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

Advantages and Disadvantages of the Care-Of Address Types

The foreign agent care-of address is considered the type used in “classical” Mobile IP, where there is both a home agent and a foreign agent. While it seems less efficient than the co-located address method, it offers some important advantages. A key one is that the same foreign agent care-of address can be used for all mobile nodes visiting that network. Datagrams for all mobile nodes on that network are sent to the foreign agent, which completes the delivery to the individual nodes. Since the mobile nodes use the foreign agent's address, no extra addresses or extra work are required for each mobile node.

The co-located care-of address has the advantage that traffic can be forwarded directly from the home agent to the mobile node. In this type of arrangement, it is possible for a Mobile IP device to travel to a foreign network where there is no Mobile-IP-aware router to act as a foreign agent. This does mean, however, that the Mobile IP implementation must include all the functions of communicating with the home agent that the foreign agent normally performs.

When co-located care-of addresses are used, an issue is how the temporary address is obtained. In many foreign networks automatic assignment of an IP address using something like DHCP may be possible, but if not, a temporary IP address would need to be assigned. Either way, some of the foreign network's limited IP address space would need to be set aside for mobile nodes, each of which would use an address while present on the network. In some cases this could lead to an address depletion issue.

Foreign agent care-of addressing is usually preferred due to its more automatic nature, when a foreign agent is present on the visited network. Considering that all datagrams will need to go through some router on the foreign network to reach the mobile node anyway, we might as well save the extra IP addresses. Co-located care-of addresses would be used when there is no foreign agent, or might be practical for long term connections even when a foreign agent is present.

Remember that the care-of address represents only the destination to which mobile node datagrams are forwarded. Foreign agents provide services other than forwarding, so it is possible for a mobile node to use a co-located care-of address even when a foreign agent is present, while continuing to take advantage of the other foreign agent services.

For more information on how datagrams are forwarded between the home agent and the mobile node's care-of address, see the topic on Mobile IP encapsulation and tunneling.

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