hetero- geneous networks and then present a comprehensive performance study of a SIP-based vertical handoff scenario in third generation (3G) networks, where we show that performance-wise SIP may not meet the expectations of the 3rd Generation Partnership Project (3GPP)-specified standards. We conclude the chapter with covering some of the auxiliary schemes that help SIP meet the standard specified requirements. 17.2 Introduction SIP [1] is gaining acceptance as an application-layer signaling protocol for Internet multimedia and telephony services, as well as for wireless Internet © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security applications. SIP is free of many of the drawbacks of network or transport layer mobility, such as suboptimal routing and protocol stack modification. The fact that SIP has been accepted as the signaling standard by 3GPP and is also capable of providing mobility support made us choose SIP as the representative application-layer mobility solution. Another advantage is that the application layer, architecturally the highest layer of operation, can function across networks, a capability that will be tremendously useful in next-generation heterogeneous wireless networks. SIP is a control protocol that allows creation, modification, and termina- tion of sessions with one or more participants. It is used for both voice and video calls, either for point-to-point or multiparty sessions. It is independent of the media transport which for example, typically uses Real-time Trans- port Protocol (RTP)/Real-time Transport Control Protocol (RTCP) over User Datagram Protocol (UDP) [2]. It allows multiple endpoints to establish media sessions with each other. This includes terminating the session, locat- ing the endpoints, establishing the session, and modifying the media session after the session establishment has been completed. Recently, SIP has gained widespread acceptance and deployment among wireline service providers for introducing new services such as VoIP,