 is not given in the INVITE request. 2.6.1.4. BYE method The BYE method is used to terminate an established session. A session is considered to be established when the response 2xx is received following the INVITE request. The BYE request is transmitted by either of the UA entities participating in the session. The UAS sends the response back 2xx if the dialog is known, or else sends the response 481 Dialog/Transaction Does Not Exist. 2.6.1.5. CANCEL method The CANCEL method is used to terminate a session which has not yet been successful. It is generated when a provisional response 1xx has been received, but not a definitive response. Upon receiving a CANCEL request, the PROXY SERVER transmits the request to the next hop (a PROXY SERVER or a UA entity) and confirms the cancellation directly to the source with the response 200 OK. A UAS receiving the CANCEL request confirms cancellation with the response 200 OK and terminates the dialog initiated by the INVITE request with a definitive negative response 487 Request Terminated. 2.6.1.6. PRACK method The PRACK method is used to acknowledge the reception of a provisional response (1xx), with the exception of the response 100 Trying. The PRACK request is transmitted when the provisional response received contains the headers CSeq and Rseq. The PRACK request must include the header RAck containing the values of the headers CSeq and Rseq of the provisional response received. VoLTE and ViLTE The provisional response is transmitted at the expiration of a timer until the reception of the PRACK request. The PRACK request is transmitted at the expiration of a timer until the reception of a 200 OK response. 2.6.1.7. UPDATE method The UPDATE method modifies the characteristics of the media in a session which has not yet been successfully established. If the session has successfully been established (the INVITE request has received a 2xx response), the modification of the characteristics of the media is negotiated by the INVITE method (reINVITE). 2.6.1.8. SUBSCRIBE method The 