.2 SIP Mobility Personal mobility is the ability to have a constant identifier across a number of devices. A sip or sips URI has exactly this property and is fundamentally supported by SIP. SIP can also support service mobility, the ability of a user to keep the same services when mobile, although some conventions and extensions have been proposed that provide this in certain architectures. Wireless and 3GPP Basic personal mobility is supported by SIP using the REGISTER method, which allows a mobile device to change its IP address and point of con- nection to the Internet and still be able to receive incoming calls. As discussed in Chapters 2 and 4, registration in SIP temporarily binds a user's AOR URI with a Contact URI of a particular device. As a device's IP address changes, regis- tration allows this information to be automatically updated in the SIP network. An end device can also move between service providers using multiple layers of registrations, in which a registration is actually performed with a Contact as an address of record with another service provider. For example, consider the user agent in Figure 9.2, which has temporarily acquired a new SIP URI with a new service provider. (The reasons for doing SO could include security, local pol- icy, NAT/firewall traversal.) The user agent then performs a double registration as shown in Figure 9.2. The first registration is with the new service provider, which binds the Contact URI of the device with the new service provider's AOR URI. The second REGISTER request is routed back to the original serv- ice provider and provides the new service provider's AOR as the Contact URI. As shown later in the call flow, when a request comes in to the original service provider's network, the INVITE is redirected to the new service pro- vider who then routes the call to the user. For the first registration message containing the device URI would be: REGISTER sip:registrar.capetown.org SIP/2.0 Via: SIP/2.0/TLS 128.5.2.1:5060;branch=z9hG4bK382112 Max-Forwards: 70 To: Nath