The Timeless Strength | Why the Purdue Model Remains Essential for ICS Security

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Figure 1: The Purdue Enterprise Reference Architecture (PERA) AKA the Purdue model

The Purdue Reference Model is a comprehensive framework for Industrial Control System (ICS)/OT network segmentation, encompassing six layers and their associated components and security controls. Developed in the 1990s and adopted by ISA-99, this model serves as a conceptual framework for ICS networks. It effectively illustrates the interconnections and interdependencies of key components within a typical ICS, dividing the architecture into two distinct zones: Information Technology (IT) and Operational Technology (OT).

The Purdue Model further divides these 6 zones into levels, beginning with level 0. Network segmentation is a critical aspect of ICS/OT networks and is emphasized within this model.

At the foundation of the Purdue Model lies OT, which encompasses the systems utilized in critical infrastructures and manufacturing to monitor and control physical equipment and operational processes. This is entirely separate from the IT, which is positioned at the top of the model. Between these two zones, a Demilitarized Zone (DMZ) is implemented to ensure separation and regulate access between them.

Within each zone, distinct layers define the industrial control components present. These layers include:

• Level 0 | Comprising sensors, pumps, valves, actuators, and motors, which transmit information to the Programmable Logic Controllers (PLCs) and serve as the sensory input for the OT system. Various types of assets exist within this level, such as energy and signaling devices.
• Level 1 | Encompassing systems responsible for monitoring and issuing commands to level 0 assets. Examples include PLCs, Remote Terminal Units (RTUs), and Intelligent Electronic Devices (IEDs). RTU-IEDs are often utilized by the power sector and perform data collection and long-distance communication, some even possessing protective functions. Distributed Control Systems (DCS) are prevalent in continuous production sectors like the chemical and food industry, offering redundancy and process-wide control.
• Level 2 | Consisting of devices that control the overall processes within the system, including Human-Machine Interfaces (HMIs) and SCADA software. HMIs provide a visual environment for human interaction with various OT assets, enabling action and process management. This level finds application in sectors such as water, gas, power, oil, and transport.
• Level 3 | Supporting the management of production workflows, level 3 includes batch management, manufacturing operations management/execution systems (MOMS/MES), and data historians. These computerized systems track and document the transformation of raw materials into finished goods, aiding decision-makers in optimizing production output. Integration with Enterprise Resource Planning (ERP) systems enhances automation capabilities.

The accuracy and relevance of the Purdue Model depend on the specific assets employed within the described framework. Additionally, the emergence of Industrial IoT (IIoT) assets may require adaptations to the model. Nonetheless, the model's segmentation and hierarchical structure offer notable advantages. With clearly defined system components and assets grouped into distinct layers, network segmentation becomes more manageable. In IIoT environments, where a three-component architecture (devices, field or cloud gateways, and services backend) may be present, the Purdue Model remains valuable in ensuring effective network segmentation and control.