N4 — SMF ⇔ UPF (PFCP Reference Point)¶

Learning objectives¶

After reading this page, you will be able to:

• Explain that N4 is the interface connecting the control-side SMF and the forwarding-side UPF.
• State N4's protocol stack (PFCP / UDP / IP) and port (8805).
• Distinguish N4's two layers of procedures (Node procedures / Session procedures).
• Explain what the PDR / FAR / QER / URR rules that the SMF distributes to the UPF do.
• Explain why N4 is the linchpin of CUPS (separation of control and forwarding).
• Explain how it corresponds to 4G (EPC) Sx (Sxa/Sxb/Sxc).

Prerequisites¶

It will go more smoothly if you understand the following first.

• The positioning of PFCP and UDP → Protocol dictionary
• The NF that terminates N4 on the control side → SMF
• The NF that terminates N4 on the forwarding side → UPF
• The overall picture of CUPS and the PDU session → PDU Session Establishment
• The concept of an interface → Interface dictionary

Why — why it is needed¶

In 5G, the control (the brain = SMF) that decides "how to set up a session and how to make it forward" and the forwarding (the pipe = UPF) that actually flows packets were clearly separated. This is CUPS (Control and User Plane Separation). However, merely separating them means the brain's (SMF's) decisions are not conveyed to the pipe (UPF). The UPF remains just an empty pipe, not knowing "which packet, to where, at what quality" to flow.

Therefore, a dedicated road by which the control-side SMF injects instructions into the forwarding-side UPF becomes necessary. This is N4. Through N4, the SMF distributes rules to the UPF: "detect this packet this way, forward it this way, at this quality, and measure the volume this way." Without N4, the SMF could not operate the UPF, CUPS would not hold, and user data would not flow a single byte.

An analogy: N4 is the radio line connecting the control tower (SMF) and the workers on the ground (UPF). The control tower does not carry the packages itself, but over the radio it fires off instructions like "that package to this lane, priority high." The ground moves exactly as instructed. The line that carries these instructions is N4.

Overview¶

N4 is the control interface connecting SMF ⇔ UPF. The protocol flowing on top is PFCP (Packet Forwarding Control Protocol), and below it is UDP over IP.

N4's procedures are broadly divided into two layers.

The core of N4 is the four kinds of forwarding rules that the SMF distributes to the UPF: PDR / FAR / QER / URR. By setting these on the UPF, the SMF finely controls the UPF's forwarding behavior.

Basic Concept — a beginner-friendly explanation¶

Let us liken N4 to the exchange of instruction sheets handed from headquarters (SMF) to a distribution center (UPF).

• First, before headquarters and the distribution center begin operations, they perform an opening procedure (PFCP Association Setup) to say "we will now work together." This is node-to-node preparation, unrelated to any specific package.
• After that, for each package (PDU session), headquarters creates an instruction sheet (N4 session) and hands it to the distribution center (PFCP Session Establishment). If the contents change, it is updated each time (Modification), and when it is no longer needed, it is discarded (Deletion).
• Furthermore, they periodically check in (Heartbeat) to confirm the other side is alive, and if there is no response, they judge it a fault.

The contents of the instruction sheet are divided into four kinds of rules. The acronyms are hard, but what they do is simple.

• PDR (detection) … identifies which rule an incoming packet matches.
• FAR (forwarding) … decides how to handle the identified packet (forward/drop/buffer, and where to send it out).
• QER (QoS) … applies quality (rate limiting, priority) to the packet.
• URR (measurement) … measures the usage of packets and reports it to the SMF (material for charging and cap management).

In other words, N4 is "the delivery path for the instruction sheets that headquarters hands to the field," and the contents of those instruction sheets are PDR/FAR/QER/URR.

Protocol / Transport¶

Architecture¶

This shows how N4 connects the SMF and the UPF, with control (N4) and data (N3/N6) separated.

flowchart LR
  gNB["gNB (base station)"]
  SMF["SMF (control)"]
  UPF["UPF (forwarding)"]
  DN[(Data Network)]

  SMF == "N4 (PFCP / UDP:8805)" ==> UPF
  gNB -- "N3 (GTP-U user data)" --> UPF
  UPF -- "N6" --> DN

  classDef ctrl fill:#efe,stroke:#3a3,stroke-width:2px;
  classDef uplane fill:#eef,stroke:#66f,stroke-width:2px;
  class SMF ctrl;
  class UPF uplane;

How to read the diagram: the thick line (==>) is N4's control signaling, and the green SMF (control side) distributes PDR/FAR/QER/URR rules to the UPF. The SMF itself does not handle user data. Real data enters the blue UPF (forwarding side) from the gNB over N3 (GTP-U), and the UPF exits to the DN over N6. As the color coding shows, the separation of control on N4, user data on N3/N6 is the linchpin of CUPS. N4's port is UDP 8805.

