Traffic Detection Function Session

Description

The TDF session is a core network concept introduced in 3GPP Release 11 as part of the Policy and Charging Control (PCC) architecture. It represents a logical association between an IP Connectivity Access Network (IP-CAN) session and a Traffic Detection Function (TDF). The TDF is a functional entity that performs application detection and reporting, as well as application control, based on rules provided by the Policy and Charging Rules Function (PCRF). The TDF session is established when the PCRF decides to activate application detection and control for a subscriber's IP-CAN session. This typically occurs during session establishment (e.g., PDN connection setup) or when the PCRF receives an indication from the TDF that application detection is required.

The TDF session is identified by one UE IPv4 address and/or IPv6 prefix, together with optionally a PDN represented by a PDN ID. This identification allows the PCRF and TDF to correlate the session with the correct user and PDN connection. The session also includes a set of Application Detection and Control (ADC) rules that are to be applied by the TDF. These rules specify which applications should be detected (e.g., based on IP address, port, protocol, or deep packet inspection) and what actions should be taken (e.g., reporting to PCRF, gating, bandwidth limitation).

The architecture involves several key interfaces. The PCRF communicates with the TDF over the Sd interface (Gx for TDF), which is based on Diameter. The TDF may be standalone or integrated with a PCEF (Policy and Charging Enforcement Function). When integrated, the TDF session may be handled within the same node as the PCEF. The TDF session is created, modified, or terminated based on PCRF decisions. For example, the PCRF may install ADC rules when a subscriber starts a session, update them during the session, or remove them upon termination.

The TDF session enables dynamic policy control for applications that may not be easily detected by traditional IP 5-tuple filters. It allows operators to apply specific charging, QoS, or gating policies for over-the-top (OTT) applications such as VoIP, video streaming, or peer-to-peer traffic. This is particularly important in networks where traffic from different applications needs to be differentiated for billing or quality of service purposes.

In terms of evolution, the TDF session definition has remained largely unchanged across 3GPP releases from Rel-11 to Rel-19. The core concept and identification parameters have been stable, reflecting the maturity of the PCC architecture. However, the surrounding specifications and implementation details may have evolved to support new features such as 5G integration, network slicing, and enhanced application detection techniques. The TDF session remains a fundamental building block for application-aware policy control in mobile networks.

Purpose & Motivation

The TDF session was created to address the growing need for application-aware policy control in mobile networks. Before its introduction, policy control was primarily based on IP 5-tuple filters (source/destination IP, port, protocol), which could not effectively identify and control specific applications, especially those using dynamic ports or encryption. This limitation made it difficult for operators to apply differentiated charging or QoS for popular OTT services.

The TDF session solves this problem by providing a standardized mechanism for the PCRF to instruct a TDF to detect specific applications and apply ADC rules. It enables operators to monetize OTT traffic, enforce fair usage policies, and ensure quality of experience for critical applications. The session concept ensures that application detection and control are tied to a specific user session, allowing per-subscriber policies.

Historically, the TDF session was introduced in Rel-11 as part of the PCC enhancements to support application detection and control (ADC). It built upon the existing Gx interface and PCRF architecture, extending them with the Sd interface and TDF functionality. This allowed for a clear separation of concerns: the PCRF makes policy decisions, while the TDF enforces them at the application level. The TDF session remains relevant in 5G networks, where application detection is even more critical due to diverse service requirements.