Cisco Systems Cisco Systems This document defines an extension to the Session Initiation Protocol (SIP). This extension allows for a periodic refresh of SIP sessions through a \%re-INVITE or UPDATE request. The refresh allows both user agents and proxies to determine whether the SIP session is still active. The extension defines two new header fields: \%Session-Expires, which conveys the lifetime of the session, and \%Min-SE, which conveys the minimum allowed value for the session timer. [STANDARDS-TRACK]