AA.3 SBI Capable HSS Discovery and Selection

23.2283GPPIP Multimedia Subsystem (IMS)Release 18Stage 2TS

AA.3.1 General

An SBI capable IMS entity (e.g. I-CSCF, S-CSCF or IMS AS) performs HSS discovery to discover an HSS that manages the user subscriptions.

The SBI capable IMS entity shall utilize the NRF to discover the SBI capable HSS instance(s) unless the information about SBI capable HSS instances is available by other means, e.g. locally configured on the SBI capable IMS entity. The HSS selection function in SBI capable IMS entities selects an SBI capable HSS instance based on the available SBI capable HSS instances (obtained from the NRF or locally configured).

An SBI capable IMS entity always selects an HSS within its own PLMN. The HSS selection should consider one of the following factors when available to the SBI capable IMS entity:

1. HSS Group ID of the IMS identity (IMPI or IMPU, or PSI).

2. IMS private identity (IMPI); e.g. the SBI capable IMS entity selects an SBI capable HSS instance based on the IMPI set the UE’s IMPI belongs to, configured locally or based on the results of a discovery procedure with NRF using the UE’s IMPI as input for HSS discovery.

3. IMS public identity (IMPU); e.g. the SBI capable IMS entity selects an SBI capable HSS instance based on the IMPU set the UE’s IMPU belongs to, configured locally or based on the results of a discovery procedure with NRF using the UE’s IMPU as input for HSS discovery.

4. Public Service Identity (PSI); e.g. the SBI capable IMS entity selects an SBI capable HSS instance based on the PSI set the received PSI belongs to, configured locally or based on the results of a discovery procedure with NRF using the received PSI as input for HSS discovery.

5. MSISDN; e.g. the SBI capable IMS entity selects an SBI capable HSS instance based on the MSISDN set the UE’s MSISDN belongs to, configured locally or based on the results of a discovery procedure with NRF using the MSISDN as input for HSS discovery.

Unless the information about the interface type to be used towards HSS is locally configured on the SBI capable IMS entity, an SBI capable IMS entity can also use the NRF to decide the type of interface (SBI vs diameter) to be used towards HSS.

The following clause describes the procedure for HSS registration in NRF, SBI capable HSS discovery and interface type selection via NRF.

AA.3.2 HSS Registration in NRF

An SBI capable HSS registers in the NRF using the Nnrf_NFManagement_NFRegister Request message as defined in TS 23.502 [94]. The NF profile of the HSS registered in NRF includes necessary information for an SBI capable IMS entity to send SBI service requests to the selected SBI capable HSS service instance.

Different SBI capable HSS instances managing different sets of IMPIs/IMPUs may be deployed in a given PLMN. In this case, the SBI capable HSS instances register in NRF using either different ranges of IMPIs/IMPUs and/or HSS Group IDs.

NOTE: In deployments where simple IMPU and IMPI ranges are not suitable to describe the IMPU/IMPI sets served by HSS instances, it is expected the HSS instances only register HSS Group IDs.

AA.3.3 HSS Discovery and Selection via NRF

AA.3.3.1 General

During the IMS procedure, an SBI capable IMS entity sends a Nnrf_NFDiscovery_Request to NRF as defined in TS 23.502 [94] to discover SBI capable HSS instances within a given PLMN. The SBI capable IMS entity may store all returned SBI capable HSS instances and their NF profiles for subsequent use, including, if applicable, supported IMPIs/IMPUs ranges, and/or HSS Group IDs. If no SBI capable HSS instance is available in the PLMN, then the NRF replies to the SBI capable IMS entity with no information. In this case, the SBI capable IMS may then attempt to communicate with the HSS using non-SBA protocols.

NOTE: The SBI enabled IMS entity can also perform HSS discovery prior to receiving an IMS request without any public identity information.

When an SBI capable IMS entity receives an NRF response that HSS supports SBI and stores the information received from the HSS, it shall make use of Nnrf_NFStatusSubscribe/Unsubscribe service operations with NRF as defined in TS 23.502 [94] to receive Nnrf_NFStatusNotify service operation for updates to the NF profiles of SBI capable HSS instances registered in NRF.

AA.3.3.2 HSS Discovery

The call flow in Figure AA.3.3.2-1 illustrates the steps to locate the HSS instance for an IMS public identity.

