The Purdue Reference Model outdated or up-to-date?

Is the Purdue Reference Model (PRM) outmoded? If I listen to the Industrial Internet of Things (IIoT) community, I would almost believe so. IIoT requires connectivity to the raw data of the field devices and our present architectures don’t offer this in an easy and secure way. So let’s look in a bit more detail to the PRM and the IIoT requirements, to see if they really conflict or can coexist side by side?

I start the discussion with the Purdue Reference Model, developed in the late 80’s. Developed in a time that big data was anything above 256 Mb and the Internet was still in its early years, a network primarily used by and for university students. PRM’s main objective was creating a hierarchical model for manufacturing automation, what was called computer integrated manufacturing (CIM) in those days. If we see the model as it was initially published we note a few things:

  • The model has nothing to do with a network, or security. There are no firewalls, there is no DMZ.
  • There is an operator console at level 1. Maybe a surprise for people who started to work in automation in the last 20 years, but in those days quite normal.
  • Level 4 has been split in a level 4A and a level 4B, to segment functions that directly interact with the automation layers and functions requiring no direct interface.
  • There is no level 0, the Process layer is what it is a bunch of pipes, pumps, vessels and some clever stuff.

Almost a decade later we had ANSI / ISA-95 that took the work of the Purdue university and extended it with some new ideas and created an international standard that became very influential for ICS design. It was the ANSI / ISA-95 standard that named the Process level, level 0. But in those days level 0 was still the physical production equipment. The ISA-95 standard says following on level 2, level 1, and level 0 : ” Level 0 indicates the process, usually the manufacturing or production process. Level 1 indicates manual sensing, sensors, and actuators used to monitor and manipulate the process. Level 2 indicates the control activities, either manual or automated, that keeps the process stable or under control.” So level 0 is the physical production equipment, and level 1 includes the sensors and actuators. It was many years later that people started using level 0 for the field equipment and their networks, all new developments with the rise of Foundation Fieldbus, Profibus, the HART protocol, but never part of the standard. It was probably the ISA 99 standard that introduced a new layer between level 4 and level 3, the DMZ. It was the vendor community that started giving it a level name, level 3.5. But level 3.5 has never been a functional level, it was an interface between two trust zones for adding security. Though often the way it was implemented contributed little to security, but it was a nice feeling to say we have a demilitarized zone between the corporate network and the control network. So far the history of the Purdue Reference Model and ISA-95 and their contribution to the levels. Now lets have a look at how a typical industrial control system (ICS) looks without immediately using names for levels.

From a functional viewpoint we can split a traditional ICS architecture in 5 main parts:

  • Production management system, which task it is to control the automation functions. Typical domain of the DCS and SCADA systems. But also compressor control (CCS), power management systems (PMS) and the motor control center (MCC) reside here. basically everything that takes care of the direct control of the production process. And of course all these systems interact with each other using a wide range of protocols, many of them with security short comings;
  • Operations management system, which task it is to optimize the production process, but also to monitor and diagnose the process equipment (for example machine monitoring functions such as vibration monitoring), and various management functions such as asset management, accounting and reconciliation functions to monitor the mass balance of process units, and emission monitoring systems to meet environmental regulations;
  • Information systems is the third category, these systems collect raw data and transform it into information to be used for historical trends or to feed other systems with information. The objective here is to transform the raw data into information and ultimately information into knowledge. The data samples stored are normally one minute snapshots, hourly averages, shift averages, daily averages, etc. An other example of an information system is custody metering system (CMS) for measuring product transfer for custody purposes.
  • The last domain of the ICS is the Application domain, for example for advanced process control (APC) applications, traditionally performing advisory functions. But overtime the complexity of running an production processes grew, response to changes in the process became more important so advisory functions were taking over the task of the process operator immediately changing the setpoints using setpoint control or controlling the valve with direct digital control functions. There are plants today that if APC would fail, the production is no longer profitable or can’t reach the required product quality levels.
  • Finally there is the Business domain, generally not part of the ICS. in the business domain we mange for example the feed stock and products to produce. It is the decision domain.
Functional model of a chemical plant or refinery

The production management systems are shown in the light blue color, the operations management, information management, and application systems in the light purple color. The model seems to comply with the models of the ANSI / ISA-95 standard. However this model is far from perfect, because an operations management function as vibration monitoring, or corrosion monitoring also have sensors that connect to the physical system. And an asset management system requires information from the field equipment. If we consider a metering system, also part of the information function, it is actually a combination of a physical system and sensors.

