Supporting mobile Internet multimedia applications requires more than just the ability to maintain connectivity across subnet changes. We describe how the Session Initiation Protocol (SIP) can help provide terminal, personal, session and service mobility to applications ranging from Internet telephony to presence and instant messaging. We also briefly discuss application-layer mobility for streaming multimedia applications initiated by RTSP. I. Introduction A large amount of effort has been

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... nded over the years on allowing computer and communication devices to continue communicating even when mobile. However, the vast majority of mobile communication devices today continue to be singleservice cellular phones. Third generation wireless systems offer the opportunity However, given the extremely high cost of spectrum 1 , other, non-licensed, means of wireless access will likely remain attractive. Thus, system design should make it easy for devices to move between different wireless networks, depending on population density, speed of movement and propagation characteristics. Here, we describe one possible architecture that allows to support a full range of mobility options, independent of the underlying technology. We primarily focus on the provision of multimedia services, but allude briefly to "data" services. We explore how application-layer support is necessary to offer more than just hand-off between base stations and subnets, as well as how it can, under some circumstances, compensate for the lack of deployment of mobile IP. The protocol at the heart of this effort is the Session Initiation Protocol (SIP), an IETF-developed signaling protocol. We begin by outlining the principles of operation of the Session Initiation Protocol (SIP) in Section II, followed by a discussion of four modes of mobility, terminal, session, personal and service mobility in Sections III through VI.