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.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (119 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the QFI (QoS Flow Identifier) was formally introduced as a core element for QoS management in the 5G System. Key specifications included its presence within QoS rules and its values being carried in 5GSM messages. Furthermore, clarifications were required due to the initial 6-bit limit for the QFI, defining its scope for identifying individual QoS Flows that require separate forwarding treatment.
- Use of identifiers for mobility between GERAN/UTRAN and 5GS TS 23.501CR0017
- QFI in N9 TS 23.501CR0062
- Partitioning of Identifier space to ensure success of Context retrieval for EPS Interworking TS 23.501CR0090
- Subscription Permanent Identifier TS 23.501CR0189
- Changed length and mapping of 5GS Temporary Identifiers TS 23.501CR0206
- Clarification to the usage of Internal-Group Identifier TS 23.501CR0262
+ 19 more changes
In Release 16, the QFI function was enhanced with new procedures for its allocation and modification, as indicated by the introduction of a "QFI allocation event" and corrections for "PDU session modification with QFI change." Furthermore, the release introduced specific semantic error checks to prevent the assignment of duplicate QFI values within a session, improving system robustness.
- QoS monitoring based on GTP-U paths TS 23.501CR1414
- Group Identifier Translation TS 29.503CR0281
- External Group Identifier in NIDD information TS 29.503CR0330
- Translation of Group Id to UE identifier list TS 29.503CR0366
- QFI allocation event TS 29.508CR0071
- Retrieval of BDT policy data for a set of BDT reference identifiers TS 29.513CR0086
+ 22 more changes
In Release 17, specific error handling was defined for scenarios where the network sets the QoS Rule Identifier (QRI) and QoS Flow Identifier (QFI) to zero. Furthermore, clarifications were made to ensure the PDU Session ID is included in QFI allocation event notifications sent by the SMF, preventing its omission.
- Identifier Translation TS 29.503CR0654
- Authorization of Group Identifier Translation TS 29.503CR0694
- Corrections on the AF related identifier TS 23.501CR3064
- Avoid including both PAP/CHAP and EAP identifiers in PDU session establishment request TS 24.501CR2941
- Clarification on the setting of packet filter identifier value TS 24.501CR3300
- Network identifier is not specified TS 24.501CR3389
+ 13 more changes
In Release 18, the primary update for the QFI function was a correction to its application for the QoS flow of non-default rules. This specific technical correction, detailed in the listed Change Request, refined the handling and identification of QoS flows that are not established by default session management procedures. No other new QFI-related functionalities or identifier expansions were introduced in this release.
- PIN identifiers TS 23.501CR4287
- Context identifier for transport mode 1 of MPQUIC functionality TS 24.193CR0136
- Protecting the N3IWF/TNGF identifier information in the REGISTRATION REJECT message TS 24.501CR5932
- Resolving the EN related to N3IWF selection based on N3IWF identifier information in the REGISTRATION REJECT message TS 24.502CR0230
- Prefixed OI/TAI Identifier FQDN for N3IWF selection TS 24.502CR0223
- N3IWF selection for non-IMS services supporting extended home N3IWF identifier configuration and slice-specific N3IWF prefix configuration TS 24.502CR0248
+ 13 more changes
In Release 19, the QFI's function was enhanced to enable QoS differentiation for traffic from Non-3GPP devices connecting behind a UE or 5G-RG, using a generic string identifier bound to each such device. This introduced new procedures for supporting, suspending, and rejecting QoS differentiation based on these Non-3GPP Device Identifiers within session management signalling. Additionally, clarifications were provided for handling multiple such identifiers and for roaming support.
- UDR enhancement supporting Device Identifier of non-3GPP Devices connecting behind a UE/5G-RG TS 23.501CR5547
- Definition of identifiers of N3GPP device behind UE/5G-RG TS 23.501CR5749
- Support of reject QoS differentiation for non-3GPP device identifier(s) TS 24.501CR6926
- Procedure update for QoS differentiation of non-3GPP device identifiers TS 24.501CR6994
- Support for AF Specific Identifier Selection in Multiple Identifiers Translation in UDM TS 29.503CR1341
- Support of satellite identifier report at call set-up TS 29.513CR0602
+ 22 more changes
Explore further
Broader topics and technologies where QFI plays a role.
Defining Specifications
3GPP specifications that define or reference QFI, 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 |
| TS 24.193 vj50 | ATSSS Procedures Specification | Rel-19 |
| TS 24.501 vj50 | 5G NAS Protocols Specification | Rel-19 |
| TS 24.502 vj20 | 5G Core Access via Non-3GPP Networks; Stage 3 | Rel-19 |
| TS 24.890 vg00 | 5G NAS Protocol for 5GS Stage 3 | Rel-16 |
| TS 26.510 vj10 | Media Delivery APIs for 5GMS and RTC Systems | Rel-19 |
| TS 26.804 vj10 | 5G Media Streaming Extensions Study | Rel-19 |
| TR 26.806 vi00 | Technical Report on Smartly Tethering AR Glasses | Rel-18 |
| TS 26.891 vg00 | Media Distribution Services in 5G System | Rel-16 |
| TR 26.926 vj00 | Traffic Models & Quality Evaluation for Media/XR in 5G | Rel-19 |
| TR 26.928 vj00 | Study on eXtended Reality (XR) in 5G | Rel-19 |
| TS 29.503 vj50 | UDM Service Based Interface Stage 3 | Rel-19 |
| TS 29.508 vj40 | 5G Session Management Event Exposure Service | Rel-19 |
| TS 29.513 vj40 | 5G PCC Signalling Flows & QoS Mapping | Rel-19 |
| TS 29.890 vg00 | CT3 5G System Technical Report | Rel-16 |
| TS 32.255 vk10 | Telecom Management; Charging for 5G Data Connectivity | Rel-20 |
| TS 32.291 vj40 | Charging Management: Service-Based Interface Protocol | Rel-19 |
| TS 37.324 vj00 | Service Data Adaptation Protocol (SDAP) | Rel-19 |
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| TS 38.415 vj10 | PDU Session User Plane Protocol | Rel-19 |
| TR 38.835 vi01 | Technical Report on XR Enhancements for NR | Rel-18 |