| Before we start putting together the pieces of the Mobile IP puzzle, we should take a closer look at the big picture. Given the components of the Mobile IP solution, how do they all work together? Earlier in the chapter, you learned that Mobile IP is a mobility protocol that allows a Mobile Node to roam across various IP subnets and access links, all the while maintaining continuous communication. In Chapter 1, "Mobile and Wireless Technologies," we identified the four basic requirements for a mobility protocol: location discovery, move detection, update signaling, and path (re)establishment. We will briefly explain how Mobile IP addresses each of these requirements, and we then examine the details in the remainder of the chapter: Location discovery In Mobile IP, two types of locations exist: the Home Network and the Foreign (Visited) Network. The type of network to which the Mobile Node is attached is central to the protocol, because each protocol results in a different type of Mobile IP handover and requires different Mobile IP signaling. The location is determined by examining Mobile IP agent advertisements, if one is received, or by examining the allocated Colocated CoA. Move detection Mobile Nodes continually engage in the process of move detection, which is the act of monitoring changes in available paths into the network. Move detection rapidly becomes cloudy because the line between Layer 2 and Layer 3 is blurred. Features like proactive movement and simultaneous association with multiple Layer 2 access points complicate matters even further. Remember that Mobile IP is a Layer 3 protocol, and in this context, move detection is the process of keeping track of changes in Layer 3 paths that the Mobile Node can use to reach the network. Looking at this further in the light that Mobile IP is a routing protocol, this means that the Mobile Node must understand, as part of move detection, when candidate routes become available or disappear. To this end, whenever movement is detected, Mobile IP uses its Mobile IP handover policy algorithm to evaluate all candidate routes and determine whether a change in routing is necessary. A change in routing is known as a Mobile IP handover. Update signaling After a Mobile IP handover has been initiated, the Mobile Node determines the type of Mobile IP signaling necessary based on its previous and new location type. This Mobile IP signaling takes the form of a Registration Request (RRQ) or a Deregistration Request. At this point, the FA, if one is being used, and the Home Agent evaluate the Registration or Deregistration Request and send either a success or failure Registration Reply (RRP) message to the Mobile Node. This signaling exchange is referred to as the Mobile IP registration process. Path (re)establishment For successful Mobile IP registrations, a tunnel is established between the CoA and Home Agent. Conversely, for successful Mobile IP deregistrations, the tunnel is removed. In either case, the routing table of both the Home Agent and FA is updated to reflect the current routing path. At this point, the Mobile Node cycles back to the move detection state, and the process begins again.
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