UPF — User Plane Function¶

Learning objectives¶

After reading this page, you will be able to:

• Explain in one sentence the role the UPF plays in the 5GC (the forwarding entity of the user plane).
• Explain what the rules (PDR/FAR/QER/URR) the UPF receives from the SMF over N4(PFCP) do.
• Diagram the interfaces the UPF terminates (N3/N4/N6/N9), along with their peers and purposes.
• Explain the positioning of the UPF as the "forwarding side" in CUPS (the separation of the control SMF and forwarding UPF).
• Explain the correspondence with the 4G (EPC) SGW-U/PGW-U (GTP-U in common, CUPS standardized via PFCP control).

Prerequisites¶

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

• Basics of SBA (Service Based Architecture) and NFs → Curriculum
• The overall picture of the PDU Session (the procedure in which the UPF actually appears) → PDU Session Establishment
• The role of the SMF (the side that controls the UPF over N4) → SMF
• The positioning of GTP-U / PFCP → Protocol dictionary

What you will learn in this chapter¶

We enter from Why (what goes wrong without the UPF), then organize What (the UPF's role, N4 rules, and APIs) with tables and Mermaid diagrams, and finally move to How (its appearance in the PDU session, comparison with EPC, and reference specs).

This chapter focuses on the UPF as an NF itself (its role, data-plane processing, and application of N4 rules). The procedural details in which the UPF appears (the call flow of PDU Session Establishment and the two-stage N4 Establishment→Modification) are covered in PDU Session Establishment (directed from "Appearance in procedures" later on this page).

Why — why the UPF is needed¶

Even after all of the 5GC's control signaling (registration, authentication, session management) is complete, if there is no one to carry the actual user data (web pages, video, the content of calls), communication does not succeed. The control-plane NFs (AMF/SMF/AUSF/UDM) only decide the "arrangements" and do not handle the packets themselves. The dedicated entity that actually forwards these user data packets is the UPF.

Why separate forwarding into a dedicated NF? This is the concept of CUPS (Control and User Plane Separation). Separating control (the brain = SMF) from forwarding (the pipe = UPF) has the following advantages:

• Can scale independently: Even when a large amount of traffic flows, you only need to reinforce the forwarding side (UPF). The control side (SMF) need not be increased.
• Can be placed at the edge for low latency: If the UPF is placed close to the user (near the base station = the edge), data can exit via a short path without traveling back and forth to the core center. Where to place the UPF (edge or center) is implementation/configuration-dependent.

Analogy: The UPF is a postal sorting and delivery center. The control side (SMF) issues a delivery instruction sheet (N4 rules) saying "carry this parcel to this destination at this priority," and the delivery center (UPF) sorts and forwards the parcels (packets) exactly as instructed. The delivery center does not decide destinations itself; it is a working unit that steadily handles a large volume of parcels according to the instruction sheet. By separating the headquarters that issues instructions (SMF) from the center that actually carries the parcels (UPF), you can add only centers when parcels increase, and placing centers close to users makes delivery faster.

Overview¶

The UPF (User Plane Function) is the entity of the 5GC's user plane (data forwarding). Following the rules received from the SMF over N4(PFCP) (PFCP = the protocol by which the SMF controls the UPF over N4), it detects, forwards, applies QoS to, and measures the usage of user data packets. It connects to the radio side (gNB) over N3(GTP-U) (GTP-U = the tunnel protocol that carries user data), to the data network side (DN: Data Network, external networks such as the internet) over N6, and to other UPFs over N9(GTP-U).

The UPF is a forwarding-dedicated NF that does not make its own decisions and acts according to the SMF's instructions (N4 rules). Unlike control-plane NFs that expose services over an SBI (Service Based Interface), the UPF has no SBI and is controlled by the SMF over the dedicated interface N4.

Basic Concept — explanation for beginners¶

What the UPF does, boiled down, is simply "look at incoming packets and send them out according to the rules." These "rules" are the four kinds of instructions passed from the SMF over N4, each with a distinct role. It becomes easier to understand with a parcel-delivery analogy.

(Basis: PDR/FAR/QER/URR are rules defined in TS 29.244 (PFCP). The IEs of each rule and the detailed order of application are to be confirmed.)

Summed up in one line, the UPF roughly repeats the processing "identify the packet (PDR) → apply quality (QER) → measure usage (URR) → send to the forwarding destination (FAR)" at high speed for all packets. This is the substance inside the user plane's "pipe." However, the strict order of application of each rule is to be confirmed against the spec (TS 29.244), and here it is shown as a conceptual flow.

Network Function details¶

We organize the UPF's main functions.

About UL CL / Branching Point / edge placement: The UL CL (Uplink Classifier) and Branching Point that distribute uplink traffic to multiple UPFs or a local DN (edge), as well as local breakout configurations that place the UPF at the edge, are all implementation/configuration-dependent in both adoption and placement. 3GPP defines the mechanism (TS 23.501), but whether it is actually used is left to operator design. This page does not present hypothetical placement examples.