Procedures that use it¶

N4 is used in a two-tier fashion: first connecting the nodes, then loading individual sessions on top.

• PFCP Association Setup: the first move of N4. The SMF and UPF establish a node-to-node association (a coordination relationship) (Node procedure). The foundation for subsequent Session procedures.
• PFCP Session Establishment: accompanying PDU session establishment, the SMF initially injects PDR/FAR/QER/URR into the UPF and creates an N4 session (Session procedure).
• PFCP Session Modification: post-establishment rule updates, such as reflecting the gNB-side N3 information or QoS changes (Session procedure).
• PFCP Session Deletion: accompanying PDU session release, deletes the N4 session (Session procedure).
• Heartbeat: node liveness monitoring (Node procedure).

In PDU Session Establishment, after UPF selection, the SMF injects rules with N4 Session Establishment. Then, once the gNB-side N3 TEID returns, it reflects it with N4 Session Modification to make the user plane connective. For the detailed flow including this two-tier Establishment→Modification, see PDU Session Establishment (this page does not repeat the procedure details).

Main Messages¶

These are representative PFCP messages that flow over N4.

For details of each message, see the Message dictionary.

Packet Analysis (Wireshark)¶

N4 is observed as PFCP, with UDP below it.

Decode highlights:

• The PFCP Message Type lets you identify the message kind (Association Setup, Session Establishment, etc.).
• In Session Establishment / Modification, the rule bodies themselves — Create PDR / Create FAR / Create QER / Create URR — are carried nested inside the IE (Information Element) of the PFCP message. This structure of "rules packed inside IEs" is key to understanding N4.
• In Association Setup / Heartbeat, you can check the SMF and UPF node information (Node ID) and the Recovery Time Stamp, etc. These are used to detect restarts after a fault.
• The details of the rules' IE composition and their application order vary by release, and some are to be confirmed.

Comparison with EPC¶

• In 4G: control/forwarding separation (CUPS) was introduced as a retrofit option, and its control interfaces were Sxa / Sxb / Sxc. The protocol flowing on top was PFCP (TS 29.244), connecting SGW-C/U, PGW-C/U, and TDF-C/U.
• In 5G: the separation of the SMF (control) and the UPF (forwarding) is made the standard architecture from the start, and its control interface is N4. The protocol flowing on top is the same PFCP (TS 29.244) as in 4G. PFCP was born in 4G's CUPS and was adopted as-is as the standard in 5G.

3GPP Specification¶

• 3GPP TS 29.244 — PFCP (Packet Forwarding Control Protocol). N4/Sx session control and rules such as PDR/FAR/QER/URR. Individual clause numbers are to be confirmed
• 3GPP TS 23.501 §4 — System architecture and reference points (definition of N4)
• 3GPP TS 23.502 — Procedure flows including N4 procedures, such as PDU Session Establishment (clause numbers vary by release, to be confirmed)

Summary¶

• N4 is the control interface connecting SMF ⇔ UPF, and its protocol is PFCP.
• The lower transport is UDP over IP, and the port is 8805. Reliability is ensured by retransmission at the PFCP application layer.
• Procedures are in two layers: Node procedures (Association Setup, Heartbeat) and Session procedures (Session Establishment/Modification/Deletion).
• The core of N4 is the rules the SMF distributes to the UPF: PDR (detection) / FAR (forwarding) / QER (QoS) / URR (measurement).
• N4 is the linchpin of CUPS (separation of control and forwarding); control is on N4, and user data is split onto N3/N6.
• It corresponds to 4G's Sx (Sxa/Sxb/Sxc); PFCP (TS 29.244) originates in 4G and is common with 5G.

Next Step¶

• SMF — the NF that terminates N4 on the control side (distributes rules to the UPF)
• UPF — the NF that terminates N4 on the forwarding side (applies PDR/FAR/QER/URR)
• PDU Session Establishment — the procedure where N4's Establishment→Modification appears
• N3 / N6 — the forwarding-side interfaces of the UPF
• Message dictionary / Interface dictionary / Protocol dictionary — to check terminology