## 5 Existing Surveys on RPL
They consider important factors such as the application goals, communication security, and node mobility within the network. To lay the groundwork for their exploration, the authors first revisit fundamental aspects of RPL. They then provide a comprehensive review of objective function implementations proposed to optimise the routing topology construction process. In addition, they uncover the principal threats prevalent in the RPL landscape and discuss proactive efforts and countermeasures devised to effectively mitigate these security challenges. The authors also highlight the dynamic nature of node mobility within RPL networks, emphasising the importance of considering mobility, and shedding light on the enhancements that have been made to the RPL framework to address the challenges associated with the movement of nodes. [PERSON] and [PERSON] [131] provided a comprehensive exploration of the RPL routing protocol, addressing both foundational aspects and practical considerations with a specific emphasis on its applications in IoT and associated challenges. The survey is divided into two main sections. The initial part introduced the survey, reviewed the related literature, and delved into the scientific background and main features of the RPL routing protocol. The subsequent part shifted focus to the latest advancements in RPL research, navigated through challenges in RPL routing, particularly within the IoT landscape, and engaged in discussions while offering additional analysis. The critical role of RPL routing in IoT environments, which are characterised by unreliability and multi-hop interference, is emphasised. The survey also reviewed various routing protocols tailored for LLNs and outlined open research issues and future directions in the field. [PERSON] et al. [132] presented a comprehensive review of the current and recent advancements in RPL. The focus extends to surveying existing approaches and protocols, aiming for a comparative analysis of the utilised objective functions. The paper summarised the characteristics of main routing metrics, facilitating a comparison that underscoresthe need for further research on the validity and effectiveness of existing metrics or the development of new ones. As part of future work, the authors proposed an investigation into RPL performance in high-density networks using two objective functions, namely MRHOF and OF0, in various topologies. [PERSON] et al. [16] focused on examining the security aspects of LLNs in the context of the IoT. They emphasised the importance of identifying and analysing security attacks on RPL, which is crucial for enabling communication among IoT devices. The paper classified attacks into three main categories: attacks against resources, attacks against the topology, and attacks against network traffic. Attacks on resources involve actions that reduce the network lifetime, while attacks against the topology aim to manipulate the network configuration. Attacks on network traffic focus on capturing and analysing a significant portion of the traffic. The paper also discussed various techniques to protect RPL topologies and highlighted the critical role of secure routing in ensuring the seamless and safe functioning of IoT networks. Additionally, the authors emphasised the need for a universal solution that can be applied to all routing attacks, both present and future. [PERSON] et al. [76] provided a detailed analysis of RPL, focusing on its usage and evaluation in several published works. The survey finds that RPL has been extensively evaluated through both test-bed experiments and simulations in various application domains. Notably, many evaluations have used prototype implementations on real embedded devices, with ContikiOS and TinyOS being the most popular implementations. The results of the survey indicate that many optional RPL functionalities are not well-supported or necessary in many scenarios, which raises questions about their relevance and impact on the protocol's simplicity and interoperability. [PERSON] et al.