S9 (application id: 16777267)

The S9 interface in the Diameter protocol plays a critical role in roaming scenarios involving a Home Public Land Mobile Network (HPLMN) and a Visited Public Land Mobile Network (VPLMN). The primary function of the S9 interface is to facilitate communication between the Home Policy and Charging Rules Function (H-PCRF) in the HPLMN and the Visited Policy and Charging Rules Function (V-PCRF) in the VPLMN. This communication is essential for ensuring consistent policy control and charging decisions when a user roams from their home network to a visited network.

The S9 interface is designed to transmit Policy and Charging Control (PCC) rules and Quality of Service (QoS) rules from the H-PCRF to the V-PCRF. These rules determine how data sessions are managed and billed while a user is roaming. The S9 interface also supports the exchange of Application Detection and Control (ADC) rules between the two PCRFs, which is necessary when both support the Application Detection and Control feature.
In addition to transmitting PCC, QoS, and ADC rules, the S9 interface enables the H-PCRF to receive reports from the V-PCRF about the status of these rules in the visited network. This allows the H-PCRF to maintain control over the user's session even when they are not in their home network.
Architecture
The architecture of the S9 interface is relatively straightforward, involving a direct connection between the H-PCRF and the V-PCRF. This connection is established using the Diameter protocol, specifically designed for authentication, authorization, and accounting (AAA) purposes in IP-based networks. The S9 interface uses Stream Control Transmission Protocol (SCTP) as the transport layer to ensure reliable message delivery between the two PCRFs.
The interface is defined by two Diameter applications: the S9 application and the Rx application. The S9 application is used to install PCC, QoS, and ADC rules generated in the HPLMN into the VPLMN. The Rx application is used when the Application Function (AF) is in the VPLMN to exchange service session information from the V-PCRF to the H-PCRF.

S9 interface workflow:

1. Session Initiation: When a user roams into a VPLMN, the V-PCRF establishes a connection with the H-PCRF over the S9 interface. The V-PCRF requests PCC rules from the H-PCRF to manage the user's data session according to their home network's policies.
2. Rule Installation: The H-PCRF generates PCC, QoS, and ADC rules based on the user's subscription and sends them to the V-PCRF over the S9 interface. These rules dictate how the user's data session should be handled in the VPLMN.
3. Session Management: The V-PCRF applies the received rules to the user's session. It monitors the session and enforces the policies specified by the H-PCRF. The V-PCRF also reports back to the H-PCRF on the status of these rules, including any changes or violations.
4. Session Termination: When the user's session ends, or they leave the VPLMN, the V-PCRF sends a termination message to the H-PCRF over the S9 interface. The H-PCRF then terminates the session on its side and clears any associated resources.

For complete technical specification of S9 interface in Diameter protocol please refer to: [3GPP TS 29.215]