[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, the SMF sends N4 Session Modification Request message to PDU Session Anchor UPF, providing DL Tunnel Info from new intermediate UPF. The SMF may also provide updated UL CN Tunnel Information. If the new intermediate UPF was added for the PDU Session, the UPF (PSA) begins to send the DL data to the new I-UPF as indicated in the DL Tunnel Info.
If the Service Request is triggered by the network, and the SMF removes the old I-UPF but does not replace it with a new I-UPF, and if UPF allocates UP tunnel endpoint information, the SMF may also include a request for the UPF to allocate a second tunnel endpoint for buffered DL data from the old I-UPF. In this case, the UPF (PSA) begins to buffer the DL data it may receive at the same time from the N6 interface.