Section: 4.9.1.2.3 Xn based inter NG-RAN handover with insertion 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 insertion of a new additional intermediate UPF is needed. If redundant transmission is performed for one or more QoS Flows of a PDU Session to be switched to the Target NG-RAN, the SMF may select two Intermediate UPFs (I-UPFs) and set up two N3 and N9 tunnels between the Target NG-RAN and the UPF (PSA) via the two I-UPFs as described in clause 5.33.2.2 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. In the case of using UL CL, the I-UPF can be regarded as UL CL and additional PSA providing local access to a DN. 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 only one UPF that acts as a PDU Session Anchor 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 one UPF in the HPLMN at the time of this handover procedure. In this case, additional insertion of an N3 terminating intermediate UPF will not have impact on the connectivity between the UPF in VPLMN and UPF in HPLMN. The presence of IP connectivity between the UPF (PDU Session Anchor) and Source NG-RAN, between the UPF (PDU Session Anchor) and Target NG-RAN and between the intermediate UPF (I-UPF) and Target NG-RAN, is assumed. (If there is no IP connectivity between UPF (PDU Session Anchor) 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 4.9.1.2.3-1. Figure 4.9.1.2.3-1: Xn based inter NG-RAN handover with insertion of intermediate UPF Steps 1-2 are the same as described in clause 4.9.1.2.2. 3a. [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). 3b. [Conditional] UPF (PSA) to SMF: N4 Session Modification Response. The UPF (PSA) sends an N4 Session Modification Response message to the SMF. The UPF 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) 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. 4a. SMF to I-UPF: N4 Session Establishment Request (Target NG-RAN Tunnel Info, CN Tunnel Info of the PDU Session Anchor) For PDU Sessions to be updated, if the UE has moved out of the service area of UPF connecting to the serving NG-RAN node, the SMF then selects an I-UPF based on UPF Selection Criteria according to clause 6.3.3 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. An N4 Session Establishment Request message is sent to the I-UPF. The CN Tunnel Info of the PDU Session Anchor, which is used to setup N9 tunnel, is included in the N4 Session Establishment Request message. 4b. I-UPF to SMF: N4 Session Establishment Response. The I-UPF sends an N4 Session Establishment Response message to the SMF. The UL and DL CN Tunnel Info of I-UPF is sent to the SMF. If SMF select two Intermediate UPFs (I-UPFs) to perform redundant transmission for a PDU session, step 4a and 4b are performed between the SMF and each I-UPF. 5. SMF to PDU Session Anchor: N4 Session Modification Request (DL CN Tunnel Info of the I-UPF). The SMF sends N4 Session Modification Request message to the PDU Session Anchor. If a different CN Tunnel Info is used on N9 in UPF (PSA), the SMF starts a timer to release the CN Tunnel for N3. Otherwise the SMF does not need to start a timer to release the CN Tunnel Info used on N3 in UPF(PSA) (i.e. CN Tunnel Info is common for both N3 and N9). 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. 6. PDU Session Anchor to SMF: N4 Session Modification Response. The PDU Session Anchor responds with the N4 Session Modification Response message after requested PDU Sessions are switched. At this point, PDU Session Anchor starts sending downlink packets to the Target NG-RAN via I-UPF. 7. In order to assist the reordering function in the Target NG-RAN, the PDU Session Anchor 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. 8. SMF to AMF: Nsmf_PDUSession_UpdateSMContext Response (N2 SM Information (UL CN Tunnel Info of the I-UPF, updated QoS parameters for accepted QoS Flows)). The SMF sends an Nsmf_PDUSession_UpdateSMContext response to the AMF. Steps 8-11 are same as steps 6-9 defined in clause 4.9.1.2.2. 12. After the timer set in step 5 expires, the SMF informs the PDU Session Anchor to remove the CN Tunnel for N3 via N4 Session Modification procedure.