ct to the system. So, SIP allows for personal mobility whether through the use of a proxy that sets up the session between the calling parties or through the use of redi- rection servers. However, the protocol defines a user only within the domain boundaries of the service provider. A user must associate with a specific proxy server that handles user authentication as well as initial traffic routing. The proxy maintains a unique account for the user, who in turn, is expected to coordinate with that same proxy irrespective of his location. This require- ment translates into unnecessary loads on the SIP server and on a particular domain. Additionally, it complicates the coordination of roaming users who must communicate with a central proxy server while roaming. Despite the possible presence of firewalls and other network restrictions on the foreign domain, roaming users are required to use the central home server instead of using the available local servers. Consequently, while URIs solve the loca- tion binding issue, they introduce the domain binding issue. Inefficient traffic routing is a direct consequence of such binding. Besides, the URI identifica- tion translates into users needing to be aware of each others' current domain associations. It also brings up the complexity of satisfying calls when initiated from regular keypad terminals. In fact, inter-domain mobility can be addressed through the use of an abstraction framework based on a unique and persistent identification mech- anism. This provides an approach that can enhance personal and terminal mobility [4] in current SIP architectures. As to session mobility, the readily available approaches like mid-call mobility [5] or enhancements to that [9] may be used. The framework we propose, which we refer to as the H-SIP, can seamlessly fit into the current SIP architecture, allowing SIP users to trans- parently roam across different SIP domains. H-SIP can be gradually deployed and can coexist with the traditional SIP infrastructure. User location and associa