session timer

RFC4028 defines the SIP Session Timer mechanism (which is a way to prevent SIP sessions hanging up indefinitely). A brief description of its operation is as follows:

SIP Session Timer is a mechanism in which a ‘heartbeat’ message sequence is used within a SIP session (a call) to protect against the unexpected loss of an endpoint leading to a SIP session continuing to remain up.
There is an initial negotiation (actually a simple offer/answer like SDP) which determines:
- If the session is to be refreshed
- How often the refresh is to be performed
- Which end does the refresh
- The refresh mechanism to be used
Two new headers are defined to support Session Timer: ‘Session Expires’ and ‘Min-SE’. These are used to define the initial offer and the subsequent answer.

One new response code (422) is defined to report offered session timer duration is too small i.e. 'Session Expires' is less than 'Min-SE' supported.

In a SIP session with Session Timer active, either end can do the refresh and this can be done with either a ‘reINVITE’ or by using the SIP ‘UPDATE’ method.

Blueworx Voice Response implements RFC4028. To control Session Timer operation, Blueworx Voice Response provides the following additional System Parameters in the VoIP SIP Signalling group:

Session Timer Enable: Allows the Session Timer to be disabled completely, enabled for inbound calls only, enabled for outbound calls only, or enabled for both directions. See Session Timer Enable.
Session Timer Minimum Session Time: Minimum allowed session time in seconds. This controls the value put in the ‘Min-SE’ header of a Session Timer offer. See Session Timer Minimum Session Time
Session Timer Maximum Session Time: Maximum allowed session time in seconds. See Session Timer Maximum Session Time.
Session Timer Inbound Refresher Default: See Session Timer Inbound Refresher Default.
Session Timer Outbound Calls Refresher Default: See Session Timer Outbound Calls Refresher Default.
Session Timer Allow Update For Refresh: This controls whether reINVITE or UPDATE are to be used for refresh. See Session Timer Allow Update For Refresh.

Session timer