Routing Datagrams to and From Mobile Nodes

This section describes how mobile nodes, home agents, and foreign agents cooperate to route datagrams to and from mobile nodes that are connected to a foreign network.

Encapsulation Types

Home agents and foreign agents support tunneling datagrams using one of the available encapsulation methods (IP in IP Encapsulation, Minimal Encapsulation, or Generic Routing Encapsulation). Mobile nodes that use a co-located care-of address can receive tunneled datagrams using any encapsulation type.

Unicast Datagram Routing

When registered on a foreign network, the mobile node chooses a default router using the following rules:

• If the mobile node is registered using a foreign agent care-of address, then the mobile node chooses its default router from among the router addresses advertised in the ICMP router advertisement portion of that agent advertisement message. The mobile node can also consider the IP source address of the agent advertisement as another possible choice for the IP address of a default router.

• If the mobile node is registered directly with its home agent using a co-located care-of address, then the mobile node chooses its default router from among those advertised in any ICMP router advertisement message that it receives. The chosen default router network prefix must match the mobile nodes externally obtained care-of address. If the mobile node's externally obtained care-of address matches the IP source address of the agent advertisement under the network prefix, the mobile node can also consider that IP source address as another possible choice for the IP address of a default router.

• If the mobile node is registered, a foreign agent that supports reverse tunnels routes unicast datagrams from the mobile node to the home agent through the reverse tunnel.

Broadcast Datagrams

When a home agent receives a broadcast datagram, it does not forward the datagram to any mobile nodes in its mobility binding list. However, the home agent does forward the datagram if a mobile node has requested forwarding of broadcast datagrams. For each registered mobile node, the home agent forwards received broadcast datagrams to the mobile node; the method depends on how the configuration of the home agent specifies categories of broadcast datagrams forwarded to mobile nodes. Broadcast datagrams over reverse tunnels are not supported.

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.

In the case of reverse tunnels, multicast datagrams are not routed through reverse tunnels. The multicast datagrams are routed as previously described.