5G Core User Plane Protocols: UPF, PFCP and UDSF

The architecture of 5G introduces a fundamental shift in how mobile networks are structured, operated, and optimized. Unlike previous generations, the 5G Core (5GC) is designed to be service-based, modular, and cloud-native, providing the flexibility and scalability required to support a wide range of use cases including enhanced mobile broadband, ultra-reliable low latency communications, and massive machine-type communications. At the heart of this architecture is the separation of the control and user planes, enabling independent scaling and simplified management. Within the user plane, key protocols and components play a critical role in the forwarding and handling of user data traffic. Among these, the User Plane Function (UPF), the Packet Forwarding Control Protocol (PFCP), and the Unstructured Data Storage Function (UDSF) are pivotal in achieving high performance, dynamic traffic steering, and service differentiation.

The User Plane Function is a core network function in the 5GC responsible for handling user data packets. It operates at the data path level and is tasked with packet routing and forwarding, Quality of Service (QoS) enforcement, traffic usage reporting, and packet inspection for lawful interception or analytics. UPFs are the execution points where user data traverses the mobile core, making them the workhorses of the user plane. Their positioning within the network can vary depending on service requirements; for instance, to meet ultra-low latency targets, UPFs can be deployed closer to the radio access network at the network edge. In contrast, for services requiring centralized data aggregation or processing, UPFs may be placed deeper within the core network. The flexibility to deploy multiple UPFs in distributed topologies is a key enabler of 5G’s network slicing and edge computing capabilities.

The control of the UPF is handled by the Session Management Function (SMF), which uses the Packet Forwarding Control Protocol to instruct the UPF on how to manage the user data flows. PFCP is a 3GPP-defined protocol used exclusively between the SMF and UPF. It enables the SMF to establish, modify, and delete Packet Detection Rules (PDRs), Forwarding Action Rules (FARs), QoS Enforcement Rules (QERs), Usage Reporting Rules (URRs), and Buffering Action Rules (BARs) in the UPF. These rules govern how the UPF processes packets for each user session. For example, a PDR might specify how to identify packets belonging to a particular session based on IP address and port, while a FAR determines the forwarding behavior for those packets, such as whether to send them to an external data network, drop them, or buffer them for policy enforcement.

PFCP sessions are tied to the lifecycle of user sessions, and changes in user mobility, policy, or QoS requirements prompt the SMF to send PFCP messages to update the UPF’s rules accordingly. These messages can include heartbeat procedures for session maintenance and recovery mechanisms to handle failovers and session relocations. PFCP operates over UDP and uses message types like Session Establishment Request, Session Modification Request, and Session Deletion Request to perform its functions. The efficiency of PFCP allows rapid reconfiguration of user plane paths, which is essential for achieving the dynamic behavior expected in 5G networks, such as seamless mobility, network slicing transitions, or multi-access edge computing (MEC) integration.

Another emerging component in the 5G user plane ecosystem is the Unstructured Data Storage Function. UDSF is not directly part of the data forwarding path like the UPF but plays a crucial supporting role in storing and retrieving context information that is unstructured in nature. The 5G architecture specifies that functions requiring persistent storage of unstructured data should leverage the UDSF rather than maintaining their own local databases. This model supports stateless design principles, facilitating function redundancy and scalability, which are central to the service-based architecture of the 5GC.

In the context of the user plane, UDSF may be used to store session-related metadata, usage records, or analytics outputs generated by the UPF or related analytics functions. For example, data collected by the UPF for application detection, traffic categorization, or user behavior profiling can be offloaded to the UDSF for later retrieval and processing by network data analytics functions (NWDAF) or charging systems. UDSF provides a unified interface for storing and retrieving such data through standardized APIs, often using RESTful methods over HTTP/2, aligning with the overall cloud-native direction of the 5G core.

The interaction among UPF, PFCP, and UDSF exemplifies the decoupled and flexible nature of the 5G architecture. UPF handles the high-speed packet forwarding and real-time enforcement of policies, PFCP provides the dynamic and programmable control channel for managing the behavior of the UPF, and UDSF offers persistent storage capabilities that decouple data from logic. This separation allows each component to be independently scaled, upgraded, and maintained, which is essential for supporting the diverse performance and operational requirements of 5G services.

The convergence of these protocols and components facilitates a user plane that is not only highly performant but also intelligent and adaptable. The ability to steer traffic to different UPFs based on slice ID, geographic location, or application type enables operators to fine-tune the user experience. Moreover, the PFCP’s ability to quickly reprogram UPF behavior in response to network events or policy changes allows 5G networks to maintain service continuity and meet stringent service-level agreements (SLAs). As new services such as augmented reality, vehicle-to-everything (V2X), and industrial IoT place unprecedented demands on mobile networks, the 5G core user plane protocols must evolve further to support tighter integration with artificial intelligence, real-time analytics, and cross-domain orchestration.

In conclusion, the combination of the User Plane Function, Packet Forwarding Control Protocol, and Unstructured Data Storage Function creates a comprehensive and future-proof foundation for the 5G core user plane. These protocols and components embody the principles of flexibility, programmability, and scalability that define the next generation of mobile networks, enabling service providers to deliver differentiated and reliable services across a wide range of verticals and use cases. As 5G matures and paves the way for 6G, the evolution of these core user plane technologies will remain central to achieving ultra-high-speed, low-latency, and intelligent connectivity for the digital age.

The architecture of 5G introduces a fundamental shift in how mobile networks are structured, operated, and optimized. Unlike previous generations, the 5G Core (5GC) is designed to be service-based, modular, and cloud-native, providing the flexibility and scalability required to support a wide range of use cases including enhanced mobile broadband, ultra-reliable low latency communications, and…