Description
The Reflective QoS Indication (RQI) is a user-plane flag critical to the operation of the Reflective QoS mechanism in 5G networks. Unlike its control-plane counterpart, the RQA, the RQI is a single-bit indicator embedded within the encapsulation headers of user data packets traveling in the downlink direction. It is typically carried in the GTP-U extension header for N3/N9 interfaces or within the frame structure on the Uu air interface, associated with the QoS Flow ID (QFI). The entity setting the RQI can be the UPF (User Plane Function) or, in some deployments, the gNB, based on policies received from the SMF/PCF.
Architecturally, the RQI operates at the intersection of the user plane and the UE's QoS policy enforcement function. The network, upon determining that a specific downlink packet belongs to a QoS Flow configured for reflective treatment, sets the RQI bit in the packet header. This packet then traverses the N3 and Uu interfaces to the UE. The UE's protocol stack, specifically its SDAP (Service Data Adaptation Protocol) layer in 5G NR or its equivalent, inspects incoming downlink packets. Detection of the RQI bit is the key triggering event.
The mechanism works through a coordinated sequence. First, the UE must have previously received a control-plane QoS rule for that QoS Flow containing the RQA attribute, providing authorization. When the UE receives a downlink packet with the RQI bit set for that authorized flow, it initiates a reflective QoS action. This action involves the UE creating a new uplink QoS rule or updating an existing one. The UE derives the parameters for this uplink rule—such as the 5QI, Allocation and Retention Priority (ARP), and potentially flow-specific filters—from the characteristics of the observed downlink packet flow. The derived rule is then installed in the UE's uplink classifier and marker, ensuring subsequent uplink packets for that application flow receive the appropriate QoS treatment without needing a new request to the SMF.
Its role is to provide a real-time, in-band signaling mechanism that prompts the UE to self-configure its uplink QoS. This decouples the dynamic activation of symmetric QoS from the slower control-plane procedures. It is essential for achieving low latency in service setup, reducing signaling load on the core network, and enabling efficient support for applications where traffic patterns and required resources are mirrored between uplink and downlink. The RQI turns the downlink data stream itself into a control channel for QoS provisioning.
Purpose & Motivation
The RQI was developed to solve the problem of latency and signaling overhead associated with establishing symmetric QoS flows via traditional core-network-centric methods. In pre-5G systems, if an application started generating downlink traffic that required a corresponding uplink QoS flow, the network would need to detect this, formulate a policy (via PCF), instruct the SMF/ PGW, and then signal the new QoS rule to the UE—a process taking hundreds of milliseconds. For real-time interactive services, this delay is unacceptable.
The creation of RQI was motivated by the need for 'zero-touch' or immediate QoS activation. By marking a downlink packet, the network can instantly instruct a properly authorized UE to establish the necessary uplink resources. This is analogous to a network 'nudge' that leverages the existing data path for control. It addresses the limitations of purely reactive, signaling-based approaches which are too slow for URLLC and critical communication use cases envisioned for 5G.
In historical context, QoS markings like DSCP in IP headers provided hints, but the UE's response was not standardized or tied to secure, network-authorized policies. The 3GPP RQI, introduced in Release 15, formalizes this hint mechanism within the 3GPP protocol stack and securely couples it with the control-plane authorization (RQA). This solves the dual problem of speed and security, allowing for rapid adaptation while maintaining network control over which flows can use reflective QoS and what resources they can claim.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (224 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the RQI (Reflective QoS Indication) function was introduced with specific procedures for its operation and interworking. The release included corrections to the Reflective QoS logic for handling downlink packets marked with the RQI and defined mechanisms for Reflective QoS in interworking scenarios. Furthermore, a feature for the temporary restriction of Reflective QoS was also standardized.
- LADN indication from UE at registration TS 24.501CR0065
- Corrections to RQoS logic when receiving DL packet with RQI TS 23.501CR0011
- Clean up on the interworking without 26 indication TS 23.501CR0023
- Homogeneous support for IMS voice over PS Session supported indication TS 23.501CR0046
- Emergency Services Support indication per RAT TS 23.501CR0170
- Temporary restriction of Reflective QoS TS 23.501CR0169
+ 37 more changes
In Release 16, the Reflective QoS Indication (RQI) function was enhanced with clarifications for its operation with Multi-Path TCP (MPTCP). The release provided specific technical clarifications on how Reflective QoS applies when MPTCP is used, ensuring consistent QoS enforcement across multiple simultaneous transmission paths. This update did not introduce new procedures or interfaces but refined the existing framework for this specific transport protocol scenario.
