termine the current bindings of the SIP URI. Signaling messages are exchanged between user agents, proxies, and redirect/location servers to locate the appropriate services or endpoints for media exchange. For reasons of scal- ability, multiple proxies are used to distribute the signaling load [3]. A session is setup between two user agents through SIP signaling messages comprised of an INVITE (messages 1, 2, 4, 7, and 8 in Figure 17.1), an OK response (messages 9-12 in Figure 17.1) and an ACK (message 13 in Figure 17.1) to the response [1]. The call setup is followed by media exchange using RTP. The session is torn down through an exchange of BYE and OK messages. The performance of SIP comprises the performance of call set-up and tear- down and performance of SIP mobility. Of these performance challenges of SIP, the mobility performance challenge is most critical. In this chapter, we will focus only on the challenge of SIP mobility and its performance. If the reader understands the underlying messages of SIP mobility and its perfor- mance modeling, the performance modeling of call set-up and tear-down will be pure applications of these models. The other aspects of SIP performance like reliability and robustness are not addressed in this chapter. 17.3 Mobility Management Mobility is the most important feature of wireless networks that makes con- tinuous service delivery possible in ubiquitous environments. Seamless service is usually achieved by supporting handoff or handover mechanisms. Hand- off is the process of changing parameters (e.g., channel) associated with the © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security current connection. It is often initiated by the movement of a mobile device or mobile host (MH), either within a network or across different networks. The former scenario initiates horizontal handoff, whereas the latter initiates verti- cal handoff. Handoff is again divided into two broad categories: hard and soft handoffs. They are characterized by "break-be