Tech-invite3GPPspecsSIPRFCs
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Content for  TS 23.122  Word version:  17.1.1

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C.3  Stage-2 flow for steering of UE in HPLMN or VPLMN after registrationWord‑p. 75
The stage-2 flow for the steering of UE in HPLMN or VPLMN after registration is indicated in figure C.3.1. The selected PLMN can be the HPLMN or a VPLMN. The AMF is located in the selected PLMN. The flow is triggered:
  • If the HPLMN UDM supports obtaining a list of preferred PLMN/access technology combinations and the SOR-CMCI, if any, or a secured packet from the SOR-AF, the HPLMN policy for the SOR-AF invocation is present in the HPLMN UDM and the SOR-AF provides the HPLMN UDM with a new list of preferred PLMN/access technology combinations and the SOR-CMCI, if any, or a secured packet for a UE identified by SUPI; or
  • When a new list of preferred PLMN/access technology combinations and the SOR-CMCI, if any, or a secured packet becomes available in the HPLMN UDM (i.e. retrieved from the UDR).
Reproduction of 3GPP TS 23.122, Figure C.3.1: Procedure for providing list of preferred PLMN/access technology combinations after registration
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For the steps below, security protection is described in TS 33.501.
0)
The SOR-AF to the HPLMN UDM: Nudm_ParameterProvision_Update request is sent to the HPLMN UDM to trigger the update of the UE with the new list of preferred PLMN/access technology combinations and the SOR-CMCI, if any, or a secured packet for a UE identified by SUPI.
1)
The HPLMN UDM to the AMF: The UDM notifies the changes of the user profile to the affected AMF by the means of invoking Nudm_SDM_Notification service operation. The Nudm_SDM_Notification service operation contains the steering of roaming information that needs to be delivered transparently to the UE over NAS within the Access and Mobility Subscription data. If the HPLMN decided that the UE is to acknowledge successful security check of the received steering of roaming information, the Nudm_SDM_Notification service operation also contains an indication that the UDM requests an acknowledgement from the UE as part of the steering of roaming information. If the SOR-CMCI was obtained, the HPLMN UDM shall include the SOR-CMCI into the steering of roaming information;
2)
The AMF to the UE: the AMF sends a DL NAS TRANSPORT message to the served UE. The AMF includes in the DL NAS TRANSPORT message the steering of roaming information received from the UDM.
3)
Upon receiving the steering of roaming information, the UE shall perform a security check on the steering of roaming information included in the DL NAS TRANSPORT message to verify that the steering of roaming information is provided by HPLMN, and:
  1. if the security check is successful and:
    • if the steering of roaming information contains a secured packet (see TS 31.115) and the service "data download via SMS Point-to-point" is allocated and activated in the USIM Service Table (see TS 31.102), the ME shall upload the secured packet to the USIM using procedures in TS 31.111.
      If the UDM has requested an acknowledgement from the UE in the DL NAS TRANSPORT message, the UE sends an UL NAS TRANSPORT message to the serving AMF with an SOR transparent container including the UE acknowledgement.
      • when the ME receives a USAT REFRESH command qualifier (see TS 31.111) of type "Steering of Roaming" it performs the procedure for steering of roaming in subclause 4.4.6 with an exception that if the UE is in automatic network selection mode, then the UE shall wait until it moves to idle mode or 5GMM-CONNECTED mode with RRC inactive indication (see TS 24.501) before attempting to obtain service on a higher priority PLMN (specified in subclause 4.4.6 bullet d); or
      • when the ME receives SOR-CMCI in the USAT REFRESH with command qualifier (see TS 31.111) of type "Steering of Roaming", the UE shall perform items a), b) and c) of the procedure for steering of roaming in subclause 4.4.6. If the UE is in automatic network selection mode it shall apply the actions in subclause C.4.2;
    • otherwise, the ME shall replace the highest priority entries in the "Operator Controlled PLMN Selector with Access Technology" list stored in the ME with the received list of preferred PLMN/access technology combinations, and delete the PLMNs identified by the list of preferred PLMN/access technology combinations from the Forbidden PLMN list and from the Forbidden PLMNs for GPRS service list, if they are present in these lists.
