Section: 4.9.1.3.3 Execution phase
Content: Figure 4.9.1.3.3-1: inter NG-RAN node N2 based handover, execution phase NOTE 1: Registration of serving AMF with the UDM is not shown in the figure for brevity. 1. S-AMF to S-RAN: Handover Command (Target to Source transparent container, List Of PDU Sessions to be handed-over with N2 SM information containing information received from T-RAN during the handover preparation phase, List Of PDU Sessions failed to be setup). Target to Source transparent container is forwarded as received from S-AMF. If DAPS Response information for one or more DRBs is received by S-RAN and indicates that DAPS handover is accepted, the execution phase for DAPS handover procedure as described in clause 4.9.1.3.3a is performed. The SM forwarding info list includes T-RAN SM N3 forwarding info list for direct forwarding or S-UPF SM N3 forwarding info list for indirect data forwarding S-RAN uses the PDU Sessions failed to be setup list and the indicated reason for failure to decide whether to proceed with the N2 Handover procedure. If the S-RAN supports and receives a reference to an Alternative QoS Profile for an accepted QoS Flow, it shall take it into account for deciding whether or not to proceed with the N2 Handover procedure (see TS 23.501[ System architecture for the 5G System (5GS) ]  [2]). 2. S-RAN to UE: Handover Command (UE container). UE container is a UE part of the Target to Source transparent container which is sent transparently from T-RAN via AMF to S-RAN and is provided to the UE by the S-RAN. 2a0. If the PLMN has configured secondary RAT usage reporting and the source NG-RAN has Secondary RAT usage data to report, the source NG-RAN node may provide RAN usage data report message (N2 SM Information (Secondary RAT usage data), Handover Flag) as in clause 4.21 to the AMF. The Handover Flag indicates to the AMF that it should buffer the N2 SM Information containing the usage data report before forwarding it. NOTE 2: This step is not shown in this figure but the secondary RAT usage data reporting procedure is shown in figure 4.21-1 in clause 4.21. 2a. - 2c. The S-RAN sends the Uplink RAN Status Transfer message to the S-AMF, as specified in TS 36.300[ Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 ]  [46] and TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [9]. The S-RAN may omit sending this message if none of the radio bearers of the UE shall be treated with PDCP status preservation. In the case of time-based handover, the S-RAN sends the Uplink RAN Early Status Transfer message to the S-AMF as specified in TS 38.413[ NG-RAN; NG Application Protocol (NGAP) ]  [10]. If there is an AMF relocation, the S-AMF sends this information to the T-AMF via the Namf_Communication_N1N2MessageTransfer service operation and the T-AMF acknowledges. The S-AMF or, if the AMF is relocated, the T-AMF, sends the information to the T-RAN via the Downlink RAN Status Transfer message, as specified in TS 36.300[ Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 ]  [46] and TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [9]. In the case of time-based handover, the T-AMF sends the Downlink RAN Early Status Transfer message to the T-RAN as specified in TS 38.413[ NG-RAN; NG Application Protocol (NGAP) ]  [10]. For Inter PLMN handover, if the target AMF has been relocated in Preparation phase, e.g. due to the inter PLMN handover, the S-AMF send this information to the indicated T-AMF, which is derived from the target AMF ID received in step 12 of clause 4.9.1.3.2. 3. Uplink packets are sent from T-RAN to T-UPF and UPF (PSA). Downlink packets are sent from UPF (PSA) to S-RAN via S-UPF. The S-RAN should start forwarding of downlink data from the S-RAN towards the T-RAN for QoS Flows or DRBs subject to data forwarding. This may be either direct (step 3a) or indirect forwarding (step 3b). 4. UE to T-RAN: Handover Confirm. After the UE has successfully synchronized to the target cell, it sends a Handover Confirm message to the T-RAN. Handover is by this message considered as successful by the UE. 5. T-RAN to T-AMF: Handover Notify. Handover is by this message considered as successful in T-RAN. 6a. [Conditional] T-AMF to S-AMF: Namf_Communication_N2InfoNotify. The T-AMF notifies to the S-AMF about the N2 handover notify received from the T-RAN by invoking the Namf_Communication_N2InfoNotify. A timer in S-AMF is started to supervise when resources in S-RAN shall be release. 