Section: 5.8.2.10 UP Tunnel Management
Content: 5GC shall support per PDU Session tunnelling on N3 between (R)AN and UPF and N9 between UPFs. If there exist more than one UPF involved for the PDU Session, any tunnel(s) between UPFs (e.g. in the case of two UPFs, between the UPF that is an N3 terminating point and the UPF for PDU Session Anchor) remains established when a UE enters CM-IDLE state. In the case of downlink data buffering by UPF, when mobile terminated (MT) traffic arrives at the PDU Session Anchor UPF, it is forwarded to the UPF which buffer the data packet via N9 tunnel. See clause 5.8.3 for more details on UPF buffering. In the case of Home Routed roaming, the SMF in HPLMN is not aware of the UP activation state of a PDU Session. When the UP connection of the PDU Session is deactivated, the SMF may release the UPF of N3 terminating point. In that case the UPF (e.g. the Branching Point/UL CL or PDU Session Anchor) connecting to the released UPF of N3 terminating point will buffer the DL packets. Otherwise, when the UPF with the N3 connection is not released, this UPF will buffer the DL packets. When the UP connection of the PDU Session is activated due to a down-link data arrived and a new UPF is allocated to terminate the N3 connection, a data forwarding tunnel between the UPF that has buffered packets and the newly allocated UPF is established, so that the buffered data packets are transferred from the old UPF that has buffered packets to the newly allocated UPF via the data forwarding tunnel. For a PDU Session whose the UP connection is deactivated and the SMF has subscribed the location change notification, when the SMF is notified of UE's new location from the AMF and detects that the UE has moved out of the service area of the existing intermediate UPF, the SMF may decide to maintain the intermediate UPF, remove the established tunnel between UPFs (in the case of removal of the intermediate UPF) or reallocate the tunnel between UPFs (in the case of reallocation of the intermediate UPF).