Section: 6.3.8 UDM discovery and selection
Content: The NF consumer or the SCP performs UDM discovery to discover a UDM instance that manages the user subscriptions. If the NF consumer performs discovery and selection, the NF consumers shall utilize the NRF to discover the UDM instance(s) unless UDM information is available by other means, e.g. locally configured on NF consumers. The UDM selection function in NF consumers selects a UDM instance based on the available UDM instances (obtained from the NRF or locally configured). The UDM selection functionality is applicable to both 3GPP access and non-3GPP access. The UDM selection functionality in NF consumer or in SCP should consider one of the following factors: 1. Home Network Identifier (e.g. MNC and MCC, realm) of SUCI/SUPI, along with the selected NID (provided by the NG-RAN) in the case of SNPN, UE's Routing Indicator and optionally Home Network Public Key identifier (e.g. in the case that Routing Indicator is not enough to provide SUPI range granularity). NOTE 1: The UE provides the SUCI to the AMF, which contains the Routing Indicator and Home Network Public Key identifier as defined in TS 23.003[ Numbering, addressing and identification ]  [19] during initial registration. The AMF provides the UE's Routing Indicator and optionally Home Network Public Key identifier to other NF consumers (of UDM) as described in TS 23.502[ Procedures for the 5G System (5GS) ]  [3]. NOTE 2: The usage of Home Network Public Key identifier for UDM discovery is limited to the scenario where the NF consumers belong to the same PLMN as AUSF. NOTE 3: In the case of SNPN and the UE provides an IMSI type SUCI to the AMF and the SUCI provided by UE or the SUPI derived from the SUCI is for an SNPN served by the AMF, the AMF uses the selected NID provided by the NG-RAN together with the selected PLMN ID (from IMSI) or the Routing Indicator provided by the UE within the SUCI for UDM selection. In the case of SNPN and the UE provides an NSI type SUCI to the AMF, the AMF uses the Home Network Identifier and Routing Indicator of SUCI/SUPI for selection of UDM. When the UE's Routing Indicator is set to its default value as defined in TS 23.003[ Numbering, addressing and identification ]  [19], the UDM NF consumer can select any UDM instance within the home network of the SUCI/SUPI. 2. UDM Group ID of the UE's SUPI. NOTE 4: The AMF can infer the UDM Group ID the UE's SUPI belongs to, based on the results of UDM discovery procedures with NRF. The AMF provides the UDM Group ID the SUPI belongs to other UDM NF consumers as described in TS 23.502[ Procedures for the 5G System (5GS) ]  [3]. 3. SUPI or Internal Group ID; the UDM NF consumer selects a UDM 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 or Internal Group ID as input for UDM discovery. 4. GPSI or External Group ID; UDM NF consumers which manage network signalling not based on SUPI/SUCI (e.g. the NEF) select a UDM instance based on the GPSI or External Group ID range the UE's GPSI or External Group ID belongs to or based on the results of a discovery procedure with NRF using the UE's GPSI or External Group ID as input for UDM discovery. In the case of delegated discovery and selection in SCP, NF consumer shall include one of these factors in the request towards SCP.