A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> 2) OF Enhancements Based on Multi-path Routing <s> The RPL routing protocol published in RFC 6550 was designed for efficient and reliable data collection in low-power and lossy networks. Specifically, it constructs a Destination Oriented Directed Acyclic Graph (DODAG) for data forwarding. However, due to the uneven deployment of sensor nodes in large areas, and the heterogeneous traffic patterns in the network, some sensor nodes may have much heavier workload in terms of packets forwarded than others. Such unbalanced workload distribution will result in these sensor nodes quickly exhausting their energy, and therefore shorten the overall network lifetime. In this paper, we propose a load balanced routing protocol based on the RPL protocol, named LB-RPL, to achieve balanced workload distribution in the network. Targeted at the low-power and lossy network environments, LB-RPL detects workload imbalance in a distributed and non-intrusive fashion. In addition, it optimizes the data forwarding path by jointly considering both workload distribution and link-layer communication qualities. We demonstrate the performance superiority of our LB-RPL protocol over original RPL through extensive simulations. <s> BIB001 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> 2) OF Enhancements Based on Multi-path Routing <s> This study proposes a link quality aware routing protocol for low-power and lossy networks. The proposed protocol primarily considers the quality of links to determine the node rank for constructing a proper destination-oriented directed acyclic graph, on which a node may have many parents. In the proposed protocol, a node selects the node with the maximum remaining energy from all of its neighbors as the next hop to guarantee that the packet has a high probability of being forwarded toward the destination. Simulation results confirm that, compared with the routing protocol based on the hop count, the proposed protocol improves the packet delivery ratio, especially for environments with a high bit error rate. Results also show that the proposed protocol can balance the energy consumption of nodes in the network. <s> BIB002 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> 2) OF Enhancements Based on Multi-path Routing <s> The devices composing Wireless Sensor Networks (WSN) are very limited in terms of memory, processing power and battery. RPL has emerged as the de facto routing standard in low-power and lossy networks. While most of the proposals focus on minimizing the global energy consumption, we aim here at designing an energy-balancing routing protocol: each node should efficiently consume the same quantity of energy to improve the network lifetime. To this end, we exploit an Expected Lifetime metric, denoting the residual time of the nodes (time until the node will run out of energy). We propose mechanisms to detect the energy-bottleneck nodes and to spread the traffic load uniformly among them. While RPL constructs a Destination-Oriented Directed Acyclic Graph (DODAG) structure, it only implements single path. We propose here to exploit its natural multipath structure. This multipath approach helps reducing the number of DODAG reconstructions that leads to instabilities and convergence problems. Simulations highlight we improve both the routing reliability and the network lifetime. <s> BIB003 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> 2) OF Enhancements Based on Multi-path Routing <s> Energy is a very scarce resource in Wireless Sensor Networks. While most of the current proposals focus on minimizing the global energy consumption, we aim here at designing an energy-balancing routing protocol that maximizes the lifetime of the most constraint nodes. To improve the network lifetime, each node should consume the same (minimal) quantity of energy. We propose the Expected Lifetime metric, denoting the residual time of a node (time until the node will run out of energy). We design mechanisms to detect energy-bottleneck nodes and to spread the traffic load uniformly among them. Moreover, we apply this metric to RPL, the de facto routing standard in low-power and lossy networks. In order to avoid instabilities in the network and problems of convergence, we propose here a multipath approach. We exploit the Directed Acyclic Graph (DAG) structure of the routing topology to probabilistically forward the traffic to several parents. Simulations highlight that we improve both the routing reliability and the network lifetime, while reducing the number of DAG reconfigurations. <s> BIB004 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> 2) OF Enhancements Based on Multi-path Routing <s> Routing protocol for low-power and lossy networks (RPL) is the standard IPv6 based routing protocol for low power, lossy Networks (LLNs) proposed by IETF. It is proposed for networks with