A Review of Research Works on Supervised Learning Algorithms for SCADA Intrusion Detection and Classiﬁcation

: Supervisory Control and Data Acquisition (SCADA) systems play a signiﬁcant role in providing remote access, monitoring and control of critical infrastructures (CIs) which includes electrical power systems, water distribution systems, nuclear power plants, etc. The growing interconnectivity, standardization of communication protocols and remote accessibility of modern SCADA systems have contributed massively to the exposure of SCADA systems and CIs to various forms of security challenges. Any form of intrusive action on the SCADA modules and communication networks can create devastating consequences on nations due to their strategic importance to CIs’ operations. Therefore, the prompt and efﬁcient detection and classiﬁcation of SCADA systems intrusions hold great importance for national CIs operational stability. Due to their well-recognized and documented efﬁciencies, several literature works have proposed numerous supervised learning techniques for SCADA intrusion detection and classiﬁcation (IDC). This paper presents a critical review of recent studies whereby supervised learning techniques were modelled for SCADA intrusion solutions. The paper aims to contribute to the state-of-the-art, recognize critical open issues and offer ideas for future studies. The intention is to provide a research-based resource for researchers working on industrial control systems security. The analysis and comparison of different supervised learning techniques for SCADA IDC systems were critically reviewed, in terms of the methodologies, datasets and testbeds used, feature engineering and optimization mechanisms and classiﬁcation procedures. Finally, we brieﬂy summarized some suggestions and recommendations for future research works.


