Using Deterministic Networks for Industrial Operations and Control

Introduction Process automation systems involve operating a piece of equipment (such as actuating and/or sensing field devices). The communication between the 'process controllers' and field devices exhibits a well-defined set of behaviors and has specific characteristics: the delivery of a control-command to a machine must be executed within the time frame specified by a controller or by an application to provide reliable and secure operation. A low or zero tolerance to latency and packet losses (among other things) is implied. The endpoints ('process controllers' and field devices) embody machine-to-machine communications to facilitate both remote and local process automation. In this document, networks that support all the characteristics of remote process automation are referred to as Operation and Control Networks (OCNs) for convenience. This document describes using DetNet to enable OCN applications since they provide mechanisms for guaranteed delay aware packet delivery, reliability, and for packet loss mitigation. This document defines the interface between an OCN application and DetNet framework. i.e., using DetNet services for communication between the controllers and the field devices. This interface is used by an application to express its network-specific requirements. This document presents the perspective of an end system. Because IP network stack is widely used by general-purpose applications and provides more connection flexibility to end systems, the scope our discussion is specific to the IP-enabled DetNet data planes . For the other type of field devices, service level proxy is assumed (section 4.1 in RFC8655). Mapping OCNs to DetNet helps with developing a better understanding of how DetNets can be used in such scenarios. To this end, the document provides a background on the type of traffic patterns in OCN applications. It proposes an interface between an application and DetNet, and a potential solution direction to support OCN traffic patterns over DetNet, as covered in .

Terminology

Operational Technology (OT):

Programmable systems or devices that interact with the physical environment (or manage devices that interact with the physical environment). These systems/devices detect or cause a direct change through the monitoring and/or control of devices, processes, and events. Examples include industrial control systems, building management systems, fire control systems, and physical access control mechanisms. Source:
process controller:

Is a logic control function used in process automation and control systems. A process controller maintains the operational requirement of a process and performs functions similar to programmable logic controllers (PLCs) but it can be either a hardware or software component. The term process controller is used through out to avoid confusion with 'network controllers' used in network infrastructures.
Industral Automation:

Mechanisms that enable machine-to-machine communication by use of technologies that enable automatic control and operation of industrial devices and processes leading to minimizing human intervention.
Control Loop:

Control loops are part of process control systems in which desired process response is provided as input to the 'process controller', which performs the corresponding action (using actuators) and reads the output values. Since no error correction is performed, these are called open control loops.
Feedback Control Loop:

A feedback loop is part of a system in which some portion (or all) of the system's output is used as input for future operations.
Industrial Control Networks:

Industrial control networks are the interconnection of equipment used for the operation, control or monitoring of machines in the industral environment. It involves a different level of communication - between fieldbus devices, digital controllers and software applications
Human Machine Interface (HMI):

An interface between the operator and the machine. The communication interface relays I/O data back and forth between an operator's terminal and HMI software to control and monitor equipment.

Acronyms

HMI: Human Machine Interface
OCN: Operations and Control Networks
PLC: Programmable Logic Control
OT: Operational Technology
OC: Operation and Control
OCN: Operation and Control Networks

Background on Industrial Control Systems An industrial control network interconnects devices used to operate, control and monitor physical equipment in industrial environments. below shows such systems' reference model and functional components. Closest to the physical equipment are field devices (actuators and sensors) that connect to the Programmable Logic Controllers (PLCs) or other types of controllers (Note: in this memo term 'process controller' will be used to differentiate from other meanings of controller) using serial bus technologies (and now Ethernet). Above those 'process controllers' are Human Machine Interface (HMI) connecting different PLCs and performing several control functions along with exchanging data with the applications. A factory floor is divided into cell-sites. The PLCs or other types of controllers are physically located close to the equipment in the cell-sites. The collection of monitoring, status and sensing data is first done on the site and then transmitted over secure channels to the data applications for aggregation and further processing. These applications can be hosted in remote cloud infrastructure, but are also often hosted within a limited domain environment, controlled by a single operator, like on-premise, at the edge or in a private cloud. Both options gain from infrastructure that scales out, has elastic compute and storage resources, so they will both be referred to as cloud in the following sections.

