QoS Rule Identifier

Description

The QoS Rule Identifier (QRI) is a fundamental element in the 5G Quality of Service (QoS) model defined from 3GPP Release 15 onwards. A QoS Rule is a set of parameters that defines how a specific Service Data Flow (SDF) or aggregate of SDFs should be treated regarding packet forwarding (e.g., priority, packet delay budget, packet error rate). Each QoS Rule configured in the User Equipment (UE) or applied by the network is associated with a unique QRI. The QRI is an integer value that serves as a handle to reference that particular rule within QoS-related signaling and operations.

Architecturally, the QRI operates within the Protocol Data Unit (PDU) Session context. During PDU Session Establishment or Modification, the Session Management Function (SMF) may create QoS Rules and assign QRIs. These rules, along with their identifiers, are then provided to the UE via the NAS SM protocol (specified in TS 24.501) and to the (R)AN via the N2 interface. The UE uses the QRI to map uplink user plane packets to the appropriate QoS Flow (identified by a QFI - QoS Flow Identifier) based on the packet filters and precedence defined in the corresponding QoS Rule. In the downlink, the UPF marks packets with a QFI, and the (R)AN uses the binding between QFI and the QoS profile (which is associated with the QoS Rules via the QRI) to apply the correct radio resource treatment.

How it works: When the SMF decides to install, modify, or remove a QoS Rule for a PDU Session, it includes the QRI in the relevant N1 (to UE) and N2 (to (R)AN) messages. For example, in a PDU Session Modification Command, the SMF may send a QoS rule operation (e.g., 'add') with a new QRI value. The UE stores this rule locally in its QoS Rules container. During uplink transmission, the UE's uplink classifier evaluates packet headers against the packet filters of all QoS Rules in order of precedence. When a match is found, the UE tags the packet with the QFI corresponding to the matched rule's QRI (via a binding known from the QoS rule). This allows consistent end-to-end QoS enforcement. The QRI's role is thus to provide a stable reference point for managing the potentially dynamic set of QoS rules per PDU Session, enabling efficient updates without ambiguity.

Purpose & Motivation

The QRI was introduced with the new 5G QoS model in Release 15 to address limitations in the EPS (4G) bearer-based QoS mechanism. In EPS, QoS was tightly coupled with EPS bearers, and rules were implicitly associated with a bearer's Traffic Flow Template (TFT). This made dynamic QoS adjustments cumbersome, as modifying filters often required bearer modification. The 5G system decouples QoS rules from the transport layer (QoS Flows), allowing more flexible and granular QoS control. The QRI exists to uniquely identify each independent QoS rule within this decoupled architecture.

The motivation stems from the need to support diverse 5G use cases—from enhanced mobile broadband (eMBB) to ultra-reliable low-latency communications (URLLC) and massive IoT—each with distinct QoS requirements. A system was needed where the network could swiftly add, modify, or remove packet detection and treatment rules without disrupting ongoing flows. The QRI provides the necessary handle for such operations. It solves the problem of unambiguous reference in signaling: when the SMF instructs the UE to update a specific rule, it uses the QRI to pinpoint exactly which rule to change. This enables fine-grained, service-driven QoS management, which is essential for network slicing and efficient resource utilization in 5G's service-based architecture.