characteristics like small packet size, lossy links, low bandwidth, low data rate and low power resources. RPL is a single path routing protocol and the existing objective functions do not support creation of multiple routing paths between source and destination. Multipath routing can be used to achieve multifold objectives, including higher reliability, increased throughput, fault tolerance, congestion mitigation and hole avoidance. In this paper, M-RPL a multi-path extension of RPL is proposed that aims to provide temporary multipath routing during congestion over a path. Congestion is primarily detected by a forwarding node by monitoring packet delivery ratio and is mitigated by providing partially disjoint multipath routing. Detailed simulation analysis of M-RPL against RPL shows that using multi-path RPL reduces congestion and increases the overall throughput. Hence M-RPL is suitable for supporting high data rates as compared to single path RPL. <s> BIB005 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> 2) OF Enhancements Based on Multi-path Routing <s> Emerging Wireless Sensor Networks (WSN) give support to a wide diversity of applications, but these networks are based on devices with limited resources, requiring communication protocols such as RPL (Routing Protocol for Low power lossy networks) which allow the routes to be fitted to certain requirements. In this paper, we design a custom energy efficient objective function and build a WSN composed of real devices, and deployed it in a real use case under two scenarios. We then analyze the impact on network behavior and validate the designed objective function, with network lifetime extension showing some promise. <s> BIB006 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> 2) OF Enhancements Based on Multi-path Routing <s> RPL is an IPv6 routing protocol for low-power and lossy networks (LLNs) designed to meet the requirements of a wide range of LLN applications including smart grid AMIs, industrial and environmental monitoring, and wireless sensor networks. RPL allows bi-directional end-to-end IPv6 communication on resource constrained LLN devices, leading to the concept of the Internet of Things (IoT) with thousands and millions of devices interconnected through multihop mesh networks. In this paper, we investigate the load balancing and congestion problem of RPL. Specifically, we show that most of packet losses under heavy traffic are due to congestion, and a serious load balancing problem exists in RPL in terms of routing parent selection. To overcome this problem, this paper proposes a simple yet effective queue utilization based RPL (QU-RPL) that significantly improves end-to-end packet delivery performance compared to the standard RPL. QU-RPL is designed for each node to select its parent node considering the queue utilization of its neighbor nodes as well as their hop distances to an LLN border router (LBR). Owing to its load balancing capability, QU-RPL is very effective in lowering the queue losses and increasing the packet delivery ratio. We verify all our findings through experimental measurements on a real testbed of a multihop LLN over IEEE 802.15.4. <s> BIB007 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> 2) OF Enhancements Based on Multi-path Routing <s> Designing routing protocols in Low power and Lossy Networks (LLNs) imposes great challenges. In emergency scenarios, the large and rapid data traffic caused by emergencies will lead to network congestion and bring about significant packet loss and delay. Routing protocol for LLNs (RPL) is the IETF standard for IPv6 routing in LLNs. The basic version of RPL uses Expected Transmission Count (ETX) as the default routing metric; it cannot solve the problem of sudden large data traffic. In this paper, we propose a congestion avoidance multipath routing protocol which uses composite routing metrics based on RPL, named CA-RPL. A routing metric for RPL that minimized the average delay towards the DAG root is proposed, and the weight of each path is computed by four metrics. The mechanism is explained and its performance is evaluated through simulation experiments based on Contiki. Simulation results show that the proposed CA-RPL reduces the average time delay by about 30% compared to original RPL when the interpacket interval is short and has almost 20% reduction in packet loss ratio. The CA-RPL can effectively alleviate the network congestion in the network with poor link quality and large data traffic and significantly improve the performance of LLNs. <s> BIB008 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> 2) OF Enhancements Based on Multi-path Routing <s> The routing protocol for low-power and lossy networks (RPL) is an internet protocol based routing protocol developed and standardized by IETF in 2012 to support a wide range of applications for low-power and lossy-networks (LLNs). In LLNs consisting of