Functions in Industrial Control Networks What is changing now is that data applications are integrating process control functions to improve automation and to make real-time decisions, programmatically. The equipment control and collection of data generated by the sensors should be possible over small or large-scale deterministic networks as illustrated in .

Converged Cloud based Industrial Control Networks One particular motivation is to provide the behavior of a field bus between the cloud and the actuators/sensors with the same assurance of reliability and latency, albeit over wide area networks (WAN). This is evident from many industrial control applications, such as factory automation , PLC virtualization , power grid operations , etc. that are now expected to operate in the cloud by leveraging virtualization and shared infrastructure wherever possible.

Connected Process-Controllers, Sensors and Actuators Control systems comprise 'process controllers', Sensors and Actuators. The data traffic essentially carries instructions that cause machines or equipment to move and do things within or at a specific time. The connectivity exists in the following manner:

A 'process controller' interfaces with the sensors and actuators. It knows an application's performance parameters which are expressed in terms of network specific requests or resources such as tolerance to packet loss, latency limits, jitter variance, bandwidth, and specification for safety. The 'process controller' knows all the packet delivery constraints.
An actuator receives specific commands from the 'process controllers'. The Deterministic network between them should support control of actuating devices remotely from the 'process controller' while meeting all the requirements (or key performance indicators - KPIs) necessary for successful command execution. The actuator participates in a closed control loop as needed.
A sensor emit periodic data from the sensors. It may intermittently provide asynchronous readings upon request from the 'process controller'. Sensors may report urgent messages regarding malfunctioning in certain equipment, cell-sites, or zones.

In many control systems there is at least one 'process controller' (or server) entity on one end and two other entities - the sensors and actuators on the other end. The communication with sensors and actuators is through the 'process controller' entity; as such data applications do not directly interact with the field devices. Neither actuators nor sensors perform decision-making tasks. This responsibility belongs to the 'process controller'.

Generalized Communication Model To describe networked process control behavior a conceptual communication model is used so that the data applications do not concern with the details of the networks realizing operations and control. We refer to this model as operation and control network (OCN). The scope of the model is

Logical reference points: identify an endpoint's role or function as sensor-point, actuation-point, or operation & control point (oc-point for short). Note: term 'oc-point' is used to avoid confusion with network controllers and term 'fd-point' is used when both type of field devices are refered to.
Interface specification: in terms of associated traffic patterns between the endpoints as described below in . The interface may be any type of network (Ethernet, IP, wireless, etc. The model assumes that the network is capable of providing network services and resources necessary of the application specific operations and control.

Depending on the design of the usecase the process 'process controller' functionality (oc-point) may reside as a software module in the data application or as a separate module. When deployed as a separate module, another connectivity interface between the data application and oc-point will be needed and is out of the scope of this document. The applications will use an communication interface between oc-point and sensor-point to receive sensory data and similarly interface between oc-point to actuation-point to execute a single or a sequence of control instructions. This abstraction provides an additional layer of protection in the sense that the traffic patterns between the reference points are well defined so any exceptions can be easily caught.

Traffic Patterns For either local or wide area, the process automation activities over the network can generate a variety of traffic patterns between the oc-point and field devices such as:

Control Loops The equipment being operated upon is sensitive to when a command request actually executes. An actuator upon receiving a command (say a function code) will immediately perform the corresponding action. For several such applications, the knowledge of a successful operation is equally critical to advance to the next steps; therefore, getting the response back in a specified time is required, leading to a knowledge of timing. These types of bounded-time request and response mechanisms are called control loops. Unlike general purpose applications, commands cannot be batched, the parameters of the command that will follow depends on the result of the previous one. Each request in control loop takes up a minimal payload size (function code, value, device or bus address) and will often fit in a single short packet. In Detnet-enabled network, it can be imagined as a small series of packets with the same flow identifier, but with different latency constraints. It is required to support control loops where each request presents its own latency constraints to the network and where commands are small sized packets.

