Re-Auth-Request

Description

The Re-Auth-Request (RAR) is a Diameter command code (258) defined within the 3GPP Policy and Charging Control (PCC) framework, specified in TS 29.212 and related specifications. It is a critical component of the Gx and Rx reference points, enabling dynamic policy control. The RAR command is initiated by the Policy and Charging Rules Function (PCRF) and sent to a Policy and Charging Enforcement Function (PCEF) over the Gx interface, or to an Application Function (AF) over the Rx interface, to trigger a re-authorization of an active IP-CAN (IP Connectivity Access Network) session. This allows the PCRF to modify session parameters in real-time without waiting for a periodic update or a trigger from the access network.

When the PCRF decides that a change in policy is required—due to factors like subscriber quota exhaustion, change in service tier, network congestion, or application request—it constructs an RAR message. This message includes Attribute-Value Pairs (AVPs) that specify the required actions, such as installing, modifying, or removing PCC rules. For the Gx interface, the RAR may contain new QoS parameters, charging rules, or gate status instructions. Upon receiving the RAR, the PCEF (typically residing in the PGW/GGSN) must apply the new rules and respond with a Re-Auth-Answer (RAA) to acknowledge the changes. Similarly, on the Rx interface, the RAR can be used to notify the AF about session modifications, enabling application-layer adaptation.

The RAR procedure is integral to the event-triggered policy control mechanism in 3GPP networks. It operates in conjunction with other Diameter commands like Credit-Control-Request (CCR) and Answer (CCA). Key AVPs in the RAR include Session-Id, Origin-Host, Origin-Realm, Destination-Host, Destination-Realm, and specific PCC rule AVPs like Charging-Rule-Install or Charging-Rule-Remove. The protocol ensures reliable delivery through the Diameter base protocol's failover and transport mechanisms. This dynamic re-authorization capability is essential for implementing advanced services like sponsored data, QoS on-demand, and congestion management, allowing the network to respond agilely to changing conditions while maintaining session continuity.

Purpose & Motivation

The RAR command was introduced to address the need for proactive, network-initiated policy updates within the PCC architecture. Prior to its standardization, policy changes often relied on client-initiated requests or static configurations, which were insufficient for real-time service control. The RAR enables the PCRF, as the central policy decision point, to enforce policy changes immediately when triggered by external events, such as a change in subscriber profile, network load, or application requirements.

This capability solved significant limitations in early policy control systems, which were largely reactive. For example, without RAR, modifying the QoS for a video stream during congestion would require waiting for the UE or AF to request a change, potentially degrading user experience. The RAR allows the network to take the initiative, ensuring that policies align dynamically with operational conditions. It is particularly important for monetization strategies like tiered services and sponsored data, where policy rules must be updated instantly based on charging events.

Historically, RAR's introduction in Release 9 coincided with the maturation of the PCC framework for LTE/EPC. It provided the missing mechanism for server-driven session modification, complementing the existing client-initiated CCR/CCA dialogues. This empowered operators to implement more sophisticated, real-time service differentiation and network optimization, moving beyond simple static policy provisioning to a dynamic, event-driven policy model that is foundational for modern mobile broadband services.