Introduction
Supervisory Control and Data Acquisition (SCADA) systems play a significant role in providing remote access, monitoring and control of critical infrastructures (CIs), which includes power systems, water distribution systems, gas plants, wastewater collection systems, etc. [1][2][3][4]. The stringent real-time requirements, growing interconnectivity, standardization of communication protocols and remote accessibility of modern SCADA systems have contributed massively to the exposure of the infrastructures to various vulnerabilities and security challenges such as sabotage, terrorism and intrusions [5][6][7][8]. Historically, when SCADA systems were initially deployed, the goal was to improve CIs' efficiency and effectiveness with little consideration of the potential future security challenges [9,10]. In fact, back in the days, SCADA security issues were majorly due to environmental challenges and equipment failures due to wear and tear. However, advancement in technology in recent years have shifted the focus to varieties of security challenges, which includes cyber intrusions and attacks [11][12][13][14]. Proprietary SCADA systems' components which includes the human machine interface (HMI), sensors, master terminal units (MTU), remote terminal ment in technology in recent years have shifted the focus to varieties of security challenges, which includes cyber intrusions and attacks [11][12][13][14]. Proprietary SCADA systems' components which includes the human machine interface (HMI), sensors, master terminal units (MTU), remote terminal units (RTU), etc. are vulnerable to different forms of intrusions [5,15,16]. Also, the various protocols being used for communication, which include Modbus, DNP3, Profibus, etc., can be remotely targeted, via cyberattacks [4]. Apart from severe operational instabilities, failures, financial losses, etc., SCADA systems vulnerabilities and security challenges can have serious devastating consequences on nations due to their strategic importance to the various CIs [9,17,18]. As explained by the authors in [19,20], documented SCADA systems cyberattacks such as the Stuxnet, Aurora, etc. have shown the grave harms that adversaries can accomplish. The ugly reality is that the attacks and intrusions that target SCADA networks and industrial control systems (ICS) are geometrically increasing in recent times. According to [21]  The prompt detection and classification of these SCADA security menaces has consistently been very challenging. With the huge cyber-presence from the various SCADA infrastructures across numerous modern ICS facilities, coupled with the voluminous data generated from the various SCADA sensors and other infrastructure, it is hugely vital to devise modern measures and models that can learn and discover irregular patterns in the SCADA system data and reach meaningful conclusions in the prediction, detection and classification of SCADA intrusions [23].
Historically, most of the proposed security models for traditional SCADA networks were based on statistical formulation theories. However, these models struggle in handling modern SCADA systems due to their complex nature. This limitation calls for better methodologies, such as data driven approaches like machine learning and deep learning methods. The data driven methods have better computing capacity to handle voluminous SCADA datasets, with huge number of features and variables. In fact, SCADA intrusion detection and classification (IDC) is currently one of the most significant areas of machine learning applications. This reflects the increasing number of publications involving machine learning models for SCADA security in recent years. Using a decade gap, and starting from the year 1991, Figure 2 presents an estimate of the number of machine learning- The prompt detection and classification of these SCADA security menaces has consistently been very challenging. With the huge cyber-presence from the various SCADA infrastructures across numerous modern ICS facilities, coupled with the voluminous data generated from the various SCADA sensors and other infrastructure, it is hugely vital to devise modern measures and models that can learn and discover irregular patterns in the SCADA system data and reach meaningful conclusions in the prediction, detection and classification of SCADA intrusions [23].
Historically, most of the proposed security models for traditional SCADA networks were based on statistical formulation theories. However, these models struggle in handling modern SCADA systems due to their complex nature. This limitation calls for better methodologies, such as data driven approaches like machine learning and deep learning methods. The data driven methods have better computing capacity to handle voluminous SCADA datasets, with huge number of features and variables. In fact, SCADA intrusion detection and classification (IDC) is currently one of the most significant areas of machine learning applications. This reflects the increasing number of publications involving machine learning models for SCADA security in recent years. Using a decade gap, and starting from the year 1991, Figure 2 presents an estimate of the number of machine learning-based SCADA security publications, whereby various machine learning models were proposed/deployed for solving and mitigating SCADA security problems including IDC. The publications' statistics in Figure 2 are acknowledged in terms of relevance using [24]. Logically, machine learning algorithms are not the definitive solutions to all SCADA security menaces. However, they present powerful set of tools that justify thoughtful consideration in dealing with intrusion menaces. Traditionally, based on the structure and characteristics of datasets, machine learning algorithms are grouped as supervised, semi-supervised and unsupervised learning. Supervised learning algorithms are designed to learn the correlations between features in the training dataset, in order to create a predictive model that has the capacity to infer annotations for another dataset with unknown annotations [23]. On the contrary, unsupervised learning algorithms do not need labelled output. Instead, the goal is to infer some underlying structure that is present within the set of input data points. Semi-supervised learning techniques fall between supervised learning and unsupervised learning [25]. Semi-supervised learning is a type of learning whereby the algorithm is trained upon a combination of labelled and unlabeled datasets.
Supervised learning is arguably the most prominent learning approach for SCADA security tasks. As illustrated in various SCADA security studies in the literature, researchers use the features in the SCADA dataset as training set to infer a model for predicting the annotations of the features in the testing dataset. This paper presents a critical review of recent research works whereby supervised learning algorithms ranging from artificial neural networks (ANN), k-nearest neighbors (k-NN), etc. were modelled for SCADA IDC.
Several articles in the literature has surveyed SCADA security from different viewpoints. Ferrag et al. [26] surveyed SCADA cyber-security challenges and discussed several solutions including data mining solutions. Similarly, Upadhyay [3] reviewed SCADA vulnerabilities and recommendations to strengthen SCADA security. the authors in [27] reviewed SCADA cyber-security risk assessment methodologies. Tariq et al. [1] reviewed cyber threats and defense mechanisms for securing SCADA-based CIs. Rakas et al. [16] focused on network-based solutions for SCADA intrusions. Yaacoub et al. [20] reviewed several security features including vulnerabilities, threats and intrusions. Rezai et al. [10] reviewed key management challenges in SCADA systems. Furthermore, the authors discussed several applications, technologies, standards and communication protocols while identifying and analyzing their limitations. Ahmim et al. [28] also presented a review on intrusion detection systems for SCADA systems. In contrast to other review works in the literature, we critically review, analyze and compare different supervised learning techniques for SCADA IDC, focusing on the methodologies, datasets and testbeds used, feature engineering and optimization mechanisms and classification procedures. The paper aims to contribute to the state-of-the-art research and recognize critical open issues. We intend to provide a comprehensive summary of research and trends for researchers working in the area of ICS/SCADA security. Finally, we briefly summarized some suggestions and recommendations for future research works.
The rest of the paper is structured as follows. Section 2 briefly describes the methodology adopted for the review. Section 3 presents a brief overview of modern SCADA architecture. In Section 4, we analyze and discuss some prominent supervised learning for SCADA security. In the Section, we analyze the processes of using supervised learning algorithms for SCADA IDC, especially in terms of datasets and testbeds used, feature engineering and optimization mechanisms and classification procedures. In Section 5, we summarized some suggestions and recommendations for future research works. Finally, the last section presents the conclusions.

