>;tag=34kdilsp3 Call-ID: 123456787@173.23.43. CSeq: 1 INVITE WWW-Authenticate: Digest realm="globe.org", nonce="8eff88df84f1cec4341ae6e5a359", gop="auth", opaque=" ", stale=FALSE, algorithm=md5 Content-Length: 0 The presence of the required WWW-Authenticate header field is required to give the calling user agent a chance to respond with the correct cre- dentials. A typical authentication exchange using SIP digest is shown in Figure 3.7. Note that the follow-up INVITE request should use the same Call-ID as the original request as the authentication may fail in some cases if the Call-ID is changed from the initial request to the retried request with the proper credentials. 5.4.3 402 Payment Required This response is a placeholder for future definition in the SIP protocol. It could be used to negotiate call completion charges. SIP Response Messages 5.4.4 403 Forbidden This response is used to deny a request without giving the caller any recourse. It is sent when the server has understood the request, found the request to be cor- rectly formulated, but will not service the request. This response is not used when authorization is required. 5.4.5 404 Not Found This response indicates that the user identified by the sip or sips URI in the Request-URI cannot be located by the server, or that the user is not currently signed on with the user agent. 5.4.6 405 Method Not Allowed This response indicates that the server or user agent has received and understood a request but is not willing to fulfill the request. An example might be a REGISTER request sent to a user agent. An Allow header field (Section 6.4.1) must be present to inform the UAC what methods are acceptable. This is differ- ent from the case of an unknown method, in which a 501 Not Imple- mented response is returned. Note that a proxy will forward request types it does not understand unless the request is targeted to the proxy server (i.e., the Request-URI is the URI of the proxy server). 5.4.7 406 Not Acceptable This response indicates that the request cannot b