A possible solution to this problem would be to distribute routes through the network to declare the node’s new location and to update the routing tables so that packets can be correctly dispatched. This might, at first, seem attractive, but it is a solution that scales very poorly since it would be necessary to retain host-specifi c routes for each mobile host. As the number of mobile hosts in the Internet increases (and the growth of web access from mobile devices such as cell phones and palm-tops is very rapid), it would become impractical to maintain such tables in the core of the Internet.
The solution developed by the IETF involves protocol extensions whereby packets targeted at a mobile host are sent to its home network (as if the host were not mobile) and passed to a static (nonmobile) node called the node’s home agent . The mobile host registers its real location with the home agent, which is responsible for forwarding the packets to the host.
If the mobile host is at home (attached to its home network), forwarding is just plain old IP forwarding, but if the host is roving, packets must be tunneled across the Internet to a care-of address where the host has registered its attachment to a foreign agent . At the care-of address (the end of the tunnel) the packets are forwarded to the mobile host.
Note that this tunneling process is only required in one direction. Packets sent by the mobile host may be routed through the network using the standard IP procedures. It is worth observing that although mobile IP can be used to address any IP mobility issue, its use within wireless LANs and mobile phone networks might be better served by linklayer (i.e., sub-IP) procedures such as link-layer handoff. These processes are typically built into the link-layer mechanisms and involve less overhead than mobile IP. Such processes do, however, require that the mobile host remains logically connected within the IP subnet to which its address belongs — it becomes the responsibility of the link layer to maintain connections or virtual connections into that subnet.
An alternative to tunneling in mobile IP might be to use source routing within IP. IPv4 has been enhanced with optional extensions to support source routing. However, since the source routing extensions to IPv4 are a relatively new development and are in any case optional, many (or even most) deployed IPv4 nodes do not support them. This means that they are not a lot of use for developing mobile IP services over existing IPv4 networks. They may be of more use in new networks that are being constructed for the first time since the Service Providers can insist on these extensions from their equipment vendors.
IPv6 offers some alternatives to tunneling for mobile IP by using the routing extension header. In this way the mobile node can establish communications with its home agent and then use information learned to directly route packets to the destination, bypassing the home agent. Since this feature is built into IPv6 and so supported by all IPv6 implementations, it makes IPv6 a popular option for mobile IP deployments.
Source of Information : Elsevier Wireless Networking Complete 2010