resource-constrained devices, the energy consumption of battery powered sensing devices during network operations can greatly impact network lifetime. In the case of inefficient route selection, the energy depletion from even a few nodes in the network can damage network integrity and reliability by creating holes in the network. In this paper, a composite energy-aware node metric (RERBDI ) is proposed for RPL; thismetric uses both the residual energy ratio (RER) of the nodes and their battery discharge index. This composite metric helps avoid overburdening power depleted network nodes during packet routing from the source towards the destination oriented directed acyclic graph root node. Additionally, an objective function is defined for RPL, which combines the node metric RERBDI and the expected transmission count (ETX) link quality metric; this helps to improve the overall network packet delivery ratio. The COOJA simulator is used to evaluate the performance of the proposed scheme. The simulations show encouraging results for the proposed scheme in terms of network lifetime, packet delivery ratio and energy consumption, when compared to the most popular schemes for RPL like ETX, hop-count and RER. <s> BIB009 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> 2) OF Enhancements Based on Multi-path Routing <s> Routing Protocol for Low Power and Lossy Network (RPL) is standardized and known as the primary solution for the last mile communication network in the smart grid. Various applications with different requirements are rapidly developed in the smart grid. The need to provide Quality of Service (QoS) for such a communication network is inevitable. In this paper, we use the benefits of virtualization and software-defined networking to present a virtual version of the RPL protocol which we name OMC-RPL (Optimized Multi-Class RPL). We present an SDN-enabled architecture consisting of a central controller and some SDN nodes. This implementation reduces the complexity and controls interactions to distribute the network states and other related information in the network. The proposed SDN-enabled architecture consists of different components including Network Link Discovery, Topology Manager, and Virtual Routing. OMC-RPL utilizes a holistic objective function including distinctive metrics related to QoS, and supports the data classification which is an essential requirement in this context. The proposed objective function considers different numbers of traffic classes by using weighting parameters. An optimization algorithm determines the best values of these coefficients. OMC-RPL is evaluated in different aspects. Simulation results show that the new idea significantly decreases both the end-to-end delay and packet loss which are the important factors of QoS. The virtualization idea is also investigated, which results in less message exchange. <s> BIB010
In order to overcome some performance issues resulting from single-path based routing in RPL, several multipath forwarding optimizations have been proposed and still other studies have proposed multi-path forwarding approaches that use composite metrics. For instance, the authors in BIB001 propose a probabilitybased load-balancing multi-path solution for RPL referred to as LB-RPL. LB-RPL achieves load balancing by having each node distributes traffic among its top k parents, in terms of rank, based on their traffic load. A parent experiencing heavy load may signal its status by delaying the broadcasting of its scheduled DIO message. This enables child nodes to remove that parent from their top k and hence, exclude it from further data forwarding. It is shown BIB001 by means of simulations that LB-RPL outperforms RPL in terms of packet delivery ratio, delay, and workload distribution. The work in BIB003 [106] highlights the advantages of incorporating multipath forwarding schemes into the RPL protocol. Intuitively, the multipath mechanisms have been proven to have a wide spectrum of benefits such as improving faulttolerance, enhancing reliability, minimizing congestion and improving QoS. The authors propose a multi-path routing mechanism based on RPL in order to allow the protocol to forward traffic to multiple preferred parents. The study asserts that a routing metric must: (1) capture the variations in link quality; (2) use energy-efficient paths to maximize the end-to-end reliability; and (3) minimize the energy expenditure for those nodes consuming the most energy (the bottleneck nodes). In this regard, a new metric is proposed, referred to as the Expected Lifetime metric (ELT) that aims to balance energy consumption among network nodes and maximize the lifetime of the bottleneck nodes. The network lifetime is defined as the time before the first node dies (runs out of energy). The ELT of a specific node is calculated by: (1) computing the throughput of that node based on its own traffic and also the