6.3.11 CHF discovery and selection

The CHF discovery and selection function is supported by the SMF, the AMF, the SMSF and the PCF. It is used by the SMF to select a CHF that manages the online charging or offline charging for a PDU Session of a subscriber. It is used by the AMF to select a CHF that manages the online charging or offline charging for 5G connection and mobility of a subscriber. It is used by the SMSF to select a CHF that manages the online charging or offline charging for the SMS over NAS transactions of a subscriber. It is used by the PCF to select a CHF that manages the spending limits for a subscriber and/or a PDU Session of a subscriber.

For the PCF to select the CHF, the address(es) of the CHF, including the Primary CHF address and the Secondary CHF address, may be:

• stored in the UDR as part of the PDU Session policy control subscription information as defined in clause 6.2.1.3 of TS 23.503 [45].
• stored in the UDR as part of the UE context policy control subscription information as defined in clause 6.2.1.3 of TS 23.503 [45].
• stored in the UDR as part of the Access and Mobility policy control subscription information as defined in clause 6.2.1.3 of TS 23.503 [45].
• locally configured in the PCF based on operator policies.
• discovered using NRF as described in in clause 6.1 of TS 32.290 [67].

NOTE 1: The operator can perform the above UDR provisioning or local configuration in a consistent manner such that the same CHF address is used for SM policy, AM policy and UE policy. If NRF discovery is used, it is up to the PCF logic (or SCP logic when working in Delegated Discovery mode) and operator configuration to guarantee the CHF address consistency.

The address(es) of the CHF shall be applicable for all services provided by the CHF.

The CHF address(es) that a stored in the UDR or configured in the PCF may be complemented by the associated CHF instance ID(s) and/or CHF set ID(s) (see clause 6.3.1.0) stored or configured in the same location.

The CHF address(es) retrieved from the UDR and possible associated CHF instance ID(s) and/or CHF set ID(s) take precendence over the locally configured CHF address(es) and possible associated CHF instance ID(s) and/or CHF set ID(s) and over the CHF address(es) discoverred by the NRF. If no CHF address(es) is received from the UDR, the PCF selects, based on operator policies, either the CHF addresse(es) provided by NRF, or the locally configured CHF address(es) and possible associated CHF instance ID(s) and/or CHF set ID(s).

If the PCF has a CHF set ID but no CHF instance ID associated to the CHF address(es) in the same location, the CHF instance within the CHF set may change. If the PCF is not able to reach the CHF address(es), it should query the NRF for other CHF instances within the CHF set.

If the PCF received a CHF set ID and a CHF instance ID associated to the CHF address(es) in the same location, the CHF service instance within the CHF may change. If an PCF is not able to reach the CHF address(es), it should query the NRF for other CHF service instances within the CHF.

In the non-roaming case it is possible to either:

• Have the SMF select the same CHF that is selected by the PCF for the PDU Session. In this case, operator policies in the PCF indicate it to provide the selected CHF address(es) and, if available, the associated CHF instance ID(s) and/or CHF set ID(s) in the PDU Session related policy information to the SMF as described in Table 6.4-1 of TS 23.503 [45] and the SMF applies the CHF address and if available, the associated CHF instance ID(s) and/or CHF set ID(s) passed from the PCF as defined in clause 5.1.8 of TS 32.255 [68] or
• Have the SMF select a CHF based on other criteria as defined in clause 5.1.8 of TS 32.255 [68].

In the Home Routed roaming case, the above text shall apply with the change that SMF is replaced by H-SMF, PCF is replaced by H-PCF, CHF is replaced by H-CHF and for b) the other criteria is defined in clause 5.1.9.2 of TS 32.255 [68].

In the non-roaming case, it is possible to either:

• Have the AMF select the same CHF that is selected by the PCF for the UE. In this case operator policies in the PCF indicate it to provide the selected CHF address(es) and, if available, the associated CHF instance ID(s) and/or CHF set ID(s) in the Access and mobility related policy information and/or in the UE Policy Association supplementary information to the AMF as described in Table 6.5-1 and Table 6.6.7-1 of TS 23.503 [45] respectively and the AMF applies the CHF address and if available, the associated CHF instance ID(s) and/or CHF set ID(s) passed from the PCF as defined in clause 5.1.3 of TS 32.256 [114] or
• Have the AMF select a CHF based on other criteria as defined in clause 5.1.3 of TS 32.256 [114].

In the roaming case, the above text shall apply with the change that PCF is replaced by H-PCF, CHF is replaced by H-CHF, Access and mobility related policy information is not relevant, UE Policy Association supplementary information is between the AMF and the V-PCF and between the V-PCF and the H-PCF and for b) the other criteria is defined in clause 5.1.5.2 of TS 32.256 [114].

NOTE 2: Clause 5.1.2 of TS 32.256 [114] describes the charging reporting requirements on the AMF in the roaming scenario.

How the CHF is selected by the SMSF is defined in clause 5.4 of TS 32.274 [118].

If the NF consumer performs discovery and selection via NRF, the CHF selection function in NF consumers selects a CHF instance based on the available CHF instances obtained from the NRF.

The CHF selection functionality in NF consumer or in SCP should consider one of the following factors:

1. CHF Group ID of the UE's SUPI; the NF Consumer selects a CHF instance based on the CHF Group ID the SUPI belongs to, which is provided by the UDM if available.

NOTE 3: The NF Consumer can also infer the CHF Group ID the UE's SUPI belongs to, based on the results of CHF discovery procedures with NRF.

2. SUPI; the NF consumer selects a CHF instance based on the SUPI range the UE's SUPI belongs to or based on the results of a discovery procedure with NRF using the UE's SUPI as input for CHF discovery.

In the case of delegated discovery and selection in SCP, the NF consumer shall include all available factors in the request towards SCP.