SIP headers 내용 정리
The Max-Forwards header field is used to indicate the maximum number of hops that a SIP request may take. The value of the header field is decremented by each proxy that forwards the request. A proxy receiving the header field with a value of zero discards the message and sends a 403 Too Many Hops response back to the originator.
Max-Forwards is a mandatory header field in requests generated by an RFC 3261 compliant UA. However, an RFC 2543 UA generally will not include the header field. The suggested initial value is 70 hops.
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 field in the response to the request. A UAS may also shorten the interval by reducing the interval.