the future. To gauge this, we must find out how many applications require mobility. "Portability" often suffices, whether or not mobility is available. Today's Internet application suite is heavily weighted toward client-server applications (e.g., the Web, e-mail, and server- based instant messaging). Client-server applications do not require the same level of mobility support as peer-to-peer applications. In fact, the major- ity of the applications are client-server-based and can be supported using application-layer mobility solutions with the exception of applications like sustained file downloads during movement. Today's servers do not usually move, nor do they initiate communication with a client that has not already located the server. If inherently peer-to-peer applications are widely used, mobility mechanisms operating at a lower layer of the protocol stack will be more attractive. 17.4 Protocols In this section, we select a representative protocol in each layer and evaluate its merits and demerits with respect to the above discussions. The selected protocols either have been standardized for handling the mobility in the cor- responding layer, or require minimal changes in the network stack and legacy infrastructure. Understanding of the working of the protocols is necessary to develop the performance models. Also, as the mobility in a real network will involve multiple mobility protocols, we selected the representative protocols to cover the scope of our models. 17.4.1 Link-Layer (Subnetwork-Layer) Mobility One example of subnetwork-layer mobility, often termed link-layer mobility, is the access mobility provided in General Packet Radio Service (GPRS) [8] or Universal Mobile Telecommunication System (UMTS) [9] networks. Packets are forwarded from the core network to the MH or user equipment (UE) in the radio access networks by setting up GPRS Tunneling Protocol (GTP) in two segments from the gateway GPRS support node (GGSN) to the MH/UE. The serving GPRS support node (SGSN) handles inter-radio network con