SIP provides a mechanism by which both user agents and proxies can determine whether a given SIP session is still active. This mechanism is referred to as a Session Timer and is described in RFC 4028 “Session Timers in SIP”. This specification defines a keepalive mechanism for SIP sessions.

In a SIP session that utilizes a session timer, UAs send periodic re-INVITE or UPDATE requests to refresh the session. The interval at which these session refreshers are sent and which UA is responsible for sending them is regotiated in the initial INVITE transaction that sets up the session. If a session refresher request is not received before the negotiated interval expires, both the UAS and UAC should send a BYE and any proxies in between can remove any state maintained for the session.

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