And then of course we have all the issues around vendor support obligations. Asset owners expect systems with a very high level of reliability, vendors have to offer this level of reliability in a challenging real-time environment by testing and validating their systems and changes rigorously. Than there is nothing worse than suddenly having to integrate with the unknown, a 3rd party solution. As a result we see systems that have a level 2 function connected to level 3, in principle exposing a critical system more and making the functionality rely on more components so reducing reliability.

So many conflicts in the model, still it has been used for over 20 years, and it still helps us to guide us in securing systems today. If we take the same model and add some basic access security controls we get the following:

At high level this represents the architecture in use today, in smaller systems the firewall between level 2 and 3 might miss, some asset owners might prefer a DMZ with two firewalls (from different vendors to enforce diversity), the level 4A hosts those functions that interface with the control network and generally different policies are enforced for these systems than required at level 4B. For example asset owners might enforce the use of desktop clients, might limit Internet access for these clients, might restrict email to internal email only, etc. All to reduce the chance on compromising the ICS security.

Despite that the reference model is not supporting all of the new functions we have in plants today – for example I didn’t discuss wireless, virtualization, and system integration topics at level 2 – the reference model still helps structure the ICS. Is it impossible to add the IIoT function to this model? Let’s have a look at what IIoT requires?

In an IIoT environment we have a three level architecture:

  • The edge tier – The edge tier is for our discussion the most important, this is where the interface is created with the sensors and actuators. Sometimes existing sensors / actuators, but also new sensors and actuators.
  • The platform tier – The platform tier processes and transforms the data and controls the direction of the data flow. So from that point also of importance for the security of the ICS.
  • The enterprise tier – This tier hosts the application and business logic that needs to provide the value. It is external to the ICS, so as long as we have a secure end to end connection between the platform tier and the enterprise tier ICS security needs to trust that the systems and data in this tier are properly secured.

The question is can we reconcile these two architectures? The answer seems to be what is called the gateway-mediated edge. This is a device that aggregates and controls the data flows from all the sensors and actuators providing a single point of connection to the enterprise tier. This type of device also performs the necessary translation for communicating with the different protocols used in ICS. The benefits of such a gateway device is the scalability of the solution together with controlling the entry and exit of the data flows from a single controllable point. In this model some of the data processing is kept local, close to the edge, allowing controlled interfaces with different data sources such as Foundation Field Bus, Profibus, and HART enabled field equipment. To implement such an interface device doesn’t require to change the Purdue reference model in my opinion, it can make use of the same functional architecture. Additionally new developments such as the APL (Advanced Physical Layer) technology, the technology that will remove the I/O layer as we know it today, will fully align with this concept.

So I am far from convinced that we need a new model, also today the model doesn’t reflect all the data flows required by today’s functions. The very clear boundaries we used to have, have disappeared with the introduction of new technologies such as virtualization and wireless sensor networks. Centralized process computer systems of the 70s, have disappeared becoming decentralized over the last 30 years and are now moving back into centralized data centers where hardware and software have lose ties. Today these data centers are primarily onsite, but over time confidence will grow and larger chunks of the system will move to the cloud.

Despite all these technology changes, the hierarchical structure of the functions hasn’t change that much. It is the physical location of the functions that changes, undoubtedly demanding a different approach to cyber security. It are the cyber security standards of today that are dated on the day they are released. The PRM was never about security or networking, it is a hierarchical functional model for the relationship between functions which is as relevant today as it was 30 years ago.

We have a world of functional layers, and between these layers data is exchanged, so we have boundaries to protect. As long as we have bi-directional control over the data flows between the functional layers and keep control over where the data resides, we protect the overall function. If there is something we need to change it are the security standards, not the Purdue reference model. But we need to be careful with the security of these gateways as the recent OSI PI security risk has learned us.

There is no relationship between my opinions and publications in this blog and the views of my employer in whatever capacity.

Sinclair Koelemij

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