Supporting mobility in IP networks is a crucial step towards satisfying the nomadic communication paradigms on the current Internet. The Session Initiation Protocol (SIP) presents one approach towards supporting IP mobility and is increasingly gaining in popularity as the next generation multimedia signaling and session establishment protocol. In this paper, we explore the design of an efficient approach to inter-domain SIP mobility in an attempt to improve personal and terminal mobility schemes. We apply a persistent identification framework to application level SIP addressing by introducing a level of indirection on top of the traditional SIP architecture. We show how this approach helps achieve efficient inter-domain authentication and call routing towards providing inter-domain mobility. This paper presents the design of H-SIP, while its implementation is described in a companion paper.