Section: 4.3.2.2.3 SMF selection
Content: 4.3.2.2.3.1 General The SMF selection function, as described in clause 6.3.2 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2], is supported by the AMF and is used to allocate an SMF that manages the PDU Session. The SMF selection function described in this clause does not apply to the selection of an SMF for Emergency services. For SMF selection for Emergency services is described in clause 5.16.4.5 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. Two main branches of deployment scenarios to consider: - Non-roaming and roaming with local breakout, see clause 4.3.2.2.3.2 - Home routed roaming, see clause 4.3.2.2.3.3 In the case of non-roaming and local breakout, there are two operational scenarios dependent on the configuration of AMF and the deployment option of NSSF in the serving PLMN. In the case of home-routed, there are two main options dependent on the operators' choices in terms of involvement of NRF, NSSF and configuration of AMF. The decision of which option to use is part of the roaming agreements. NOTE: The use of NSI ID and the use of multiple NRFs in the network are optional and depend on the deployment choices of the operator. 4.3.2.2.3.2 Non-roaming and roaming with local breakout Figure 4.3.2.2.3.2-1: SMF selection for non-roaming and roaming with local breakout scenarios This procedure may be skipped altogether if SMF information is available in the AMF by other means (e.g. locally configured); otherwise: - when the serving AMF is aware of the appropriate NRF to be used to select NFs/services within the corresponding Network Slice instance based on configuration or based on the Network Slice selection information received during Registration, only steps 3 and 4 in the following procedure are executed as described in Figure 4.3.2.2.3.2-1; - when the serving AMF is not aware of the appropriate NRF to be used to select NFs/services within the corresponding Network Slice instance, all steps in the following procedure are executed as described in Figure 4.3.2.2.3.2-1. 1. The AMF invokes the Nnssf_NSSelection_Get service operation from the NSSF in serving PLMN with the S-NSSAI of the Serving PLMN from the Allowed NSSAI or Partially Allowed NSSAI requested by the UE, PLMN ID of the SUPI, TAI of the UE and the indication that the request is within a procedure of PDU Session establishment in either the non-roaming or roaming with local breakout scenario. 2. The NSSF in serving PLMN selects the Network Slice instance, determines and returns the appropriate NRF to be used to select NFs/services within the selected Network Slice instance and optionally may return a NSI ID corresponding to the Network Slice instance. 3. AMF queries the appropriate NRF in serving PLMN by issuing the Nnrf_NFDiscovery_Request including at least the S-NSSAI of the Serving PLMN for this PDU Session from the Allowed NSSAI or Partially Allowed NSSAI, PLMN ID of the SUPI, DNN and possibly NSI ID if the AMF has stored an NSI ID for the S-NSSAI of the Serving PLMN for this PDU Session from the Allowed NSSAI or Partially Allowed NSSAI. NOTE: The list of parameters for SMF selection is defined in clause 6.3.2 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. See also clause 5.34.3 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2] for I-SMF selection. 4. The NRF in serving PLMN provides to the AMF, e.g. FQDN or IP address, of a set of the discovered SMF instance(s) or Endpoint Address(es) of SMF service instance(s) in Nnrf_NFDiscovery_Request response message and possibly an NSI ID for the selected Network Slice instance corresponding to the S-NSSAI for subsequent NRF queries. 4.3.2.2.3.3 Home routed roaming The selection of the SMF in VPLMN is performed in the same way as for non-roaming and roaming with local breakout (see clause 4.3.2.2.3.2). The selection of the SMF in HPLMN is performed by means of one of two main options. Which of these two options to use is decided based on Service Level Agreements between the operators. NOTE 1: The procedures described in this clause are not limited to SMF selection but can be used to discover and select any NF/NF service in the HPLMN part of a Network Slice instance. In the first option, requiring the use of NSSF in both the VPLMN and the HPLMN, the selection of the SMF in HPLMN is performed by means of the procedure depicted in Figure 4.3.2.2.3.3-1. Figure 4.3.2.2.3.3-1: Option 1 for SMF selection for home-routed roaming scenarios 1. Based on the operator's configuration, if the AMF is not aware of the appropriate NRF to be used to select NFs/services in the HPLMN, the AMF invokes the Nnssf_NSSelection_Get service operation from the NSSF in VPLMN with the VPLMN S-NSSAI from the Allowed NSSAI or Partially Allowed NSSAI requested by the UE for this PDU Session, the HPLMN S-NSSAI that maps to the VPLMN S-NSSAI, PLMN ID of the SUPI, the TAI of the UE and the indication that the request is within a procedure of PDU Session establishment in the home-routed roaming scenario. 