SUBSCRIPTIONS THAT INDICATE THE PRESENCE OF APPLICATION SERVERS

1 . A system comprising:

a serving network element of an IP Multimedia Subsystem (IMS) network configured to communicate with a primary application server to provide a service for a session, to determine that the primary application server is unavailable, to failover to a secondary application server to provide the service, and to initiate a subscribe request to a presence server requesting to be notified when the primary application server becomes available,

the serving network element further configured to receive a notification from the presence server that the primary application server has become available, and to switch back to the primary application server to provide the service after receiving the notification.

2 . The system of claim 1 , wherein:

the serving network element is further configured to generate the subscribe request as a Session Initiation Protocol (SIP) SUBSCRIBE request.

3 . The system of claim 1 , wherein:

the serving network element comprises a Serving Call Session Control Function (S-CSCF).

4 . The system of claim 1 , wherein:

the serving network element is further configured to include a Session Initiation Protocol (SIP) address for the application server in the subscribe request.

5 . The system of claim 1 , wherein:

the serving network element is further configured to include a name for the service provided by the primary application server in the subscribe request.

6 . The system of claim 1 , wherein:

the serving network element is further configured to identify a Session Initiation Protocol (SIP) address for the presence server, and to initiate the subscribe request to the identified SIP address.

7 . The system of claim 1 , wherein:

the serving network element is further configured to analyze the notification to identify a Session Initiation Protocol (SIP) address, and to determine that the SIP address belongs to the primary application server.

8 . A method comprising:

operating a serving network element of an IP Multimedia Subsystem (IMS) network to communicate with a primary application server to provide a service for a session;

detecting that the primary application server is unavailable;

failing over to a secondary application server to provide the service;

initiating a subscribe request from the serving network element to a presence server requesting that the serving network element be notified when the primary application server becomes available;

receiving a notification from the presence server at the serving network element indicating that the primary application server has become available; and

switching back to the primary application server to provide the service after receiving the notification.

9 . The method of claim 8 , further comprising:

transmitting the subscribe request comprises initiating a Session Initiation Protocol (SIP) SUBSCRIBE request.

10 . The method of claim 8 , wherein:

the serving network element comprises a Call Session Control Function (CSCF).

11 . The method of claim 8 , further comprising:

including a Session Initiation Protocol (SIP) address for the application server in the subscribe request.

12 . The method of claim 8 , further comprising:

including a name for the service provided by the primary application server in the subscribe request.

13 . The method of claim 8 , further comprising:

identifying a Session Initiation Protocol (SIP) address for the presence server; and

initiating the subscribe request to the identified SIP address.

14 . The method of claim 8 , further comprising:

analyzing the notification to identify a Session Initiation Protocol (SIP) address; and

determining that the SIP address belongs to the primary application server.

15 . A system comprising:

a presence server configured to communicate with network elements of an IP Multimedia Subsystem (IMS) network,

the presence server configured to receive a subscribe request from a serving network element that serves a session, wherein the subscribe request from the serving network element requests that the serving network element be notified when a primary application server becomes available to provide a service for the session,

the presence server further configured to receive a status message published from the primary application server indicating that the primary application server is available, and to send a notification to the serving network element that the primary application server has become available as a response to the subscribe request.

16 . The system of claim 15 , wherein:

the presence server is further configured to access the status message to identify a Session Initiation Protocol (SIP) address for the application server, and to include the SIP address of the application server when notifying the serving network element.

17 . The system of claim 15 , wherein:

the presence server is further configured to notify the serving network element by initiating a Session Initiation Protocol (SIP) NOTIFY to the serving network element.

18 . The system of claim 15 , wherein:

the presence server is further configured to access the status message to identify a name for a service provided by the application server, and to include the name for the service when notifying the serving network element.

19 . The system of claim 15 , wherein:

the presence server is further configured to access the status message to identify a reason why the application server became unavailable, and to include the reason when notifying the serving network element.

20 . The system of claim 15 , wherein:

the presence server is further configured to identify the unavailable application server based on a Session Initiation Protocol (SIP) address included in the subscribe request.