cates when the called user agent may be available again. The header field can also be included by proxy and redirect servers in responses if a recent registration was removed with a Retry-After header field indicating when the user may sign on again. The contents of the header field can be either an integer number of seconds or a SIP date. A duration parameter can be used to indicate how long the resource will be available after the time specified. Examples of this header field are shown in Table 6.7. 6.1.12 Subject The optional Subject header field is used to indicate the subject of the media session. It can be used by user agents to do simple call screening. The contents of the header field can also be displayed during alerting to aid the user in deciding whether to accept the call. The compact form of this header field is S. Some examples are: Subject: More good info about SIP S: Are you awake, yet?? SIP: Understanding the Session Initiation Protocol Table 6.7 Examples of Retry-After Header Field Header Field Meaning Retry-After: 3600 Request can be retried again in 1 hour Retry-After: Sat, 21 May 2000 08:00:00 GMT Request can be retried after the date listed Retry-After: 3600 Request can be tried after 1 hour Retry-After: Mon, 29 Feb 2000 13:30:00 GMT Request can be retried after the ; duration=1800 specified date for 30 minutes 6.1.13 Supported The Supported header field is used to list one or more options implemented by a user agent or server. It is typically included in responses to OPTIONS requests. If no options are implemented, the header field is not included. If a UAC lists an option in a Supported header field, proxies or UASs may use the option during the call. If the option must be used or supported, the Require header field is used instead. Table 6.8 shows the current set of defined feature tags. An example of the header field is: Supported: rel100 6.1.14 Timestamp The Timestamp header field is used by a UAC to mark the exact time a request was generated in some numerical time format. A UAS must e