2. If slicing configuration information for the S-NSSAI in the HPLMN is not available (e.g. the NSSF has no cached information), the NSSF of the VPLMN invokes the Nnssf_NSSelection_Get service operation from NSSF of the HPLMN according to the PLMN ID of SUPI by including the HPLMN S-NSSAI. 3. The NSSF in HPLMN may include the NSI ID, if needed, for the Network Slice instance in HPLMN selected for the corresponding S-NSSAI of the HPLMN in the Nnssf_NSSelection_Get response. The NSSF in HPLMN also includes the appropriate hNRF to be used to select NFs/services within HPLMN in the Nnssf_NSSelection_Get response. 4. The serving NSSF includes in the Nnssf_NSSelection_Get response all the information that has been received from the NSSF in HPLMN when responding to the AMF. 5. The AMF queries the target vNRF using the Nnrf_NFDiscovery_Request by including PLMN ID of the SUPI, DNN, HPLMN S-NSSAI, the hNRF and possibly an HPLMN NSI ID for the selected Network Slice instance corresponding to the HPLMN S-NSSAI if available in the AMF (obtained from the HPLMN NSSF in steps 3 and 4 or cached from a previous H-NSSF query). 6. The NRF in serving PLMN identifies NRF in HPLMN (hNRF) based on the information provided by the NSSF in the serving PLMN and it invokes the Nnrf_NFDiscovery_Request service from hNRF according the procedure in Figure 4.17.4-1 to get the expected SMF instance(s) deployed in the HPLMN. As the vNRF in VPLMN triggers the "NF Discovery" on behalf of the AMF, the NRF in the VPLMN shall not replace the information of the NF, i.e. AMF ID, in the Nnrf_NFDiscovery_Request message it sends to the hNRF. 7-8. The hNRF provides to the AMF, via vNRF, the information e.g. FQDN or IP address, of a set of the SMF instance(s) in Nnrf_NFDiscovery_Request response message and possibly an NSI ID for the selected Network Slice instance corresponding to the S-NSSAI of the HPLMN for subsequent NRF queries. When the NSSF is not deployed in HPLMN then the AMF in VPLMN relies on either the configuration to obtain the NRF in HPLMN or on the option below. The second option for the selection of the SMF in HPLMN is performed by means of the procedure depicted in Figure 4.3.2.2.3.3-2. Figure 4.3.2.2.3.3-2: Option 2 for SMF selection for home-routed roaming scenarios 1. Based on the operator's configuration, the AMF queries the vNRF with PLMN ID of the SUPI, PLMN ID of the serving PLMN, DNN, the HPLMN S-NSSAI that maps to the S-NSSAI from the Allowed NSSAI or Partially Allowed NSSAI of the Serving PLMN the UE has requested, the hNRF and if applicable and available, an HPLMN NSI ID (if the AMF has stored an hNRF and if applicable and available, an HPLMN NSI ID for the selected Network Slice instance corresponding to the S-NSSAI of the HPLMN) and DNN. 2. The vNRF queries, on behalf of the AMF in VPLMN, the hNRF identified by means of the PLMN ID of the SUPI (if no hNRF is received from the AMF, the hNRF is locally determined in the vNRF based on information received in step 1). The NRF in VPLMN requests "NF Discovery" service from hNRF according the procedure in Figure 4.17.4-1 to get the expected SMF instance(s) deployed in the HPLMN. As the NRF in the serving PLMN triggers the "NF Discovery" on behalf of the AMF, the NRF in the VPLMN shall not replace the information of the NF, i.e. AMF ID, in the Nnrf_NFDiscovery_Request message it sends to the hNRF. Depending on the available information and based on configuration, the hNRF may either execute steps in 3(A) or in 3(B). 3(A) The hNRF provides to the AMF, via vNRF, the information e.g. FQDN or IP address, of a set of the discovered SMF instance(s) and possibly an NSI ID for the selected HPLMN part of the Network Slice instance corresponding to the S-NSSAI of the HPLMN for subsequent NRF queries in Nnrf_NFDiscovery_Request response message(steps 3a and 3b). 3(B) The hNRF queries, on behalf of the AMF, an appropriate local NRF in HPLMN (e.g. a slice level NRF); this local NRF provides the IP address or the FQDN of expected SMF instance(s) and possibly an NSI ID for the selected HPLMN part of the Network Slice instance corresponding to the S-NSSAI of the HPLMN for subsequent NRF queries (steps 3a and 3b) that the hNRF returns, via vNRF, to the AMF (steps 3c and 3d).