Multicast Datagram Routing

To receive multicasts, a mobile node joins the multicast group in one of the following ways:

• If a multicast router exists on the visited subnet, the mobile node uses this local multicast router. If the mobile node is using a co-located care-of address, it uses this address as the source IP address of its Internet Group Management Protocol (IGMP) messages. Otherwise, it uses its home address.

• If the mobile node's home agent is a multicast router, the mobile node can join groups using a bidirectional tunnel to its home agent. The mobile node tunnels IGMP messages to its home agent. The home agent then forwards multicast datagrams down the tunnel to the mobile node.

A mobile node that sends datagrams to a multicast group also has the following options:

• Send directly on the visited network

• Send through a tunnel to its home agent

Multicast routing depends on the IP source address. Therefore, a mobile node that sends multicast datagrams directly on the visited network uses a co-located care-of address as the IP source address. Similarly, a mobile node that tunnels a multicast datagram to its home agent uses its home address as the IP source address of both the multicast datagram and the encapsulating datagram. This second option assumes that the home agent is a multicast router.