Periodicity Sensors emit data at regular intervals; i.e., there may be more tolerance to variations in jitter between the measurement intervals. Usually, 'process controllers' or applications listening to sensor data are programmed to tolerate and record intermittent losses or delay variations upto certain number of times. Therefore, time criticality is not always high. Notably, industrial software now increasingly rely on sensor data collection to monitor the state and behavior of the entire shop floor. Thus, the number of sensors are growing and the combined traffic volume generated by sensors is expected to be very high. In fact will contribute to a large percentage of ocn traffic. Moreover, the periodicity of each sensor will also vary based on the equipment. It is required that network capacity is planned appropriately for the periodic traffic generated from the different sensors. The periodic interval should also be preserved in the network because any variations could provide false indications that the equipment is misbehaving.

Ordering In real-time process control communications, out of order processing of related messages will lead to costly failures of operations. For example, Messages such as request and reply, or a sequence of commands to different endpoints may be related in some way. therefore, both time constraints and order must be preserved. The network should be capable of supporting sporadic on-demand short-term flows. This does not imply instantaneous resource provisioning, instead it would be more efficient if the provisioned resources could be shared for such asynchronous traffic patterns. Another consideration with ordering is that both actuators and sensors are low-resource devices. They can not buffer multiple packets and execute them in order while maintaining the latency bounds of each command execution. This means the network must pace packets that may arrive early.

Urgency Besides latency constrained and periodic messages, sensors also report failures as fault notifications, such as pressure valve failure, abnormally high humidity, etc. These messages must be delivered with utmost urgency and immediately.

Communication Patterns Control systems follow a specific communication discipline. The field devices (sensors and actuators) are always controlled, i.e., interact with the system through 'process controllers' in the following manner:-

Sensor to 'process controller': data emitted at periodic interval providing status/health of the environment or equipment. The traffic volume for this communication is determined by the payload size of each sensor data and the interval. These are a kind of synchronous Detnet flows but with much higher time intervals; still the inter-packet gap should be minimum.
'Process controller' to/from actuator: the commands/instructions to write or read. Actuators generally do not initiate a command unless requested by the 'process controller'. Actuators will often execute a command, read the corresponding result, and send that in response to the original write command. The traffic profile will be balanced in both directions due to requests/ response behavior. These are like asynchronous flows but without the observation interval constraint.

Gap Analysis Today, most of the operations and control solutions are split approaches. This means that the 'process controller' is on-premises close to the equipment, sensor data is first collected on-site, and then bulk transmitted to the enterprise cloud for further processing. To support delivering remote instructions to the machines over wide area networks using Deterministic Network data plane architecture and corresponding data plane DetNet over IP mechanisms apply as discussed in . Later in additional asks from DetNet are covered.

Deterministic Networks Relevance

Note: This section's text and explanation on DetNet can be removed.

A Simple DetNet-Enabled IP Network, Ref. RFC8939 | Appl. | +----------+ ............ ........... +----------+ | Service |<-: Service :-- DetNet flow --: Service :->| Service | +----------+ +----------+ +----------+ +----------+ |Forwarding| |Forwarding| |Forwarding| |Forwarding| +--------.-+ +-.------.-+ +-.---.----+ +-------.--+ : Link : \ ,-----. / \ ,-----. / +......+ +----[ Sub- ]----+ +-[ Sub- ]-+ [Network] [Network] `-----' `-----' |<--------------------- DetNet IP --------------------->| ]]> illustrates a DetNet-IP dataplane divided into two sublayers and is specified in . The forwarding sub-layer is responsible for resource allocation and explicit path functions, whereas the service sublayer provides packet replications, sequence numbering, and other functions. Within the Detnet nodes, resources are allocated a priori for a flow. The DetNet-enabled end systems originate traffic encapsulated with Detnet forwarding and service sub-layers; otherwise relay node will create those based on information received from the end system (via traffic classification). The DetNets support both asynchronous (by allocating resources for the observation interval) and synchronous (with repeating schedules) flow behaviors (Section 4.3.2 in ). The granularity of traffic treatment is at the flow level specified by 6-tuple information, including DSCP. Realistically, leveraging DetNets for Operations and Control (OCN) traffic patterns directly depends on how DetNets will provide network knowledge to the applications.

