UPF（User Plane Function，用户面功能）： ts 29.244 23.501
1. Protocol Stack
Figure 4.1-1: Control Plane stack over Sxa, Sxb, Sxc and combined Sxa/Sxb and N4
Figure 4.1-2: User Plane stack over Sxa, Sxb, combined Sxa/Sxb and N4
Figure 5.2.1-1: Packet processing flow in the UP function