traffic of its children; (2) multiplying the average number of retransmission by the calculated traffic; (3) computing the time ratio required for transmission based on the sending data rate; (4) computing the energy consumption based on the transmission power of the radio only; and, finally, (5) calculating the ELT as the ratio between the node's remaining energy and the energy calculated in the previous step. Based on the ELT calculated value, the bottleneck nodes are first identified and advertised along the topology, then a multipleparents, energy-balanced topology is constructed, in which the traffic is balanced among parents with careful consideration of bottleneck nodes BIB003 BIB004 . The proposed protocol is evaluated using WSNet  and compared to RPL considering the metrics of: residual energy, the ETX-using-hysteresis objective function, and a linearly combined metric of ETX and residual energy. The experimental results indicate that the proposed multipath ELT has almost the same reliability as ETX although selecting the paths with maximum residual energy. Multipath ELT was also found to enhance routing stability through preventing sudden changes in the parent weight in comparison with standard RPL BIB003 . In BIB005 , the authors again highlighted the issue of RPL being single path routing protocol and the incapacity of standard objective functions to provide multipath routing. The ultimate goal of the study is to provide RPL with multipath routing capabilities that will enable the protocol to react efficiently to the congestion BIB005 . Thus, the authors propose an extension referred to as a multi-path RPL (M-RPL) that provides temporary multiple paths during congestion. In M-RPL, the packet delivery ratio (PDR) is used by the forwarding nodes to detect congestion. If a forwarding node on a routing path detects that the PDR has decreased below a specific threshold, the node sends a notification to its children, by means of DIO messages, informing them of congestion. Each child node that hears the congestion advertisement message starts multipath routing by splitting its forwarding rate in half. Thereafter, only every second packet is sent to its original congested parent while the others are forwarded to any other parent from its parent list BIB005 . The proposed protocol is evaluated using Cooja and compared to RPL with MRHOF in terms of energy consumption, latency, and throughput. Their simulation results show that M-RPL has better throughput and lower per-bit energy consumption than RPL, due its splitting mechanism. The results also indicated that while the delay of M-RPL is initially comparable to RPL, this changes when congestion begins. Initially M-RPL experiences greater delay as multiple paths are introduced but when the network stabilizes; M-RPL gets to outperform RPL in terms of delay BIB005 . The work in BIB008 proposes a multi-path forwarding approach based on a composite metric. The authors point out that the two single-metric RPL's objective functions are vulnerable in scenarios where a sudden increase in traffic volume introduces a congestion, resulting in significant delay and packet loss. The authors propose a congestion avoidance multipath routing protocol, referred to as CA-RPL, whose primary goal is to enable the network to react quickly and reliably to sudden events. They have designed a composite routing metric based on the ContikiMac duty cycle protocol with the aim of minimizing the average delay towards the DODAG root, referred to as DELAY ROOT. Under this metric, a node saves time by first learning the wakeup phase of its candidate parents and then sending the packets to the first awake parent BIB008 . CA-RPL is a composite multi-path routing metric that combines the new proposed DELAY ROOT with the number of received packets and ETX to calculate the path weights. Cooja with Contiki operating system is used to compare the proposed protocol with standard RPL in terms of latency, packet loss ratio, throughput and the packet reception number (PRN) of the DODAG root per unit time. The experimental results illustrate that the proposed protocol relieves network congestion and enhancing the PRN by up to 50%, the throughput by up to 34%, packet loss by up to 25%, and average delay by 30% compared to RPL. The authors in BIB002 reported that the ETX metric used in RPL is inefficient in quantifying the quality of links as it only "reflects the quality of a single link". To overcome this issue, the study proposes a link quality aware routing protocol for LLNs referred to as LQA-RPL. LQA-RPL calculates the rank of a node based on the quality of links to all its neighbors, which is derived from the ETX and defined as the expected probability of unsuccessful transmissions. If a node has more than one parent in its parent set, the node uses multi-path routing by selecting the parent with the maximum residual energy to act as next-hop