Reflective QoS Indication

Description

The Reflective QoS Indication (RQSI) is a parameter defined in earlier 3GPP releases, primarily within the context of IP Multimedia Subsystem (IMS) and Policy and Charging Control (PCC) for 3G and 4G networks. Unlike the 5G RQI which is a user-plane marker, the RQSI is an application-layer indicator exchanged within Session Initiation Protocol (SIP) and Session Description Protocol (SDP) messages during IMS session establishment (e.g., VoIP call setup). Its purpose is to signal from the IMS Application Server (AS) or the UE to the PCC architecture that the requested media flow requires network-initiated QoS resource allocation.

Architecturally, the RQSI operates in the service layer. When an IMS endpoint (UE or network server) includes an SDP offer or answer for a media stream (audio, video), it can include the 'gpp:rqsi' media-level attribute. This attribute acts as a request or instruction to the underlying bearer network. The P-CSCF (Proxy-Call Session Control Function), which acts as the IMS entry point, intercepts this SIP/SDP signaling. Upon detecting the RQSI parameter, the P-CSCF informs the Policy and Charging Rules Function (PCRF in 4G) via the Rx interface. The PCRF then uses this as a trigger to generate appropriate PCC rules, which are pushed to the Packet Gateway (PGW) and, ultimately, lead to the establishment of a dedicated bearer with specific QoS (e.g., a Guaranteed Bit Rate bearer) for that media flow.

How it works involves a cross-layer interaction. The application (IMS) expresses its QoS need via RQSI in SDP. The P-CSCF performs service-level authorization and translates this application requirement into a network resource request via the Rx interface. The PCRF, as the central policy decision point, creates a PCC rule that maps the SDP media description (IP addresses, ports) to a specific QoS class identifier (QCI) and potentially bit rates. This rule is then enforced by the PGW and the radio access network, ensuring the media flow receives the contracted QoS. The RQSI itself is not seen by the user plane; it is purely a service-to-control-plane trigger.

Its role in legacy networks was to enable IMS applications to dynamically request guaranteed QoS from the packet-switched domain, a critical requirement for voice and video over LTE (VoLTE). It provided a standardized mechanism for the IMS layer to communicate its needs to the PCC framework, bridging the gap between application signaling and bearer management. This was essential for achieving consistent quality in all-IP communication services.

Purpose & Motivation

The RQSI was created to address a key challenge in early LTE/EPC deployments: enabling IMS-based real-time communication services like VoLTE to reliably request and receive appropriate network resources. In initial data-only networks, all traffic used default bearers with best-effort QoS, which was unsuitable for delay-sensitive voice. There was no standard way for the IMS application to explicitly ask the EPC for a dedicated, QoS-guaranteed bearer.

The problem it solved was the lack of coupling between application session signaling (SIP) and network resource setup. Without RQSI, network-initiated QoS for IMS relied on static provisioning or non-standard methods, leading to interoperability issues and poor user experience. The RQSI parameter, introduced around 3GPP Release 11, provided a standardized 'hook' within the SDP that the P-CSCF could recognize and act upon, triggering the dynamic PCC procedures.

Historically, it represents the evolution towards application-aware networking in mobile systems. It motivated the tighter integration between the service layer (IMS) and the policy control layer (PCC). While its functional goal—triggering network QoS for an application flow—is similar to the 5G RQI, the mechanisms are vastly different: RQSI uses application-layer signaling (SIP) to trigger core-network control-plane (PCRF) actions, whereas 5G RQI uses user-plane marking to trigger UE-local actions. RQSI is thus a precursor concept that highlighted the need for dynamic QoS but relied on a more centralized, signaling-heavy architecture.

Key Features

• An SDP media-level attribute ('gpp:rqsi') used in IMS signaling.
• Signals a request for network-initiated QoS for a specific media flow.
• Intercepted by the P-CSCF during SIP session establishment.
• Triggers PCRF policy decision and dedicated bearer setup via PCC.
• Used primarily for VoLTE and ViLTE service assurance.
• Operates at the application-to-control-plane boundary.

Evolution Across Releases

Defining Specifications