fore-make" and "make-before- break" connections. In hard handoffs, current resources are released before new resources are used while in soft handoffs, both existing and new resources are used during the handoff process. For a soft handoff, the MH should be capable of communicating to multiple antennas through multiple interfaces. Existing mobility protocols have mainly been designed for network, trans- port, and application layers, and the majority of existing literature refers to the inherent mobility support provided as the subnetwork layer mobility. We will evaluate candidate protocols in each layer for their merits and demerits with respect to such important design issues as the scope of mobility, the set of applications supported, and the objectives of mobility protocols. A layer-by-layer classification of existing mobility protocols is shown in Figure 17.2, which also shows the scope or range of operation of these pro- tocols. Only the transport and application layer protocols maintain the end- to-end semantics of a connection between communicating hosts. Most of the mobility protocols are limited to operate on a single layer only, SO they are transparent to the other layers. In the following, the protocol design issues in each layer are discussed in more detail. Mobile host Router Correspondent host Example Application Transport TCP-Migrate Network Mobile IP Network Subnetwork Subnetwork FIGURE 17.2 A layer-by-layer classification of the mobility protocols. © 2009 by Taylor & Francis Group, LLC SIP-Based Mobility Management and Its Performance Evaluation 17.3.1 Subnetwork Layer Subnetwork layer mobility is transparent to network and upper layers because an MH changes its point of attachment using solely layer two (link layer) mech- anisms. This makes the deployment of these mechanisms easy from the per- spectives of the network and end-to-end hosts, since all affected components are confined to the mobile host and its immediately adjacent subnetwork, with no changes to routing or correspondent hosts. But 