DetNet related Considerations and Dependencies A DetNet-aware node should express the network requirements as part of either forwarding sublayer or service sublayer. doesnot specify the interface to how those sublayers are mapped. This can be a non-trivial task, as an OCN-application, will first need some way to request its DetNet service provider or the operator. The DetNet operation is expected to allocate resources and return a mapping - possibly a DSCP (DetNet Qos) for each pair. This could be become a tedius scaling problem as the number of 'process controller'-device pairs start to grow or keep changing. Given that only DSCP is available, field-device pair can pose issues such as:

How can application request the proper network-resource for each command?
How can an application receive periodic data from sensors and with what interval?
What are the ways to differentiate a less sensitive (periodic) updates from urgent alarms.
Or how to differentiate data received from a sensor vs an actuator (with stringent latency requirements) and process them accordingly?

These issues are described below in more detail.

Operator vs Application view The DetNet data plane is designed with a network-operator-centric approach. The operator's view on dealing with large-scale networks is being discussed in . In order to use resources efficiently, flow aggregation is done. The operators in industrial control networks are not necessarily network experts; they simply need an application side tool to dispatch their request to the Deterministic networks' entry point. Especially, an OCN application may need many 'process controller'-field-device (ctrl-flddev) pairs to the be mapped to different aggregates. As the number of ctrl-flddev pairs grow, their variable traffic profiles can become hard to manage. The Detnet service provisioning internals should be transparent to an OCN application. This can be achieved with a common signaling or user-to-network interface between the applications and DetNet.

Flow reservation and classification Inside the DetNet, flow identification is done using IP header and DSCP information. These flow identifiers are then used by DetNet nodes to provide the corresponding traffic treatment. For a variety of commands and sensor data, latency or interval parameters will vary and DSCP maybe limited in expressing all the possible requirements. Encoding requirements into each packet explicitly can provide deterministic scheduling even in an otherwise link that can be congested when used with non-deterministic flows.

Split Traffic flows One of the most constrained design elements in today's industrial control systems is that data from the sensors is collected on-site and often aggregated before transporting to the cloud. Historical reasons for this approach do not apply anymore. Due to growth in sensor data, it now requires a much larger on-site storage infrastructure which is expensive. Applications also expect real-time streaming telemetry data. Although latency constraints are not as strict as for control loops, sensor data need to preserve periodicity (), thus could use DetNet service support. Leveraging DetNet could eliminate split traffic flows by collecting the sensor data by the applications. This also allows 'process controller's to be run and operated from the cloud platforms where much more powerful compute capabilities and available.

Provisioning for variety of Traffic flows Different operational scenarios have different constraints; even commands within the same application will have different time requirements.

Different types of latency bounds will be required between a 'process controller' and an actuator pair based on the type of end-equipment and precision requirements. Out-of-order message processing may lead to failures and shutdown of operations. Messages may also be correlated. Therefore, time constraints may be applied on a single message or on a group of messages.
Similarly, each sensor-'process controller' pair may come with its own interval requirement. Sensors emit data at regular interval but this type of information may not always be time-constrained. The gaps between the period can provide an indication to the 'process controller' about communication or other problems.
Additionally, some faults and alarm messages are urgent reports and must be marked and transmitted accordingly.

It is not clear if all these variations can be predictably resolved without any additional information offered to the DetNet forwarding plane. For example, if two independent OCN flowlets (that is, ordered group of packets that are related at process control logic) with variable bounded latency are classified to the same DetNet flow, they will receive the same treatment, regardless if one has the shorter latency than the other and may end up behind a flowlet with longer latency value. On the other hand, if an OCN flowlet have packets with different latency values, they could end up in different DetNet flow and may not reach destination in a specific order.

Security Operations and control networks also have split security boundaries. They have been designed to be air-gapped or secure by separation. Current systems have strict admission control, ingress and egress policies. From network layer security perspective, how DetNet-enabled network deals with security falls in the , the end systems expect those mechanisms in place. In particular if additional information is distributed for datapath decisions, integrity protection as per Section 7.2 of . The border gateways and firewalls will be more prone to errors related to provisioning churns if the system is dynamic or continuously changing. The transport layer deals with the end-to-end encryption. It should evolve to incorporate additional IoT-friendly(lightweight) protocols such as COAP, MQTT and their encryption mechanisms.

