# Multiple Instances QoS Routing in RPL: Application to Smart Grids +
## 2 Related Work
### RPL Proposed Metrics and Modifications  
Many researchers are active around RPL in order to adapt it to different Internet of Things applications. Moreover many critical analyses were made to highlight the gaps concerning reliability and adequate metrics in a SG environment [10; 18; 19]. \(ETX\) in \(MRHOF\)[15] and \(HC\) (Hop Count) in \(OF0\)[16] are the two main metrics used in the objective functions. \(ETX\) finds paths with the fewest expected number of transmissions (including retransmissions) required to deliver a packet all the way to its destination [17]. Although \(ETX\) is reliable and widely used as a metric in wireless sensor networks, it does not take directly into account the latency which is critical in some SG applications [20]. \(ETX\) is not energy aware, thus for a link with few re-transmissions, \(ETX\) will keep sending packets on it without taking the decrease of battery nodes level into account. \(HC\) only takes the number of hops into consideration to calculate the best path which is not always satisfactory in LLN. In [21] several routing metrics were proposed to be used for path calculation in LLN, i.e., the Throughput, Node Energy, Latency, Link reliability with the LQL (Link Quality Level) or \(ETX\) metric. An energy-based objective function for RPL that uses the remaining energy as the main routing metric was proposed in [22]. It achieves a better load balancing compared to \(ETX\) and increases the network lifetime but with a lower delivery ratio. In [23], the authors proposed NL-OF, an objective function based on a non linear length that construct DODAGs from roots to nodes such that the non linear length is the smallest possible. They evaluated it using Cooja while considering three QoS parameters: End-to-end delay, packet loss and jitter. In [24] two MAC aware routing metrics were proposed to be used in RPL: R-metric and Q-metric. R-metric extends \(ETX\) by considering packet losses due to the MAC contention. Q-metric provides load balancing by selecting the lightest parent in terms of traffic load by solving an optimization problem and mainly considering reliability, transmission and reception power consumption. ETT-LB was proposed in [25]. It is based on the ETT (Expected Transmission Time) metric [26], which extends \(ETX\) by considering the link transmission rate and packet size, adding to it the Expected Delay Time (EDT), which is the average link load at a node in order to achieve load balancing. In [27]\(L^{2}AM\) metric was proposed. It is based on an combination of both data reliability (defined by \(ETX\)) and the nodes residual energy. Although their solution extended the network lifetime, it remains not adapted to a network with heterogeneous applications in terms of criticality and powered/battery nodes. Fuzzy logic metric combination was also considered in several works [28; 29; 30] in order to be used for RPL. They combined several metrics like end-to-end delay, \(HC\), link quality and battery level. In [31] two combinations of two metrics were proposed: lexical and additive. In the lexical combination, the second metric is inspected only if the first one leads to equal paths, while in the additive combination the paths are calculated based on a different cost given to each metric. Multiple instances in RPL and QoS were studied in many works [9; 32; 33]. Yet, these works limit the number of instances to two and don't take into consideration the drawbacks of the used metrics (\(ETX\) and \(HC\)) concerning the energy efficiency and end-to-end delay. As a conclusion, a single routing metric cannot assure traffic differentiation in a SG since different applications require different QoS levels. In addition, in a multiple instance environment, the chosen objective function/metric has to guarantee the QoS requirements of the concerned SG application, which to the best of our knowledge has not been proposed yet. This is why we propose \(OFQS\) with its multi-objective metric \(mOFQS\) taking account of these requirements and improving the communication in the SG.