nits Media type not available Incompatible media format Attribute not understood Session description parameter not understood Multicast not available Unicast not available Insufficient bandwidth Miscellaneous warning Table 6.19 Example of WWW-Authenticate Header Field Header Field Meaning WWW-Authenticate: : Digest HTTP digest challenge realm="example.com", nonce="9c8e88df84f1cec4341ae6e5a359", opaque=" ", stale=FALSE, algorithm=MD5 response with a PRACK method, as described in Section 4.1.12. The RSeq header field contains a reliable sequence number that is an integer randomly initialized by the UAS. Each subsequent provisional response sent reliably for this dialog will have a monotonically increasing RSeq number. The UAS matches the reliable sequence number and CSeq from the RACK in a PRACK request to a sent response to confirm receipt and stop all retransmissions of the response. An example is: RSeq: 2345263 SIP: Understanding the Session Initiation Protocol 6.4 Message Body Header Fields These header fields contain information about the message body or entity. 6.4.1 Allow The All ow header field is used to indicate the methods supported by the user agent or proxy server sending the response. The header field must be present in a 405 Method Not Allowed response and should be included in a positive response to an OPTIONS request. An example is: Allow: INVITE, ACK, BYE, INFO, OPTIONS, CANCEL 6.4.2 Content-Encoding The Content-Encoding header field is used to indicate that the listed encoding scheme has been applied to the message body. This allows the UAS to determine the decoding scheme necessary to interpret the message body. Multi- ple listings in this header field indicate that multiple encodings have been used in the sequence in which they are listed. Only encoding schemes listed in an Allow-Encoding header field may be used. The compact form is e. Exam- ples include: Content-Encoding: text/plain e: gzip 6.4.3 Content-Disposition The Content-Disposition header field is used to describe the function of a me