 server.) The expires parameter indicates how long the URI is valid and is also only used in registrations. The parameter either contains an integer number of seconds or a date in SIP form (see Section 6.1.4). Examples are shown in Table 6.3. The Contact header field may contain a feature tag [4], which can be used to indicate the capabilities of the device identified by the Contact URI. For example, the feature tag isfocus is used to indicate that the URI in the Contact header field is a Conference URI, and that the dialog is associates with a focus. A focus is a SIP user agent that hosts a particular instance of a con- ference, called a "bridge" or MCU in other protocols. The presence of the isfocus feature tag can be used by a SIP user agent that supports advanced Table 6.3 Examples of Contact Header Fields Header Field Meaning Contact: sip:bell@telephone.com A single SIP URI without a display name. Contact: Lentz k<h.lentz@petersburg.edu> A display name with the URI is enclosed in < >; the display name is treated as a token and ignored. Contact: M. Faraday <faraday@effect.org>, Two URIs are listed, the second being "Faraday" <mailto:faraday@pop.effect.org> a non-SIP URI with a display name en- closed in quotes. m: <morse@telegraph.org;transport=tcp> The compact form of the header field expires= "Fri, 13, Oct 1998 12:00:00 GMT " is used for a single URI. The URI contains a port number and a URI parameter contained within the <>. An expires header field parameter uses a SIP date enclosed in the quotes. SIP: Understanding the Session Initiation Protocol conferencing features to invoke certain call control operations [5] or subscribe to the conference package [6]. Other feature tags are listed in Table 6.4. The compact form is m. 6.1.5 The command sequence CSeq header field is a required header field in every request. The CSeq header field contains a decimal number that increases for each request. Usually, it increases by 1 for each new request, with the exception of CANCEL and ACK requests, which use the CSeq nu