## 4 Resource Constrained IoT and Vulnerability to Cyber-Attacks  
The emerging IoT paradigm is considered a natural evolutionary step of the Internet that can create a worldwide infrastructure interconnecting living things, machines, and computing devices to each other [52]. IoT is a network of heterogeneous physical devices capable of co-existing with the current internet infrastructure and is gaining rapid popularity and adoption [53]. The internet digitizes a wide range of services but remains mainly human-centric. However, the IoT concept is to have smart sensors collaborating directly with each other without any human involvement. IoT is a vision of pervasive computing where smart devices seamlessly connect. In the IoT context, smart devices are known as those devices that usually include processing, memory, and communication resources. An illustrative diagram of a typical IoT ecosystem is given in Figure 2, presenting the components commonly found in an IoT system. The plant represents the physical environment that is to be monitored, such as agriculture fields, manufacturing plants, power grid generators, home appliances, and human beings [54]. The IoT devices that are generally deployed near the plant can be classified as IoT end devices or IoT gateways. IoT end devices host the sensors, processing unit, and a low-powered communication module [55]. Whereas the IoT gateways, depicted as hubs, act as aggregators for the data generated by the IoT sensors and also provide the first level of data connectivity between the sensing nodes and the rest of the network [56]. Both IoT end devices and IoT gateways are generally known to be resource-constrained devices within the IoT ecosystem as opposed to the conventional IT networks in which such devices are known to have abundant amounts of resources [57]. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Survey** & **Thereats at** & **Discussion** & **ML-** & **Enabled** & **Review on** & **Discussion** & **Discussion on IDS** & **Feature on IDS** & **Este** & **Este** & **Explainable** & **Performance Analysis of Feature Selection** \\
**Papers** & **IoT Layers** & **on ML-** & **Enabled** & **DIs** & **Datasets** & **InDS** & **Lightweight** & **IcDIs** & **niques** & **Seletion** & **AI** & **Selection Methods** \\ \hline [49] & **✓** & **✓** & **✓** & **x** & **✓** & **x** & **x** & **x** & **✓** & **✓** \\ \hline [50] & **✓** & **✓** & **x** & **x** & **x** & **x** & **x** & **x** & **✓** & **x** \\ \hline [51] & **x** & **x** & **✓** & **x** & **✓** & **✓** & **x** & **x** & **x** & **x** & **x** \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._  
Figure 2: Generic IoT architecture, encompassing end devices and gateways as resource-constrained components. IoT is building a worldwide infrastructure that will influence all facets of our lives, including agriculture, mining, health services, manufacturing energy production, energy distribution, and transportation. The rate of IoT devices is increasing rapidly, encompassing multiple domains including home appliances, healthcare, and transportation [58]. IoT is one of the fastest-growing technologies and is employed in many areas to facilitate the needs of consumer and industrial networks [59]. Selected application domains of IoT are presented and discussed in Figure 3, which includes healthcare, manufacturing, logistics, and agriculture and transportation domains. Other popular domains include environment monitoring control and management, reducing loss of electricity in the energy sector, and defense. Table 2 provides an outlook of several existing IoT end and gateway devices that are manufactured by five different manufacturers, where the information in this Table is prepared based on the publicly available information from the official company's website. The portrayed devices are being classified as potential resource-constrained IoT devices due to a limit in one of their resources, namely the CPU's clock rate, memory capacity, and limited power source.