Section: 4.3.5.1 Change of SSC mode 2 PDU Session Anchor with different PDU Sessions
Content: The following procedure is triggered by SMF in order to change the PDU Session Anchor serving a PDU Session of SSC mode 2 for a UE when neither multi-homing nor UL CL applies to the PDU Session. This procedure releases the existing PDU Session associated with an old PDU Session Anchor (i.e. UPF1 in figure 4.3.5.1-1) and immediately establishes a new PDU Session with a new PDU Session Anchor (i.e. UPF2 in figure 4.3.5.1-1) to the same DN. Figure 4.3.5.1-1: Change of SSC mode 2 PSA for a PDU Session 1. The SMF determines that the serving UPF needs to be changed due to events that may benefit from such change. 1a. If the UPF (PSA) cannot connect to the target DNAI that SMF received from SM-PCF, the SMF invokes Nsmf_PDUSession_SMContextStatusNotify Request (target DNAI information) service operation to the AMF. The SMF also indicates the SMF selection is expected. The target DNAI information is used for SMF selection which can control UPF connecting to that DNAI at next PDU session establishment towards the same DNN and S-NSSAI. Due to it is for SMF selection, the AMF stores the target DNAI information received from SMF. The target DNAI information is not transferred outside, e.g. to support the UE context transfer between AMFs for AMF relocation. 2. The PDU Session Release procedure is initiated as described in clause 4.3.4. The SMF sends an N1 SM Information to the UE via the AMF by invoking Namf_Communication_N1N2MessageTransfer as described in Step 3b of clause 4.3.4.2. The PDU Session Release Command message in N1 SM Information contains the PDU Session ID and Cause indicating that a PDU Session re-establishment to the same DN is required. 3. Upon reception of PDU Session Release Command with Cause indicating that a PDU Session re-establishment to the same DN is required as sent in step 2, the UE generates a new PDU Session ID and initiates PDU Session Establishment procedure as described in clause 4.3.2.2. Then, the AMF selects an SMF as described in clause 6.4.2 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2] and the SMF can select a new UPF (i.e. UPF2) for the re-established PDU Session of SSC mode 2. If the AMF has received target DNAI information from old SMF (i.e. SMF1), for the PDU Session toward same DNN and S-NSSAI the AMF selects the SMF using the stored target DNAI information. The AMF includes the target DNAI in the Nsmf_PDUSession_CreateSMContext Request and deletes the stored target DNAI information. The SMF selects the new PDU Session Anchor using the target DNAI.