Materials and Methods
In this paper, the authors conduct a comprehensive state-of-the-art review of recent research studies whereby supervised learning techniques were modelled for SCADA IDC. The state-of-the-art research was conducted using popular databases, which include MDPI database, IEEE Explore database, springer database, google scholar database, Wiley online library, ACM digital library, Elsevier database, etc., in order to search for relevant publications. The methodology and criteria used to include a published paper in this work include, but are not limited to, a focus on solving SCADA intrusion detection problem using supervised learning model. For the publication time span, the focus was mainly on recent research works, from the 2018 to 2021. Multiple searches were performed between January 2021 and July 2021. By focusing on recent trends on supervised learning algorithms for SCADA security, the paper intends to provide help to researchers working in the field of ICS security.

Brief Overview of Modern SCADA Architecture
Typical modern SCADA system combines hardware components and software programs that operate in a pervasive manner. The components are interconnected using varieties of wired and wireless communication standards. Major architectural components of a modern SCADA system include: field devices, RTUs, MTUs, programmable logic controllers (PLCs), intelligent electronic devices (IEDs), HMI, Historian, etc. [5,16]. Figure 3 presents the simplified architectural structure of a SCADA system. A typical SCADA network collects data from the field devices (sensors, actuators, etc.), which directly engage with the CIs physical equipment such as pumps and valves [29]. The RTUs, PLCs, IEDs assist in retrieving real-time data from the field devices, which control and monitor the actions of the CIs' process [30]. Modern day RTUs, IEDs, etc. are technologically advanced. They send the received data to the MTUs for analysis and processing [3]. Basically, MTUs are the central monitoring station and they assist in the distribution of control commands and data from field devices to control centers. The overall control of the operation is conducted in the control centers, which consist of computers, databases, servers, HMIs, etc. The statuses of the monitored and controlled physical processes are presented on the HMI consoles [2]. Moreover, HMIs present a graphical display of various emergency notifications, such as alerts and warnings, which allow operators to interact with the systems [7,31]. Historians are databases that store the various data gathered by the SCADA systems. The stored data in the historians are used for purposes such as auditing and analysis. As SCADA networks are increasingly adopting certain technologies, vulnerabilities, intrusions and cyberattacks are increasingly becoming major challenges [32,33]. Yadav and Paul [19] explained that, in order to support effective remote access, some of the SCADA nodes are connected to the internet, which exposes them to varieties of network based attacks. In recent years, it has become increasingly challenging to protect SCADA systems using traditional methods. As viable alternatives, data mining and analytics methodologies which include machine learning and deep learning models are continuously being proposed as they present exciting advantages, such as high performance, high speed of execution and effectiveness. Generally, data mining and analytics involve the process of learning and identifying patterns or trends in datasets [36].

Supervised Learning for SCADA Security
Supervised learning algorithms are aimed at training labelled input data for a particular output [25,37]. The algorithms are trained to detect some underlying patterns between the input dataset and the output labels, which allows them to successfully label unlabeled dataset [31]. Based on the mode of the learning task, supervised learning algorithms are basically categorized into regression and classification. While the output for the regression category takes continuous values and contains an interval on the real line, the output for classification type takes categorical values and they are tagged class labels. Popular supervised learning algorithms include k-NN, ANN, random forest (RF), Bayesian networks (BN), decision tree (DT), etc. Supervised learning algorithms have been consistently and successfully proposed in solving various non-linear problems and fields, such as bioinformatics, handwriting recognition, spam detection, and SCADA IDC. In the literature, various SCADA security researchers have proposed and modelled numerous supervised learning algorithms such as ANN, NB, etc. for SCADA IDC. As explained in [5], data mining and analytics approach for SCADA IDC involves three major processes: Testbed design/dataset generation, feature engineering/preprocessing and prediction/classification/detection.