- UPF Selection influenced by the indication of the identity/identities of 5G AN N3 User Plane capability TS 23.501CR0862
- QoS monitoring based on GTP-U paths TS 23.501CR1414
- Handling of IAB-indication to 5GC TS 23.501CR1901
- Assistance indication for WUS grouping TS 23.501CR2053
- Service Gap control in 5GS, activation with IE and indication flag TS 24.501CR0975
- Strictly periodic registration timer indication for MICO mode TS 24.501CR1030
+ 56 more changes
In Release 17, the RQI (Reflective QoS Indication) function was not specifically mentioned as being updated in the provided change request titles or grounding context. The listed enhancements for Release 17 instead focused on other indication mechanisms, such as the introduction of a Paging Cause Indication for Voice Service to assist NG-RAN with RAN-based paging and the support of a Paging Subgrouping Support Indication. Therefore, based solely on the provided materials, no new changes for the RQI function itself are described for this release.
- Support for Paging Early Indication TS 23.501CR3319
- Multi-USIM UE support indications in 5GS TS 24.501CR3121
- ProSe policy provisioning start and stop indications TS 24.501CR3127
- ECS address support indication and provisioning in ePCO TS 24.501CR2977
- UE's handling of the indication of country of UE location TS 24.501CR3219
- Paging Early Indication with Paging Subgrouping Assistance TS 24.501CR3803
+ 46 more changes
In Release 18, the enhancement for the Reflective QoS Indication (RQI) function specifically introduced a **capability signalling limitation**. This change addresses how the UE indicates its support for Reflective QoS to the network, ensuring the signalling is properly constrained as defined within the UE's 5GMM Core Network Capability. The update provides necessary clarifications to prevent potential misinterpretation or over-provisioning of this QoS feature during session establishment.
- CN based MT communication capability indication TS 23.501CR4081
- Equivalent SNPNs usage in 5GMM-CONNECTED mode with RRC inactive indication TS 24.501CR4839
- User plane positioning capability indication TS 24.501CR5015
- Instructing a UE to reconnect to the network upon receiving an indication of a change in the RAN timing synchronization status TS 24.501CR5061
- UE capability indication to the network for A2X TS 24.501CR5008
- Indication of support for reconnection to the network due to RAN timing synchronization status change via the 5GMM capability IE TS 24.501CR5056
+ 40 more changes
In Release 19, the specification included a correction to the reflective QoS function, addressing an identified issue in its implementation or procedural description. This update ensures proper handling of the RQI mechanism, which allows the User Plane Function to mark downlink packets to instruct the UE to apply reflective QoS for corresponding uplink traffic. The correction clarifies UE or network behavior to maintain consistent QoS flow mapping based on the received indications.
- I-SMF selection/insertion based on local offloading allowed indication TS 23.501CR5604
- I-SMF selection/insertion based on local offloading allowed indication TS 23.501CR5871
- KI#1, Support the PDU Set Information Marking Support Indication TS 23.501CR5673
- Indication of Rate-Adaptable QoS Flows to NG-RAN TS 23.501CR6194
- Update of UE parameters update transparent container with Disaster Roaming Enabled Indication in EPS TS 24.501CR6952
- Clarification on NAS LP-WUS Subgroup Support Indication meaning TS 23.501CR6139
+ 15 more changes
Explore further
Broader topics and technologies where RQI plays a role.
Defining Specifications
3GPP specifications that define or reference RQI, 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 24.501 vj50 | 5G NAS Protocols Specification | Rel-19 |
| TS 24.502 vj20 | 5G Core Access via Non-3GPP Networks; Stage 3 | Rel-19 |
| TS 24.554 vj40 | 5G Proximity Services (ProSe) Protocols | Rel-19 |
| TS 24.890 vg00 | 5G NAS Protocol for 5GS Stage 3 | Rel-16 |
| TS 29.890 vg00 | CT3 5G System Technical Report | Rel-16 |
| TS 37.324 vj00 | Service Data Adaptation Protocol (SDAP) | Rel-19 |
| TS 38.415 vj10 | PDU Session User Plane Protocol | Rel-19 |