Section: 4.9.1.3.2 Preparation phase
Content: Figure 4.9.1.3.2-1: Inter NG-RAN node N2 based handover, Preparation phase 1. S-RAN to S-AMF: Handover Required (Target ID, Source to Target transparent container, SM N2 info list, PDU Session IDs, intra system handover indication). NOTE 1: When applicable the message includes the selected NID, see TS 38.413[ NG-RAN; NG Application Protocol (NGAP) ]  [10] for the IE that includes the selected NID. The T-AMF, ensures that the selected NID is forwarded to SMF. Source to Target transparent container includes NG-RAN information created by S-RAN to be used by T-RAN and is transparent to 5GC. It also contains for each PDU session the corresponding QoS flows/DRBs information subject to data forwarding. It may also contain DAPS Information if DAPS handover is supported by S-RAN and S-AMF and DAPS handover is requested for one or more DRBs as described in TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [9]. It may also contain the time-based handover parameters when time-based trigger condition is used. All PDU Sessions handled by S-RAN (i.e. all existing PDU Sessions with active UP connections) shall be included in the Handover Required message, indicating which of those PDU Session(s) are requested by S-RAN to handover. The SM N2 info includes Direct Forwarding Path Availability if direct data forwarding is available. Direct Forwarding Path Availability indicates whether direct forwarding is available from the S-RAN to the T-RAN. This indication from S-RAN can be based on e.g. the presence of IP connectivity and security association(s) between the S-RAN and the T-RAN. If the source NG RAN and target NG RAN support RACS as defined in TS 23.501[ System architecture for the 5G System (5GS) ]  [2], the Source to Target transparent container need not carry the UE radio access capabilities (instead the UE Radio Capability ID is supplied from the CN to the T-RAN). However, if the source NG-RAN has knowledge that the target NG-RAN might not have a local copy of the Radio Capability corresponding to the UE Radio Capability ID (i.e. because the source NG-RAN had itself to retrieve the UE's Radio Capability from the AMF) then the source NG-RAN may also send some (or all) of the UE's Radio Capability to the target NG-RAN (the size limit based on local configuration). In the case of inter-PLMN handover, when the source and target NG-RAN support RACS as defined in TS 23.501[ System architecture for the 5G System (5GS) ]  [2] and the source NG-RAN determines based on local configuration that the target PLMN does not support the UE Radio Capability ID assigned by the source PLMN, then the source NG-RAN includes the UE radio access capabilities in the Source to Target transparent container. 2. T-AMF Selection: When the S-AMF can't serve the UE anymore, the S-AMF selects the T-AMF as described in clause 6.3.5 on "AMF Selection Function" in TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. 3. [Conditional] S-AMF to T-AMF: Namf_Communication_CreateUEContext Request (N2 Information (Target ID, Source to Target transparent container, SM N2 information list, PDU Session IDs), UE context information (SUPI, Service area restriction, Allowed NSSAI for each Access Type and Partially Allowed NSSAI if available, Tracing Requirements, LTE M Indication, the list of PDU Session IDs along with the corresponding SMF information and the corresponding S-NSSAI(s), PCF ID(s), DNN, UE Radio Capability ID and UE Radio Capability Information, N2 Notify URI). If the subscription information includes Tracing Requirements, the old AMF provides the target AMF with Tracing Requirements. If the old AMF was a consumer of UE related NWDAF services, the old AMF includes information about active analytics subscriptions, i.e. the Subscription Correlation ID(s), NWDAF identifier(s) (i.e. Instance ID or Set ID), Analytics ID(s) and associated Analytics specific data in the Namf_Communication_UEContextTransfer request. Usage of the analytics information by the new AMF is specified in TS 23.288[ Architecture enhancements for 5G System (5GS) to support network data analytics services ]  [50]. In inter PLMN handover case, UE context information includes HPLMN S-NSSAIs corresponding to the Allowed NSSAI for each Access Type and Partially