N4 Interface
The reference point between the SMF and the UPF using PFCP for session establishment, modification, and deletion of user-plane forwarding rules.
N4 is the link between the SMF and the UPF, and it's how the control plane actually programs the data plane. It uses PFCP (Packet Forwarding Control Protocol, over UDP), and the SMF uses it to install, change, and tear down the rules that govern each PDU session on the UPF.
Those rules come as a small family of PFCP structures, the main ones being:
- PDRs (Packet Detection Rules) — how to match incoming packets to a session/flow.
- FARs (Forwarding Action Rules) — what to do with matched packets (forward, drop, buffer, encapsulate).
- QERs and URRs — QoS enforcement and usage reporting/measurement.
If you came from 4G CUPS, N4 is the direct heir to the Sx interface, and PFCP here is essentially the evolved version of Sx's PFCP. It's the exact boundary where SMF decisions become packet behaviour — so when sessions establish but traffic doesn't flow, N4/PFCP is the seam to inspect. When the rules it installed are wrong, PFCP session debugging reads them back; N2, N3 and N4 covers how the interface sits against its neighbours.
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