Mobile IP

Mobile networking is a subject that is becoming increasingly important as portable devices such as personal digital assistants (PDAs) and notebook computers are becoming more powerful and less expensive, coupled with people's need to be connected whenever and wherever they are. The link between the portable device and the fixed communication network can be wireless or wired. If a wireless link is used, the device can utilize a radio or infrared channel. Of these two alternatives, radio channels can traverse longer distance without the line-of-sight requirement, but introduce electromagnetic interference and are often subject to federal regulations (e.g., Federal Communication Commission, or FCC). Infrared channels are often used in shorter distances. A wireless connection enables a user to maintain its communication session as it roams from one area to another, providing a very powerful communication paradigm. In this section we look at a simple IP solution for mobile computers.

Mobile IP allows portable devices called mobile hosts (MHs) to roam from one area to another while maintaining the communication sessions. One requirement in mobile IP is that a legacy host communicating with an MH and the intermediate routers should not be modified. This requirement implies that an MH must continuously use its permanent IP address even as it roams to another area. Otherwise, existing sessions will stop working and new sessions should be restarted when an MH moves to another area. The basic mobile IP solution is sketched in Figure 8.29.

The mobile IP routing operates as follows:

• When a correspondent host (CH) wants to send a packet to an MH, the CH transmits the standard IP packet with its address as the source IP address and the MH's address as the destination IP address. This packet will be intercepted by the mobile host's router called the home agent (HA), which keeps track of the current location of the MH. The HA manages all MHs in its home network that use the same address prefix. If the MH is located in the home network, the HA simply forwards the packet to its home network.

• When an MH moves to a foreign network, the MH obtains a care-of address from the foreign agent (FA) and registers the new address with its HA. The care-of address reflects the MH's current location and is typically the address of the FA. Once the HA knows the care-of address of the MH, the HA can forward the registration packet to the MH via the FA.

Home agent

FIGURE 8.29 Routing for mobile hosts

Home agent

Correspondent host

FIGURE 8.29 Routing for mobile hosts

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