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 Concepts and General Operation 1 2 Mobile IP Agent Discovery, and Agent Advertisement and Solicitation Messages

Just as most of us have only a single address used for our mail, most IP devices have only a single address. Our traveling consultant, however, needs to have two addresses; a normal one and one that is used while he is away. Continuing our earlier analogy, the Mobile-IP-equipped notebook our consultant carries needs to have two addresses as well:

• Home Address: The “normal”, permanent IP address assigned to the mobile node. This is the address used by the device on its home network, and the one to which datagrams intended for the mobile node are always sent.
  
  
• Care-Of Address: A secondary, temporary address used by a mobile node while it is 'traveling” away from its home network. It is a normal 32-bit IP address in most respects, but is used only by Mobile IP for forwarding IP datagrams and for administrative functions. Higher layers never use it, nor do regular IP devices when creating datagrams.

The care-of address is a slightly tricky concept. There are two different types, which correspond to two distinctly different methods of forwarding datagrams from the home agent router.

This is a care-of address provided by a foreign agent in its Agent Advertisement message. It is, in fact, the IP address of the foreign agent itself. When this type of care-of address is used, all datagrams captured by the home agent are not relayed directly to the mobile node, but indirectly to the foreign agent, which is responsible for final delivery. Since in this arrangement the mobile node has no distinct IP address valid on the foreign network, this is typically done using a layer two technology. This arrangement is illustrated in Figure 129.

In our consultant analogy, this type of care-of address is like forwarding from the London post office to the Tokyo post office. The London personnel would take a letter for John Smith sent to his London address, and repackage it for delivery to “John Smith, care of the Tokyo post office”. The Tokyo post office (or John Smith himself) would need to worry about the last leg of the delivery.

This is a care-of address assigned directly to the mobile node using some means external to Mobile IP. For example, it may be assigned on the foreign network manually, or automatically using DHCP. In this situation, the care-of address is used to forward traffic from the home agent directly to the mobile node. This was the type of address shown in Figure 128.

In our consultant analogy, this is like John Smith obtaining a temporary address for his use while in Tokyo. The London post office would forward directly to his Tokyo address. They would not specifically send it to the Tokyo post office (though of course that PO would handle the mail at some point).

Mobile IP Concepts and General Operation 1 2 Mobile IP Agent Discovery, and Agent Advertisement and Solicitation Messages