Datasets Generation Mechanism Overview
The most important element for the utilization of supervised learning algorithms in SCADA security studies is the deployed dataset/testbed. For the effective application of the various supervised learning algorithms, it is important to have sufficient dataset for the algorithm's training process. Researchers use the features in the SCADA data as training dataset to induce a model for predicting the feature instances in the testing dataset. In the literature, different types of datasets have been deployed in the supervised learning based SCADA security studies. While several authors used various open source datasets such as the popular Knowledge Discovery and Data Mining (KDD99) dataset [38], UNSW-NB15 dataset [39], iTrust SWAT dataset [40,41], etc., others have modelled different SCADA imitation testbeds using hardware devices and/or software simulators for generating datasets. A detailed review of cyberattack simulation tools was conducted by Nazir et al. [42]. During testbed simulations and dataset generation, diverse attack scenarios are modelled to create well-structured and balanced datasets that mimic real life SCADA intrusion event situations. Some of the popular cyberattacks are denial of service (DoS), false data injections, man-in-the-middle (MITM), etc. [30,43]. To generate a dataset, the authors in [44] and [45] used the popular open source network simulator, Network simulator 2 to model a SCADA system. The authors in [6] used Omnet++ simulator to model a SCADA system. In a related work, Queiroz et al. [46] used SCADASim for building SCADA system. A detailed review of SCADA simulators were done by Mathioudakis et al. [47]. Using a different approach for generating SCADA dataset, the authors in [48] used virtual host Nova and PLC by HoneyD. Similarly, Yang et al. [49] set up several SUN servers and workstations in their security work. The authors in [50] simulated a testbed involving both hardware and software tools which include Allen Bradley PLC controller, Ethernet network, Nmap, Nessus, MetaSploit, etc. to generate SCADA dataset. Branisavljevic et al. [51] used the Belgrade sewage system as testbed to generate SCADA dataset. Yadav [7] presented a detailed review and comparison of several SCADA testbeds developed by various researchers.
Due to the scarce availability of resources among other factors, numerous authors in the literature make use of open source data in their SCADA security studies. The first open source intrusion detection dataset was generated in February 1998 by Defense Advanced Research Project Agency (DARPA), in the MIT Lincoln Laboratory and they initially created the KDD98 dataset [52]. The dataset containing simulated intrusions is made up of network traffic and audit log files. The KDD98 dataset was used for creating the KDD99 dataset. Of all the public datasets used in supervised learning algorithms based SCADA security studies, KDD99 is the most deployed dataset. The dataset was generated from five weeks of experimentation to generate training and testing datasets. Furthermore, the KDD99 attack vectors are categorized as DoS, Probe, Root 2 Local (R2L) and User 2 Root (U2R) [38]. The authors in [48,49,[53][54][55] and several others deployed KDD99 dataset in their SCADA security studies. However, the dataset has been heavily criticized for having a lot of duplicate and redundant records. To improve on some of the KDD99 dataset's limitation, the NSL-KDD dataset was developed by Tavallaee et al. in 2009 [56]. The NSL-KDD dataset does not contain a huge amount of duplicate and redundant packets. Another public dataset, popular among supervised learning algorithm-based SCADA security studies, is the SWAT dataset developed by the iTrust Centre for Research in Cyber Security, Singapore University of Technology and Design (SUTD) [41]. The SWAT testbed framework for the dataset generation is a water distribution system, which is made up of a six-stage process that imitates an actual water treatment facility. For the dataset generation, operation was run for 11 consecutive days, seven days out of the 11 days were run as normal operation while the remaining four days were run under attack scenarios. The authors in [13,[57][58][59][60][61] and several others used the SWAT dataset in their research works. Another dataset that is popular within supervised learning algorithms based SCADA security studies is the Mississippi State University (MSU) gas pipeline system SCADA dataset [33]. Table 1 presents a comparison of some popular open source datasets that are commonly deployed in studies involving supervised learning techniques for SCADA IDC. Five datasets were developed: Power system datasets, Gas pipeline datasets, Energy Management System (EMS) dataset, New gas pipeline datasets and Gas pipeline and Water storage tank datasets. The three Power system datasets comprises electric transmission system normal, disturbance, control, cyber-attack behaviors data. The EMS dataset consist of a voluminous anonymized log file that are recorded over 30 days interval. The Gas pipeline, Gas pipeline and water storage tank and New gas pipeline datasets is made up of packets captured from control devices and the HMI in a gas pipeline testbed. The Gas Pipeline and Water Storage Tank datasets has additional packet data captured from a water storage tank.
Some of the attacks include data injection, relay setting change and remote tripping command injection.

