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 126.96.36.199-1) and immediately establishes a new PDU Session with a new PDU Session Anchor (i.e. UPF2 in figure 188.8.131.52-1) to the same DN.
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 184.108.40.206. 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.
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 220.127.116.11. Then, the AMF selects an SMF as described in TS 23.501, clause 6.3.2 and the SMF can select a new UPF (i.e. UPF2) for the re-established PDU Session of SSC mode 2.