PFCP

PFCP is what makes 5G's split between control and user plane actually work. The SMF decides how a user's packets should be handled; the UPF is the box that handles them; PFCP, on the N4 interface, is how the SMF programs the UPF. It installs, updates and removes the forwarding rules using a small set of rule types — Packet Detection Rules to match traffic, Forwarding Action Rules for what to do with it, plus QoS Enforcement and Usage Reporting rules — and the UPF reports back usage and events.

This is the CUPS architecture (Control and User Plane Separation) that became central in 5G: you can scale and place user-plane capacity independently of the control plane, putting UPFs at the edge for low latency while the SMF stays central. PFCP also carries the heartbeats that keep the N4 association alive and the session reports that drive charging. If a UPF isn't forwarding a user's traffic correctly, the PFCP exchange that set up its rules is the first thing to inspect.