Allowed NSSAI, without Allowed NSSAI and Partially Allowed NSSAI of source PLMN. The target AMF may determine the Allowed NSSAI and Partially Allowed NSSAI based on the HPLMN S-NSSAIs received in step 3, or else the target AMF queries the NSSF by invoking Nnssf_NSSelection_Get service operation with the HPLMN S-NSSAIs and PLMN ID of SUPI. Based on the query result returned from the NSSF, the target AMF determines whether the AMF re-allocation need be triggered and if needed reselects another target AMF. In this case the target AMF selected by the S-AMF is the Initial AMF. And another reselected target AMF is the final target AMF (i.e. T-AMF). The S-AMF initiates Handover resource allocation procedure by invoking the Namf_Communication_CreateUEContext service operation towards the target AMF. When the S-AMF can still serve the UE, this step and step 12 are not needed. If target AMF re-allocation is needed, e.g. due to the inter PLMN handover, the initial AMF invokes Namf_Communication_CreateUEContext request (SUPI, Target 5GAN Node ID, Source to Target Transparent Container, 5GS MM Context, PDU Session ID and its associated S-NSSAI of the VPLMN value for each PDU Session, the corresponding S-NSSAI of HPLMN value for home routed PDU Session(s), Allowed NSSAI, N2 Notify URI) to the selected final target AMF. The N2 Notify URI is the N2 Notify URI of the source AMF, which is used by the T-AMF to send N2 handover notify to the S-AMF. If the information about active analytics subscriptions are received from S-AMF, it is also included them in the Namf_Communication_CreateUEContext request. If Service area restrictions are available in the S-AMF, they may be forwarded to the T-AMF as described in clause 5.3.4.1.2 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. If both Home and Visited PCF ID(s) are provided by the S-AMF, the T-AMF contacts the (V-) PCF identified by the (V-)PCF ID. If the (V-)PCF identified by the (V-)PCF ID is not used or there are no PCF ID(s) received from the S-AMF, the T-AMF may select the PCF(s) as described in clause 6.3.7.1 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2] and according to the V-NRF to H-NRF interaction described in clause 4.3.2.2.3.3. The T-AMF informs the S-AMF that the PCF ID is not used, as defined in step 12 and then the S-AMF terminates the AM Policy Association with the PCF identified by the PCF ID. 4. [Conditional] T-AMF to SMF: Nsmf_PDUSession_UpdateSMContext (PDU Session ID, Target ID, T-AMF ID, N2 SM Information). For each PDU Session indicated by S-RAN, the AMF invokes the Nsmf_PDUSession_UpdateSMContext Request to the associated SMF. However, if the S-NSSAI associated with PDU Session is not available in the T-AMF, the T-AMF does not invoke Nsmf_PDUSession_UpdateSMContext for this PDU Session. PDU Session ID indicates a PDU Session candidate for N2 Handover. Target ID corresponds to Target ID provided by S-RAN in step 1. SM N2 Info includes the Direct Forwarding Path Availability if the direct data forwarding is available between the S-RAN and the T-RAN and has been inserted by the S-RAN. If the (T-)AMF detects that the UE moves into a non-allowed area based on Service area restrictions, the (T-)AMF notifies each NF consumer which has subscribed for UE reachability event (e.g. SMFs corresponding to the list of PDU Sessions received in UE Context from (S-)AMF via Namf_EventExposure_Notify that the UE is only reachable for regulatory prioritized services. 5. [Conditional] SMF checks if the Target ID is within the service area of the UPF connecting to NG-RAN. If UE has moved out of the service area of the UPF connecting to NG-RAN, SMF selects a new intermediate UPF according to clause 6.3.3 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2], if available. If redundant transmission is performed for one or more QoS Flows of the PDU Session, the SMF selects two new Intermediate UPFs to support the redundant transmission based on two N3 and N9 tunnels between the T-RAN and the UPF (PSA) as described in clause 5.33.2.2 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. In this case, step 6c and 6d are performed between SMF and each T-UPF. 6a. [Conditional] SMF to UPF (PSA): N4 Session Modification Request. If the SMF selects a new UPF to act as intermediate UPF for the PDU