 telepresence, video conferencing; within the enterprises for instant messaging and collaboration; and among mobile carriers for push-to-talk services. Industry acceptance of SIP as the protocol of choice for converged communications over IP networks is thus highly likely. As shown in Figure 17.1, a SIP infrastructure consists of user agents, regis- tration servers, location servers, and SIP proxies deployed across a network. Request Redirect Response service Media Location service Registrar Proxy Proxy Proxy agent agent FIGURE 17.1 SIP architecture. © 2009 by Taylor & Francis Group, LLC SIP-Based Mobility Management and Its Performance Evaluation A user agent is a SIP endpoint that identifies services such as controlling session setup and media transfer. User agents are identified by SIP Uniform Resource Identifiers (URIs), which are unique HTTP-like URIs of the form sip: user@domain. All user agents register their IP addresses with a SIP registrar server (which can be colocated with a SIP proxy). Details of SIP can be found in [1]. SIP defines a set of messages, such as INVITE, REGISTER, REFER, etc., to setup sessions between the user agents. These messages are routed through SIP proxies that are deployed in the network. The Domain Name Server (DNS) service records help in finding SIP proxies responsible for the destination domain. A session or dialog is setup between two user agents following a client-server interaction model, where the requesting user agent client (UAC) interacts with the target user agent server (UAS). A logical entity formed by concatenating a UAC and a UAS that keeps all the dialog information and intercepts all the messages within a dialog to participate in the same, is known as a back- to-back user agent (B2BUA). All requests from an originating UAC, such as an INVITE are routed by the proxy to an appropriate target UAS, based on the target SIP URI included in the Request-URI field of the INVITE mes- sage. Proxies may query location and redirect servers for SIP service discovery or to de