nation of these two [16]) can be used. These approaches implement mid-call mobility by sending binding updates directly to correspondent nodes without going through home agents. Mobile IP (MIP) [7], however, uses home agents to forward traffic, which creates triangular routing issues. An enhanced version of MIP is MIPv6 [17] that avoids triangular routing and implements route optimization. As to the simultaneous mobility issue, discussed lately in [18], it is left for a future paper to offer a secure framework for simultaneous mobility in the context of H-SIP. A second scenario is that of a SIP roaming-aware approach such as the one proposed by double user agent servers [19] that mimics the roaming solution employed in telecommunication environments. In other words, a user who is roaming outside his home domain, registers with a for- eign server. The latter consults the user's home server for redirection, authentication, and billing, and proceeds to process the user's transac- tions. Correspondent users trying to communicate with the roaming user will have to go through his home proxy server, which in turn, redirects them to the foreign proxy where the user is currently located. Hence, sig- nificant signaling overhead results primarily due to the nature of the SIP URI. The URI is composed of a domain part, like in r_user@hdomain, thus forcing the calls directed to this user to go through the hdomain proxy server first. The data flow for this scenario is presented in Fig- ure 10.2b. We argue that this approach is inefficient as it introduces unnecessary overhead and load on the original server. In the two scenarios above, the use of URIs to identify users and the inherent dependence of the URI on a particular domain, complicates message routing. One solution is to abstract the actual identifier eliminating per-call coordina- tion to minimize the signaling traffic in highly mobile environments. © 2009 by Taylor & Francis Group, LLC SIP Mobility Technologies and the Use of Persistent Identifiers 10.3 Host Identifica