A SIP session timer has been proposed to limit the time period over which a stateful PROXY must MAINTAIN state information. In the initial INVITE request, a Session-Expires HEADER field indicates a timer interval after which stateful proxies may discard state information about the session. User agents must TEAR down the call after the expiration of the timer. The caller can send re-INVITEs to refresh the timer, ENABLING a “keep alive” mechanism for SIP. This solves the problem of how long to store state information in cases where a BYE request is lost or misdirected.

A SIP session timer has been proposed to limit the time period over which a stateful proxy must maintain state information. In the initial INVITE request, a Session-Expires header field indicates a timer interval after which stateful proxies may discard state information about the session. User agents must tear down the call after the expiration of the timer. The caller can send re-INVITEs to refresh the timer, enabling a “keep alive” mechanism for SIP. This solves the problem of how long to store state information in cases where a BYE request is lost or misdirected.