Architecture¶

The UPF is positioned in the 5GC's user plane and is the "forwarding entity" controlled by the control-plane SMF over N4. Thanks to CUPS, the point that control (SMF) and forwarding (UPF) are separated is the star of this chapter.

flowchart LR
  gNB["gNB (base station)"]
  SMF["SMF (control)"]
  UPF["UPF (forwarding)"]
  UPF2["other UPF"]
  DN[(Data Network)]

  SMF -- "N4 (PFCP) control" --> UPF
  gNB == "N3 (GTP-U)" ==> UPF
  UPF == "N9 (GTP-U)" ==> UPF2
  UPF == "N6" ==> DN

How to read the diagram: The SMF instructs the UPF on forwarding rules (PDR/FAR/QER/URR) over N4(PFCP), but the SMF itself does not handle user data (the thin control line). The actual data (bold lines) enters the UPF from the gNB over N3(GTP-U), and the UPF exits to the DN over N6. When passing through multiple UPFs, the UPFs are connected by N9(GTP-U). Control (N4) and forwarding (N3/N6/N9) being separated is CUPS.

Note that whether to place the UPF at the edge (near the base station) or at the center, and whether to branch with a UL CL/Branching Point, is implementation/configuration-dependent; the diagram above is the basic form.

Interface¶

The main interfaces terminated by the UPF and their roles.

You can also refer to the individual pages for each interface (N3 / N4 / N6 / N9). For the list, see the Interface dictionary, and for details of GTP-U / PFCP see the Protocol dictionary.

Appearance in procedures¶

In PDU Session Establishment, the UPF appears as the forwarding entity. The SMF performs UPF selection and sets rules such as PDR/FAR/QER on the UPF via N4 Session Establishment (PFCP). At this time the UPF-side N3 TEID is assigned, and subsequently the gNB-side N3 TEID is reflected via N4 Session Modification, so that the uplink and downlink GTP-U tunnels are aligned and the user plane becomes connected.

In other words, the UPF receives the rules "forward this way" from the SMF and prepares; data begins to flow at the point when the TEID exchange is complete.

To avoid duplication, please refer to PDU Session Establishment for the detailed call flow (message order, IEs, state transitions) including this two-stage N4 Establishment→Modification. This page does not repeat the procedural details.

Comparison with EPC¶

• In 4G: User-plane forwarding was handled by the SGW-U / PGW-U (the user-plane portions of the S-GW/P-GW). Initially control and forwarding were unified, but late in 4G, CUPS (separation into PGW-C/PGW-U, etc., with the Sx interface) was introduced as an option.
• In 5G: The forwarding function was consolidated into the UPF, and it became a standard architecture in which the SMF (control) and UPF (forwarding) are separated from the start. Its control interface is N4(PFCP). The user-plane tunnel uses the same GTP-U as 4G over N3/N9.

Release differences¶

Anything uncertain is marked to be confirmed and not asserted.

• Rel-15: Defines the UPF's basic functions (N3/N4/N6/N9 termination, N4 rule application, QoS application, usage measurement, CUPS standardization).
• Rel-16: Enhancement of user-plane functions accompanying extensions such as Ethernet PDU / TSC (Time-Sensitive Communication). The specific section numbers and scope of incorporation are to be confirmed.
• Rel-17 onward: Further extensions continue. Specific items and section numbers are to be confirmed.

3GPP Specification¶

Summary¶

• The UPF is the entity of the 5GC's user plane (data forwarding), a forwarding-dedicated NF that forwards packets exactly as instructed by the SMF.
• It receives PDR (detection) / FAR (forwarding) / QER (QoS) / URR (usage measurement) from the SMF over N4(PFCP) and applies them to all packets.
• Its terminating interfaces are N3 (gNB, GTP-U), N4 (SMF, PFCP), N6 (DN), and N9 (other UPFs, GTP-U). It has no SBI.
• Through CUPS, control (SMF) and forwarding (UPF) are separated, allowing the UPF alone to be scaled and placed at the edge independently (placement is implementation/configuration-dependent).
• It corresponds to the 4G SGW-U/PGW-U; GTP-U is common, and control was standardized as CUPS over N4(PFCP).
• UL CL/Branching Point, edge placement, and deep inspection such as DPI are implementation/configuration-dependent.

Next Step¶

• PDU Session Establishment — the procedure in which the UPF appears as the forwarding entity (details of N4 rule setting and GTP-U tunnel establishment)
• SMF (the command tower that controls the UPF over N4) → SMF
• PCF (the source of QoS/charging policy) → PCF
• NF dictionary / Interface dictionary / Protocol dictionary — for checking terminology, N3/N4/N6/N9, and GTP-U/PFCP