Feature Engineering and Optimization Mechanism
In SCADA security studies involving the use of data mining and analytics methodologies for the prediction, detection and classification of intrusions, the use of feature engineering mechanism for the voluminous dataset(s) is highly important as they have significant impact on the results [4]. The authors in [32] explained that feature engineering tools, such as feature extraction, weighting, selection, reduction, etc. assist in improving the performances of classifiers. Further, considering the problem of imbalanced datasets which is very common with SCADA datasets, feature engineering tools assist in making the classification tasks computationally easier. In the literature, different models and algorithms have been proposed as feature engineering tools in SCADA security studies involving supervised learning algorithms. Waghmare et al. [73] deployed principal component analysis (PCA) for feature reduction for a SCADA dataset. The authors in [74] used PCA, generalized hebbian algorithm, independent component analysis (ICA), singular value decomposition and self-organizing map as feature reduction tool in their SCADA security study. The authors in [75] also used PCA, ICA and canonical correlation analysis for a similar task. In [76], the authors used models based on function code, time factor, etc. as feature extraction tools. In another study, information gain and singular value decomposition was used as feature engineering tools by the authors in [77]. Similarly, InfoGainAtributeEval, information gain and filter based approach were used as feature engineering tools in [78]. For improved detection and classification, several authors deployed meta-heuristic algorithms such as genetic algorithm (GA), particle swarm optimizer (PSO), etc. as feature engineering and optimization tools. The authors in [32] used GA as feature weighting tool. Similarly, the authors in [59] used GA as feature engineering tool for ANN in classifying SWAT dataset. The authors in [57] used PSO for optimizing the parameters for back-propagation neural network. In related works, the authors in [79,80] used PSO as feature engineering tools in separate security studies involving supervised learning algorithms.

Classification Mechanism
Various prominent supervised learning algorithms such as SVM, NB, RF, etc. have been proposed in several intrusion prediction, detection and classification of SCADA datasets and other problems that needed to be addressed. In some of the reviewed articles, while some authors proposed singular supervised learning models [32,57], several authors [81][82][83][84] ensemble two or more models, with the aim of achieving improved performances. Moreover, some authors compared several supervised learning models on specific dataset(s), in order to establish the best possible model for the analyzed SCADA system and testbed. Additionally, some authors do the comparative study to prove that the individually developed supervised learning model has the capacity to achieve exceptional results within specific context. In this section, some of the widely deployed supervised learning algorithms are discussed.

