PFCP (N4 interface): SMF ↔ UPF communication

PFCP — The Control Protocol for UPF

The Packet Forwarding Control Protocol operates on the N4 interface between the SMF and UPF. It is the mechanism by which the control plane programs the user plane forwarding behavior. PFCP carries three main session message types: Session Establishment Request to create new forwarding rules for a PDU session, Session Modification Request to update rules during handover or QoS changes, and Session Deletion Request to remove all rules when the session ends. PFCP runs over UDP port 8805 and includes its own reliability mechanisms with sequence numbers and retransmission timers.