Summary of Gaps

Application view (: An OCN application is unaware of how DetNet services are provisioned. A common UNI between the applications and DetNet-enabled network needs to be added to the current framework to better map the expectations better.
Security (): of process control related metadata to be used by network must be secured.
Traffic behavior ( and ): Within the same DetNet flow, classified via 6-tuple, additional information/metadata must be supported so that dynamic traffic patterns can be scheduled deterministically.
Split traffic (): Leveraging DetNet should eliminate split traffic flows by direct collection of sensor data by the applications. This also allows controllers to be run and operated from the cloud platforms where much more powerful compute capabilities are available.

DetNet Potential Approach Remote process automation presents different types of traffic profiles and to deal with them within the DetNet framework, we discuss few possibilities. The DetNet UNI will enable applications to convey specific requirements to DetNet-aware Network. Note, that it is just an interface and blind to the internal implementation of such networks. The DetNet architecture does not describe how DetNet-aware node can design DetNet sub-layers. But even from the view of an end system the separation between forwarding and service sublayer functions should be maintained. This means, the DSCP should not be overloaded and DetNet-IP forwarding layer should be extended.

Application association to Forwarding sub layer Applications should convey specific resource requirements to the DetNets they connect to. There are two potential options: (a) The DetNet Relay-node performs translation and binding to one of the DetNet services in the DetNet; or (b) or carry the application defined data over DetNet as is and enable processing on transit nodes.

Encapsulation OCN applications are expected to be IP based end stations. (MPLS DetNet will not apply). It is also reasonable to assume that the data plane is IPv6 and Extension Headers are used for support in DetNet. The end system network requirement is expressed as 'OCN flow QoS'. Each packet carries its own unique OCN-QoS. The meta data to be transmitted to DetNet are: This can be implemented using the HBH extension header option.

Operation and Control Network Option (OCNO) The OCN Option (OCNO) is a hop-by-hop option that can be included in IPv6 for OCN traffic control extensions.

Explicit Traffic Control HBH Options

Option Type:

8-bit identifier of the type of option. The option identifier for the OCN Option (0x??) to be allocated by the IANA. First two bits will be 00 (skip over this option and continue processing the header.)

Option Length:

8-bit unsigned integer. Multiple of 8-octets.

OCN Function Flags:

Some flags require metadata, others dont. So process flags in order, if the flag is off, following metadata will not be present.

Flag	Description
U	send message immediately. its an alarm
P	periodic packet (intervals in ~ms)
F	part of flowlet. see Nonce and seq
L	bounded latency spec provided
R	Reliability with no packet loss tolerance
V	Delay variation with no packet loss tolerance

Flowlet nonce:

16-bit. identifies that a packet is associated to group of packets and shares fate. as an example, an application can set same nonce for a set of an actuator and sensors. when set to 0,flow id flowid is set to same value in related flows. when flow id is also 0, no relationship exists.

Flowlet sequence:

8-bit. sequence to be used for ordering with in flowlets.

Bound Latency Spec:

32-bit. Encodings, to be defined.
16-bit (upper bound), 16-bit (lower-bound). This field will provide upper and lower latency bounds describing the the latency bounds in milliseconds corresponding to the packet.

Delay Variation Spec:

16-bit. for synchronous stream, delay variation tolerance in ms.

OCNO Operation

An interface from 'process controller' to DetNet +-----+ +----------+ ............ ........... |Appl.| | OCNO-EH :--UNI-->: Service :-- DetNet -: Service : +-----+ +----------+ +----------+ +----------+ |IPv6 | | Ipv6 | |Forwarding| |Forwarding| +-----+ +--------.-+ +---.------+ +----------+ : : OCN scope : : : +..............+ : :--------------------------------------------: extended scope ]]>

OCNO Extension Header Signaling The current definition of OCNO only covers application to DetNet unidirectional interface. It is possibile to extend OCNO EH for conveying errors from DetNet to the 'process controller' as well - for example, when service violation happened in the DetNet, Relay node will set error flag in OCNO EH. Field devices are considered resource constrained and are not expected to insert or process extension headers. Due to flow aggregation, it is upto the DetNet operator to design mapping between an application and the DetNet. Two different options of carrying hop-by-hop options are considered. 1. EH is inserted by application and Relay node performs mapping to DetNet flow. 2. if the DetNet dataplane is IPv6, then EH can be carried all the way to last Relay node, which acts as gateway for the fld device and performs EH specific functions.

IANA Considerations To request an option code.

Security Considerations See section on security above.

Acknowledgements