2118~r_user Invite 200 OK 21 -r_user@fdomain Invite 2118~r_user@fdomaih 200 OK 200 OK MEDIA REGISTER traffic INVITE traffic FIGURE 10.2 SIP traffic flow (a) With no roaming logic (b) With traditional roaming logic (c) With proposed roaming logic. through his central home proxy (triangle routing) as depicted in Fig- ure 10.2a, despite the availability of a local proxy server in the foreign domain (foreign server). This results into significant delays that are not accepted for time-sensitive applications. Even with SIP mobility man- agement (SIPMM) [5,6] support (personal, terminal and session mobil- ity) enabled, the same scenario occurs. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security SIP mobility allows a user to roam between subnets and domains main- taining accessibility and session continuation using pre-call and mid-call mobility signaling. With pre-call signaling, the mobile user will re-REGISTER with the home proxy anytime his IP address changes. With mid-call signaling, the mobile user will negotiate an address change with the correspondent user while the session is in progress using re- INVITE messages. Mid-call mobility assumes a session is already in progress between the calling parties. Inefficient pre-call traffic routing and service centralization are obvious limitations that users roaming in these traditional and mobile SIP environments have to suffer from. This is the same case also for Mobile IP with location registers (MIP- LR) [14,15], whereas here, the SIP proxy servers are replaced with location registers. We argue that our proposed approach to roaming and inter-domain mobility in general, can significantly enhance the SIP personal and terminal mobility performance. Additionally, since our approach addresses SIP personal and terminal mobility, we can improve the pre-call portion of any SIP session mobility scheme while other fea- tures like mid-call mobility can remain unchanged. For mid-call mobil- ity, current proposals like MIP-LR, SIPMM, or a combi