relay node to the DODAG root BIB002 . LQA-RPL is evaluated and compared to RPL with hop count in terms of packet delivery ratio, energy consumption, and network lifetime. The reported results indicated that LQA-RPL outperforms RPL in terms of PDR, which is attributed to the higher number of candidate parents. It is also shown BIB002 that LQA-RPL can balance energy consumption due to its capacity to distribute the traffic among multiple candidate parents based on residual energy, prolonging the network lifetime. The work in BIB006 has reported a new energy-based OF that proposes selecting the preferred parent based on the remaining energy with a hysteresis value of 5% to reduce network frequent changes. The remaining energy is obtained by polling each node to check its battery voltage, which is claimed to be a good indicator of remaining energy BIB006 . The proposed OF is evaluated using two testbed deployments (basic and extended), and compared to RPL objective functions (OF0, MRHOF) in terms of packet loss, delay, energy consumption and network churn. In both testbeds, a sink and seven sensor nodes are deployed in a building area and differentiated by node position, distance between nodes, RF interference and noise. Based on the obtained results, it is shown that the proposed OF has improved network lifetime by up to 40% compared to RPL's OFs. The authors also claim that their OF lowered the delay in the Basic Deployment compared to MRHOF, which was expected, and to OF0, which was not. In explaining the latter case, they observe that OF0 suffers from excessive churn and frequent changes in the network topology resulting in higher delays and more energy expenditure. Pertaining to packet loss, the reported results show that the proposed energy-based OF is superior to RPL OF0 but is outperformed by RPL MRHOF. The authors in BIB010 propose an optimization for RPL referred to as Optimized Multi-Class RPL (OMC-RPL) based on virtualization and software-defined networking techniques. The study asserts that standard RPL faces two significant issues when offering QoS. The first is the absence of a holistic and comprehensive objective function. For example, an objective function may enhance delay but at the cost of higher energy consumption as all packets overuse the same paths with the minimum delay. The second issue is that RPL does not support a mechanism for data classification, which is critical component in ensuring the QoS BIB010 . Thus, a holistic objective function that supports multiple data classes is needed BIB010 . The steps of OMC-RPL are as follows: first, the nodes send the information required to construct the virtual DODAG to the SDN controller, using onehop communication; then the SDN controller calculates the ranks of nodes in the network for each traffic class using a custom weighted-metric objective function BIB010 . The main parameters of the proposed objective function are the Propagation Delay (PD), Node Congestion (NC) and Link Congestion (LC). Energy is considered as a secondary parameter and is thus incorporated into the objective function in a way that it can be removed or considered as desired BIB010 . The weight values of the objective function parameters were found using the Particle Swarm Optimization (PSO) algorithm. OMC-RPL is simulated with four different classes of traffic and compared to standard ETX-RPL in terms of end-to-end delay, packet loss, network lifetime and traffic overhead. OMC-RPL then outperforms RPL in terms of end-toend delay for the class of traffic that requires minimum delay and likewise performs better than RPL in terms of PDR with the class of traffic that requires reliability. It is also found BIB010 that OMC-RPL reacts better to network failures since it can use a backup parent to replace a failed one. OMC-RPL outperforms RPL in terms of network lifetime by up to 41% and shows a better fairness in energy distribution by about 18%. The study also reports that incorporating the SDN controller with OMC-RPL reduces the number of exchanged control packets compared to both OMC-RPL and standard RPL by approximately 62% and minimizes the energy consumption by more than 50% compared to standard RPL BIB010 . In BIB009 , the authors propose a new composite energy-aware routing metric, RERBDI, which aims at enhancing the energy consumption of LLN nodes. The study considers the Battery Discharge Index (BDI) and the Residual Energy Ratio (RER) of nodes for taking the routing decision. The study also defines a new objective function, referred to as OFRBE, which combines the new proposed metric with ETX for calculating the rank and selecting the preferred parent. The study mentioned that using RER as a primary routing metric favors paths with a higher average residual energy. BDI was introduced as an additional cost function to favor paths that do not include nodes whose battery energies have been depleted or overburdened