e testbed. We show a comparison of the registra- tion process for roaming (H-SIP) and non-roaming (no H-SIP) environments. ruser is located in fdomain, ece.unm.edu. For the no H-SIP case, we tested with hdomain being either cnri.reston. (Reston, Virginia) or istec.org (Panama). Again, registration here assumes the basic digest authentication with the proxy server. In our model, the UA refreshes its registration with the proxy/registrar continuously (we used a registration TTL of one minute). If we estimate the server's average processing time of the REGISTER request including digest authentication by a, then from Figure 10.5a, the average registration time t A as seen by ruser is given by, (10.1a) C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security hdomain r_user fdomain 132.151.9.104 / Handle 129.24.27.152 129.24.27.106 168.77.202.59 system REGISTER <r_user@hdomain> 401 Unauthorized Authorization 200 OK REGISTER 401 Unauthorized Authorization 200 OK REGISTER <2118r_user> 401 Unauthorized Lookup <2118r_user> Authorization Update <2118r_user> 200 OK REGISTER 401 Unauthorized Authorization 200 OK FIGURE 10.5 REGISTER message flow A. With no roaming logic C. With proposed roaming logic H-SIP. since, t1t2t=a+2y (10.1b) where ti is time consumed by the ith REGISTER, and y is the average round- trip communication delay between ruser and the registering SIP server. Obvi- ously, t A includes a relatively significant communication delay as a result of the presumably large geographical separation between the roaming user and the home SIP server. Now, if we consider H-SIP, then from Figure 10.5c, the average registration time tc as seen by ruser is given by, (10.2a) 1<i<n thatxtu (10.2b) (10.2c) C 2009 by Taylor & Francis Group, LLC SIP Mobility Technologies and the Use of Persistent Identifiers where t'1 is time to perform the ith REGISTER, X is the average handle resolution delay and U is the average handle update delay. First, we note that the round-trip delay y is negligible in this 