Managing Group Memberships
Although any host can send traffic to a multicast group, applications that want to participate in a distributed multicast group as listeners must inform the local multicast routers of the groups that they want to listen for. This is done using IGMP messages that state the multicast group address that the host wants to participate in.
In addition, multicast routers also use IGMP messages to query the local network for hosts that have active memberships in multicast groups that are being routed to the local network. This is basically the entire function of IGMP: routers asking for hosts that are listening for multicast traffic, and hosts telling routers which groups they are listening for.
All IGMP messages have certain requirements. First of all, they must all have the Time-to-Live value of the IP datagram set to “1 hop”, thereby preventing them from being forwarded by any routers. In addition, RFC 2236 states that all IGMP messages should set the IP datagram to use the IP Router Alert Option, as discussed in Router Alert in Chapter 2.
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