## 5 Existing Surveys on RPL  
Several surveys have explored routing solutions for LLNs, with a particular focus on RPL. These surveys have shed light on the core intricacies of RPL and its deployment across diverse environments, providing valuable insights into the unique characteristics and inherent limitations of the protocol. Although these efforts have significantly contributed to the understanding of RPL, there is still a need for further research. A comprehensive examination of existing RPL surveys, presented in Table 7, highlights two essential aspects that require more exploration. Firstly, routing metrics that impact delivery reliability and efficiency within RPL objective functions in LLNs and MLLNs need to be comprehensively \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**RPL Objective Functions** & **R1** & **R2** & **R3** & **R4** \\ \hline [PERSON] et al. [118] & ✓ & ✗ & ✗ & ✗ \\ [PERSON] et al. [119] & ✓ & ✗ & ✗ & ✗ \\ [PERSON] et al. [120] & ✓ & ✗ & ✗ & ✗ \\ [PERSON] and [PERSON] [121] & ✓ & ✗ & ✗ & ✗ \\ [PERSON] et al. [122] & ✓ & ✗ & ✗ & ✓ \\ [PERSON] et al. [123] & ✓ & ✓ & ✗ & ✓ \\ [PERSON] and [PERSON] [124] & ✓ & ✓ & ✗ & ✓ \\ [PERSON] et al. [125] & ✓ & ✗ & ✗ & ✓ \\ [PERSON] et al. [126] & ✓ & ✗ & ✗ & ✓ \\ [PERSON] and [PERSON] [127] & ✓ & ✗ & ✗ & ✓ \\ \hline \hline \end{tabular}  
* considered; �—not considered; R1—malicious threats; R2—link quality; R3—energy level; R4—perform within node constraints.  
\end{table} 
Table 6: Analysis of RPL objective function solutions for MLLNs. explored. Secondly, further investigation is required into the path-finding methodology employed by RPL solutions, and their ability to make optimal decisions globally in real time while adapting to dynamic changes in the network. This underscores the need for a more thorough exploration of these aspects to provide a holistic understanding of reliability issues in RPL, encompassing various influencing factors and potential security threats in LLNs. Our survey aims to offer a detailed understanding of these crucial factors, including both malicious and non-malicious factors, providing valuable insights into RPL's routing metrics and path-finding methods. [PERSON] et al. [128] presented an in-depth survey and comparative analysis aiming to address the weaknesses of RPL. The discussion begins by providing an introduction to LLNs, their communication technologies, and the routing requirements based on the RPL standard. The survey intends to assess whether RPL extensions effectively overcome previously reported limitations related to its core operations, including objective functions, routing maintenance, and downward routing primitives. In conclusion, the survey underscores the need for further research efforts, particularly in addressing challenges hindering the adoption of the standard in large-scale deployments. The identified research directions aim to propel the understanding and improvement of RPL in real-world applications. [PERSON] et al. [129] conducted a systematic review of RPL-based routing protocols, providing technical insights and evaluations of various RPL implementations and optimisation approaches found in the literature. The review assessed the current state of RPL, emphasising its growing interest and relevance in IoT applications. The analysis showed a shift in focus over the years, from early concerns about energy-saving improvements to later considerations of additional functionalities and core design enhancements. The authors recognised RPL as a widely accepted IoT routing protocol and highlighted research trends, including industrial applications, cross-layer design, and security-enabled RPL. The authors also proposed a more flexible framework to address features such as congestion control and mobility, fostering interoperability among various RPL adaptations documented in the literature. [PERSON] et al. [130] focused on optimising the network topology in the context of RPL.