Re-Auth-Answer

Description

The Re-Auth-Answer (RAA) is a command within the Diameter base protocol (RFC 6733) and is specifically utilized in the 3GPP Policy and Charging Control (PCC) framework defined in TS 29.212. It functions as the definitive response from a Diameter server, such as a Policy and Charging Rules Function (PCRF), to a previously received Re-Auth-Request (RAR) command. The RAR/RAA exchange is a server-initiated procedure, allowing the network to dynamically modify the parameters of an ongoing IP-CAN (IP Connectivity Access Network) session without waiting for a trigger from the client side.

When the PCRF decides to modify policy or charging rules for a subscriber (e.g., due to a change in subscription, application detection, or network conditions), it sends a RAR command to the Policy and Charging Enforcement Function (PCEF) located in the gateway (e.g., PGW, TDF, or SMF in 5G). The RAR contains the new rules or instructions within Attribute-Value Pairs (AVPs). Upon receiving and successfully applying these instructions, the PCEF must respond with a Re-Auth-Answer (RAA). This RAA message contains a Result-Code AVP indicating success (e.g., DIAMETER_SUCCESS) or a specific error code if the request could not be fulfilled.

The RAA message is not merely an acknowledgment; it is a mandatory part of the Diameter transaction state machine. Its absence or the receipt of an error code informs the PCRF that the policy update failed, which may trigger corrective actions like session termination or alternative rule provisioning. The message includes the Session-Id AVP to correlate it with the specific session and the RAR. Within the PCC architecture, this mechanism is fundamental for real-time service control, enabling features like on-demand QoS modification, gating control for application flows, and mid-session charging updates.

Purpose & Motivation

The RAA message exists to provide a reliable, acknowledged mechanism for network-initiated policy updates within a standardized online charging and policy control framework. Prior to PCC and Diameter, policy control was often static or relied on proprietary signaling, making real-time, per-session adjustments complex and vendor-dependent. The Diameter protocol's RAR/RAA command pair was adopted to solve this by providing a robust, request-response model for server-driven session re-authorization.

Its creation was motivated by the need for operators to dynamically control user experiences and network resources based on evolving conditions. For example, an operator might want to boost a user's bandwidth during a video call or apply new spending limits without interrupting the session. The RAA provides the necessary confirmation that these critical instructions have been received and enacted by the enforcement point. This closed-loop communication is essential for billing accuracy, service assurance, and implementing advanced business models like sponsored data or tiered QoS offerings.

Key Features

• Mandatory response to a Re-Auth-Request (RAR) command in the Diameter protocol.
• Carries a Result-Code AVP indicating success or failure of the policy update.
• Uses the Session-Id AVP for precise correlation with a specific user session.
• Enables server-initiated, mid-session modification of PCC rules.
• Fundamental to the real-time operation of the 3GPP PCC architecture.
• Ensures reliable delivery and application of dynamic policy and charging instructions.

Evolution Across Releases

Defining Specifications