Support Vector Machine (SVM)
Proposed by Vapnik in the early 90s, SVM is a popular supervised learning method that has consistently been deployed in varieties of classification and regression studies [85]. Most SCADA IDC tasks are binary classification tasks, which makes SVM models suitable for the task. SVMs create an hyperplane, or sets of hyperplanes, in a high-dimensional feature space, which optimally separates the training patterns based on the classes [4,84]. SVMs are robust to high dimensional data and they are well known to have good generalization ability. Even with data that are non-linearly separable in feature space, SVMs has the capacity to perform exceptionally well. Commonly used kernel methods for SVMs are linear, polynomial and sigmoid [32]. The two popular variation of SVMs: one class support vector machines (OCSVM) [86] and support vector data description (SVDD) [87] have been widely proposed by various authors in numerous SCADA security studies. Schuster et al. [88] and Yasakethu et al. [89] deployed OCSVMs in constructing models for the detection of intrusions in a network data. The authors in [90,91] modelled K-Means clustered OCSVM for the classification of SCADA intrusions. Similarly, OCSVM model was also modelled in [92,93] and [51]. Lee et al. [94] developed an OCSVM based detection model with the capacity to learn header-based whitelist and payload and experimented it on a testbed dataset. Maglaras et al. [35] modeled a OCSVM model for detecting cyberintrusions in a designed small SCADA testbed. Prisco and Duitama [95] also proposed OCSVM for detecting intrusions on SCADA network. In a related work, Fang et al. [96] modelled a support vector regression for predicting a SCADA monitoring data. Terai et al. [97] and Waghmare et al. [73] developed SVM models for detecting intrusions and achieved remarkable results. Wang et al. [85] developed a SVM model for classifying faults in SCADA power system dataset. In a comparative study, the authors in [98] modelled several supervised learning, which include OCSVM, DT, k-NN, etc. for classifying SCADA dataset. From the study, OCSVM presented the best result. In a related work, the authors in [99] did a comparative task using SVM and RF models for classifying two different datasets. Inoue et al. [58] also compared the effectiveness of OCSVM model with deep neural network model in classifying the SWAT dataset. Similarly, the authors in [100] did a comparison work using OCSVM and SVDD models in classifying SCADA system intrusion. For improved classification performance, the author in [101,102] did a hybrid task involving the ensemble of SVDD and kernel principal component analysis (KPCA) for the classification of SCADA intrusions using the MSU gas pipeline dataset.

Artificial Neural Network (ANN)
ANN is a supervised learning algorithm that is based on the structural framework of the neurons and they have the learning and processing capacity that is similar to a small scale human brain [103]. Similar to SVM, ANNs are quite robust to high dimensional data and they have good generalization ability. The simplest form of ANNs are the perceptrons and they are very useful for the classification of patterns that are linearly separable [84]. In recent years, ANN has been consistently deployed in various SCADA security studies. Shalyga et al. [59] combined GA and ANN for classifying the SWAT SCADA dataset. Neha et al. [104] similarly developed a classification model based on a hybrid salp swarm optimization with ANN. The authors evaluated the developed model using the KDD99 dataset. Also, Demertzis et al. [105] presented an ANN based detection model using 3 SCADA datasets for the evaluation. The authors in [44] used weighted particle based cuckoo search optimization based artificial neural network model for classifying SCADA intrusions. Reuter et al. [31] proposed an artificial neural network based anomaly detection for the CSE-CIC-IDS 2017 dataset. Li et al. [106] modelled a back propagation neural network for analyzing the health assessment for fault and intrusion using wind turbines SCADA dataset. Likewise, Zhang [107] proposed an ANN prediction model with the capacity to generate warning and alarm for wind turbine using a stored SCADA data. With regards to ensemble models, Kosek et al. [108] achieved remarkable results from the ensemble of multiple ANNs for a photovoltaic SCADA dataset. Similarly, Yan et al. [109] developed a multilayer neural network and RF algorithms for detecting intrusions in a wind turbine SCADA dataset.

Decision Tree (DT)
DTs are tree based models. DT is a popular supervised learning method that has consistently been used in varieties of classification and regression studies [110]. Among the popular machine learning models, DT is one of the simplest to understand, visualize and evaluate. As they are non-parametric, outliers do not have much effect on DT models and they also perform reasonably well with linearly inseparable data. Depending on the splitting criteria, popular variation of the DT algorithms include: ID3, C4.5, C5.0 and CART [12]. In recent years, DT has been consistently deployed in various SCADA security studies. McNabb et al. [111] developed a classification model based on DT for predicting intrusions in a SCADA and wide area measurement system dataset. Upadhyay et al. [112] developed a DT classification model for classifying intrusions into power system SCADA network. Mrabet et al. [113] developed a DT based model for detecting intrusive signatures in phase measurement unit dataset. In relation to the ensemble model, Al-Asiri et al. [114] used WEKA to model a couple of DTs in classifying the MSU gas pipeline dataset. Similarly, Siddavatam et al. [115] developed a DT and RF models for detecting intrusions in SCADA traffic. In a comparative study, Swetha and Meena [116] developed a DT, k-NN and SVM models for classifying the KDD99 dataset. From the study, the best results were achieved using DT and SVM. However, it is well-known that DTs struggle in handling high dimensional data as it takes quite a considerable time to build the tree model. Moreover, without proper pruning DTs can easily overfit, a reason several researchers usually opt for the ensemble models of DT, well known as Random Forests.

