UE Route Selection Policy

Description

The UE Route Selection Policy (URSP) is a cornerstone of the 5G system's ability to support diverse services with vastly different requirements through network slicing and concurrent access to multiple data networks. It is a structured policy, defined in 3GPP TS 23.503, that the Policy Control Function (PCF) provisions to the User Equipment (UE) via the Access and Mobility Management Function (AMF). The URSP consists of a prioritized list of URSP rules. Each rule contains two main components: a Traffic Descriptor and a list of Route Selection Descriptors (RSDs). The Traffic Descriptor defines the set of application traffic to which the rule applies. It can use criteria such as Destination Internet Protocol (IP) addresses/prefixes, port numbers, protocol IDs, Domain Name System (DNS) queries, or, most importantly, an OS/Application ID that directly identifies a specific application on the UE. The Route Selection Descriptor(s) then define the network resources that should be used for matching traffic. A single RSD can specify a combination of a Data Network Name (DNN), a Single-Network Slice Selection Assistance Information (S-NSSAI), and a Session and Service Continuity (SSC) mode. The UE's operating system or policy enforcement function evaluates these rules in priority order for each new application flow or socket connection. When a match is found, the UE uses the information in the RSD to establish a Protocol Data Unit (PDU) Session with the appropriate DNN and network slice, or to route the traffic over an existing matching PDU Session. This allows a single UE to have multiple active PDU Sessions—for example, one for enhanced Mobile Broadband (eMBB) to a public internet DNN, and another for Ultra-Reliable Low-Latency Communication (URLLC) to a private industrial network slice—with applications automatically mapped to the correct session. The URSP mechanism shifts significant intelligence to the UE, enabling dynamic, application-aware network resource selection without requiring constant network intervention for every flow.

Purpose & Motivation

URSP was created to address the fundamental challenge of efficiently supporting the 5G vision of a single network infrastructure catering to massively diverse use cases, from broadband to IoT to critical communications. Pre-5G networks primarily associated a UE with a single default data connection (PDN connection in 4G). While some form of dedicated bearers could provide different quality of service (QoS), the selection was network-controlled and limited in scope, lacking deep application awareness. URSP solves this by providing a flexible, programmable policy framework that allows the network operator to instruct the UE on how to map its myriad applications to the appropriate network slices and data networks. This enables simultaneous connectivity to multiple logical networks, optimal resource utilization, and seamless user experience where, for instance, a vehicle's telematics uses a low-latency slice while its infotainment uses a high-bandwidth slice. It was motivated by the need for automation and scalability; manually configuring each application on the UE or having the network micro-manage every flow is impractical. URSP empowers the UE to make intelligent local routing decisions based on operator-defined policies, which is essential for realizing the full economic and technical potential of network slicing and 5G service-based architecture.