## 6 Results and Evaluation
### IoT-LAB Realistic Scenario  
In this study, using IoT-LAB realistic testbed we provide two uniform and random network topologies to study RPL objective function. OF0 and ETX are investigated with respect to Control Traffic Overhead, Packet Delivery Ratio, Network Latency, and Power Consumption. We measured the RPL performance metrics simultaneously to select the best routes and optimize the network packet \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value** \\ \hline Num. of nodes & 1000 nodes \\ Network Topologies & Randomly distributed \\ Experiment duration (S) & 1300 \\ Operating system & Contiki \\ Frequency & IEEE 802.15.4 radio \\ Mac protocol & Contiki MAC \\ Startup delay (s) & 65 \\ Data rate transfer & 200-byte payload every 30 s \\ The number of the border router & One border router \\ DIO Interval Minimum(ms) & 12 \\ DIO Interval Doublings(ms) & 8 \\ RPL Mode Of Operation & storing mode \\ Use Authentication & No \\ Path Control Size & 0 \\ DIO Redundancy Constant & 10 \\ Max Rank Increase & 1792 \\ Minimum Hop Rank Increase & 256 \\ OF & OF0 \& ETX \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Cooja simulator RPL configuration Parameters. delivery ratio without consuming more energy. The results of both uniform and random IoT-LAB topology are expressed in the following section. \(\blacksquare\) Control Traffic Overhead  
Figure 7 illustrates the RPL Control Traffic messages (DIO, DIS, DAO) with uniform network topology using OF0 and ETX. In general, the low-density networks provide fewer control messages than dense networks. When RPL protocol uses OF0 to calculate the route to the destination, it provides too many control messages rather than using ETX. This scenario is considered as a medium scale network that experienced a high control overhead to build the DODAG routes, which is used to propagate the routing tables between nodes. We note that DIO messages are the dominant control messages between all RPL messages as they are used to update and construct the routing tables. The increasing of control messages reflects the network instability and the increasing of the collision and congestion between packets. This collision increases the network delay and consumes the network resources. The RPL scenario, which uses OF0 to calculate the routes, consumes more control traffic overhead than scenario using ETX. Figure 8 illustrates the RPL Control Traffic messages (DIO, DIS, DAO) with a random network topology using OF0 and ETX that indicates higher control messages than uniform topology. Figure 9 shows the comparison between random and uniform topology with OF0 and ETX of total control traffic for 140 nodes. ETX outperforms OF0 in route calculation under both uniform and random topology while random networks need higher control messages due to collisions and retransmissions. Around 90000 packets are exchanged between 140 nodes for two hours experiments using OF0 and under random nodes distribution. In small densities, the control overhead metric is similar in both uniform and random distribution. Figure 8: IoT-LAB testbed control traffic overhead. Figure 7: IoT-LAB testbed control traffic overhead with uniform network topology. Packet Delivery Ratio (PDR)  
We calculate the average packet delivery ratio with a uniform and random testbed topology using OF0 and ETX. PDR value is decreased while increasing the number of nodes and PDR metric for ETX is slightly higher than OF0 which reflects the ETX higher efficiency. Figure 10 shows the PDR comparison between uniform and random testbed topology under OF0 and ETX as using ETX with a uniform topology outperforms the random topology because the random topologies always have many collisions and packet congestion that result in more retransmission. However, for a high network density, a lot of packet loss happened due to interference as nodes send many messages to some multiple destinations with the same minimum rank. From this result, ETX cannot be considered as the best solution for the high-density networks with poor link quality.