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.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (98 CRs across 6 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the Intermediate User Plane Function (I-UPF) was introduced as part of the architectural separation of the user plane from the control plane, enabling independent scalability and flexible deployments. This function is managed via the N4 interface and is involved in procedures like PDU Session handover and user plane security enforcement between the NG-RAN and the SMF. Its introduction supports key features such as interworking with EPS and the handling of local area data networks.
- Architectural solution for User Plane (UP) Security policy and User Plane Integrity Protection TS 23.501CR0066
- User Plane management to support interworking with EPS TS 23.501CR0122
- N4 User Plane Path TS 23.501CR0171
- Correction on Control Plane protocol stacks TS 23.501CR0240
- Providing AF with information on the N6 User Plane tunnelling information TS 23.501CR0265
- Clarification on the PDU Session handover procedure with the User Plane Security Enforcement TS 23.501CR0682
+ 2 more changes
In Release 16, the I-UPF's role was enhanced to support new functionalities introduced for Access Traffic Steering, Switching, and Splitting (ATSSS), requiring updates to N4 rules to enable ATSSS-LL functionality in the UPF. Furthermore, the release defined more precise control for traffic offload and local traffic switching by an I-UPF when it is under the control of an inserted Intermediate SMF (I-SMF). These enhancements also included mechanisms for the SMF to transfer N4 information to the I-SMF to facilitate this local traffic switching.
- Introduction of data transfer in Control Plane CIoT 5GS Optimisation TS 23.501CR0889
- Introduction of ATSSS Support TS 23.501CR0735
- Support of Steering Functions for ATSSS TS 23.501CR0740
- General description of solution 1 in 23.725 for user plane redundancy TS 23.501CR0753
- UL CL/BP controlled by I-SMF TS 23.501CR0848
- ATSSS-SMF and UPF selection TS 23.501CR0761
+ 46 more changes
In Release 17, enhancements for the Intermediate User Plane Function (I-UPF) were primarily defined through its associated control function, the I-SMF. Key updates included refinements to DNAI-based I-SMF selection and removal procedures, ensuring the I-SMF can be dynamically managed based on the user's proximity to specific Data Network Access Points. Furthermore, the architecture supported user plane remote provisioning of UEs and integrated considerations for user plane latency requirements in edge relocation scenarios.
- Applying thresholds to Load-Balancing steering mode in ATSSS TS 23.501CR2590
- KI #1-1, I-SMF selection TS 23.501CR2634
- Enabling restricted PDU Session for remote provisioning of UE via User Plane TS 23.501CR2709
- User Plane Remote Provisioning of UEs if PLMN as ON TS 23.501CR2802
- Edge relocation considering user plane lantecy requirement TS 23.501CR2804
- Partial ATSSS rule update by using ATSSS rule ID TS 23.501CR2886
+ 8 more changes
In Release 18, specific enhancements were made for the Intermediate User Plane Function (I-UPF) concerning ATSSS capabilities and user plane management. The release introduced procedures for determining the ATSSS capabilities of a Multi-Access PDU Session when the UE supports MPQUIC, impacting the functionality located in the UPF. Furthermore, it included user plane function selection for UPEAS and involved a restructuring of the user plane management clause within the specifications.
In Release 19, the enhancements for the Intermediate User Plane Function (I-UPF) primarily focus on its role within architectures involving an Intermediate SMF (I-SMF) for local offloading and ATSSS support. Key updates include refined procedures for I-SMF selection and insertion based on local offloading policies, clarifications on UPF selection for the ATSSS feature, and enabling the subscription of UPF events via the I-SMF. Furthermore, the release introduced specific handling for transport level marking based on PDU Set Importance and N6 delay measurement in scenarios where an I-SMF is inserted.
- Exposure enhancements for static UE IP address assignment and 5G VN group's User Plane Security Policy TS 23.501CR5492
- Control Plane and User Plane Protocol stacks involving the MWAB node TS 23.501CR5561
- I-SMF selection/insertion based on local offloading allowed indication TS 23.501CR5604
- Local Offloading handling at I-SMF TS 23.501CR5744
- Local Offloading handling at I-SMF TS 23.501CR5825
- PCF's awareness of I-SMF insertion for Local Offloading Management TS 23.501CR5833
+ 13 more changes
In Release 20, the I-UPF's functionality was enhanced to support new mitigation actions based on analytics for abnormal user plane traffic. This allows the network to implement specific countermeasures when such traffic is detected, improving security and reliability. The update leverages the existing, separated user plane architecture where the UPF interacts with control plane functions like the SMF.
- Mitigation actions based on New Abnormal user plane traffic Analytics TS 23.501CR6507
Explore further
Broader topics and technologies where I-UPF plays a role.
Defining Specifications
3GPP specifications that define or reference I-UPF, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.501 vk00 | 5G System Architecture Stage 2 | Rel-20 |
| TR 23.726 vg00 | SMF/UPF Topology Enhancements in 5G | Rel-16 |
| TR 28.833 vi01 | Technical Report on 5G LAN-type Service Management | Rel-18 |
| TS 29.892 vg00 | Study on User Plane Protocol in 5GC | Rel-16 |