Session and the different CN Tunnel Info need be used, the SMF sends N4 Session Modification Request message to UPF (PSA). The SMF provides the CN Tunnel Info (on N9) if the CN Tunnel Info is allocated by the SMF and UL Packet detection rules associate the CN Tunnel Info (on N9) to be installed on the UPF (PSA). If redundant transmission is performed for one or more QoS Flows of the PDU Session and the different CN Tunnel Info need be used, the SMF provides two CN Tunnel Info (on N9) to the UPF (PSA) if the CN Tunnel Info is allocated by the SMF and indicates to the UPF (PSA) one of the CN Tunnel Info is used as redundancy tunnel of the PDU Session. 6b. [Conditional] UPF (PSA) to SMF: N4 Session Modification Response. The UPF (PSA) sends an N4 Session Modification Response message to the SMF. If the UPF (PSA) allocates CN Tunnel Info (on N9) of UPF (PSA), it provides CN Tunnel Info (on N9) to the SMF. If redundant transmission is performed for one or more QoS Flows of the PDU Session, the UPF (PSA) provides two CN Tunnel Info (on N9) of UPF (PSA) to the SMF and indicates the SMF that one CN Tunnel Info is used as redundancy tunnel of the PDU Session as described in in clause 5.33.2.2 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. The UPF (PSA) associate the CN Tunnel Info (on N9) with UL Packet detection rules provided by the SMF. 6c. [Conditional] SMF to T-UPF (intermediate): N4 Session Establishment Request. If the SMF selects a new intermediate UPF, i.e. the target UPF (T-UPF), for the PDU Session and if CN Tunnel Info is allocated by the T-UPF, an N4 Session Establishment Request message is sent to the T-UPF, providing Packet detection, enforcement and reporting rules to be installed on the T-UPF. The CN Tunnel Info (on N9) of UPF (PSA) for this PDU Session, which is used to setup N9 tunnel, is also provided to the T-UPF. 6d. T-UPF (intermediate) to SMF: N4 Session Establishment Response. The T-UPF sends an N4 Session Establishment Response message to the SMF with DL CN Tunnel Info and UL CN Tunnel Info (i.e. N3 tunnel info). The SMF starts a timer to release the resource of S-UPF, which is to be used in step 13a of the Execution Phase. 7. SMF to T-AMF: Nsmf_PDUSession_UpdateSMContext Response (PDU Session ID, N2 SM Information, Reason for non-acceptance). If at step 5 the SMF has determined that the UPF connecting to NG-RAN can still be used after the handover or has selected a new intermediate UPF, the SMF includes in the Nsmf_PDUSession_UpdateSMContext response the N2 SM Information containing the N3 UP address and the UL CN Tunnel ID of the UPF, the QoS parameters, TSCAI and the User Plane Security Enforcement information for the Target NG-RAN. If redundant transmission is performed for one or more QoS Flows of the PDU Session, two UL CN Tunnel Info are included in the N2 SM Information. If the N2 SM information received at step 4 does not include the Direct Forwarding Path Availability and the SMF knows that there is no indirect data forwarding connectivity between source and target, the N2 SM Information includes a Data forwarding not possible indication. If the SMF indicated that Direct Forwarding is available in step 4, the SMF should further include a "Direct Forwarding Path Availability" indication in the N2 SM information container. The SMF sends the Alternative QoS Profiles, QoS monitoring configuration for congestion information or ECN marking for L4S indicator for QoS Flow (see TS 23.501[ System architecture for the 5G System (5GS) ]  [2]) to the Target NG-RAN on a per QoS Flow basis, if available. If the SMF failed to find a suitable I-UPF at step 5, the SMF does not include any N2 SM Information regarding the PDU Session to avoid establishment of radio resources at the target NG-RAN. Instead of that, the SMF provides a reason for non-acceptance. The SMF decides to (based on local policies) either: - trigger re-establishment of PDU Session. After handover procedure, SMF sends N1 message to the UE via the AMF by invoking Namf_Communication_N1N2MessageTransfer containing the cause indicating PDU Session re-establishment is required for the UE; or - keep the PDU Session (the User Plane connection being deactivated); or - release the PDU Session after handover procedure. If the SMF has received notification from (T-)AMF that the UE is only reachable for regulatory prioritized services, the SMF does not include any N2 SM info regarding the PDU Session for non-regulatory prioritized services to avoid establishment of radio resources at the target NG-RAN. If the SMF receives notification from (T-)AMF that UE is only reachable for regulatory prioritized service after this step via Namf_EventExposure_Notify, the SMF deactivates the PDU Session after handover procedure finish if the PDU Session is not for regulatory prioritized services. 