record the traffic.

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I am seeing a lot of traffic on my multicast group. The traffic is LLC messages (Logical Link Control) ... a LOT of this stuff ... um ... is this a by-product of doing multicast? x –  Xofo Nov 23 '09 at 5:03
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In short, you need the networking equipment that forwards traffic between the client nodes to take care of this. Those vary depending on the network topology but would normally be:

  1. Ethernet switches
  2. IP routers.
  3. Switch / router (implements functionality of a switch & router)

There are multicast control protocols such as IGMP but of course the source nodes and/or intermediate nodes (e.g. switches) must comply to these control protocols.

And YES multicast result in more network traffic : this is why plain Ethernet hubbing is practically extinct and additions to IEEE Ethernet such as VLANs are prevalent nowadays.

This is probably best addressed on some other sites (maybe this SO-style site PacketDrop).

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@Xofo: does my answer satisfy you? –  jldupont Dec 9 '09 at 2:02
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LLC packets means you probably have sub-netting on your local segment, usually this doesn't mean extra packets though. You should change the network to a full class C if you want to remove LLC. On regular packets LLC or SNAP adds a 8-byte header.


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