Section: 4.9.1.2.2 Xn based inter NG-RAN handover without User Plane function re-allocation
Content: This procedure is used to hand over a UE from a source NG-RAN to Target NG-RAN using Xn when the AMF is unchanged and the SMF decides to keep the existing UPF. The UPF referred in this clause 4.9.1.2.2 is the UPF which terminates N3 interface in the 5GC for non-roaming or local breakout roaming scenario, V-UPF which terminates N3 interface in 5GC for home routed roaming scenario. The SMF referred in this clause 4.9.1.2.2 is the V-SMF for home routed roaming scenario. The presence of IP connectivity between the Source UPF and Target NG-RAN is assumed. The call flow is shown in figure 4.9.1.2.2-1. Figure 4.9.1.2.2-1: Xn based inter NG-RAN handover without UPF re-allocation 1a. If the PLMN has configured secondary RAT usage reporting, the source NG-RAN node during the handover execution phase may provide RAN usage data Report (N2 SM Information (Secondary RAT usage data), Handover Flag, Source to Target transparent container) to the AMF. The source NG-RAN node shall provide this only when the Target NG-RAN has confirmed handover over Xn interface. The Handover Flag indicates to the AMF that it should buffer the N2 SM Information containing the usage data report before forwarding it. 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 NG-RAN provides the UE's UE Radio Capability ID to the Target NG-RAN. 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 Xn signalling as defined in TS 38.423[ NG-RAN; Xn Application Protocol (XnAP) ]  [72] . 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 shall provide the UE radio access capabilities to the Target NG-RAN and shall not send the UE Radio Capability ID. If, as permitted in TS 38.423[ NG-RAN; Xn Application Protocol (XnAP) ]  [72], the Target NG-RAN during the handover preparation received the UE radio access capabilities but did not receive the UE Radio Capability ID, NG-RAN shall proceed with handover using the received UE radio access capabilities. If the Target NG-RAN received both the UE radio access capabilities and the UE Radio Capability ID, then the Target NG-RAN shall use any locally stored UE radio access capability information corresponding to the UE Radio Capability ID. If none are stored locally, the Target NG-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 Target NG-RAN chooses not to request them, then the Target NG-RAN shall proceed with the UE radio access capabilities sent by the source NG-RAN. The Target NG-RAN shall not use the UE radio access capability information received from the source NG-RAN for any other UE with the same the UE Radio Capability ID. 1b. Target NG-RAN to AMF: N2 Path Switch Request (List of PDU Sessions To Be Switched with N2 SM Information, List of PDU Sessions that failed to be established with the failure cause given in the N2 SM information element, UE Location Information, established 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 included in the N2 SM information). The Target NG-RAN sends an N2 Path Switch Request message to an AMF to inform that the UE has moved to a new Target cell and provides a List Of PDU Sessions To Be Switched. AN Tunnel Info for each PDU Session to be switched is included in the N2 SM Information. If redundant transmission is performed for one or more QoS Flows in the PDU Session, two AN Tunnel Info are provided by the Target NG-RAN and the Target NG-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 serving PLMN ID is included in the message. The Target NG-RAN shall include the PDU Session in the PDU Sessions Rejected list: - If none of the QoS Flows of a PDU Session are accepted by the Target NG-RAN; or - If the corresponding network slice is not supported in the Target NG-RAN; or - When the NG-RAN cannot set up user plane resources fulfilling the User Plane Security Enforcement with a value Required, the NG-RAN rejects the establishment of user plane resources for the PDU Session. If the NG-RAN cannot set up user plane resources fulfilling the User Plane Security Enforcement with a value Preferred, the NG-RAN establishes the user plane resources for the PDU session and shall include the PDU Session in the PDU Sessions Modified list. PDU Sessions Rejected contains an indication of whether the PDU session was rejected because User Plane Security Enforcement is not supported in the Target NG-RAN. Depending on the type of target cell, the Target NG-RAN includes appropriate information in this message. For the PDU Sessions to be switched to the Target NG-RAN, the N2 Path Switch Request message shall include the list of accepted QoS Flows. For each QoS Flow accepted with an Alternative QoS Profile as specified in TS 23.501[ System architecture for the 5G System (5GS) ]  [2], the N2 SM Information shall include a reference to the fulfilled Alternative QoS Profile. The NG-RAN includes the PDU Set Based Handling Support Indication in N2 SM information as defined in clause 5.37.5.3 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. 2. AMF to SMF: Nsmf_PDUSession_UpdateSMContext Request (N2 SM information received from T-RAN in step 1b and N2 SM Information from source NG-RAN (Secondary RAT usage data), UE Location Information, UE presence in LADN service area). The N2 SM Information here from source NG-RAN is the one buffered at step 1a when applicable. The AMF sends N2 SM information by invoking the Nsmf_PDUSession_UpdateSMContext request service operation for each PDU Session in the lists of PDU Sessions received in the N2 Path Switch Request. The Nsmf_PDUSession_UpdateSMContext Request contains either an indication that the PDU Session Is To Be Switched (together with information on the N3 addressing to use and on the transferred QoS flows) or an indication that the PDU Session is to be Rejected (together with a rejection cause). If the AMF, based on configuration, as described in clause 5.43.4 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2], is aware that satellite backhaul category has changed due to the handover and needs to be updated to the SMF, the AMF includes the new satellite backhaul category and indicates the satellite backhaul category change as described in clause 5.43.4 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. If the AMF, based on configuration, as described in clause 5.43.2 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2], is aware that the UE is accessing over a gNB using GEO satellite backhaul and GEO Satellite ID needs to be updated to the SMF, the AMF may include the latest GEO Satellite ID as described in clause 5.43.2 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. For a PDU Sessions to be switched to the Target NG-RAN, upon receipt of the Nsmf_PDUSession_UpdateSMContext request, the SMF determines whether the existing UPF can continue to serve the UE. If the existing UPF cannot continue to serve the UE, steps 3-11 of clause 4.9.1.2.3 or 4.9.1.2.4 are performed depending on whether the existing UPF is a PDU Session Anchor. Otherwise, the following steps 3 to 6 are performed if the existing UPFs can continue to serve the 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" to the SMF. 