ly gaining in popularity as the next generation multimedia signaling and session establishment protocol. It is anticipated that the SIP infrastructure will be extensively deployed all over the Internet and it has already been accepted as the signaling protocol of preference for many multimedia frameworks. In this paper, we explore the use of persistent identifiers to provide a secure and efficient approach to inter- domain SIP mobility. We will discuss the issues associated with SIP session mobility and the different approaches used to provide persistent identification that expedites mobility; some of which are explicitly applied to SIP. We will C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security later illustrate SIP mobility and security through the implementation of an identification framework to application-level SIP addressing by introducing a level of indirection on top of the traditional SIP architecture. We refer to our approach as the Handle SIP (H-SIP). H-SIP leverages the current SIP archi- tecture abstracting any domain binding from users. Our approach to mobility is both secure and user-controlled and has shown through our inter-domain authentication and call routing experiments to be as scalable as current solu- tions for static networks and more scalable and reliable in the presence of mobility. 10.1 Introduction SIP [1] and H.323 [2] are among the most widely adopted protocols for IP telephony. While SIP and H.323 have different architectural components, the ability of these two protocols to coexist, and the simpler implementation and open collaboration of SIP makes it the signaling protocol of choice for advanced multimedia communications signaling. This fact is evidenced by its acceptance into the Third Generation Partnership Project (3GPP) as a sig- naling protocol for establishing real-time multimedia sessions. The protocol is continuously gaining in popularity and deployment and has been adopted by service providers like Verizon and Sprint to provide IP tel