IGMP Messages
Before a host can receive and process multicast datagrams, it has to do two things: it must inform the local network interface card that it wishes to accept and process network frames for specific multicast groups, and it must inform any multicast routers on the local network that it wishes to receive the IP packets for those multicast groups as well.
How the application communicates with the local network interface card is a function of the specific IP implementation in use on the local system (the process is discussed earlier in this chapter in Section 4.1.2). However, the mechanisms used by IP to inform the multicast routers on the local network of its desire to participate in certain multicast groups is a function of the Internet Group Management Protocol (this process is discussed earlier in Managing Group Memberships).
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