P-CSCF recovery and reregistration

1. A method performed in an IP multimedia subsystem (IMS) of a telecommunications network, the method comprising:

detecting a loss of a secure association between a P-CSCF of the IMS and a user equipment (UE) served by the P-CSCF;

at the P-CSCF:

sending a reregistration request to the UE;

receiving a SIP REGISTER message from the UE; and

performing P-CSCF registration with the UE including establishing a new secure association with the UE.

2. The method according to claim 1 , wherein the step of detecting the loss of a secure association is performed at the P-CSCF.

3. The method according to claim 2 , wherein said detecting the loss of a secure association between the P-CSCF and the UE comprises:

receiving a message addressed to the UE;

during processing of the message, determining that a security association for the UE is not available;

and comprising, subsequent to the P-CSCF registration with the UE, the P-CSCF sending the message addressed to the UE to the UE.

4. The method according to claim 1 , wherein the P-CSCF is a node of a distributed P-CSCF.

5. The method according to claim 1 , wherein sending the reregistration request is performed in response to a failure of a second P-CSCF.

6. The method according to claim 1 , and comprising, at the P-CSCF prior to sending the reregistration request, retrieving a security token associated with the secure association, wherein the reregistration request contains the security token.

7. The method according to claim 6 , wherein the security token is or is derived from any one or more of:

a pseudorandomly generated number;

a key value generated during a previous registration of the UE;

a CK value generated during a previous registration of the UE;

an IK value generated during a previous registration of the UE; or

a quantity digitally signed using a private key of the P-CSCF or a second P-CSCF.

8. The method according to claim 6 , wherein retrieving the security token comprises sending a request for the security token associated with the secure association to a database of the telecommunications network, and receiving a message containing the security token from the database.

9. The method according to claim 1 , wherein the reregistration request is contained within a SIP MESSAGE or SIP NOTIFY message.

10. A method in a user equipment (UE) connected to a telecommunications network, the method comprising:

after a loss of a secure association between the UE and a Proxy Call Session Control Function (P-CSCF) of the telecommunications network, receiving a reregistration request from the P-CSCF;

in response to receiving the reregistration request, sending a SIP REGISTER message to the P-CSCF; and

performing P-CSCF registration with the P-CSCF including establishing a new secure association with the P-CSCF.

11. The method according to claim 10 , wherein the reregistration request contains a security token and comprising, prior to sending the SIP REGISTER message, authenticating the P-CSCF using the security token.

12. The method according to claim 11 wherein the security token is or is derived from any one or more of:

a pseudorandomly generated number;

a key value generated during a previous registration of the UE;

a CK value generated during a previous registration of the UE;

an IK value generated during a previous registration of the UE; or

a quantity digitally signed using a private key of the P-CSCF.

13. The method according to claim 11 , wherein the step of authenticating the P-CSCF using the token comprises comparing the security token to a stored token.

14. The method according to claim 13 , wherein the security token and the stored token are each or are each derived from any one or more of:

a pseudorandomly generated number;

a key value generated during a previous registration of the UE;

a CK value generated during a previous registration of the UE;

an IK value generated during a previous registration of the UE; or

a quantity digitally signed using a private key of the P-CSCF.

15. An apparatus configured to operate as a Proxy Call Session Control Function (P-CSCF) in a telecommunications network, the apparatus comprising:

a transceiver configured to communicate with user equipments (UEs); and

a reregistration processor configured to:

in response to detection or notification of a loss of a secure association between the P-CSCF and one of the UEs, send a reregistration request to the UE;

receive, via the transceiver, a SIP REGISTER message from the UE; and

perform P-CSCF registration with the UE including establishing a new secure association with the UE.

16. The apparatus according to claim 15 , wherein the apparatus comprises a secure association processor configured to detect the loss of a secure association between the P-CSCF and the UE.

17. The apparatus according to claim 16 , wherein the secure association processor is configured to detect the loss of a secure association by:

receiving, via the transceiver, a message addressed to the UE;

during processing of the message, determining that the secure association is not available; and

the reregistration processor is configured to send the message addressed to the UE to the UE subsequent to performing P-CSCF registration with the UE.

18. The apparatus according to claim 15 , wherein the reregistration processor is configured to, prior to sending the reregistration request, retrieve a security token associated with the secure association between the P-CSCF and the UE, and the reregistration processor is configured to include the token in the reregistration request.

19. The apparatus according to claim 18 , wherein the apparatus comprises a second transceiver configured to communicate with a database, and the reregistration processor is configured to retrieve the security token by sending a request for the security token to the database via the second transceiver, and receiving a message containing the security token from the database via the second transceiver.