      If the UDM has requested an acknowledgement from the UE in the DL NAS TRANSPORT message, the UE sends an UL NAS TRANSPORT message to the serving AMF with an SOR transparent container including the UE acknowledgement.
      If the UE is in automatic network selection mode and the selected PLMN is a VPLMN, then:
      • if the UE is configured with the SOR-CMCI or received the SOR-CMCI over N1 NAS signalling, the UE shall apply the actions in subclause C.4.2; or
      • the UE shall wait until it moves to idle mode or 5GMM-CONNECTED mode with RRC inactive indication (see TS 24.501) before attempting to obtain service on a higher priority PLMN as specified in subclause 4.4.3.3 by acting as if timer T that controls periodic attempts has expired.
    If the selected PLMN is a VPLMN and the UE has an established emergency PDU session then the UE shall attempt to perform the PLMN selection subsequently after the emergency PDU session is released, if the UE is in automatic network selection mode.
    If the UDM has not requested an acknowledgement from the UE then steps 5 is skipped; and
  2. if the selected PLMN is a VPLMN, the security check is not successful and the UE is in automatic network selection mode, then the UE shall wait until it moves to idle mode or 5GMM-CONNECTED mode with RRC inactive indication (see TS 24.501) before attempting to obtain service on a higher priority PLMN as specified in subclause 4.4.3.3 by acting as if timer T that controls periodic attempts has expired, with an exception that the current PLMN is considered as lowest priority. If the selected PLMN is a VPLMN and the UE has an established emergency PDU session then the UE shall attempt to perform the PLMN selection after the emergency PDU session is released.
    If the UDM has not requested an acknowledgement from the UE then step 5 is skipped;
4)
void;
5)
The AMF to the HPLMN UDM: If the UL NAS TRANSPORT message with an SOR transparent container is received, the AMF uses the Nudm_SDM_Info service operation to provide the received SOR transparent container to the UDM. If the HPLMN decided that the UE is to acknowledge successful security check of the received steering of roaming information in step 1, the UDM verifies that the acknowledgement is provided by the UE.
If the present flow was invoked by the HPLMN UDM after receiving from the SOR-AF a new list of preferred PLMN/access technology combinations or a secured packet for a UE identified by SUPI using an Nudm_ParameterProvision_Update request, and the HPLMN UDM verification of the UE acknowledgement is successful, then the HPLMN UDM informs the SOR-AF about successful delivery of the list of preferred PLMN/access technology combinations, or of the secured packet to the UE, using Nsoraf_SoR_Info (SUPI of the UE, successful delivery); and
6)
The HPLMN UDM to the SOR-AF: Nsoraf_SoR_Info (SUPI of the UE, successful delivery). If the HPLMN policy for the SOR-AF invocation is present and the HPLMN UDM received and verified the UE acknowledgement in step 5, then the HPLMN UDM informs the SOR-AF about successful delivery of the list of preferred PLMN/access technology combinations, or of the secured packet to the UE.
If the selected PLMN is a VPLMN and:
  • the UE in manual mode of operation encounters security check failure of SOR information in DL NAS TRANSPORT message; and
  • upon switching to automatic network selection mode the UE remembers that it is still registered on the PLMN where the security check failure of SOR information was encountered;
the UE shall wait until it moves to idle mode or 5GMM-CONNECTED mode with RRC inactive indication (see TS 24.501) before attempting to obtain service on a higher priority PLMN as specified in subclause 4.4.3.3, by acting as if timer T that controls periodic attempts has expired, with an exception that the current registered PLMN is considered as lowest priority. If the selected PLMN is a VPLMN and the UE has an established emergency PDU session then the UE shall attempt to perform the PLMN selection after the emergency PDU session is released.
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