The SIP-event notification mechanism allows a SIP entity to request notification from remote nodes indicating that certain standardised events have occurred. Examples of such of events are changes in presence states, changes in registration states, changes in Subscription authorization policies (see TS 23.141
) and other events that are caused by information changes in e.g. Application Servers or S-CSCF.
It shall be possible to either fetch relevant information once or monitor changes over a defined time. It shall be possible for a user to subscribe to events related to his/her own subscription (e.g. when the user subscribes to his own registration state) or to events related to other users' subscription (an example is when a watcher subscribes to presence information of a presentity, see TS 23.141
The S-CSCF is not mandated to stay in the path after the initial SubscribeEvent request and ACK has been exchanged, if the S-CSCF does not execute any functions for the subsequent requests and responses of the dialog. The example, in Figure 5.8a
below, assumes that the S-CSCF does not want to execute any functions for the subsequent requests.
The UE initiates a subscription to an AS requesting notification of any changes in specified information stored in the control of the AS
The P-CSCF remembers (from the registration process) the next hop CSCF for this UE, i.e., the SubscribeEvent is forwarded to the S-CSCF in the home network.
The S-CSCF invokes whatever service logic procedures are appropriate for this request.
The S-CSCF applies regular routing procedures and forwards the request to the next hop.
The AS acknowledges the SubscribeEvent request.
The S-CSCF forwards the acknowledgement to the P-CSCF.
The P-CSCF forwards the acknowledgement to the UE.
As soon as the AS sends an acknowledgement to accept the subscription, the AS sends an EventNotification message with the current information the UE subscribed to. The EventNotification is sent along the path set-up by the SubscribeEvent dialog to the P-CSCF allocated to the UE. Further notifications, if monitor of changes was requested, sent by the AS is sent along the same path.
The P-CSCF forwards the EventNotification to the UE.
The UE acknowledges the EventNotification.
The P-CSCF forwards the acknowledgement to the AS.