header field [14] is an optional header field in REGISTER requests. It can be thought of as a Record-Route mechanism REGISTER requests, which establishes a route set that is valid for the dura- tion of the registration. The Path header field may be inserted by a proxy, which forwards a REGISTER request to a registrar server. The registrar copies the Path header field into the 200 OK response to the REGISTER, which then provides the route set information to the user agent that is registering. In a mobile network, the Path header field can be used to discover and inform the user agent of the proxies can be used to populate preloaded Route header fields. An example is: Path: <sip:proxy2.another.provider.com;lr> 9.4.3 Other P-Headers In addition, some headers that are specific to 3GPP have been defined [15]. These so-called P-headers (which stands for proprietary, preliminary, or pri- vate) are defined in syntax only in an informational RFC per the SIP change process [16]. Examples of the use of these P-headers are given in the call flow of Section 10.7. They are listed in Table 9.2. SIP: Understanding the Session Initiation Protocol Table 9.2 3GPP P-Headers Header Field P-Associated-URI Lists other URIs associated with the user P-Called-Party-ID Lists the URI of the called party P-Visited-Network-ID Identifies the visited network P-Access-Network-Info Identifies the access network P-Charging-Function-Addresses Contains charging information P-Charging-Vector More charging information Source: [15]. 9.5 Future of SIP and Wireless It is clear that as IP networks become increasingly wireless, SIP will often be util- ized over wireless networks. It is well suited for such use for the reasons dis- cussed in this chapter: it has both built-in mobility support when Mobile IP is not used, and can also be used with Mobile IP depending on the wireless net- work design. Additional work on authentication and roaming will likely be done with SIP as the extensions developed for the 3GPP architecture are too specific to be useful in