Section: 4.2.2.2.3 Registration with AMF re-allocation
Content: When an AMF receives a Registration request, the AMF may need to reroute the Registration request to another AMF, e.g. when the initial AMF is not the appropriate AMF to serve the UE. The Registration with AMF re-allocation procedure, described in figure 4.2.2.2.3-1, is used to reroute the NAS message of the UE to the target AMF during a Registration procedure. Figure 4.2.2.2.3-1: Registration with AMF re-allocation procedure The initial AMF and the target AMF register their capability at the NRF. 1. If the UE is in CM-IDLE State, steps 1 and 2 of figure 4.2.2.2.2-1 have occurred and the (R)AN sends the Registration request message within an Initial UE message to the initial AMF. If the UE is in CM-CONNECTED state and triggers registration procedure, the NG-RAN sends Registration request message in the Uplink NAS Transport message to the serving AMF which is initial AMF. The AMF may skip step 2-3. 2. If the AMF needs the SUPI and/or UE's subscription information to decide whether to reroute the Registration Request or if the Registration Request was not sent integrity protected or integrity protection is indicated as failed, then AMF performs steps 4 to 9a or to 9b of figure 4.2.2.2.2-1. 3a. [Conditional] If the initial AMF needs UE's subscription information to decide whether to reroute the Registration Request and UE's slice selection subscription information was not provided by old AMF, the AMF selects a UDM as described in clause 6.3.8 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. 3b. Initial AMF to UDM: Nudm_SDM_Get (SUPI, Slice Selection Subscription data). The initial AMF request UE's Slice Selection Subscription data from UDM by invoking the Nudm_SDM_Get (see clause 5.2.3.3.1) service operation. UDM may get this information from UDR by Nudr_DM_Query(SUPI, Slice Selection Subscription data). For a Disaster Roaming Registration, the AMF may provide the indication of Disaster Roaming service to the UDM. 3c. UDM to initial AMF: Response to Nudm_SDM_Get. The AMF gets the Slice Selection Subscription data including Subscribed S-NSSAIs. UDM responds with slice selection subscription data to initial AMF. For a Disaster Roaming Registration, the UDM responds with the slice selection subscription data for a Disaster Roaming service to initial AMF based on the local policy and/or the local configuration as specified in clause 5.40.4 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. 4a. [Conditional] Initial AMF to NSSF: Nnssf_NSSelection_Get (Requested NSSAI, [Mapping Of Requested NSSAI], Subscribed S-NSSAI(s) with the default S-NSSAI indication, [NSSRG Information], TAI, Allowed NSSAI for the other access type (if any), [Mapping of Allowed NSSAI], PLMN ID of the SUPI). If there is a need for slice selection, (see clause 5.15.5.2.1 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]), e.g. the initial AMF cannot serve all the S-NSSAI(s) from the Requested NSSAI permitted by the subscription information, the initial AMF invokes the Nnssf_NSSelection_Get service operation from the NSSF by including Requested NSSAI, optionally Mapping Of Requested NSSAI, Subscribed S-NSSAIs with the default S-NSSAI indication, [NSSRG Information], Allowed NSSAI for the other access type (if any), Mapping of Allowed NSSAI, PLMN ID of the SUPI and the TAI of the UE. If the AMF needs to indicate the NSSF to return the Configured NSSAI to obtain network slice configuration when it receives from the UDM an indication that subscription has changed for the UE (see step 14b of clause 4.2.2.2.2) to ensure that the information returned by the NSSF includes the new Configured NSSAI for the UE which can be used to update UE network slicing configuration (see step 21 of clause 4.2.2.2.2), the AMF indicates to the NSSF that the AMF needs a Configured NSSAI by providing the Default Configured NSSAI Indication as described in clause 5.15.5.2.1 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. The AMF includes, if available, the NSSRG Information for the S-NSSAIs of the HPLMN, defined in clause 5.15.12 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2], including information whether the UE has indicated support of the subscription-based restrictions to simultaneous registration of network slices and whether the UDM has indicated to provide all subscribed S-NSSAIs for non-supporting UEs. If the UE context includes Partially Allowed NSSAI, then the AMF includes the S-NSSAIs of the Partially Allowed NSSAI in the Allowed NSSAI and includes the corresponding HPLMN S-NSSAI in the Mapping Of Allowed NSSAI. 4b. [Conditional] NSSF to Initial AMF: Response to Nnssf_NSSelection_Get (AMF Set or list of AMF addresses, Allowed NSSAI for the first access type, [Mapping Of Allowed NSSAI], [Allowed NSSAI for the second access type], [Mapping of Allowed NSSAI], [NSI ID(s)], [NRF(s)], [List of rejected (S-NSSAI(s), cause value(s))], [Configured NSSAI for the Serving PLMN], [Mapping Of Configured NSSAI]). The NSSF performs the steps specified in point (B) in clause 5.15.5.2.1 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. The NSSF returns to initial AMF the Allowed NSSAI for the first access type, optionally the Mapping Of Allowed NSSAI, the Allowed NSSAI for the second access type (if any), optionally the Mapping of Allowed NSSAI and the target AMF Set or, based on configuration, the list of candidate AMF(s). The NSSF may return NSI ID(s) associated to the Network Slice instance(s) corresponding to certain S-NSSAI(s). The NSSF may return the NRF(s) to be used to select NFs/services within the selected Network Slice instance(s). It may return also information regarding rejection causes for S-NSSAI(s) not included in the Allowed NSSAI. The NSSF may return Configured NSSAI for the Serving PLMN and possibly the associated mapping of the Configured NSSAI. If the NSSRG information was included in the request, the NSSF provides the Configured NSSAI as described in clause 5.15.12 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. NOTE 1: The NRF(s) returned by the NSSF, if any, belong to any level of NRF (see clause 6.2.6 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]) according to the deployment decision of the operator. 