3. Multicast group subscription/termination protocols

3.1 Context

A protocol is needed to enable multicast terminals to announce their membership of broadcast groups to their local multicast router. In this way, the multicast routing protocol will know the existence and location of the members of a broadcast group, so that it knows where to broadcast multicast packets. This subscribe/unsubscribe protocol, enabling IP terminals to join and leave multicast groups, is the Internet Group Management Protocol (IGMP) in IPv4, and the Multicast Listener Discovery (MLD) protocol in IPv6.

These two protocols define signaling between receiving terminals and routers, but also between multicast routers connected to the same subnet. The scope of both protocols is limited and local to a subnet. As soon as a new group subscription...