8. AMF supervises the Nsmf_PDUSession_UpdateSMContext Response messages from the involved SMFs. The lowest value of the Max delay indications for the PDU Sessions that are candidates for handover gives the maximum time AMF may wait for Nsmf_PDUSession_UpdateSMContext Response messages before continuing with the N2 Handover procedure. At expiry of the maximum wait time or when all Nsmf_PDUSession_UpdateSMContext Response messages are received, AMF continues with the N2 Handover procedure (Handover Request message in step 9). NOTE 2: The delay value for each PDU Session is locally configured in the AMF and implementation specific. 9. T-AMF to T-RAN: Handover Request (Source to Target transparent container, N2 MM Information, N2 SM Information list, Tracing Requirements, UE Radio Capability ID). If the subscription information includes Tracing Requirements, the target AMF provides the target RAN with Tracing Requirements in the Handover Request. T-AMF determines T-RAN based on Target ID. T-AMF may allocate a 5G-GUTI valid for the UE in the AMF and target TAI. Source to Target transparent container is forwarded as received from S-RAN. N2 MM Information includes e.g. security information and Mobility Restriction List if available in the T-AMF. N2 SM Information list includes N2 SM Information received from SMFs for the T-RAN in the Nsmf_PDUSession_UpdateSMContext Response messages received within allowed max delay supervised by the T-AMF mentioned in step 8. T-AMF provides the UE Radio Capability ID to T-RAN if RACS is supported. If the UE Radio Capability ID is included in the Handover Request message and no UE radio access capabilities are provided in the Source to Target transparent container, when there is no corresponding UE radio capabilities set for UE Radio Capability ID at T-RAN, T-RAN shall request the T-AMF to provide the UE radio capabilities set corresponding to UE Radio Capability ID to the T-RAN. If the Source to Target transparent container contains the UE radio access capabilities and the T-RAN did not receive the UE Radio Capability ID from the T-AMF, NG-RAN shall proceed with handover using the received UE radio access capabilities. If the T-RAN received both the UE radio access capabilities and the UE Radio Capability ID, then the T-RAN shall use any locally stored UE radio access capability information corresponding to the UE Radio Capability ID. If none are stored locally, the T-RAN may request the full UE radio access capability information from the core network. If the full UE radio access capability information is not promptly received from the core network, or the T-RAN chooses not to request them, then the T-RAN shall proceed with the UE radio access capabilities sent by the source RAN node. The T-RAN shall not use the UE radio access capability information received from the source RAN node for any other UE with the same the UE Radio Capability ID. 10. T-RAN to T-AMF: Handover Request Acknowledge (Target to Source transparent container, List of PDU Sessions to Hand-over with N2 SM information, List of PDU Sessions that failed to be established with the failure cause given in the N2 SM information element, PDU Set Based Handling Support Indication included in the N2 SM information). Target to Source transparent container includes a UE container with an access stratum part and a NAS part. The UE container is sent transparently via T-AMF, S-AMF and S-RAN to the UE. If DAPS handover is supported by the T-RAN and T-AMF and the DAPS Information for one or more DRBs had been received in the Source to Target Transparent Container, the T-RAN includes the DAPS Response information in the Target to Source Transparent Container as described in TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [9]. T-RAN