## 4 A Routing Protocol for LLNs (RPL)
### RPL Objective Functions for Non-Malicious LLNs
#### 4.2.1 Single Metric Method  
The first group of objective functions typically uses a single routing metric with a single metric method to select the best path. The single routing metric can belong to node or link metrics in this group. The objective function solutions with a node metric in this group include the OFO [81], energy total consumed (ENTOT) [90], and energy-based objective function (Energy-OF) [91]. OF0 [81] selects the shortest paths in terms of hops. The latter two objective functions [90] and [91] are based on energy metrics, typically measured by the remaining node energy or energy consumption. Thus, the path with less energy cost is selected. In contrast, the objective function solutions that select the best paths based on a single link metric include the default MRHOF [85] discussed in Section 4.1, the average delay (AVG-DEL) [92], the received signal strength indicator-based (RSSI-based) solution [93], and the elaborated cross-layer RPL objective function (ELITE) [94]. [PERSON] [90] proposed ENTOT, which uses energy consumption as a routing metric. ENTOT aims to balance the energy around the network by considering the energy consumption of each node along the path. The total energy consumption of a path is equal to the sum of the energy consumption of all the nodes forming this path. As a result, ENTOT selects the path with less energy consumption as the optimal path to reach the DODAG root. Simulation results show high performance for ENTOT in terms of energy consumption and network lifetime compared to MRHOF [85]. [PERSON] et al. [91] also proposed a similar idea to [90] and introduced Energy-OF. Energy-OF uses the node's remaining energy as a routing metric. The Energy-OF aims to (i) prolong the network lifetime and (ii) distribute energy equally between the nodes by selecting the node with the highest remaining energy as a parent node. The Energy-OF approach uses a real-time battery level estimation model proposed in [95] to compute the node's remaining energy. The battery model uses the current energy consumption of the node during each node state and its duration to estimate the node's remaining energy. The experiments show that Energy-OF prolongs the network lifetime and distributes energy almost evenly between nodes, i.e., energy balanced, compared with MRHOF [85]. Although [90; 91] reduce energy consumption and provide energy balance in RPL, they select routing paths based on the node's energy level, which only addresses the routing reliability issues caused by power constraints and ignore other reliability issues that may cause by link quality. [PERSON] et al. [92] proposed AVG_DEI to minimise the average delay towards the DODAG root. The AVG_DEL assumes that (i) the DAG root runs with a 100% duty cycle, and (ii) the nodes run with very low and different duty cycles, e.g., under 1%, at the media access control (MAC) layer [96]. The term duty cycle describes all forms of periodically switching off some functions, leaving them on only for a certain percentage of the time. In LLN, wireless radio transceivers often consume high power when they are switched on. Thus, nodes usually (i) switch off their radio transceivers as long as possible to save power and (ii) use a periodic wake-up to turn on their radio and listen for packet transmission. Nodes with long sleeping intervals cause longer delays compared to nodes with short sleeping intervals. The AVG_DEL results show a significant decrease in the end-to-end, mainly in the faraway nodes from the DAG root, i.e., leaf nodes, compared to MRHOF [85]. [PERSON] et al. [93] proposed RSSI-based IPv6 that uses RSSI as a routing metric to improve the transmission performance in RPL. The RSSI-based IPv6 aims to (i) select stable links using the RSSI metric and (ii) dynamically change the length of IPv6 fragments. The RSSI metric estimates the packet loss rate in the links, while the dynamic fragmentation reduces the Frame Error Rate (FER) [97] in that link. This makes RSSI-based IPv6 more suitable for transmitting long IPv6 packets in RPL.