QoS Flow Identifier

Description

The QoS Flow Identifier (QFI) is a 6-bit field (values 0-63) that serves as the primary handle for QoS management in the 5G user plane. Defined extensively in 3GPP TS 23.501 (system architecture) and TS 38.415 (NG user plane protocol), a QoS Flow is the finest granularity of QoS differentiation in a 5G PDU Session. Each QoS Flow is characterized by a set of QoS parameters, such as 5G QoS Identifier (5QI), Allocation and Retention Priority (ARP), Guaranteed Flow Bit Rate (GFBR), and Maximum Flow Bit Rate (MFBR). The QFI uniquely tags all packets belonging to a specific QoS Flow between the User Plane Function (UPF) and the gNodeB (gNB).

Architecturally, the QFI is assigned by the Session Management Function (SMF) during QoS Flow establishment or modification. The SMF determines the need for a new QoS Flow based on policy requests from the Policy Control Function (PCF) or application function, or from the UE via QoS Rules. Once assigned, the QFI is communicated to the gNB and UPF. In the data plane, the QFI is encapsulated within the General Packet Radio Service Tunneling Protocol for the user plane (GTP-U) header on the N3 (gNB-UPF) and N9 (inter-UPF) interfaces, specifically in the PDU Session Container extension header. This allows every intermediate node to identify the flow without deep packet inspection.

How it works involves consistent mapping throughout the data path. When the UPF receives uplink data, it classifies packets to a QoS Flow based on Packet Detection Rules (PDRs), marks them with the corresponding QFI in the GTP-U header, and forwards them to the gNB. The gNB uses the QFI to map the packets to the appropriate Data Radio Bearer (DRB) for over-the-air transmission, applying the corresponding QoS profile (scheduling, link-layer configuration). In the downlink, the gNB may also mark the QFI based on UE QoS Rules. This end-to-end tagging ensures that the QoS treatment is consistent across the radio and core network segments, enabling features like reflective QoS and efficient support for network slicing.

Purpose & Motivation

The QoS Flow Identifier (QFI) was introduced in 3GPP Release 15 as a cornerstone of the new 5G QoS model, designed to overcome limitations of the 4G Evolved Packet System (EPS) bearer model. In LTE, QoS was tied to EPS bearers, which were relatively static constructs linking a QoS Class Identifier (QCI) to a specific tunnel (GTP TEID). This model was inefficient for sessions requiring multiple concurrent services with dynamic QoS needs, as each new QoS requirement often necessitated a new bearer, increasing signaling overhead and radio resource complexity.

QFI solves this by decoupling the QoS logic from the transport tunnel. A single PDU Session (analogous to an EPS bearer) can support multiple QoS Flows, each identified by a QFI and with independent QoS characteristics. This allows for dynamic addition, modification, or removal of QoS Flows within an existing session with minimal signaling. The primary motivation was to support 5G's diverse service landscape—from massive IoT to critical communications—where a single device (e.g., an autonomous vehicle) might simultaneously run navigation, sensor data, and infotainment applications, each with vastly different latency, reliability, and bandwidth needs.

Furthermore, QFI enables more flexible network slicing. Different slices can employ different mappings between QFIs and radio resources. It also facilitates edge computing and local breakout scenarios by providing a clear QoS identifier that is understood by both the core and access network. By being a small, in-band header field, QFI allows for efficient, scalable packet processing in high-speed user plane functions, which is critical for meeting 5G's performance targets. It represents a shift towards a more fluid, service-driven QoS architecture.