Figure AA.3.3.2-1: HSS discovery and selection

Steps 1 – 2 may be performed, any time after power up, e.g. in the scenario where only a single HSS instance or HSS group is deployed, and in the scenario where an operator has IMPI/IMPU ranges that are registered in NRF as described in clause AA.3.2. In this case there is no need for any IMPU to perform the 2 steps.

1. An SBI capable IMS entity may discover the SBI capable HSS instances available in the PLMN via Nnrf_NFDiscovery_Request.

2. The NRF provides the SBI capable IMS entity with the HSS instances and/or any HSS Group IDs registered in the PLMN. If no SBI capable HSS instance and/or any HSS Group ID is available in the PLMN, then the NRF will reply to the SBI capable IMS entity with no information about available SBI capable HSS instances.

3. The SBI capable IMS entity receives an IMS procedure related to a given IMS user (IMPI or IMPU depending on the procedure).

Steps 4 – 6 may be performed, e.g. if the SBI capable IMS entity did not retrieve and store information about HSS instances and/or HSS Group IDs registered in the PLMN at an earlier stage (performed steps 1-2).

4. An SBI capable IMS entity may discover the SBI capable HSS instances available in the PLMN via Nnrf_NFDiscovery_Request.

5. The NRF provides the SBI capable IMS entity with the HSS instances and/or any HSS Group IDs registered in the PLMN. If no SBI capable HSS instance and/or any HSS Group ID is available in the PLMN, then the NRF will reply to the SBI capable IMS entity with no information about available SBI capable HSS instances.

6. The SBI capable IMS entity stores the result of the NRF discovery, if any is received. The IMS capable entity may store the received response for future use, otherwise the IMS entity must perform the query in response to each IMS request it receives.

If the SBI capable IMS entity received no results at all, the procedure is exited, and the SBI capable IMS entity may use Diameter interfaces to interact with an HSS.

NOTE 1: The SBI capable IMS entity can request the NRF to be notified of updates in the SBI capable HSS instances/ HSS Group IDs registered in NRF by using a Nnrf_NFManagement_NFStatusSubscribe service operation.

Steps 7 – 10 are performed only if the SBI capable IMS entity cannot locate an HSS instance corresponding to the IMS public identity based on stored information.

7. The SBI capable IMS entity sends to NRF an Nnrf_NFDiscovery_Request with the IMS public identity received in step 3.

8. NRF may query the UDR, via the Nudr_GroupIDmap service, for the HSS Group ID corresponding to the IMS public identity.

9. If requested the UDR returns the HSS-IMS Group ID to NRF.

10. NRF locates HSS instance(s) corresponding to the HSS Group ID. NRF returns and provides and them to the SBI capable IMS entity the HSS instance(s) profile which includes the HSS Group ID.

NOTE: In indirect communication with delegated discovery as specified in TS 23.501 [93], the SBI capable IMS entity does not interact directly with the NRF. In this case, the SBI capable IMS entity is not informed about the HSS Group ID the IMS identity belongs to.

11. The SBI capable IMS entity selects the HSS instance.

12. The SBI capable IMS entity can then start interaction with the selected HSS instance.

AA.3.3.3 Handling of HSS Group ID in IMS Procedures

The HSS group ID may be transported in SIP signalling related to the following procedures:

– Initial IMS Registration procedure.

– IMS Terminating session establishment procedure.

– IMS Originating session establishment procedure.

An SBI enabled I-CSCF receiving an HSS Group ID during the NRF-based HSS discovery procedure should include the HSS Group ID for transportation to the next hop in subsequent SIP signalling related to the above procedures.

In addition, every IMS node receiving an HSS Group ID in SIP signalling (e.g. S-CSCF and IMS AS) should store it as part of the UE information, and should use the received HSS Group ID to select an SBI capable HSS. Additionally, an IMS node receiving an HSS Group ID in SIP signalling should include it in subsequent SIP signalling related to the above procedures.

The HSS Group ID in SIP signalling is specific to the UE served by this IMS and shall not be sent to any other party involved in the session/communication (e.g. to the terminating side and/or outside the HPLMN).

Annex AB (informative):
Support of IMS in SNPN