Multicast question

[bellocarico]

I was reading about Multicast, and I thought to myself: doeas multicast traffic actually divide services per port like unicast does?

I can't see any reference in any document I've got.

Also, the IGMP group report fro mhost to router, does notify the IP multicast (group) address, but I can't find anything related to the port.

If it is true that a single host can receive informations from the same Multicast IP on 2 or more different ports, how can I than "leave" the transmission for a single port keeping the other(s)?

Could you please clarify?

Regards

rs232

[pkhatri · Accepted solution]

Multicast traffic tends to use UDP (RTP-based traffic, for example) and therefore, you will have multicast traffic destined to UDP ports on the receivers.

The IGMP Membership Report does not have any information on ports and it does not need to. The IGMP Querier (router) does not really care which port it is destined for- it only cares that there is a receiver on the local segment.

On a host, applications will request that the IP layer listens to a particular multicast address. Therefore, when a packet is received destined for that multicast address, the IP layer will pass it up to that application.

If multiple applications request membership of the same multicast group, then you can't selectively leave the membership for that port/socket. The reason for this is:

- there is no port information exchanged via IGMP, not even IGMPv3

- routers do not route based on transport layer ports, they only route based on destination IP.

Hope that helps - pls rate the post if it does.

Paresh