Random Forest (RF)
Developed by Breiman, RF is an efficient supervised learning algorithm that is widely used in various classification studies [81]. RF is an ensemble method that are constituted by a set of tree structured classifiers. RFs operate by training a couple of DTs and they return the class with the majority over all the trees in the ensemble [109]. Tamy et al. [81] explained that DTs are combinations of sets of tree predictors whereby individual tree depends on the values of a random vector that is sampled independently with the same distribution for the trees that makes up the forest. Rakhra et al. [110] explained that RFs are collections of multiple DTs whereby the challenge of overfitting which is popular with singular DTs is solved by a voting method, in which the most voted class is the final result for the target observation. Generally, RFs are well-known to be fast, scalable and robust. RFs have been proposed numerously by various authors for SCADA IDC. Alhaidari and AL-Dahasi [83] modelled three supervised learning algorithms: RF, J48 and Naive Bayes for detecting DoS intrusion patterns in SCADA system. Using the KDD99 dataset for the evaluation of the three modelled algorithms, RF presented the best result. Similarly, Choubineh et al. [117] modelled five learners; DT, RF, Naive Bayes, BayesNet and OneR for the classification of gas pipeline SCADA dataset. In the study, the authors deployed cost-sensitive learning with fisher's discriminant analysis to overcome the class imbalance problem. In another related work, the authors in [118] modelled several learners which include RF, SVM, Naive Bayes, OneR, J48, NNge for the detection of intrusions in SCADA network. From the study, RF and NNge presented the best results. Tamy et al. [81] also did a comparative study involving RF, Naïve Bayes, SVM, and J48 Tree using MSU gas pipeline dataset. The authors in [99] also did a comparison work involving two supervised learning algorithms; RF and SVM for the classification of two SCADA datasets. Similarly, Hink et al. [14] successfully explored different learners including RF, SVM, etc. on a SCADA power system dataset.

Bayesian Networks (BN)
BN, otherwise known as belief or bayes network is a probabilistic graphical model that represents variable sets whose conditional probability relationship are represented as a directed acyclic graph with nodes and edges [119,120]. BNs have been explored in varieties of SCADA security studies. Huang et al. [121] proposed a BN model for the security assessment of SCADA network. In a similar study, Shin et al. [122] also modelled BNs for the security evaluation of nuclear SCADA systems. Zhang et al. [123] developed a BN model for evaluating the probability of successful intrusions into SCADA system. In the study, the authors considered six intrusion scenarios which have disturbing effects of tripping circuit breakers. In a related work, the authors in [124] developed two BN models for intrusion procedure illustration and for evaluating the probability of successful intrusions into SCADA network. In the study, the authors considered four intrusion scenarios for the SCADA system cyber-components. Likewise, the scenarios have the effect of tripping circuit breakers. Despite the numerous successes achieved using BN for SCADA security, Borujeni et al. [119] argued that their implementations involve huge computational requirements, especially in fields, such as modern SCADA/ICS systems as there are vast number of nodes involved. BN are known to perform poorly with high dimensional datasets.

