. The P-CSCF is used as the default outbound proxy server for a UE outside its home network. The Interrogating CSCF queries the HSS to determine the proper Service CSCF. The I-CSCF also can do hiding of the S-CSCF network by removing or encrypting Via header fields. The Serving CSCF provides the services for the subscriber. It identifies the user's service profile and privileges. The 3GPP plans to exclusively use IPv6 addresses. This is due to the number of 3GPP subscribers envisioned and the fact that with Mobile IP, each device may use more than one IP address at a time. SIP RFC 3261 includes full support for IPv6 addresses, and an extension to SDP [9] adds IPv6 support to The 3GPP also uses signaling compression [10] to CO impress SIP messages transmitted over a wireless link. This is primarily done to minimize latency rather than bandwidth savings. The use of signaling compression with SIP is described in [11], which defines a parameter comp=sigcomp that can be used in Via header fields and as a URI parameter that can be used, for example, in a Contact header field. The 3GPP uses the adaptive multirate (AMR) [12] codec for the audio encoding. Wireless and 3GPP 9.4 3GPP Header Fields Some SIP header fields have been developed based on 3GPP requirements and are discussed in the next sections. They are listed here instead of the main listing of SIP header fields in Chapter 6 since their use is not well stan- dardized outside the 3GPP architecture. In the future, however, their use may be standardized within the IETF and they may be used more widely. 9.4.1 Service-Route The Service-Route header field [13] can be used in a 2xx response to a REGISTER request. It can be used by a registrar server to provide to the regis- tering UA URIs to include in a preloaded Route header field in future requests. The Service-Route URIs are only valid for the duration of the registration and should be updated when the registration is refreshed. An example is: Service-Route: sip:proxy23.service.provider.com;lr> 9.4.2 Path The Path 