5. [Conditional] Initial AMF to old AMF: Namf_Communication_RegistrationStatusUpdate (failure cause ). If the UE was in CM-IDLE and another AMF is selected, the initial AMF sends a reject indication to the old AMF telling that the UE Registration procedure did not fully complete at the initial AMF. The old AMF continues as if the Namf_Communication_UEContextTransfer had never been received. 6a. [Conditional] Initial AMF to NRF: Nnrf_NFDiscovery_Request (NF type, AMF Set). If the initial AMF does not locally store the target AMF address and if the initial AMF intends to use direct reroute to target AMF or the reroute via (NG-R)AN message needs to include AMF address, then the initial AMF invokes the Nnrf_NFDiscovery_Request service operation from the NRF to find a proper target AMF which has required NF capabilities to serve the UE. The NF type is set to AMF. The AMF Set is included in the Nnrf_NFDiscovery_Request. 6b. [Conditional] NRF to AMF: Response to Nnrf_NFDiscovery_Request (list of (AMF pointer, AMF address, plus additional selection rules and NF capabilities)). The NRF replies with the list of potential target AMF(s). The NRF may also provide the details of the services offered by the candidate AMF(s) along with the notification end-point for each type of notification service that the selected AMF had registered with the NRF, if available. As an alternative, it provides a list of potential target AMFs and their capabilities and optionally, additional selection rules. Based on the information about registered NFs and required capabilities, a target AMF is selected by the initial AMF. If the security association has been established between the UE and initial AMF, to avoid a registration failure, the initial AMF shall forward the NAS message to the target AMF by executing step 7(A). NOTE 2: The security context in the initial AMF is not transferred to the target AMF if initial AMF forward the NAS message to the target AMF via (R)AN. In this case the UE rejects the NAS message sent from target AMF as the security context in the UE and target AMF are not synchronized. NOTE 3: Network slice isolation cannot be completely maintained in case the AMF reallocation is executed by step 7(A). If the initial AMF is not part of the target AMF Set and is not able to get a list of candidate AMF(s) by querying the NRF with the target AMF Set (e.g. the NRF locally pre-configured on AMF does not provide the requested information, the query to the appropriate NRF provided by the NSSF is not successful, or the initial AMF has knowledge that the initial AMF is not authorized as serving AMF etc.) then the initial AMF shall forward the NAS message to the target AMF via (R)AN executing step 7(B) unless the security association has been established between the UE and initial AMF; the Allowed NSSAI, optionally the Partially Allowed NSSAI and the AMF Set are included to enable the (R)AN to select the target AMF as described in clause 6.3.5 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. 7(A). If the initial AMF, based on local policy and subscription information, decides to forward the NAS message to the target AMF directly, the initial AMF invokes the Namf_Communication_N1MessageNotify to the target AMF, carrying the rerouted NAS message. The Namf_Communication_N1MessageNotify service operation includes AN access information (e.g. the information enabling (R)AN to identify the N2 terminating point, CAG Identifier(s) of the CAG cell) and the complete Registration Request message in clear text as specified in TS 33.501[ Security architecture and procedures for 5G System ]  [15] and the UE's SUPI and MM Context if available. If the initial AMF has obtained the information from the NSSF as described at step 4b, that information except the AMF Set or list of AMF addresses is included. The target AMF then updates the (R)AN with a new updated N2 termination point for the UE in the first message from target AMF to RAN in step 8. 7(B). [Conditional] if the UE was in CM-IDLE, if the initial AMF, based on local policy and subscription information, decides to forward the NAS message to the target AMF via (R)AN unless the target AMF(s) are returned from the NSSF and identified by a list of candidate AMF(s), the initial AMF sends a NGAP Reroute NAS Request message to the (R)AN (step 7a). The NGAP Reroute Request NAS message includes the information about the target AMF and the complete Registration Request message. If the initial AMF has obtained the information as described at step 4b, that information is included. The (R)AN sends the Initial UE message to the target AMF (step 7b) indicating reroute due to slicing including the information from step 4b that the NSSF provided. NOTE 4: Step 7B is not supported if the UE was in CM-CONNECTED i.e. the NGAP Uplink NAS Transport message was received at step 1. 8. After receiving the Registration Request message transmitted at step 7(A)a or step 7(B)b, the target AMF continues with the Registration procedure from step 4 until 22 of figure 4.2.2.2.2-1 (with the target AMF corresponding to the new AMF), which includes the UE context retrieved from old AMF. If the 5G security context is received from the initial AMF, the target AMF continue using that one instead of the 5G security context the target AMF may have retrieved from the old AMF. If the initial AMF decides to forward the NAS message to the target AMF (step 7(A), the first message from the target AMF to (R)AN (either Initial Context Setup Request, or Downlink NAS Transport) contain the AMF name of the initial AMF and target AMF UE NGAP ID.