# An Energy Efficient Routing Approach for Cloud-Assisted Green Industrial IoT Networks
## 3 Related Works  
### QoS Provision in Resource-constrained Networks  
The extensive use of the IoT concept has contributed to the presence of RPL, which enables smart devices to interact or communicate in the surrounding ambient at anytime and anywhere. Recently, many studies have been conducted on the various aspect of RPL optimization, such as congestion control [23], data aggregation [24], security and mobility [25; 26], and OF design [27]. While RPL significantly satisfies the requirements of LLN for IoT based applications, to be fit in the IIoT concept, some aspects are crucial, such as energy efficiency and reliable routing. Due to the unique characteristic of wireless networks, it is difficult to define QoS parameters, and also poses additional challenges for QoS support. Therefore, to overcome these challenges, the authors in [28; 29] provide different solutions for various application domains. There are two common approaches for QoS provisioning one is layered, and the other is cross-layered. In the individual layer, QoS provisioning depends upon layer capability. The layered approach provides the QoS with protocols that operate merely in each layer of the communication protocol stack, whereas the cross-layered approach achieves the QoS provisioning through the interaction of multiple layers concurrently. However, there is no variation between both approaches in terms of QoS support. The authors in [30] have conformed the widespread use of QoS provisioning with service differentiation compared to deterministic based QoS provisioning approaches. Therefore, most protocols follow the classification of packets based on traffic type, and packets from different classes are processed according to their requirements by tuning the MAC layer parameters. The traditional methods based on the resource reservation, route recovery during topological changes, and end-to-end path discovery are not suitable in LLNs for several reasons.  
Based on this understanding, in the IIoT networks, it is essential to facilitate prioritization among different traffic classes when designing routing algorithms. This characteristic could be considered through different mechanisms. For instance, a priority queue system could be applied to differentiating the traffic with different QoS requirements. On the contrary, the mechanism implemented for QoS provisioning can exploit the multipath routing system. The authors in [31] presented a comprehensive survey on multipath routing protocols for QoS assurance. The study primarily focused on real-time multipath routing techniques for wireless multimedia sensor networks. In terms of QoS, a recent study shows that unlike single-path routing protocols, multipath routing protocols are suitable for WSNs. In fact, multipath routing protocols reduce the possibility of packet loss while utilizing redundant paths to the root [32]. Therefore, this mechanism can be used for a reliable-sensitive network where reliable information forwarding is essential, whereas real-time applications where delay-sensitivity is considered as basic QoS support this system may violate the specific application requirement. Multipath forwarding optimization has been proposed in [33] to minimize the average delay towards the root node based on the composite metric. The proposed work points out that default single-metric-based OFs are vulnerable in situations where congestion occurs due to a sudden increase in traffic volume, resulting in packet loss and delay. In this context, multipath forwarding techniques are desirable in LLN environments to achieve the desired QoS. However, some issues should be considered carefully while designing a routing protocol. Some of the issues about multipath approaches are high complexity, fragmentation, and overhead. Moreover, the high overheads incurred by multipath techniques in terms of communication and computation is unsuitable for devices with limited resources as well as for large-scale networks like the IIoT.