In the past, we were used to making phone calls from home or from the office. Public pay phones allowed us to make phone calls while on the road. Today, the use of mobile phones is common and we make phone calls from almost anywhere and in any life situation. The use of notebook computers, wireless networks, and portable devices is expanding, and we can imagine having smart devices and using them from wherever we are. If these devices are to use IP as a transport protocol, we need Mobile IP to make this work. We expect our device to remain connected when we move around and change our point of attachment to the network, just as we are used to roaming from one cell to the next with our mobile phones today. For example, suppose you have a PDA with an 802.11 (wireless) interface and a General Packet Radio Service (GPRS) interface. In your hotel room, you are connected to the network through your wireless interface; when you leave your room and go out to the street, you switch automatically to GPRS without losing your connection. All the applications running on your PDA stay up. Isn't this cool? This section about Mobile IP explores the mechanisms needed and shows how IPv6 is ready for this challenge.
With IPv4 and IPv6 alike, the prefix (subnet address) changes depending on the network to which we are attached. When a mobile node changes its point of access to the network, it needs to get a new IP address, which disrupts its TCP or UDP connections. RFC 3344, "IP Mobility Support for IPv4," describes Mobile IP concepts and specifications for IPv4. Using Mobile IP with IPv4 has certain limitations, though, which make it unsuitable for the requirements in a global network. One reason is the limited address space. If we even imagine only smart phones having an IP address, the number of addresses required globally to cover the number of devices far exceeds the IPv4 address space available. The other reason is that IPv6 and the use of Extension headers offers the possibility to optimize routing in a mobile world, and this is really needed if we talk about mobility for large masses of devices. The fact that IPv6 uses Neighbor Discovery (instead of ARP like IPv4) makes IPv6 more independent of the Link layer. Mobile IPv6 takes the experience from Mobile IPv4 and makes use of the advanced features of IPv6.
This chapter describes how Mobile IPv6 works and how it is suited to provide the foundation for mobile services of tomorrow. First I explain the most important terms that will be used throughout the chapter, then I provide an overview of the functionality, and after this I dive into the technical details of the protocol: the new headers, messages, options, processes, and communications. So take a deep breath and read on.