A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> Suppression Mechanism Inefficiency (Section IV-C-2) <s> The diversity of applications that current and emerging Wireless Sensor Networks (WSNs) are called to support imposes different requirements on the underlying network with respect to delay and loss, while at the same time the WSN imposes its own intricacies. The satisfaction of these requirements highly depends on the metric upon which the forwarding routes are decided. In this view, the IETF ROLL group has proposed the RPL routing protocol, which can flexibly work on various routing metrics, as long as they hold specific properties. The system implementer/user is free to decide whether to use one or multiple routing metrics, as well as the way these metrics can be combined. In this paper, we provide ways to quantify the routing metrics so that they can be combined in an additive or lexical manner. We use extensive simulation results to evaluate the impact of several routing metrics on the achieved performance. <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> Suppression Mechanism Inefficiency (Section IV-C-2) <s> This paper proposes a new framework to improve the power efficiency of the IPv6 routing protocol for low-power and lossy networks (RPL). The new framework makes it possible for the root node of a destination-oriented directed acyclic graph (DODAG root) to determine which new device will join the DODAG and its parent DODAG node, which is the parent node for the new device on the DODAG tree, based on the proposed composite value. This composite value is based on the distance between a new device and a potential parent DODAG node, the number of child DODAG nodes, which are the DODAG nodes directly under the potential parent DODAG node, and the hop-count between the DODAG root and the new device. The device having the lowest composite value to a potential parent DODAG node is chosen by the DODAG root, and the new device joins the DODAG by connecting itself directly under the parent DODAG node. When this framework is applied to RPL, the longevity of the devices on the DODAG increases dramatically, becoming as much as 10 times longer. In addition, by changing the weights of each metric of the composite value, we can adjust the composite value appropriately depending on the features of various wireless sensor networks. <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> Suppression Mechanism Inefficiency (Section IV-C-2) <s> To design a routing protocol for applications over low-power and lossy networks (LLNs), the IETF ROLL Working Group standardized the IPv6 Routing Protocol for LLNs (RPL), which organizes nodes in a LLN into a tree-like topology called Destination Oriented Directed Acyclic Graph (DODAG). RPL shows good scalability and fast network setup. However, it may suffer from severe unreliability due to the selection of suboptimal routes with low quality links. To optimize the reliability of RPL routes, this paper proposes a stability metric based routing protocol named sRPL for reliable routing and data collection in LLNs. We introduce a new routing metric for RPL called stability index (SI), which exploits stability characteristics of RPL nodes to select more stable routes. In addition, we present a passive and lightweight network layer technique to measure the bi-directional expected transmission count (ETX) for wireless links in LLNs. As a use case of SI, we combine SI metric with ETX metric to make routing decisions. Simulation results show that sRPL can improve packet delivery rate of RPL routing protocol by 20%. <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> Suppression Mechanism Inefficiency (Section IV-C-2) <s> Low-power wireless networks play an important role in the Internet of Things. Typically, these networks consist of a very large number of lossy and low-capacity devices, challenging the current state of the art in protocol design. In this context the Trickle algorithm plays an important role, serving as the basic mechanism for message dissemination in notable protocols such as RPL and MPL. While Trickle's broadcast suppression mechanism has been proven to be efficient, recent work has shown that it is intrinsically unfair in terms of load distribution and that its performance relies strongly on network topology. This can lead to increased end-to-end delays (MPL), or creation of sub-optimal routes (RPL). Furthermore, as highlighted in this work, there is no clear consensus within the research community about what the proper parameter settings of the suppression mechanism should be. We propose an extension to the Trickle algorithm, called adaptive-k, which allows nodes to individually adapt their suppression mechanism to local node density. Supported by analysis and a case study with RPL, we show that this extension allows for an easier configuration of Trickle, making it more robust to network topology. <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> Suppression Mechanism Inefficiency (Section IV-C-2) <s> Wireless sensor network protocols very often use the Trickle algorithm to govern information dissemination. For example, the widely used IPv6 Routing Protocol for Low-Power and Lossy Networks (RPL) uses Trickle to emit control packets. We derive an analytical model of Trickle to take into account multiple redundancy constants and the common lack of synchronization among nodes. Moreover, we demonstrate message count unfairness when Trickle uses a unique global redundancy constant because nodes with less neighbors transmit more often. Consequently, we propose a heuristic algorithm that calculates a redundancy constant for each node as a function of its number of neighbors. Our calculated redundancy constants reduce unfairness among nodes by distributing more equally the number of transmitted messages in the network. Our analytical model is validated by emulations of constrained devices running the Contiki Operating System and its IPv6 networking stack. Furthermore, results very well corroborate the heuristic algorithm improvements. <s> BIB005
Slow convergence time due to the listen-only period. Listen-only period (Section IV-C-1) Unrealistic MAC protocol with 100% duty-cycle is used for simulation experiments. Fast convergence time, however, moderate in lossy networks as there is listen-only period in the subsequent intervals. BIB004 adaptive-k Routing maintenance Allows each node to tune its redundancy factor dynamically based on the number of its neighbors The number of DIOs may not reflect correctly the number of neighbors. Slow convergence time due to the listen-only period. BIB005 Trickleoffset Adding more a complexity by introducing two new configuration parameters. Slow convergence time due to the listen-only period. In BIB002 , the authors highlighted the fact that relying on hopcount only in calculating the ranks of nodes may result in constructing paths characterized by long physical distances. As transmitter energy consumption is directly proportional to the square of the distance between communicating nodes, that may lead to routes that suffer from higher power consumption rates. The authors propose a new composite metric based on the distance between the node and its potential parent, the number of children that the potential parent has, and the hop count metric. The new framework is compared to OF0 and to the Karkazis BIB001 composition metric in terms of device longevity and power consumption. It is shown that the proposed framework manages to decrease significantly power consumption and enhance the longevity of the DODAG. The instability and unreliability issues of RPL are considered in BIB003 . The authors report that RPL may suffer from frequent route changes that may affect network performance negatively. They assert that even though several metrics are defined for RPL, there is not a metric that represents the stability of nodes. Thus, a new stability metric, referred to as Stability Index (SI), is proposed, to overcome this issue. The new metric relies on the transmission rate of control messages to estimate the stability of links. The SI is measured at each node by adding up the weighted number of DIO, DIS and DAO control messages transmitted during a specified interval (the Hearing Window) and dividing the sum by the size of the interval BIB003 . The weighting is used to give each type of control message a different importance. The study suggests combining the new metric with ETX to boost further the protocol reliability. The proposed and combined metrics are evaluated using NS2 simulations and compared to the RPL with hop-count and ETX metrics, in terms of control message overhead, latency and packet delivery rate BIB003 . It is shown that the new composite metric reduces significantly the CDF of control plane overhead by up to 90% and the average number of transmissions by up to 50% compared to RPL hop count and ETX. In addition, the simulation results indicate that SI-RPL and SI-ETX-RPL outperform both ETX-RPL and HC-RPL in terms of packet delivery rate and that the amount of enhancement depends on the size of the hearing window. On the other hand, SI-RPL and SI-ETX-RPL have slightly longer latency compared to HC-RPL, as they prefer more stable and reliable paths at the cost of more hops.