e token and use this information about accepting the INVITE or rejecting the call. This approach is suitable for a clearinghouse model of VoIP carrier interconnection. SIP: Understanding the Session Initiation Protocol SIP phone Proxy server 1 Proxy server 2 INVITE 407 Proxy Authentication required INVITE INVITE 100 Trying 407 Proxy 407 Proxy Authentication required Authentication required INVITE INVITE 100 Trying 100 Trying Figure 6.3 Multiproxy authentication example. Table 6.15 Example of Proxy-Authorization Header Field Header Field Meaning Proxy-Authorization: Digest This digest authorization header field username="Customer1" , contains the credentials of Customer1; realm="company.com", the nonce was supplied by the SIP nonce="9c8e88df84f1cec4341ae6e5a359" server located at the URI specified; the opaque=" uri="sip:user@company.com" esponse="e56131d19580cd833064787ecc" response contains the encrypted user- name and password; no opaque string is present An example is: P-OSP-Auth-Token: 3b8a40c10b4930ff19a85766c15182a34048d9398b834d6 ;realm="carrier.com" SIP Header Fields 6.2.16 P-Asserted-Identity The P-Asserted-Identity header field [17] is used between trusted intermediaries (proxies) to assert the identity of a user agent that has been authenticated using some means such as those described in Section 3.8. A UA receiving a request from a proxy that it trusts will typically render the value in a P-Asserted-Identity header field to the user as a "Verified Caller ID" as opposed to a From header value which is unverified. A proxy receiving a P-Asserted-Identity from another proxy that it does not trust will remove the header field. An example is: P-Asserted-Identity: <sip:user@example.com> 6.2.17 P-Preferred-Identity The P-Preferred-Identity header field [17] is used by a user agent to tell a trusted intermediary which identity it would prefer be asserted on its behalf when more than one identities are associated with that user agent. An example is: P-Preferred-Identity: <sip:alternate@example.com> 6.2.18 Max-Fo