# Sigma Routing Metric for RPL Protocol
## 8 Conclusions  
In this paper, the adaptation and implementation of a routing metric for the objective function in the RPL protocol was proposed. Once the appropriate state-of-the-art metric was defined, it was possible to observe the recent contributions in the subject by other researchers, which allowed the analysis of the operation of the RPL protocol to understand and modify the protocol, by adapting the link state metric in its objective function.  
LLN scenarios were configured with parameters close to reality; this is how the SIGMA-ETX metric was implemented, guaranteeing its operation by comparing its implementation with the performance of the objective functions OF0, MRHOF, and PH-ETX with SIGMA-ETX. This demonstrated that SIGMA-ETX is more efficient with networks containing large number of nodes per unit of area in the LLN routing in terms of quality of service. In addition, the results of SIGMA-ETX were compared with the results of other research, in terms of latency, detailing the results at the 15 th hop. Significant differences could be noted against OF0 (23.33%), MRHOF (56.45%) and PHETX (83.33%).  
The packet delivery ratio of SIGMA-ETX is slightly better than PH-ETX, and much greater than OF0 and MRHOF, demonstrating the validity of SIGMA-ETX. Putting 15 th hop results in context, the difference with OF0 is 47.44%, with MRHOF reaching 57.69% and with PH-ETX reaching 87.18% against SIGMA-ETX regarding packet delivery.  
Regarding energy consumption, results show OF0 reaches 51.11% of SIGMA-ETX's results, MRHOF reaches 74.19%, and PH-ETX reaches 85.19%. The prevented packet losses resulted in less retransmissions and also lower power consumption. Particularly, the results in a 100-node scenario shows evidence that OF0 only achieves 70.83% of SIGMA-ETX's lifetime performance, MRHOF only 77.08%, and PH-ETX 87.50%.  
On the other hand, we suggest that SIGMA-ETX could be combined with other metrics to calculate a better route. This can be done because ContikiRPL was designed to easily allow this kind of implementation. Additionally, the ContikiRPL code can be found in SourceForge and GitHub.  
In the future, it is necessary to research the relationship between other types of Quality of Service (QoS) metrics (e.g., link reliability and others), SIGMA-ETX, and the useful life of the node, because the complexity of the processes and the variability of these types of networks require the contrast of SIGMA-ETX, as opposed to metrics in specific applications of LLNs. Estimating the relation of these aspects will allow future researchers to define parameters to use this result. Additionally, we are working to apply load balance in SIGMA-ETX to improve the functionality of RPL and to apply a Software Defined Network (SDN) to RPL networks.  
Authors would like to thank the editor and the reviewers for their valuable comments and suggestions to improve the quality of the paper. This work was funded by Universidad del Norte and Universidad Simon Bolivar.  
[PERSON] and [PERSON] were involved in the design, data analysis and data interpretation. [PERSON], [PERSON] and [PERSON] carried out the design model and computer simulations. Computer simulations and data interpretations were carried out by [PERSON] and [PERSON] developed data collection and interpretation.  
The authors declare no conflict of interest.