ted Media Type response must be returned. The compact form is C. Specific MIME types that are commonly use are listed in Table 6.20. Content indirection [28] can be used to provide a URI in place of actual MIME message body. An example is: Table 6.20 Common Content-Types Present in SIP Requests and Responses Content-Type application/sdp SDP in INVITE, ACK, or UPDATE requests [22] message/sipfrag SIP fragment in NOTIFY in refer subscription [23] application/xml+dialog XML dialog [24] application/xml+conf XML conference info [25] application/cpim CPIM [26] text/plain Plain text text/html HTML text application/isup Encapsulated ISUP in INVITE, BYE, or INFO [27] SIP: Understanding the Session Initiation Protocol Content-Type: message/external-body access-type="URL"; RL="http://www.example.com/" The compact form is C. Examples are: Content-Type: application/sdp C: text/html 6.4.7 Expires The Expires header field is used to indicate the time interval in which the request or message contents are valid. When present in an INVITE request, the header field sets a time limit on the completion of the INVITE request. That is, the UAC must receive a final response (non-1xx) within the time period or the INVITE request is automatically canceled with a 408 Request Timeout response. Once the session is established, the value from the Expires header field in the original INVITE has no effect-the Session-Expires header field (Section 6.2.19) must be used for this purpose. When present in a REGISTER request, the header field sets the time limit on the URIs in Con- tact header fields that do not contain an expires parameter. Table 4.3 shows examples of the Expires header field in registration requests. The header field is not defined for any other method types. The header field may contain a SIP date or a number of seconds. Examples include: Expires 60 Expires: Fri, 15 Apr 2000 00:00:00 GMT 6.4.8 MIME-Version The MIME-Version header field is used to indicate the version of MIME protocol used to construct the message body. SIP, like H