K-Nearest Neighbors (k-NN)
k-NN is a distance based supervised learning algorithm which is well known for its simplicity. k-NN's simplicity comes from the fact that it is easy to interpret and its low computation time [125]. Also, they are generally referred to as lazy learners. They use data to classify unforeseen data points by evaluating the distances from the neighbor points [126,127]. They can be deployed for both classification and regression tasks. They are well-suited for multi-modal classes and multi-label applications. In classification and regression tasks, k-NN's performance depend on the choice of the 'k' value. Usually, in different studies, computationally expensive methods such as cross validation are normally used for choosing effective 'k' value. k-NNs have been proposed numerously by various authors for SCADA IDC. Gumaei et al. [128] developed a SCADA intrusion detection model that is based on a k-NN model. In the study, the authors used correlation-based feature selection method as feature reduction technique for the numerous power system SCADA datasets deployed. The authors in [125] modelled five algorithms: k-NN, DT, RF, AdaBoost and Naive Bayes for classifying intrusion into power systems SCADA network.
Similarly, Robles-Durazno et al. [82] modelled k-NN, SVM, DT, ANN and Naive Bayes for detecting intrusions in a simulated water system testbed. The authors realized the testbed using Festo MPA Control Process Rig. From the experimentation, KNN and SVM presented the best results. Also, Phillips et al. [4] modelled a couple of classifiers including k-NN, SVM, DT for detecting anomalous traffic in SCADA systems. Similarly, Arora et al. [127] modelled k-NN, RF, SVM, ANN, DT and Naïve Bayes for detecting intrusions on a SCADA/ICS benchmark dataset. From the study, the best accuracy results were achieved using k-NN and RF. Table 2 presents the comparison of recently proposed supervised learning based approaches for SCADA systems IDC. Apart from providing key details on the dataset/testbed used, the CI domain and the feature engineering techniques used, Table 2 presents a brief key analysis on the strength and drawbacks from the methodologies. Some of the models proposed in the articles reviewed in Table 2 are developed based on several criteria such as the resources available to the researchers, the specific goal(s) the researchers aims to achieve, etc. Some of these points determine the specific testbed or dataset, choice of methodology procedures, optimization technique(s) deployed, etc. in the research works. Furthermore, the factors that determine the classification and computational performances of the supervised learning algorithms depend on varieties of factors such as the characteristics of the deployed dataset, the pre-processing procedures, the computer and processor specifications, the choice of the learning model(s) parameters, etc. Hence, in different comparative studies involving different supervised learning models (as depicted in Table 2), there are contrasting results' performances from some of the prominent supervised learning models.

Suggestions and Recommendations for Future Works
Despite the numerous successes achieved from the application of several supervised learning algorithms in different SCADA IDC studies, there have been several challenges and open issues that can be considered in future research to improve detections and classifications of intrusions. Considering that SCADA IDC significantly depend on the deployed dataset(s), the current use of publicly available datasets, most of which are obsolete and irrelevant with regards to current and future attack and intrusion trends have continued to show irrelevancy in modern-day SCADA IDC. With the geometric increase in technologically advanced SCADA cyber-intrusions globally, modelling mitigations and solutions to an outdated problem is inapt. Further, for the current SCADA systems threat environments, the various simulation testbed set ups do not inclusively reflect modernday network traffic including some of the modelled footprint attacks. Therefore, future research works should focus on realistic modern day SCADA system testbeds with modernday attack scenarios for experimentation and evaluations. Furthermore, as ensemble methods have shown significant trends in producing effective SCADA IDC results, future researches should dedicate more attention on hybrid intrusion detections and classifications models. Last, a high intrusion detection and classification performances are obviously important. However, if the modelled single or ensemble supervised learning algorithms are computationally complex and they take too much time to detect intrusions, severe damage could have been done on the deployed CIs before the security measures produce results. Therefore, future works should always consider the computational complexities of developed models.

Conclusions
Security is a major issue in modern-day SCADA system operations as the networks are constantly under high threats of sophisticated intrusions and attacks. Modern-day SCADA systems security measures are expected to cope with intrusions with adequate and efficient mitigations methods that meet the present and future SCADA security demands. This paper reviewed recent state-of-the-art security research works whereby supervised learning algorithms were implemented for SCADA IDC. Through extensive research and analysis, the paper addressed and compared the methodologies applied in using supervised learning for SCADA systems security in terms of the datasets and testbeds used, feature engineering and optimization mechanisms and classification procedures deployed. Furthermore, open issues and challenges for using supervised-learning techniques for SCADA security are discussed. Some future recommendations for future research works were presented as well.