Once a mobile node has recognized that it's on a foreign network and has acquired a care-of address, it needs to alert a home agent on its home network and request that the home agent forward its IP traffic. The registration process involves four steps:
The mobile node requests the forwarding service by sending a registration request to the foreign agent that the mobile node wants to use.
The foreign agent relays this request to the mobile node's home agent.
The home agent either accepts or denies the request and sends a registration reply to the foreign agent.
The foreign agent relays this reply to the mobile node.
A key concern with the registration procedure is security. Mobile IP is designed to resist two types of attacks:
A node may pretend to be a foreign agent and send a registration request to a home agent so as to divert traffic intended for a mobile node to itself.
A malicious agent may replay old registration messages, effectively cutting the mobile node from the network.
The technique that's used to protect against such attacks involves the use of message authentication and the proper use of the identification field of the registration request and reply messages.