Charles E. Perkins, Sun Microsystems
ecent years have seen an explosive growth both in the number of laptop and notebook computers sold, and in the number of nodes connected to the Internet and the World Wide Web. The notebook computers are themselves ever more powerful, equal in processing capability to many systems sold as desktop workstations. I n fact, the future growth of the Internet is likely to be fueled in large part by these very notebook computers, since they account for the part of the computer market that is growing fastest. Along with these trends, we also see the steady growth of the market for wireless communications devices. Such devices can only have the effect of increasing the options for making connections to the global Internet. Mobile customers can find a wide array of such wireless devices available. There are numerous varieties of radio attachments and infrared devices; of course, communications by way of the cellular telephone network is always an option for those willing to pay the fees.
covery mechanisms of mobile IP are described in detail. Following that, the mechanisms are described by which a mobile computer is located. Next, the available tunneling mechanisms are shown, which the home agent uses to forward datagrams from the home network to the mobile computer. Having covered the details of the base mobile IP specification, we then describe further protocol messages which help to decrease the inefficiency associated with inserting the home agent in the routing path of data destined for mobile computers. This route optimization is still a topic for further work within the IETF. Finally, we summarize and discuss the current problems facing mobile IP, as well as a few areas of active protocol development.
MOBILITY PORTABILITY vs. These trends are motivating a great deal of interest in making sure that mobile wireless computers can attach to the Internet and remain attached to the Internet even as they move from place to place, establishing new links and moving away from previously established links. Early on, it was apparent that solving the problem at the network layer (say, by modifying IP [l], the Internet Protocol, itself) would provide major benefits, including application transparency and the possibility of seamless roaming. Application transparency is almost required for all reasonable solutions, because it is unacceptable to force mobile users to buy all new mobile-aware applications. Seamless roaming, while not yet mandatory, is nonetheless expected to register very high on the scale of user convenience factors once the physical wireless means for continued connectivity are widely deployed. Moreover, seamless roaming provides application transparency. Mobile IP is the only current means for offering seamless roaming to mobile computers in the Internet. It has recently progressed along the ladder to standardization within the Internet Engineering Task Force (IETF), and its specification is now available as Request for Comments (RFC) 2002 . Related specifications are available as RFCs 2003-2006. This article follows the logical outline indicated below. We first describe the problem that is solved by mobile IP in the next section. In the second section there is a list of terminology and an overview of mobile IP. In the third section, the dis-
obile I P can be thought of as the cooperation of three major subsystems. First, there is a discovery mechanism defined so that mobile computers can determine their new...