nodes BIB009 . Hence, the protocol avoids selecting paths in which some nodes have low residual energy even though the average residual energy is high. The Cooja simulator is used to compare the proposed scheme to standard RPL in terms of PDR, network lifetime and energy consumption. It is found that the RER metric outperforms ETX in terms of energy consumption by avoiding paths with lower average residual energy. However, there is still a chance that nodes with a low power profile could be selected as preferred parents, which may decrease network lifetime BIB009 . This situation is improved in RERBDI, which favors paths with higher average residual energy, while avoiding ones that include nodes with very low energy BIB009 . The new metric enhances network lifetime compared to RPL with hop-count, ETX and RER, but is slightly outperformed in terms of PDR, as it does not consider link quality BIB009 . Finally, it was shown that the proposed objective function has exceled at both the network lifetime and the PDR compared to RPL with ETX and Hop count. Summary and Insights: The implicit signaling through delayed DIO proposed in BIB001 has no extra overhead, however, a lost DIO might easily be misinterpreted as delayed, giving a false indication of higher workload at some nodes. In addition, the long transmission periods of Trickle's DIOs may cause slow recovery. Moreover, the protocol may suffer from the herding effect problem by always changing parent set members BIB007 .In BIB004 , because several parameters must be exchanged (i.e. data rate, retransmission count, throughput, transmission power and residual energy) to calculate the rank, this approach increases the size of DIO messages increasing the risk of fragmentation. This represents a problem in LLNs when multipath routing is used as two fragments belonging to the same packet may take different paths, increasing the probability of errors and packet loss. In addition, the monotonicity property does not hold for the ELT metric; hence, the study proposes to use ETX to build the DODAG and the ELT to calculate the rank of nodes. This would introduce an extra complexity to an already complex protocol  . The study in BIB005 suggests that each child node must report its current forwarding rate to its parent node by means of DAO messages to calculate the PDR. Apart from being an optional feature in RPL, only used when downward paths are needed, DAO messages are costly in term of overhead and energy consumption as they are transmitted in end-to-end fashion. The proposed protocol in BIB008 is based on ContikiMac that assumes that all nodes have similar wakeup intervals, which may not hold in all LLNs scenarios  . In addition, many additional fields are carried in the DIO message, which increases the risk of fragmentation. An obvious issue with the proposed protocol in BIB002 is that it is compared to RPL with hop count, even though the problem statement focuses on explaining the unsuitability of ETX metric to quantify the reliability of links. Thus, it would seem more logical to compare the proposed combined metric with ETX as both quantify link reliability  . Another issue is that the reported metric, if implemented according to the algorithm shown in the study, would violate the monotonicity property of rank, potentially resulting in a loop-prone DODAG topology, an issue that must be avoided. A noticeable issue related to the study in BIB006 is that only seven nodes are used for the evaluation which is very small to display the advantages of the proposed OF. In addition, while the increased delay in OF0 was attributed to the high churn, no justification was provided as to explain why OF0 experiences a higher churn given the perceived stability of the hop-count metric. In BIB010 , it is assumed that all nodes are within the range of the SDN controller so that the messages can be communicated by onehop; however, this is unrealistic in the majority of cases. In addition, while RPL uses a Trickle timer for communicating control packets, it is unclear what mechanism is used by the NONSDN-based OMC-RPL (OMC-RPL without SDN controller) for communicating such messages. Furthermore, even for SDN-based OMC-RPL, the reporting interval to the SDN is not quoted, although it could have a big effect on the control plane overhead. The decadic logarithm (i.e. log with base 10) is proposed in BIB009 to calculate the BDI (an additional cost inversely proportional to the node's residual energy). The calculated additional cost, based on the node's initial and residual energies, will be very small compared to the node's residual energy so will have no significant effect on the composite metric's final cost. Although, the study suggests using different weights to adjust the influence of metrics involved, restricting the weights to be within the interval [0, 1] limits the extent to which the influence of BDI can be adjusted.