6b. [Conditional] S-AMF to T-AMF: Namf_Communication_N2InfoNotify ACK (N2 SM Information (Secondary RAT usage data)). The S-AMF acknowledges by sending the Namf_Communication_N2InfoNotify ACK to the T-AMF. The N2 SM Information here is the one buffered at step 2a0 when applicable. 6c. [Conditional] S-AMF to SMF: Nsmf_PDUSession_ReleaseSMContext Request (SM Context ID, N2 SM Information (Secondary RAT Usage Data)). If the PDU Session(s) is not accepted by the T-AMF (e.g. S-NSSAI associated with the PDU Session is not available in the T-AMF), S-AMF triggers PDU Session Release procedure as specified in clause 4.3.4.2 after the S-AMF is notified for the reception of N2 Handover Notify in step 6a. 7. T-AMF to SMF: Nsmf_PDUSession_UpdateSMContext Request (Handover Complete indication for PDU Session ID, UE presence in LADN service area, N2 SM Information (Secondary RAT usage data)). The N2 SM Information here is the one received at step 6b when applicable. Handover Complete indication is sent per each PDU Session to the corresponding SMF to indicate the success of the N2 Handover. When an Nsmf_PDUSession_UpdateSMContext Response message arrived too late during the handover preparation phase (see step 8 of clause 4.9.1.3.2), or the PDU Session with SMF involvement is not accepted by T-RAN, Nsmf_PDUSession_UpdateSMContext Request (SM Context ID, Operation Type) is sent to the corresponding SMF allowing the SMF to deallocate a possibly allocated N3 UP address and Tunnel ID of the selected UPF. A PDU Session handled by that SMF is considered deactivated and handover attempt is terminated for that PDU Session. In the case that the AMF determines that the PDU Session is related to a LADN then the AMF provides the "UE presence in LADN service area". If the AMF does not provide the "UE presence in LADN service area" indication and the SMF determines that the DNN corresponds to a LADN, then the SMF considers that the UE is OUT of the LADN service area. The SMF takes actions for the LADN PDU Session as defined in clause 5.6.5 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2] based on the "UE presence in LADN service area" indication. For each QoS Flow for which the SMF has received a reference to the fulfilled Alternative QoS Profile in step 10 of clause 4.9.1.3.2, the SMF notifies the PCF and the UE as described in TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. 8a. [Conditional] SMF to T-UPF (intermediate): N4 Session Modification Request. If new T-UPF is inserted or an existing intermediate S-UPF is re-allocated, the SMF shall send N4 Session Modification Request indicating DL AN Tunnel Info of T-RAN to the T-UPF. 8b. [Conditional] T-UPF to SMF: N4 Session Modification Response. The T-UPF acknowledges by sending N4 Session Modification Response message to SMF. 9a. [Conditional] SMF to S-UPF (intermediate): N4 Session Modification Request. If UPF is not re-allocated, the SMF shall send N4 Session Modification Request indicating DL AN Tunnel Info of T-RAN to the S-UPF. 9b. [Conditional] S-UPF to SMF: N4 Session Modification Response. The S-UPF acknowledges by sending N4 Session Modification Response message to SMF. 10a. [Conditional] SMF to UPF (PSA): N4 Session Modification Request. For non-roaming or local breakout roaming scenario, the SMF sends N4 Session Modification Request message to PDU Session Anchor UPF, UPF (PSA), providing N3 AN Tunnel Info of T-RAN or the DL CN Tunnel Info of T-UPF if a new T-UPF is inserted or an existing intermediate S-UPF is re-allocated. If redundant transmission is performed for one or more QoS Flows of the PDU Session, two N3 AN Tunnel Info of T-RAN or two DL CN Tunnel Info of two T-UPFs are provided and the SMF indicates to the UPF (PSA) one of the AN/CN Tunnel Info is used as redundancy tunnel of the PDU Session. If the existing intermediate S-UPF terminating N9 toward the H-UPF (PDU Session Anchor) is re-allocated for the home routed roaming scenario, the V-SMF invokes an Nsmf_PDUSession_Update Request (DL CN Tunnel Info) service operation toward the H-SMF. In the case of the S-UPF acts as a UL CL or BP, the SMF indicates only one of the PDU Session Anchors to send the "end marker" packets. To ensure the "end marker" is the last user plane packet on the old path, the SMF should modify the path on other PDU Session Anchors before it indicates the PDU Session Anchor to send the "end marker" packets. If T-UPF is not inserted or an existing intermediate S-UPF is not re-allocated, step 10a and step 10b are skipped. 