Mobile IP can be summed up in one, albeit long, sentence:
In a nutshell, Mobile IP allows a user to roam across various IP subnets and access links, all the while maintaining continuous communication.
To understand the function of Mobile IP, a great analogy to use is the U.S. Postal Service. Postal mail is sent to you by placing a letter (the packet payload) in an envelope addressed to you (IP header). The letter arrives at your local post office and is routed to you at your Home Address, as shown in Figure 2-1. When you move, you tell the local post office (Home Agent) to forward packets to your new location (Care-of Address [CoA]). Now, when a letter addressed to your Home Address arrives at your local post office, it can now be readdressed (tunneled) to your new location (CoA). The letter then arrives at the post office that services your new location (Foreign Agent [FA]) and is delivered to you at your new location (CoA), as shown in Figure 2-2. This mail delivery is done with no effort (and usually even knowledge) by the original sender of the letter (Correspondent Node [CN]).
Figure 2-1. Postal Service to Your Home
Figure 2-2. Postal Service When You Move
We now dive into what Mobile IP really is by dissecting this statement: Mobile IP is a dynamic routing protocol where end devices signal their own routing updates and dynamic tunnels eliminate the need for host route propagation.
Mobile IP Is a Dynamic Routing Protocol\'85
Although saying that Mobile IP is a dynamic routing protocol is a controversial statement, we stand by it. Some people call Mobile IP an address-management protocol, but Mobile IP still relies on existing address-management techniques, either static or dynamic. The core protocol does not attempt to control how IP addresses are managed or distributed. Extensions are available to distribute IP addresses to Mobile Nodes, but this is more an economy of signaling than a core feature. In the same way that the Point-to-Point Protocol (PPP) is not referred to as an address-management protocol simply because it is capable of distributing addresses, neither should Mobile IP.
Mobile IP has also been called an application. While some Mobile IP deployments resemble applications with a centralized "server" and clients on the edge, many other deployment scenarios resemble a classic routing infrastructure.
Finally, some say that Mobile IP is not a routing protocol because it is not capable of building a full topology on its own. While this is probably the most credible argument against it, we would argue that this is not a required feature of a routing protocol.
We maintain that Mobile IP is a dynamic routing protocol. To appreciate this statement, consider the characteristics of a dynamic routing protocol. The essence of a dynamic routing protocol is that it dynamically alters routing tables as routes (and reachability) change, and does not change how routing is done per se. It detects network topology changes and adapts by choosing best available paths and updating the routing table accordingly. The best available path is determined within the context of the dynamic routing protocol and varies from protocol to protocol because each is designed to meet specific needs. For example, some protocols are designed for scalability, while others are designed for fast convergence. These design decisions impact the choice of the best available path. Both Open Shortest Path First (OSPF) and Border Gateway Protocol (BGP) are dynamic routing protocols, but they each use different methods for selecting the best path.
In this light, Mobile IP is designed for mobility. It adapts to network topology changes. It selects best available path routes and inserts them into the routing table. Sounds like any other dynamic routing protocol!
…Where End Devices Signal Their Own Routing Updates…
Mobile IP is designed to provide highly scalable host routing for clients in a mobile (and usually wireless) environment. A Mobile Node can have one or more links attaching it to the network, and each link can have metrics associated with it. Using these metrics and link availability information, a Mobile Node informs the network through routing updates of the best path through which it should be reached, as shown in Figure 2-3. Because this path selection method is unlike any other, we propose calling it edge intelligent.
Figure 2-3. Mobile IP Uses Routing Updates
The idea of making all routing decisions at the edge might seem ill conceived, but it works well in a mobile environment for two reasons. First, on wireless networks, Mobile Nodes attach and detach rapidly, making it complex for the network to keep track of them. Imagine using a standard Interior Gateway Protocol (IGP) in an environment where hundreds of neighbors are on a single link and hundreds of topology changes occur every second. Moving much of the responsibility to the Mobile Node allows the network to scale more effectively. Second, because the speed and latency of wireless networks are often dramatically different from the fixed infrastructure, choosing a best path based solely on the access link is usually effective. Just as BGP's path vector approach provides a usable solution to inter-AS routing, Mobile IP's edge-intelligent design provides an effective solution to mobility routing.
…and Dynamic Tunnels Eliminate the Need for Host Route Propagation
Unlike other routing protocols, Mobile IP builds its own links; but don't expect a Home Agent to jump out of the rack and start laying fiberthey are all logical links. These logical links are known as tunnels. Tunneling usually connects two similar networks through a dissimilar network. For example, tunneling can carry a protocol like AppleTalk across a network that is not capable of routing AppleTalk.
However, as most other protocols are being replaced by IP, tunneling is finding more use linking similar routing domains across a dissimilar one. For example, an enterprise might use tunneling to link private networks in two remote sites across the Internet. This allows traffic with private addresses to be routed across a public network.
Mobile IP uses the same concept by tunneling across a routing domain that does not know how to route to the Mobile Node's current location, as shown in Figure 2-4. With Mobile IP, Mobile Nodes maintain a constant IP address as they move around the network. Supporting this with traditional routing protocols would require a host route for each Mobile Node. Every time a Mobile Node moves, the host route would have to be updated and the routing protocol would have to reconverge. This can work for a small number of Mobile Nodes with infrequent mobility, but when the frequency of routing updates increases, traditional routing protocols can fail.
Figure 2-4. Mobile IP Uses Tunnels for Routing
Instead, Mobile IP inserts host routes in at most two devicesthe Home Agent and possibly a FAand uses tunneling to deliver traffic across the network. This eliminates the need to propagate a host route across the network every time a Mobile Node moves, which would clearly not scale. In essence, Mobile IP uses tunnels to create an overlay routing domain, thus isolating frequent host route changes from the existing routing protocols.