 To contains the URI of the UA entity which needs to be registered. The header From contains the URI of the entity performing the registration. This entity is generally identical to that of the header To. A UA entity may receive a redirect (3xx) or failure (4xx) response, whose header Contact contains the place where the registrations need to be sent. 2.6.1.2. INVITE method The INVITE method is used by a user agent client (UAC) to establish a dialog or a session. The definitive (positive or negative) responses need to be acknowledged by the ACK request. The INVITE request may contain a message body describing the media that the UAC entity wants to establish. If this description is absent, it needs to be added to the ACK request. The response 200 OK contains the description of the media that the user agent server (UAS) wants to establish. The media are established with the response 200 OK (if the INVITE request contains the description of the media) or with the ACK request (otherwise). A successful INVITE request establishes a dialog between two UA entities, which continues until a BYE request is sent by one of the parties to terminate the session. The dialog is identified by the header Call - ID and the parameter tag of the headers To (parameter specified by the caller) and From (parameter specified by the callee). The headers To and From are respectively specified with the identities of the callee and the caller. Within the dialog, it is possible for each UA entity to transmit one or more requests, with each request initializing a transaction. It is possible to transmit a new INVITE request (reINVITE) within a dialog, for an established session, to update the characteristics of the media. If the reINVITE request is declined, the session continues with the previous characteristics. Signaling Protocols 2.6.1.3. ACK method The ACK method is used to acknowledge a definitive response (2xx, 3xx, 4xx, 5xx and 6xx) to the INVITE request. The ACK request may contain a message body describing the media if this description