10b. [Conditional] UPF (PSA) to SMF: N4 Session Modification Response. The UPF (PSA) sends N4 Session Modification Response message to SMF. In order to assist the reordering function in the T-RAN, the UPF (PSA) sends one or more "end marker" packets for each N3 tunnel on the old path immediately after switching the path, the source NG-RAN shall forward the "end marker" packets to the Target NG-RAN. At this point, UPF (PSA) starts sending downlink packets to the T-RAN, via T-UPF if a new T-UPF is inserted or an existing intermediate S-UPF is re-allocated. In the case of home routed roaming scenario, the H-SMF responds with the Nsmf_PDUSession_Update Response service operation to V-SMF once the H-UPF (PDU Session Anchor) is updated with the UL Tunnel Info of the T-UPF. When there are multiple UPFs(PSA), step 10a and step 10b are performed for each UPFs(PSA). 11. SMF to T-AMF: Nsmf_PDUSession_UpdateSMContext Response (PDU Session ID). SMF confirms reception of Handover Complete. If indirect data forwarding applies, the SMF starts an indirect data forwarding timer, to be used to release the resource of indirect data forwarding tunnel. 12. The UE initiates Mobility Registration Update procedure as described in clause 4.2.2.2.2. The target AMF knows that it is a Handover procedure and therefore the target AMF performs only a subset of the Registration procedure, specifically the steps 4, 5 and 10 in the Registration procedure for the context transfer between source AMF and target AMF are skipped. The target AMF, based on the S-NSSAIs subject to Network Slice-Specific Authentication and Authorization status information from source AMF in step 3 of clause 4.9.1.3.2, may decide to skip step 25 in the Registration procedure (i.e. NSSAA procedure) or whether to perform it if the status is pending. 13a. [Conditional] SMF to S-UPF (intermediate): N4 Session Release Request. If there is a source intermediate UPF, the SMF initiates resource release, after timer in step 6 or indirect data forwarding timer expires, by sending an N4 Session Release Request (Release Cause) to source UPF. This message is also used to release the indirect data forwarding resource in S-UPF. 13b. S-UPF to SMF: N4 Session Release Response. The S-UPF acknowledges with an N4 Session Release Response message to confirm the release of resources. In the case of indirect data forwarding, the resource of indirect data forwarding is also released. 14a. AMF to S-RAN: UE Context Release Command (). After the timer in step 6a expires, the AMF sends UE Context Release Command. 14b. S-RAN to AMF: UE Context Release Complete (). The source NG-RAN releases its resources related to the UE and responds with a UE Context Release Complete () message. 15a. [Conditional] SMF to T-UPF: N4 Session Modification Request. If indirect forwarding applies and UPF is re-allocated, after timer of indirect data forwarding expires, the SMF sends N4 Session Modification Request to T-UPF to release the indirect data forwarding resource. 15b. [Conditional] T-UPF to SMF: N4 Session Modification Response. The T-UPF acknowledges with an N4 Session Modification Response message to confirm the release of indirect data forwarding resources. If the AMF is subscribed to Mobility Event by other NFs, the AMF notifies the event to the corresponding NFs by invoking the Namf_EventExposure_Notify service operation as described in clause 4.15.4.2. Upon reception of the Namf_EventExposure_Notify with an indication that UE is reachable only for regulatory prioritized service, the SMF deactivates the PDU Session if the service of the PDU Session is not regulatory prioritized. For home routed roaming case, the V-SMF triggers the deactivation of the PDU Session, in addition, the H-SMF refrains from sending downlink signalling if the signalling is not related to regulatory prioritized service upon receiving the notification.