s shown in Figure 10.8, show that the Handle System can efficiently replace the DNS system. On top of addressing the domain resolution itself, our approach minimizes the signaling traffic needed by a roaming user to join the SIP infrastruc- ture and be ready to initiate calls. The user is efficiently utilizing the ser- vices of a local server with no need for per-call coordination with his home server. If the home server is located in another continent per se, the round trip times for registration/redirect messages from the roaming user to home proxy/correspondent user, respectively, become significant. The proposed approach optimizes the association, authentication, and call routing times for the roaming user by guaranteeing that the mobile user will always be addressed through the closest available server in his vicinity. 10.5 Future Work This paper used globally unique handles to identify SIP users. It is also pos- sible to use a set of aliases of such handles that can in turn translate into © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security handles. Such a service would be administered by yet another service provider using the handle protocol to administer and coordinate the general use of aliases in the system. This may be done using registrylike features for particular handles, and allows the provider to service users that opt into this service. Since handles can also point to other handles, certain intermediate handles may be provided to systems that choose to run private handle servers in a way similar to a Private Branch eXchange (PBX). The advantage of having a distributed resolution infrastructure that is also domain-independent, trans- lates into users being able to run smaller SIP servers that communicate with the Handle System to expedite routing. You could even envision a cellular-like behavior for SIP systems in which users are able to use the resources of many smaller SIP servers along a user's roaming path. All these ideas need to be investigated as well as 