Multicast routing protocols enable the forwarding of IP multicast datagrams throughout a TCP/IP internet. Generally these algorithms forward the datagram based on its source and destination addresses. Additionally, the datagram may need to be forwarded to several multicast group members, at times requiring the datagram to be replicated and sent out multiple interfaces.
The state of multicast routing protocols is less developed than the protocols available for the forwarding of IP unicasts. Three experimental multicast routing protocols have been documented for TCP/IP. Each uses the IGMP protocol (discussed in Section [4.4]) to monitor multicast group membership.