Intermediate User Plane Function

Description

The Intermediate User Plane Function (I-UPF) is a critical user plane element within the 5G Core Network (5GC), standardized from 3GPP Release 16. It represents a specific deployment instance of the UPF that is not the session anchor point. Instead, the I-UPF is inserted into the user plane path between the Radio Access Network (RAN) – or other access network – and an Anchor UPF (A-UPF). It is typically collocated with or near the point of access to enable local data processing and routing.

Architecturally, the I-UPF is controlled by an Intermediate Session Management Function (I-SMF) via the N4 interface. The I-SMF establishes an N4 session with the I-UPF, provisioning it with Packet Detection Rules (PDRs), Forwarding Action Rules (FARs), Usage Reporting Rules (URRs), and QoS Enforcement Rules (QERs) specific to the local traffic steering needs. The I-UPF performs standard UPF functions like packet inspection, anchoring, and forwarding, but within a localized context. Its key operational role is to act as the first point of user plane termination from the access network (via N3 or N9' interfaces) and to forward traffic upstream towards the A-UPF (via the N9 interface) or to locally breakout the traffic to a Data Network (DN) via an N6 interface if deployed locally.

The I-UPF's functionality is central to several 5G paradigms. In mobility scenarios, especially when a User Equipment (UE) moves from a 3GPP to a non-3GPP access (like WLAN), an I-UPF can be inserted to provide a local user plane path without changing the A-UPF, maintaining session continuity. For edge computing, the I-UPF provides the low-latency connection to local Application Servers. It also plays a vital role in the UL CL (Uplink Classifier) and BP (Branching Point) functionalities defined for ATSSS, allowing traffic to be steered or split over multiple accesses or paths. By processing and routing traffic locally, the I-UPF offloads the core transport network, reduces end-to-end latency, and enables efficient service delivery tailored to the UE's geographical location.

Purpose & Motivation

The I-UPF was introduced in Release 16 to address the need for a more flexible and distributed user plane architecture in 5G, which was a limitation in earlier core network designs. In the initial 5G Release 15 architecture, a PDU Session was typically anchored at a single UPF. While this simplified management, it forced all user plane traffic, even that destined for a local server, to traverse the core network to the anchor point and back, creating the 'trombone effect'. This was inefficient for low-latency applications, edge computing, and localized services.

The I-UPF concept solves this by enabling a chain of UPFs. It allows network operators to deploy UPFs at the network edge, close to users and application servers. The I-UPF handles local traffic breakout, while the A-UPF remains the stable anchor for the session, ensuring IP address preservation and continuity during mobility. This is particularly important for use cases like factory automation, augmented reality, and efficient fixed-mobile convergence, where milliseconds of latency matter. The I-UPF, under the control of the I-SMF, provides the topological agility needed to optimize the data path dynamically based on the UE's location, the service requirements, and the network conditions, addressing the rigid user plane routing of previous generations.

Key Features

• Provides local user plane processing and traffic breakout close to the access network
• Controlled by an I-SMF via the N4 interface for session-specific rules
• Supports N3 (access) and N9 (inter-UPF) interfaces, and optionally a local N6 interface
• Enables UL CL (Uplink Classifier) and BP (Branching Point) functionalities for ATSSS
• Reduces latency and core network transport load by localizing traffic routes
• Essential for enabling Multi-Access Edge Computing (MEC) and low-latency services

Evolution Across Releases

Defining Specifications