creates List Of PDU Sessions failed to be setup and reason for failure (e.g. T-RAN decision, S-NSSAI is not available, unable to fulfil User Plane Security Enforcement) based on T-RAN determination. The information is provided to the S-RAN. The N2 SM information in the List Of PDU Sessions to Hand-over, contains per each PDU Session ID T-RAN N3 addressing information i.e. N3 UP address and Tunnel ID of T-RAN for the PDU Session. If redundant transmission is performed for one or more QoS Flows of the PDU Session, the T-RAN provides two AN Tunnel Info for the PDU Session in the N2 SM information. The T-RAN indicates to the SMF one of the AN Tunnel Info is used as the redundancy tunnel of the PDU session as described in clause 5.33.2.2 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. If only one AN Tunnel Info is provided by the Target NG-RAN for the PDU session, the SMF may release these QoS Flows by triggering PDU Session Modification procedure as specified in clause 4.3.3 after the handover procedure. The N2 SM information may also include: - an Indication whether UP integrity protection is performed or not on the PDU Session based on User Plane Security Enforcement information received in N2 SM information in step 9. - if the PDU Session has at least one QoS Flow subject for data forwarding, N3 UP address and Tunnel ID of T-RAN for receiving forwarded data. The T-RAN provides data forwarding addresses for each data forwarding tunnel which it decided to setup. - For each QoS Flow accepted with an Alternative QoS Profile (see TS 23.501[ System architecture for the 5G System (5GS) ]  [2]), the Target NG-RAN shall include a reference to the fulfilled Alternative QoS Profile. - For each accepted QoS Flow, QoS Flows status (active/not active) for QoS monitoring configuration for congestion information, established QoS Flows status (active/not active) for ECN marking for L4S. - PDU Set Based Handling Support Indication as described in clause 5.37.5.3 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. 11a. AMF to SMF: Nsmf_PDUSession_UpdateSMContext Request (PDU Session ID, N2 SM response received from T-RAN in step 10). For each N2 SM response received from the T-RAN (N2 SM information included in Handover Request Acknowledge), AMF sends the received N2 SM response to the SMF indicated by the respective PDU Session ID. If no new T-UPF is selected, SMF stores the N3 tunnel info of T-RAN from the N2 SM response if N2 handover is accepted by T-RAN. The SMF/UPF allocates the N3 UP address and Tunnel IDs for indirect data forwarding corresponding to the data forwarding tunnel endpoints established by T-RAN. If a PDU Session is indicated as a rejected PDU Session by the Target NG-RAN with an indication that the PDU session was rejected because User Plane Security Enforcement is not supported in the Target NG-RAN and the User Plane Enforcement Policy indicates "Required" as described in clause 5.10.3 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2], the SMF triggers the release of this PDU Session. In all other cases of PDU Session rejection, the SMF can decide whether to release the PDU Session (possibly triggering the re-establishment of the PDU Session as described in step 5) or to deactivate the UP connection of this PDU Session. If some of the QoS Flows of a PDU Session are not accepted by the Target NG-RAN, the SMF shall initiate the PDU Session Modification procedure to remove the non-accepted QoS Flows from the PDU Session(s) after the handover procedure is completed. 11b. [Conditional] SMF to T-UPF: N4 Session Modification Request (T-RAN SM N3 forwarding Information list, indication to allocate DL forwarding tunnel(s) for indirect forwarding) If the SMF selected a T-UPF in step 6a, the SMF updates the T-UPF by providing the T-RAN SM N3 forwarding information list by sending a N4 Session Modification Request to the T-UPF. If indirect forwarding applies based on indication from the S-RAN and the UPF is re-allocated and if the SMF decides to setup the indirect forwarding tunnel on the same T-UPF, the SMF also requests in the N4 Session Modification Request message to the T-UPF, to allocate DL forwarding tunnel(s) for indirect forwarding. Indirect forwarding may be performed via a UPF which is different from the T-UPF, in which case the SMF selects a T-UPF for indirect forwarding. 