agent, it may not be possible for a carrier network to use this field to route emergency traffic, for example. An example is: SIP Header Fields Priority: emergency 6.2.12 Privacy The Privacy header field [14] is used by a UAC to request varying degrees and types of privacy. Currently defined tags include critical, header, id, session, user, or none. An example follows: Privacy: header; user critical 6.2.13 Proxy-Authorization The Proxy-Authorization header field is to carry the credentials of a user agent in a request to a server. It can be sent in reply to a 407 Proxy Authentication Required response containing challenge information, or it can be sent first without waiting for the challenge if the form of the chal- lenge is known (e.g., if it has been cached from a previous call). The authentica- tion mechanism for SIP digest is described in Section 3.9. A proxy receiving a request containing a Proxy-Authorization header field searches for its own realm. If found, it processes the entry. If the credentials are correct, any remaining entries are kept in the request when it is forwarded to the next proxy. An example of this is in Figure 6.3. Examples are shown in Table 6.15. 6.2.14 Proxy-Require The Proxy-Require header field is used to list features and extensions that a user agent requires a proxy to support in order to process the request. A 420 Bad Extension response is returned by the proxy listing any unsupported feature in an Unsupported header field. Because proxies by default ignore header fields and features they do not understand, the use of a Proxy- Require header field is needed for the UAC to be certain that the feature is understood by the proxy. If the support of this option is desired but not required, it is listed in a Supported header field instead. An example is: Proxy-Require: timer 6.2.15 P-OSP-Auth-Token The P-OSP-Auth-Token header field [15] is used to transport an Open Settlements Protocol (OSP) token [16] with a SIP INVITE request. A gateway or proxy server receiving a token can verify th