Example of Proxy-Authenticate Header Field Header Field Meaning Proxy-Authenticate Digest HTTP digest challenge header field realm="example.com" nonce="9c8e88df84f1cec4341ae6e5a359" opaque=" stale=FALSE, algorithm=MD5 SIP: Understanding the Session Initiation Protocol 6.3.8 Warning The Warning header field is used in a response to provide more specific infor- mation than the response code alone can convey. The header field contains a three-digit warning code, a warning agent that indicates what server inserted the header field, and warning text enclosed in quotes used for display purposes. Warning codes in the 1xx and 2xx range are specific to HTTP [2]. The SIP standard defines 12 new warning codes in the 3xx class. The breakdown of the class is shown in Table 6.17. The complete set of defined warning codes is listed in Table 6.18. Examples are: Warning: 302 proxy "Incompatible transport protocol" Warning: 305 room132.hotel.com: 5060 "Incompatible media type" 6.3.9 WWW-Authenticate The WWW-Authenticate header field is used in a 401 Unauthorized authentication challenge by a user agent or registrar server to a UAC. It con- tains the nature of the challenge SO that the UAC may formulate credentials in a Proxy-Authorization header field in a subsequent request. SIP supports HTTP digest authentication mechanisms. Examples are shown in Table 6.19. 6.3.10 RSeq The RSeq header field [10] is used in provisional (1xx class) responses to INVITES to request reliable transport. The header field may only be used if the INVITE request contained the Supported: rel100 header field. If pres- ent in a provisional response, the UAC should acknowledge receipt of the Table 6.17 SIP Warning Codes Warning Code Range Error Type 31x, 32x SDP keywords Network services 34x, 35x, 36x Reserved for future use OoS parameters Reserved Miscellaneous SIP Header Fields Table 6.18 SIP Warning Code List Warning Code Description Incompatible network protocol Incompatible network address formats Incompatible transport protocol Incompatible bandwidth u