11c. [Conditional] T-UPF to SMF: N4 Session Modification Response (T-UPF SM N3 forwarding Information list). The T-UPF allocates Tunnel Info and returns an N4 Session Modification Response message to the SMF. The T-UPF SM N3 forwarding info list includes T-UPF N3 address, T-UPF N3 Tunnel identifiers for forwarding data 11d. [Conditional] SMF to S-UPF: N4 Session Modification Request (T-RAN SM N3 forwarding Information list or T-UPF SM N3 forwarding Information list, indication to allocate DL forwarding tunnel(s) for indirect forwarding). If the UPF is re-allocated, this message includes the T-UPF SM N3 forwarding info list. If the UPF is not re-allocated, this message includes the T-RAN SM N3 forwarding info list. If indirect forwarding applies based on indication from NG-RAN and UPF allocates tunnel identities, the SMF indicates in the N4 Session Modification Request message to the S-UPF to allocate DL forwarding tunnel(s) for indirect forwarding. Indirect forwarding may be performed via a UPF which is different from the S-UPF. 11e. [Conditional] S-UPF to SMF: N4 Session Modification Response (S-UPF SM N3 forwarding Information list). The S-UPF allocates Tunnel Info and returns an N4 Session establishment Response message to the SMF. The S-UPF SM N3 forwarding Information list includes S-UPF N3 address, S-UPF N3 Tunnel identifiers for DL data forwarding. 11f. SMF to T-AMF: Nsmf_PDUSession_UpdateSMContext Response (N2 SM Information). The SMF sends an Nsmf_PDUSession_UpdateSMContext Response message per PDU Session to T-AMF. The SMF creates an N2 SM information containing the DL forwarding Tunnel Info to be sent to the S-RAN by the AMF. The SMF includes this information in the Nsmf_PDUSession_UpdateSMContext response. The DL forwarding Tunnel Info can be one of the following information: - If direct forwarding applies, then the SMF includes the T-RAN N3 forwarding information the SMF received in step 11a. - If the indirect forwarding tunnel is setup in step 11b or 11d, then the SMF includes the T-UPF or S-UPF DL forwarding information containing the N3 UP address and the DL Tunnel ID of the UPF. 12. [Conditional] T-AMF to S-AMF: Namf_Communication_CreateUEContext Response (N2 information necessary for S-AMF to send Handover Command to S-RAN including Target to Source transparent container, PDU Sessions failed to be setup list, N2 SM information (N3 DL forwarding Information, PCF ID), [Target AMF ID]). T-AMF supervises the Nsmf_PDUSession_UpdateSMContext Response message from the involved SMFs. At expiry of the maximum wait time or when all Nsmf_PDUSession_UpdateSMContext Response messages are received, T-AMF sends the Namf_Communication_CreateUEContext Response to the S-AMF. The PDU Sessions failed to be setup list includes the List Of PDU Sessions failed to be setup received from target RAN in step 10 and the Non-accepted PDU session List generated by the T-AMF. Non-accepted PDU Session List includes following PDU Session(s) with proper cause value: - Non-accepted PDU Session(s) by the SMF(s); - Non-accepted PDU Session(s) by the AMF due to no response from the SMF within maximum wait time; and - Non-accepted PDU Session(s) by the AMF due to non-available S-NSSAI in the T-AMF, which is decided at step 4. The Target to Source transport container is received from the T-RAN. The N2 SM Information is received from the SMF in step 11f. If target AMF re-allocation is executed in step 3, the selected final target AMF, i.e. T-AMF, invoke Namf_Communication_CreateUEContext Response (Cause, N2 information necessary for S-AMF to send Handover Command to S-RAN including Target to Source transparent container, N2 SM information (PDU Sessions failed to setup list, N3 DL forwarding Information), PCF ID, PCF reselected indication, target AMF ID) to the initial AMF. The cause indicates whether the Relocate UE Context (hand-Over) succeeded or failed. If the target NG RAN has rejected the Handover Request in step 10, the cause indicates a failure due to RAN rejection. The target AMF ID is used for S-AMF to transfer RAN Status to T-AMF directly. Based on the receiving Namf_Communication_CreateUEContext Response, the initial AMF invokes Namf_Communication_CreateUEContext Response towards S-AMF.