St (application id: 16777349)

The St interface is a crucial component within the Policy and Charging Control (PCC) architecture. Specifically, it facilitates communication between the Policy and Charging Rules Function (PCRF) and the Traffic Steering Support Function (TSSF). The primary role of the St interface is to convey traffic steering control information from the PCRF to the TSSF, allowing the network to dynamically manage and steer user traffic based on pre-defined policies.

The St interface resides between two key network elements:

• PCRF (Policy and Charging Rules Function): The PCRF is responsible for making policy decisions based on a user's subscription details, service requirements, and real-time network conditions. These decisions include which traffic flows should be prioritized, which applications need special handling, and how bandwidth should be allocated.
• TSSF (Traffic Steering Support Function): The TSSF acts on the instructions received from the PCRF. It enforces the traffic steering policies within the network, particularly within the (S)Gi-LAN environment, which includes various network elements responsible for processing user data traffic.

The primary purpose of the St interface is to ensure that traffic management policies created by the PCRF are enforced by the TSSF. This allows the network to efficiently manage resources and deliver the appropriate Quality of Service (QoS) based on the type of traffic and the user’s subscription. The St interface supports both the installation of new policies and the modification or removal of existing ones.

St interface workflow:

1. Session Establishment: When an IP-CAN (IP Connectivity Access Network) session is initiated or during its lifetime, the PCRF determines if traffic steering is necessary. If required, the PCRF will establish an St session with the TSSF by sending a TDF-Session-Request (TSR) message with the Request-Type AVP set to "0" (initial request).
2. Traffic Steering Policy Provisioning: The PCRF sends the TSR message to the TSSF, including specific AVPs such as the Framed-IP-Address, ADC-Rule-Name, and TDF-Application-Identifier. These AVPs contain details about the user’s IP address, the traffic flows to be managed, and the steering policies to be applied.
3. Policy Enforcement: Upon receiving the TSR, the TSSF enforces the policies by applying the traffic steering rules to the data flows within the network. This could involve prioritizing certain types of traffic, rerouting data through specific paths, or applying QoS adjustments.
4. Policy Update: If an active St session already exists, the PCRF may update the traffic steering rules by sending another TSR with the Request-Type AVP set to "1" (update request). This update may involve adding new rules, modifying existing ones, or removing obsolete policies.
5. Policy Modification and Removal: The PCRF can modify existing policies or deactivate them as needed. This is achieved by sending updated TSR messages with instructions on the changes to be applied. The TSSF responds by adjusting its traffic handling in line with the new instructions.
6. Session Termination: When the IP-CAN session concludes, the St session is terminated. The TSSF stops enforcing the steering rules, and normal network traffic handling resumes.

For complete technical specification of St interface in Diameter protocol please refer to: [3GPP TS 29.212]