Once a mobile node has completed agent discovery, it knows whether it is on its home network or a foreign network. If on its home network it communicates as a regular IP device, but if on a foreign network it must activate Mobile IP. This requires that it communicate with its home agent so information and instructions can be exchanged between the two. This process is called home agent registration, or more simply, just registration.

The main purpose of registration is to actually start Mobile IP working. The mobile node must contact the home agent and tell it that it is on a foreign network and request that datagram forwarding be turned on. It also must let the home agent know its care-of address so the home agent knows where to send the forwarded datagrams. The home agent in turn needs to communicate various types of information back to the mobile node when registration is performed. Note that the foreign agent is not really involved in registration, except perhaps to relay messages, as we will see.

Successful registration establishes what is called in the standard a mobility binding between a home agent and a mobile node. For the duration of the registration, the mobile node's regular home address is tied to its current care-of address and the home agent will encapsulate and forward datagrams addressed to the home address over to the care-of address. The mobile node is supposed to manage its registration and handle various events using several actions:

• Registration: The mobile node initiates a registration when it first detects it has moved from its home network to a foreign network.
  
  
• Deregistration: When the mobile node returns home, it should tell the home agent to cancel forwarding, a process called deregistration.
  
  
• Reregistration: If the mobile node moves from one foreign network to another, or if its care-of address changes, it must update its registration with the home agent. It also must do so if its current registration is about to expire, even if it remains stationary on one foreign network.

Each registration is established only for a specific length of time, which is why regular reregistration is required whether the device moves or not. Registrations are time-limited to ensure that they do not become stale. If, for example, a node forgets to de-register when it returns home, the datagram forwarding will eventually stop when the registration expires.

To perform registration, two new message types have been defined in Mobile IP: the Registration Request and the Registration Reply. Each of these does what you would expect from its name. Interestingly, these are not ICMP messages like the ones used in agent discovery; they are User Datagram Protocol (UDP) messages. Thus, technically speaking, registration is performed at a higher layer than the rest of Mobile IP communication. Agents listen for Registration Requests on well-known UDP port #434, and respond back to mobile nodes using whatever ephemeral port the node used to send the message.

The formats of these two messages are described later in this topic.