## 4 A Routing Protocol for LLNs (RPL)
### RPL Objective Functions for Non-Malicious LLNs
#### 4.2.2 Combined Metrics Method
Simulation results show that PER-HOP ETX selects better paths with high PDR and low latency compared to MRHOF [85] and OF0 [81]. Although PER-HOP ETX combines a link metric, i.e., ETX, and a node metric, i.e., hop count, it does not take into consideration the routing reliability issues caused by power constraints. [PERSON] et al. [102] proposed ELT, an energy-balancing objective function that distributes energy consumption evenly among nodes to improve network lifetime. This objective function uses the ELT routing metric to estimate the time until a node's energy is depleted, i.e., residual time. The ELT metric is computed based on (i) forwarding traffic, (ii) ETX, (iii) energy drained, and (iv) residual energy. Thus, the path with the least energy constraint is selected as the best path to the DODAG root. Moreover, ELT provides two additional mechanisms to enhance RPL performance. The first mechanism allows for the detection of energy-bottleneck nodes and distributes traffic load evenly between the nodes. The second one allows nodes to select multiple preferred parents rather than just a single parent, as in the original RPL [1], to reduce maintenance costs in case of link failure. The experimental results show that ELT prolongs the network lifetime, avoids bottlenecks, and maintains a stable set of active parents. [PERSON] et al. [103] proposed L2 AM that takes into consideration link quality and energy consumption. L2 AM aims to balance energy by considering energy consumption so that each node consumes the same amount of energy. Therefore, the author proposed an Exponential Lifetime Cost (ELC) routing metric that takes into account (i) transmission power on the link and (ii) node residual energy. The ELC is then combined with the ETX metric in such a way that the weight of residual energy increases exponentially as the residual battery capacity decreases. Simulation results demonstrate that L2 AM selects parent nodes with high energy, prolonging the network lifetime without affecting link reliability. [PERSON] and [PERSON] [104] proposed a new objective function called I-RPL. This objective function utilises five different routing metrics, which include the child number of a parent, candidate parent number, hop count, ETX, and energy consumption. I-RPL improves the calculation of path ETX to avoid selecting paths with long single-hop links. The simulation results indicate that I-RPL outperforms OF0 [81] and MRHOF [85] in terms of PDR, end-to-end delay, and energy consumption. [PERSON] and [PERSON] [105] proposed CB-RPL that combines three routing metrics: ETX, hop count, and remaining energy. This objective function incorporates coordinator nodes responsible for handling control packets in the network. The coordinator nodes help to reduce network traffic and conserve energy by optimising control packet transmissions. The simulation results demonstrate that CB-RPL performs better than MRHOF [85] and OF0 [81] in various performance metrics such as PDR, delay, energy consumption, and throughput. Although the objective function solutions [102; 103; 104; 105] combine link quality and node's energy metrics for path selection, they do not consider the issue of how to make global, i.e., end-to-end, optimum decisions for path selection in real time.