ful INVITE request establishes a dialog between the two user agents, which continues until a BYE is sent by either party to end the session, as described in Section 4.1.3. A UAC that originates an INVITE to establish a dialog creates a globally unique Call- ID that is used for the duration of the call. A CSeq count is ini- tialized (which need not be set to 1, but must be an integer) and incremented for each new request for the same Call-ID. The To and From headers are popu- lated with the remote and local addresses. A From tag is included in the INVITE 100 trying 180 ringing 200 OK sdp UAS sdp UAC Media session Figure 4.1 INVITE with no SDP message body. SIP Request Messages INVITE, and the UAS includes a To tag in any responses, as described in Section 4.3. A To tag in a 200 OK response to an INVITE is used in the To header field of the ACK and all future requests within the dialog. The combina- tion of the To tag, From tag, and Call-ID is the unique identifier for the dialog. An INVITE sent for an existing dialog references the same Call-ID as the original INVITE and contains the same To and From tags. Sometimes called a re-INVITE, the request is used to change the session characteristics or refresh the state of the dialog. The CSeq command sequence number is incre- mented SO that a UAS can distinguish the re-INVITE from a retransmission of the original INVITE. If a re-INVITE is refused or fails in any way, the session continues as if the INVITE had never been sent. A re-INVITE must not be sent by a UAC until a final response to the initial INVITE has been received-instead, an UPDATE request can be sent, as described in Section 4.1.13. There is an addi- tional case where two user agents simultaneously send re-INVITES to each other. This is handled in the same way with a Retry-After header. This condition is called glare in telephony, and occurs when both ends of a trunk group seize the same trunk at the same time. An Expires header in an INVITE indicates to the UAS how long the call request is valid. For examp