Mobile IP Registration

When the mobile node receives an agent advertisement, the mobile node registers through the foreign agent, even when the mobile node might be able to acquire its own co-located care-of address. This feature enables sites to restrict access to mobility services. Through agent advertisements, mobile nodes detect when they have moved from one subnet to another.

Mobile IP registration provides a flexible mechanism for mobile nodes to communicate their current reachability information to their home agent. The registration process enables mobile nodes to perform the following tasks:

• Request forwarding services when visiting a foreign network

• Inform their home agent of their current care-of address

• Renew a registration that is due to expire

• Deregister when they return home

Registration messages exchange information between a mobile node, a foreign agent, and the home agent. Registration creates or modifies a mobility binding at the home agent, associating the mobile node's home address with its care-of address for the specified lifetime.

The registration process also enables mobile nodes to:

• Register with multiple foreign agents

• Deregister specific care-of addresses while retaining other mobility bindings

• Discover the address of a home agent if the mobile node is not configured with this information

Mobile IP defines the following registration processes for a mobile node:

• If a mobile node is registering a foreign agent care-of address, the mobile node registers using that foreign agent.

• If a mobile node is using a co-located care-of address, and receives an agent advertisement from a foreign agent on the link on which it is using this care-of address, the mobile node registers using that foreign agent (or another foreign agent on this link).

• If a mobile node uses a co-located care-of address, the mobile node registers directly with its home agent.

• If a mobile node returns to its home network, the mobile node deregisters with its home agent.

These registration processes involve the exchange of registration requests and registration reply messages. When registering using a foreign agent, the registration process takes the following steps, which the subsequent illustration depicts:

1. The mobile node sends a registration request to the prospective foreign agent to begin the registration process.

2. The foreign agent processes the registration request and then relays it to the home agent.

3. The home agent sends a registration reply to the foreign agent to grant or deny the request.

4. The foreign agent processes the registration reply and then relays it to the mobile node to inform it of the disposition of its request.

Figure 1-4 Mobile IP Registration Process

When the mobile node registers directly with its home agent, the registration process requires only the following steps:

• The mobile node sends a deregistration request to the home agent.

• The home agent sends a registration reply to the mobile node, granting or denying the request.

Network Access Identifier (NAI)

AAA servers, in use within the Internet, provide authentication and authorization services for dial-up computers. These services are likely to be equally valuable for mobile nodes using Mobile IP when the nodes are attempting to connect to foreign domains with AAA servers. AAA servers identify clients by using the Network Access Identifier (NAI). A mobile node can identify itself by including the NAI in the Mobile IP registration request.

Since the NAI is typically used to identify the mobile node uniquely, the mobile node's home address is not always necessary to provide that function. Thus, it is possible for a mobile node to authenticate itself, and be authorized for connection to the foreign domain, without even having a home address. To request that a home address be assigned, a message containing the mobile node NAI extension can set the home address field to zero in the registration request.

Mobile IP Message Authentication

Each mobile node, foreign agent, and home agent supports a mobility security association between the various Mobile IP components, indexed by their security parameter index (SPI) and IP address. In the case of the mobile node, this address is its home address. Registration messages between a mobile node and its home agent are authenticated with the Mobile-home authentication extension. In addition to Mobile-home authentication, which is mandatory, you can use the optional Mobile-foreign agent and Home-foreign agent authentications.

Mobile Node Registration Request

A mobile node registers with its home agent using a registration request message so that its home agent can create or modify a mobility binding for that mobile node (for example, with a new lifetime). The foreign agent can relay the registration request to the home agent. However, if the mobile node is registering a co-located care-of address, then the mobile node can send the registration request directly to the home agent.

Registration Reply Message

A mobility agent returns a registration reply message to a mobile node that has sent a registration request message. If the mobile node is requesting service from a foreign agent, that foreign agent receives the reply from the home agent and subsequently relays it to the mobile node. The reply message contains the necessary codes to inform the mobile node about the status of its request, along with the lifetime granted by the home agent, which can be smaller than the original request. The registration reply can also contain a dynamic home address assignment.

Foreign Agent Considerations

The foreign agent plays a mostly passive role in Mobile IP registration. A foreign agent adds all registered mobile nodes to its visitor table. It relays registration requests between mobile nodes and home agents, and, when it provides the care-of address, de-encapsulates datagrams for delivery to the mobile node. It also sends periodic agent advertisement messages to advertise its presence.

Home Agent Considerations

Home agents play an active role in the registration process. The home agent receives registration requests from the mobile node (perhaps relayed by a foreign agent), updates its record of the mobility bindings for this mobile node, and issues a suitable registration reply in response to each. The home agent also forwards packets to the mobile node when the mobile node is away from its home network.

Dynamic Home Agent Discovery

In some cases, the mobile node might not know its home agent address when the mobile node attempts to register. If the mobile node does not know its home agent address, the mobile node can use dynamic home agent address resolution to learn the address of its home agent. In this case, the mobile node sets the home agent field of the registration request to the subnet-directed broadcast address of the mobile node's home network. Each home agent that receives a registration request with a broadcast destination address rejects the mobile node's registration by returning a rejection registration reply. By doing so, the mobile node can use the home agent's unicast IP address indicated in the rejection reply when the mobile node next attempts registration.