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 Overview, History and Motivation 1 2 3 Mobile IP Addressing: Home and "Care-Of" Addresses

Mobile IP Functions

An important difference between Mobile IP and our mail forwarding example is one that represents the classic distinction between people and computers: people are smart and computers are not. When our consultant is traveling in Tokyo, he always knows he's in Tokyo and that his mail is being forwarded. (Well, assuming he goes easy on the sake, but that's a different story. J) He knows to go deal with the Tokyo post office to get his mail. The post office in London knows what forwarding is all about and how to do it. The traveler and the post offices all can communicate easily using the telephone.

In contrast, in the computer world, when a device travels using Mobile IP, things are more complicated. Suppose our consultant flies to Tokyo, turns on his notebook and plugs it in to the network. When the notebook is first turned on, it has no clue what is going on. It has to figure out that it is in Tokyo. It needs to find a foreign agent in Tokyo. It needs to know what address to use while in Tokyo. It needs to communicate back with its home agent back in London to tell it that it is in Tokyo and to start forwarding datagrams. Furthermore, it must accomplish its communication without any “telephone”.

To this end, Mobile IP includes a host of special functions that are used to set up and manage datagram forwarding. To see how these support functions work, we can describe the general operation of Mobile IP as a simplified series of steps:

1. Agent Communication: The mobile node finds an agent on its local network by engaging in the Agent Discovery process. It listens for Agent Advertisement messages sent out by agents and from this can determine where it is located. If it doesn't hear these messages it can ask for one using an Agent Solicitation message.
   
   
2. Network Location Determination: The mobile node determines whether it is on its home network or a foreign one by looking at the information in the Agent Advertisement message.

If it is on its home network it functions using regular IP. To show how the rest of the process works, let's say the device sees that it just moved to a foreign network. The remaining steps are:

3. Care-Of Address Acquisition: The device obtains a temporary address called a care-of address. This either comes from the Agent Advertisement message from the foreign agent, or through some other means. This address is used only as the destination point for forwarding datagrams, and for no other purpose.
   
   
4. Agent Registration: The mobile node informs the home agent on its home network of its presence on the foreign network and enables datagram forwarding, by registering with the home agent. This may be done either directly between the node and the home agent, or indirectly using the foreign agent as a conduit.
   
   
5. Datagram Forwarding: The home agent captures datagrams intended for the mobile node and forwards them. It may send them either directly to the node or indirectly to the foreign agent for delivery, depending on the type of care-of address in use.

Datagram forwarding continues until the current agent registration expires. The device can then renew it. If it moves again, it repeats the process to get a new care-of address and then registers its new location with the home agent. When the mobile node returns back to its home network, it deregisters to cancel datagram forwarding and resumes normal IP operation.

The following topics look in more detail at the functions summarized in each of the steps above.

Mobile IP Overview, History and Motivation 1 2 3 Mobile IP Addressing: Home and "Care-Of" Addresses