SIP headers 내용 정리
The Session-Expires header field is used to specify the expiration time of the session. To extend the session, either UA can send a re-INVITE or UPDATE with a new Session-Expires header field.
At the expiration of the interval in the Session-Expires, either UA may send a BYE and call-stateful proxies may destroy any state information. A proxy may shorten the expiration time by reducing the interval in the header field as it proxies the request.
A UAS confirms the session timer by including the Session-Expires header filed in the response to the request. A UAS may also shorten the interval by reducing the interval.