## 4 A Routing Protocol for LLNs (RPL)
### RPL Objective Functions for Non-Malicious LLNs
#### 4.2.3 Fuzzy Logic Method
FLEOF aims to address the limitations of RPL's main objective functions, OF0 and MRHOF. Although FLEOF shows promise, it is still at the conceptual stage and has not been implemented yet. Future research will focus on conducting extensive simulation evaluations to determine FLEOF's effectiveness compared to other related works. These fuzzy logic solutions [112; 113; 114; 115] take into account link and node metrics to improve routing reliability, in addition to making global optimum decisions for path selection. However, the main drawback of the fuzzy logic method is the complete dependence on human knowledge and expertise [116; 117]. Fuzzy logic relies heavily on human knowledge and expertise to determine the membership functions and fuzzy rules. This can lead to bias and subjectivity in the decision-making process. For example, the author of OF-Fuzzy [112] considers the ETX value small if it is three or less, while any ETX value of 12 or bigger is considered a high value and builds the fuzzy rules based on these considerations. In contrast, the author of OF-FL [113] considers the ETX value small if it is ten or less, and if it is 80 or more, it counts as a high value of ETX. Furthermore, a set of fuzzy rules has been built based on these given values. Moreover, the fuzzy logic method is not flexible; it can only support the use of predefined rules, the if-then conditions, which makes the method only suitable for specific network scenarios and can not adapt to the network changes as human intervention may be \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Optimisation Method** & **RPL Objective Functions** & **R1** & **R2** & **R3** & **R4** & **R5** \\ \hline \multirow{8}{*}{Single Metric Method} & OF0 [81] & ✗ & ✗ & ✗ & ✗ & ✗ \\  & ENTOT [90] & ✗ & ✓ & ✗ & ✗ & ✗ \\  & Energy-OF [91] & ✗ & ✓ & ✗ & ✗ & ✗ \\  & MRMFO [85] & ✓ & ✗ & ✗ & ✗ & ✗ \\  & AVG, Del [92] & ✓ & ✗ & ✗ & ✗ & ✗ \\  & RSSI-based IPv6 [93] & ✓ & ✗ & ✗ & ✗ & ✗ \\  & ELITE [94] & ✓ & ✗ & ✗ & ✗ & ✗ \\ \hline \multirow{8}{*}{Combined Metrics Method} & WRF-RPL [98] & ✗ & ✓ & ✗ & ✗ & ✗ \\  & CA-OF [99] & ✓ & ✗ & ✗ & ✗ & ✗ \\  & QU-RPL [100] & ✓ & ✗ & ✗ & ✗ & ✗ \\  & CAQL [101] & ✓ & ✗ & ✗ & ✗ & ✗ \\  & PER-HOP ETX [54] & ✓ & ✗ & ✗ & ✗ & ✗ \\  & ELT [102] & ✓ & ✓ & ✗ & ✗ & ✗ \\  & L2 AM [103] & ✓ & ✓ & ✗ & ✗ & ✗ \\  & I-RPL [104] & ✓ & ✓ & ✗ & ✗ & ✗ \\  & CB-RPL [105] & ✓ & ✓ & ✗ & ✗ & ✗ \\ \hline \multirow{8}{*}{Fuzzy Logic Method} & CMOF [108] & ✓ & ✗ & ✓ & \(\bigcirc\) & ✗ \\  & FMOF [109] & ✓ & ✗ & ✓ & \(\bigcirc\) & ✗ \\  & FAHP-OF [110] & ✓ & ✗ & ✓ & \(\bigcirc\) & ✗ \\  & QPt-FLOE [111] & ✓ & ✗ & ✓ & \(\bigcirc\) & ✗ \\  & OF-FUuzzy [112] & ✓ & ✓ & ✓ & \(\bigcirc\) & ✗ \\  & OF-FL [113] & ✓ & ✓ & ✓ & \(\bigcirc\) & ✗ \\  & OF-EC [114] & ✓ & ✓ & ✓ & \(\bigcirc\) & ✗ \\  & FLEOF [115] & ✓ & ✓ & ✓ & \(\bigcirc\) & ✗ \\ \hline \hline \end{tabular} \(\bigcirc\)—considered; ✗—not considered; \(\bigcirc\)—can be improved; RI—link quality; R2—energy level; R3—use optimisation method to make global optimum decision; RA—dynamic adaptation to network changes in terms of topology, link quality, and energy levels; R5—malicious threats.  
\end{table} 
Table 4: Analysis of RPL objective functions for non-malicious LLNs.