ephony, instant messaging, and other data services. It is anticipated that SIP will be widely deployed by operators and enterprises, thus populating the Internet with SIP infrastructural components. The widespread deployment and adoption of SIP has resulted in the need for an inter-domain mobility scheme for SIP. SIP [1] is a signaling and control protocol for handling multimedia sessions, allowing for the establishment and termination of media streams between two or more participants. SIP works in concert with other multimedia protocols due to the independence of the protocol from the underlying transport mech- anisms and session types. SIP architecture allows for its deployment as a centralized system, a distributed system, or a combination of both. SIP architecture is also proposed as an efficient candidate that can be reused to provide personal, terminal, and session mobility [3-6] with a readily avail- able infrastructure. This avoids the redundancy introduced by simultaneous deployment with Mobile IP [7]. The successful reuse of SIP to simultaneously support both multimedia communications and mobility leverages the issues emanating from SIP users roaming* across multiple SIP domains. These issues are highlighted through a brief overview of SIP functionality. SIP handles user location through the use of a proxy/location server that accepts user registration requests and updates the respective user location in a *Throughout this paper, roaming is defined as the SIP inter-domain roaming, i.e., the migration of a user between different SIP domains. twe will use the terms SIP proxy, SIP server, SIP registrar interchangeably. © 2009 by Taylor & Francis Group, LLC SIP Mobility Technologies and the Use of Persistent Identifiers location repository. The protocol inherently implements location independence through the use of the uniform resource identifiers (URI) [8], which directly offer personal mobility. A URI acts as a location-independent identifier abs- tracting the actual physical location of a user with respe