mber of the INVITE request to which it refers. The CSeq count is used by UASs to determine out-of-sequence requests or to differentiate between a new request (different CSeq) or a retransmission (same CSeq). The CSeq header field is used by UACs to match a response to the request it references. For example, a UAC that sends an INVITE request then a CANCEL request can tell by the method in the CSeq of a 200 OK response if it is a response to the invitation or cancellation request. Examples are shown in Table 6.5. Each user agent maintains its own command sequence number space. For example, consider the case where user agent 1 establishes a session to user agent 2 and initializes its CSeq count to 1. When user agent 2 initiates a request (such as a INVITE or INFO, or even BYE) it will initialize its own CSeq space, totally independent of the CSeq count used by user agent 1. The examples of Chapter 10 show this behavior of CSeq. 6.1.6 Date The Date header field is used to convey the date when a request or response is sent. The format of a SIP date is based on HTTP dates, but allows only the Table 6.4 Boolean Feature Tags Feature tag Meaning attendant Attendant, human or automata automata Nonhuman image Supports images message Supports messaging Supports text voicemail Is a voicemail server isfocus Is a focus, a conference server SIP Header Fields Table 6.5 CSeq Header Field Examples Header Field Meaning CSeq: 1 INVITE The command sequence number has been initialized to 1 for this initial INVITE CSeq: 432 REFER The command sequence number is set to 432 for this REFER request CSeq: 6787 INVITE If this was the first request by the user agent for this dialog then either the CSeq was initialized to 6787, or the previous request generated for this Call-ID (either an INVITE or other request) would have had a CSeq of 6786 or lower. preferred Internet date standard referenced by RFC 1123 [7]. To keep user agent date and time logic simple, SIP only supports the use of the GMT time zone. This allows time entries that are store