t>:<port>. 2 00 OK. 3xx (redirectional response): information used to indicate the user's new location or alternative services. 3.6.2.2.2 Header fields 4xx (request failure): a response used by a UAS or A header field is composed as <header> : <field>. a server to indicate that the request cannot be processed There are 44 header fields defined in RFC 3261: Accept, due to authorization failure, authentication failure, ac- Accept-Encoding, Accept-Language, Alert-Info, Allow, count issue, requesting itself, or other problems not re- Authentication-Info, Authorization, Call-ID, Call-Info, lated to the server. Example: 400 Bad Request Contact, Content-Disposition, Content-Encoding, indicates that the server does not understand the request. Content-Language, Content-Length, Content-Type, CSeq, 5xx (server failure): a response used by a UAS or Date, Error-Info, Expires, From, In-Reply-To, a server to indicate that the request cannot be processed Max-Forwards, Min-Expires, MIME-Version, Organiza- due to the server's problem. Examples include 500 tion, Priority, Proxy-Authenticate, Proxy-Authorization Server Internal Error and 501 Not Proxy-Require, Record-Route, Reply-To, Require, Retry- Implemented. After, Route, Server, Subject, Supported, Timestamp, 6xx (global failure): a response used to indicate that To, Unsupported, User-Agent, Via, Warning, and the response will fail in all locations and thus the request WWW-Authenticate. The most common ones are in- should not be delivered. troduced here: A simple message flow for call setup and tear down is Call-ID: used to uniquely identify a call. Example: illustrated in Figure 3.6-4. The SIP request message is CALL- ID :t315fde3-68te-33uyr@test.com composed as follows: method name (e.g., INVITE), ad- Contact: used to carry a URI that identifies the dress, header fields, and message body. Each response resource requested or the request originator. Example: message consists of a code (e.g., 200 OK), header fields, Contact: sip: johnsmith@test.com and message body. Note that th