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. If a PDU Session is rejected 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 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. For the PDU Session(s) that do not have active N3 UP connections before handover procedure, the SMF(s) keep the inactive status after handover procedure. If the UE moves into a non-Allowed Area, the AMF also notifies via Namf_EventExposure_Notify to each NF Consumer (e.g. SMFs of the established PDU Sessions) which has subscribed for UE reachability event, that the UE is only reachable for regulatory prioritized services. The SMF then deactivates the PDU session if this PDU Session is not for emergency service. 3. SMF to UPF: N4 Session Modification Request (AN Tunnel Info) For PDU Sessions that are modified by the Target NG-RAN, the SMF sends an N4 Session Modification Request message to the UPF. The SMF may notify the UPF that originated the Data Notification to discard downlink data for the PDU Sessions and/or to not provide further Data Notification messages. Depending on the network deployment, the CN Tunnel Info of UPF used for connection to Target NG-RAN and connection to Source NG-RAN may be different, e.g. due to Source and Target NG-RAN are in different IP domains. In this case the SMF may ask the UPF to allocate new CN Tunnel Info, providing the target Network Instance. 4. UPF to SMF: N4 Session Modification Response (CN Tunnel Info) For the PDU Sessions that are switched, the UPF returns an N4 Session Modification Response message to the SMF after requested PDU Sessions are switched. Tunnel identifiers for UL traffic are included only for PDU Sessions whose user plane resources are not being released and was requested by the SMF. If redundant transmission is performed for one or more QoS Flows of a PDU Session and different CN Tunnel Info were requested by the SMF, the UPF allocates two different CN Tunnel Info and indicates the SMF that one CN 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]. For the PDU Sessions that are deactivated, the UPF returns an N4 Session Modification Response message to the SMF after the N3 (R)AN tunnel information is released. 5. In order to assist the reordering function in the Target NG-RAN, the UPF (as specified in clause 5.8.2.9 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]) sends one or more "end marker" packets for each N3 tunnel on the old path immediately after switching the path. The UPF starts sending downlink packets to the Target NG-RAN. 6. SMF to AMF: Nsmf_PDUSession_UpdateSMContext Response (N2 SM information) The SMF sends an Nsmf_PDUSession_UpdateSMContext response (N2 SM Information (CN Tunnel Info, updated CN PDB for the accepted QoS Flows, Updated TSCAIs for the accepted QoS Flows)) to the AMF for PDU Sessions which have been switched successfully. The CN Tunnel Info of UPF send to AMF is used to setup N3 tunnel. If redundant transmission is performed for one or more QoS Flows of a PDU Session, two CN Tunnel Info are sent and the SMF indicates to the Target NG-RAN one of the CN 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]. The SMF sends an Nsmf_PDUSession_UpdateSMContext response without including the CN Tunnel Info to the AMF for the PDU Sessions for which user plane resources are deactivated or released and then the SMF releases the PDU Session(s) which is to be released using a separate procedure as defined in clause 4.3.4. For each accepted GBR QoS Flow of Delay-critical resource type, the dynamic CN PDB and TSCAI may be updated and sent to the Target NG-RAN by the SMF. The SMF may update the CN PDB and TSCAI in the response or using a separate PDU Session Modification procedure, based on local configuration. If the Source NG-RAN does not support Alternative QoS Profiles (see TS 23.501[ System architecture for the 5G System (5GS) ]  [2]) and the Target NG-RAN supports them, the SMF sends the Alternative QoS Profiles (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. NOTE: Step 6 can occur any time after receipt of N4 Session Modification Response at the SMF. 7. AMF to NG-RAN: N2 Path Switch Request Ack (N2 SM Information, Failed PDU Sessions, UE Radio Capability ID). Once the Nsmf_PDUSession_UpdateSMContext response is received from all the SMFs, the AMF aggregates received CN Tunnel Info and sends this aggregated information as a part of N2 SM Information along with the Failed PDU Sessions in N2 Path Switch Request Ack to the Target NG-RAN. If none of the requested PDU Sessions have been switched successfully, the AMF shall send an N2 Path Switch Request Failure message to the Target NG-RAN. If the UE Radio Capability ID is included in the N2 Path Switch Request Ack message, when there is no corresponding UE radio capabilities set for UE Radio Capability ID at the Target NR-RAN, the Target NG-RAN shall request the AMF to provide the UE radio capabilities set corresponding to UE Radio Capability ID to the Target NG-RAN. 8. By sending a Release Resources message to the Source NG-RAN, the Target NG-RAN confirms success of the handover. It then triggers the release of resources with the Source NG-RAN. 9. [Conditional] The UE may initiate Mobility Registration Update procedure if one of the triggers of registration procedure applies as described in clause 4.2.2.2.2. In this case, only steps 1, 2, 3, 17 and 21 in clause 4.2.2.2.2 are performed. For the mobility related events as described in clause 4.15.4, the AMF invokes the Namf_EventExposure_Notify service operation. 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.