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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Mobility Support (Mobile IP)

Mobile IP Addressing: Home and "Care-Of" Addresses 1 2 3 4 5 Mobile IP Home Agent Registration and Registration Messages

When a mobile node is first turned on, it cannot assume that it is still “at home” the way normal IP devices do. It must first determine where it is, and if it is not at home, begin the process of setting up datagram forwarding from its home network. This process is accomplished by communicating with a local router serving as an agent, through the process called agent discovery.

Agent discovery encompasses the first three steps in the simplified five-step Mobile IP operational summary I gave in the topic on general operation. The main goals of agent discovery include the following:

• Agent/Node Communication: Agent discovery is the method by which a mobile node first establishes contact with an agent on the local network to which it is attached. Messages are sent from the agent to the node containing important information about the agent; a message can also be sent from the node to the agent asking for this information to be sent.
  
  
• Orientation: The node uses the agent discovery process to determine where it is. Specifically, it learns whether it is on its home network or a foreign network by identifying the agent that sends it messages.
  
  
• Care-Of Address Assignment: The agent discovery process is the method used to tell a mobile node the care-of address it should use, when foreign agent care-of addressing is used.

Mobile IP agents are routers that have been given additional programming to make them “Mobile IP aware”. The communication between a mobile node and the agent on its local network is basically the same as the normal communication required between a device on an IP network and its local router, except more information needs to be sent when the router is an agent.

Mobile IP Addressing: Home and "Care-Of" Addresses 1 2 3 4 5 Mobile IP Home Agent Registration and Registration Messages