## 2 Review Methodology  
The methodology adopted for conducting this survey reflects the approach considered for executing an extensive review process on FS-enabled lightweight IDS for IoT, where the review process addresses two categories of research publications, i.e., existing survey papers and related technical papers. The adopted methodology aims for thorough and unbiased coverage of the available research publications to ensure credibility and reliability of reported findings. The methodology systematically identifies, selects, and critically appraises relevant research studies, followed by synthesizing the findings to provide a comprehensive overview of existing literature. An article selection criterion is devised that consists of journal articles and conference papers published in one of the most popular scientific databases, namely IEEExplore, ScienceDirect, Springer Link, and ACM, while the search process is also augmented with papers found through Google Scholar.  
The four scientific databases, namely IEEEXplore, ScienceDirect, Springer Link, and ACM, are the main sources from which relevant survey and technical papers are selected for the proposed study. However, to avoid bias in limiting the search space towards the four highlighted scientific databases, the Google Scholar search engine is considered to further expand the search process towards other sources having relevant publications. As a result, some additional relevant research papers are considered for the study that are found in other scientific databases, namely Wiley, MDPI, Taylor & Francis, Sage Journals, and court-ws. Hence, all survey and technical papers considered for this proposed study are peer reviewed and published papers.  
A two-phase article selection process followed by their study and analysis is performed as depicted in Figure 1. In Phase I, the process focuses on identifying and analyzing existing and relevant survey papers of lightweight IDS in the IoT environment, with a focused search process on FS-enabled lightweight IDS. This preliminary search process of Phase-I reveals the availability of limited survey works performed in lightweight IDS for IoT, whereas the identified works also lack in covering a comprehensive survey manuscript on FS-enabled lightweight security solutions tailored for resource-constrained IoT systems. This outcome is followed by extracting the motivation needed for producing the proposed study. In Phase-II, the process aims to study technical papers in the domain of FS-enabled lightweight IDS for the IoT ecosystem, where techniques adopted for building FS-enabled lightweight security solutions are investigated, compiled, and reported. FS-enabled IDS is identified in this work as being the fundamental approach adopted by researchers for building lightweight IDS for IoT environments. By that, search results are refined by including only those articles that present an IDS framework having FS-enabled lightweight capabilities to define the research direction. In this second phase, the search for articles also includes backward snowballing for identification and inclusion of more relevant papers in the technical papers set, where it defines a process of analyzing the reference list of each technical paper for identifying and inclusion of other relevant works.  
Subsequently, a refinement approach is applied over both phases that consist of removing non-English literature, duplications, and irrelevant literature. Lastly, the search keywords used in Phase-II encompass the following: (\"Internet of Things\" OR \"IoT\")  
Figure 1: Two-phase review methodology adopted for the survey on FS-enabled lightweight IDS in IoT.  
AND (\"Machine Learning\" OR \"ML\" OR \"Feature Selection\" OR \"FS\") AND (\"Intrusion Detection System\" OR \"IDS\") AND (\"Lightweight\" OR \"Light-weight\"), whereas the keywords (\"Survey\" OR \"Review\" OR \"Overview\") are added to the previously mentioned set for search processes executed in Phase-I.