Flow Identifier

Description

The Flow Identifier (FID) is a fundamental parameter within the 3GPP Policy and Charging Control (PCC) architecture. It serves as a unique label for an IP flow or a service data flow, which is a set of IP packet flows matching a specific service data flow template. The FID is used by network elements like the Policy and Charging Rules Function (PCRF) and the Policy and Charging Enforcement Function (PCEF) to identify and apply the correct policy and charging rules to a user's traffic. The PCRF defines these rules in PCC rules, each associated with a specific FID, dictating parameters such as QoS class, bitrates, and charging methods.

In operation, when a user initiates a service (e.g., a video stream), the PCRF authorizes the required resources and installs corresponding PCC rules in the PCEF (typically located in the PDN Gateway). Each PCC rule contains an FID, a service data flow template (defining the IP packets via 5-tuple filters), and the associated policy control (QoS) and charging control instructions. The PCEF then uses the FID to map incoming user plane packets to the correct set of rules for enforcement. This allows the network to differentiate, for example, between a high-priority VoIP flow and a best-effort web browsing flow for the same user.

The FID's role is critical for enabling advanced service-aware networking. It provides the linkage between the high-level service authorization in the control plane and the granular packet-level treatment in the user plane. By uniquely identifying flows, operators can implement sophisticated traffic management, ensure QoS for real-time services, and apply accurate charging based on service type, content, or application. The FID is a key enabler for network slicing and edge computing scenarios, where different slices or applications require distinct traffic handling policies.

Purpose & Motivation

The FID was introduced to address the need for granular, flow-based policy and charging control in IP-based mobile networks. Prior to its introduction, policy control was often coarse-grained, applied per user or per Access Point Name (APN), which was insufficient for the diverse range of services with different QoS and charging requirements emerging with mobile broadband. The proliferation of real-time services like VoIP and video streaming necessitated a mechanism to identify and treat individual application flows differently within a user's session.

Its creation was motivated by the evolution towards all-IP networks in 3GPP Release 8 (EPS). The PCC architecture was standardized to provide a unified framework for service-aware policy and charging. The FID is a core component of this framework, solving the problem of how to uniquely reference and manage the multitude of concurrent data flows a single user might generate. It allows operators to move beyond flat-rate billing and best-effort delivery, enabling service differentiation, optimized resource usage, and new business models like sponsored data or quality-on-demand.

By providing a stable identifier for a flow throughout its lifetime, the FID ensures consistent policy application even if the underlying packet filter parameters (like source IP) change due to network events. It abstracts the specific packet characteristics from the policy rules, simplifying management and enabling dynamic policy updates without redefining the entire flow template.

Key Features

• Uniquely identifies a service data flow within a user's PDN connection
• Core parameter in PCC rules for binding policy and charging instructions to a specific flow
• Enables granular QoS enforcement (e.g., guaranteed bitrate, priority) on a per-flow basis
• Supports flow-based charging, including differentiated rating and reporting
• Allows dynamic installation, modification, and removal of flow-specific policies by the PCRF
• Facilitates traffic detection and gating control for specific applications or services

Evolution Across Releases

Defining Specifications