Section: 4.9.1.2.4 Xn based inter NG-RAN handover with re-allocation of intermediate UPF
Content: This procedure is used to hand over a UE from a Source NG-RAN to a Target NG-RAN using Xn when the AMF is unchanged and the SMF decides that the intermediate UPF (I-UPF) is to be changed. In the case of using UL CL, the I-UPF can be regarded as UL CL and additional PSA provides local access to a DN, the simultaneous change of UL-CL and the additional PSA is described in clause 4.3.5.7. In the case of using Branching Point, the I-UPF can be regarded as BP. It is assumed that the PDU Session for the UE comprises of a UPF that acts as a PDU Session Anchor and an intermediate UPF at the time of this Handover procedure for non-roaming and local breakout roaming scenario. In the case of home routed roaming scenario, the PDU Session of the UE comprises of at least one UPF in the VPLMN and UPF in the HPLMN which acts as a PDU Session Anchor at the time of this handover procedure. The Source UPF referred in this clause 4.9.1.2.4 is the UPF which terminates N3 interface in the 5GC. The presence of IP connectivity between the Source UPF and Source NG-RAN, between the source UPF and Target NG-RAN and between the Target UPF and Target NG-RAN, is assumed. (If there is no IP connectivity between source UPF and Target NG-RAN, it is assumed that the N2-based handover procedure in clause 4.9.1.3 shall be used instead). The call flow is shown in figure 4.9.1.2.4-1. Figure 4.9.1.2.4-1: Xn based inter NG-RAN handover with intermediate UPF re-allocation Steps 1-4 are same as steps 1-4 described in clause 4.9.1.2.3 except that the I-UPF in clause 4.9.1.2.3 is replaced by Target UPF. 5. [Conditional] The SMF sends N4 Session Modification Request message to the PDU Session Anchor. The DL CN Tunnel Info of the Target UPF is included in this message. If redundant transmission is performed for one or more QoS Flows of the PDU Session, the SMF provides two DL CN Tunnel Info (for N9) to the UPF (PSA) and indicates to the UPF (PSA) one of the DL CN Tunnel Info is used as redundancy tunnel of the PDU Session. In the case of home routed roaming, if the N9 terminating V-UPF, which is connected to with home UPF, is changed, the V-SMF invokes an Nsmf_PDUSession_Update Request (DL CN Tunnel Info) service operation toward the H-SMF. 6. [Conditional] The SMF associated with the PDU Session Anchor responds with the N4 Session Modification Response message. In the case of home routed roaming, the H-SMF responds with the Nsmf_PDUSession_Update Response service operation toward the V-SMF once H-UPF is updated with the DL Tunnel Info of the T-UPF. At this point, PDU Session Anchor starts sending downlink packets to the Target NG-RAN via Target UPF. Steps 7-11 are same as steps 7-11 described in clause 4.9.1.2.3 except that the I-UPF in clause 4.9.1.2.3 is replaced by Target UPF. If the Source UPF acts as a UL CL or BP, the SMF indicates to 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. 11. The timer is started in step 4 if the source UPF is not the PSA UPF. When this timer is expired, the SMF initiates Source UPF(s) Release procedure by sending an N4 Session Release Request (Release Cause). 12. The Source UPF(s) acknowledges with an N4 Session Release Response message to confirm the release of resources.