4. Routing Considerations
This section describes routing behavior and considerations for mobile nodes, foreign agents, and home agents in Mobile IP.
4.1. Mobile Node Considerations
4.1.1. Sending Datagrams While Away from Home
When a mobile node is away from home, it uses its care-of address as the source address for outgoing datagrams (unless using reverse tunneling). This ensures that replies can be routed back properly.
4.1.2. Receiving Datagrams While Away from Home
Datagrams sent to the mobile node's home address are intercepted by the home agent and tunneled to the care-of address.
4.2. Foreign Agent Considerations
4.2.1. Decapsulation
Foreign agents MUST decapsulate tunneled datagrams addressed to their care-of addresses and deliver them to the appropriate mobile nodes.
4.2.2. Handling Broadcast Datagrams
Foreign agents MAY forward broadcast datagrams from registered mobile nodes.
4.3. Home Agent Considerations
4.3.1. Intercepting Datagrams
Home agents MUST intercept datagrams addressed to registered mobile nodes and tunnel them to the current care-of address.
4.3.2. ARP and Proxy ARP
Home agents MUST use proxy ARP or gratuitous ARP to intercept datagrams destined for mobile nodes that are away from home.
4.3.3. Encapsulation
Home agents MUST encapsulate intercepted datagrams using one of the encapsulation methods: IP-in-IP [14], Minimal Encapsulation [16], or GRE [13].
4.4. Tunnel Management
4.4.1. Tunnel MTU Considerations
Encapsulation reduces the effective MTU available for user data. Proper tunnel MTU management is critical to avoid fragmentation issues.
4.4.2. Tunnel Soft State
Tunnels are soft-state entities that exist as long as there is an active mobility binding.
4.5. Multicast Considerations
Mobile nodes may wish to participate in multicast communications. This requires special handling when the mobile node is away from home.
4.6. Broadcast Datagrams
Mobile IP does not inherently support broadcasting to mobile nodes, but implementations may provide limited support through foreign agents.