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Let's say I have a multicast group 239.0.0.1.

If A and B are members of this group and listening on port 6000, I can send data to (239.0.0.1, 6000) and both A and B will receive. Now, if A is listening on port 6000 and B on port 7000, how to send data to both the members in a single transmission ?

If the above scenario is not possible, then does it mean that a multicast group is not just identified by its multicast group address but by the 2-tuple multicast group address and port number ? I could not find such definition for a multicast group anywhere.

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  • If you make the question a general multicast question, I can answer it. I have seen similar setups where there are several sources sending to the same group but different ports. – Daniel Dib Dec 8 '14 at 5:14
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Subscribing to a multicast group and listening on a multicast address and port are two different things. If you are subscribed to a multicast group all traffic with that destination address should be sent to your host. It is another step to listen on a specific port and process that traffic.

In your example, traffic with destination 239.0.0.1:6000 would be sent to both hosts but since only host A has an application that is listening, only host A will do anything with that traffic.

It is not possible to send to all port numbers or send to the * port. No such concept exists in TCP/UDP. If you were using a different protocol, maybe there is something like that.

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  • So, if I have a setup as I mentioned, then I have to send twice, one for (239.0.0.1, 6000) and other for (239.0.0.1, 7000). Is that correct ? Also, is it that no two applications in a same PC can subscribe to same multicast group ? (because different processes cannot listen on same port.) – Ezio Dec 8 '14 at 21:53
  • "Also, is it that no two applications in a same PC can subscribe to same multicast group ? (because different processes cannot listen on same port.)" - I just found that this is possible with "SOL_SOCKET" option. – Ezio Dec 8 '14 at 22:15
  • Yes, if you want to send to two different ports you must send twice. – jamesbtate Dec 12 '14 at 2:09

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