# A Survey on Congestion Control for RPL-Based Wireless Sensor Networks
## 1 Introduction  
IoT (Internet of Things) is one of the key technologies enabling the next industrial revolution. As the IoT industry evolves rapidly, IoT standardization has been done by more than 100 working groups over the world. The IETF (Internet Engineering Task Force) also has many working groups (WGs) focusing on IoT standardization for resource-constrained devices and networks. For instance, the 6 lo (IPv6 over networks of resource-constrained Nodes) WG defines methods for adapting IPv6 to constrained node networks. The ROLL (routing over low-power and lossy networks) WG focuses on routing issues for resource-constrained devices. The CoRE (constrained restful environments) WG was chartered to provide a framework for resource-oriented applications intended to run on constrained IP networks. The ACE WG focuses on authentication and authorization mechanisms in IoT and the 6 TiSCH (IPv6 over the TSCH mode of IEEE 802.15.4e) WG issues on adding the TSCH (time-slotted channel hopping) mode to IEEE 802.15.4e. In this survey, we are focused on RPL (IPv6 routing protocol for low power and lossy networks) [1] standardized by the ROLL WG. In a low-power and lossy network (LLN), routers typically operate with constraints on processing power, memory, and energy. Therefore, LLNs are characterized by high loss rates, low data rates, and instability. RPL was designed with the objective to meet routing requirements that the IETF ROLL working group defined for four LLN application domains: urban LLNs, industrial LLNs, home automation LLNs, and building automation LLNs [2; 3; 4; 5], but RPL aims to be useful in a wide range of LLN application domains. For wide applicability, RPL separates packet processing and forwarding from routing optimization objectives such as minimizing energy, minimizing latency, and satisfying constraints. In compliance with the standard, RPL has been enhanced, optimized, and implemented to meet the requirements for the target applications. Depending on the application scenario, the issues to be resolved are different. One of the common issues is congestion. In a large LLN, even low-rate traffic generated by each node can cause congestion at areas where the traffic is concentrated. Congestion causes packet losses and delay in the first place. Packet losses in LLNs also may affect reliability and robustness of the applications if end-to-end congestion control and packet recovery is not ensured by the upper layer protocol due to resource limitations. In addition, congestion wastes energy and shortens the network lifetime. Load-balancing is a relevant issue to the network lifetime and congestion. Nonuniform energy depletion due to unbalanced workload distribution will shorten the overall network lifetime. Load-balancing can be utilized to alleviate congestion. Recently, several insightful surveys on RPL have been proposed. [PERSON] et al. surveyed the current status of RPL based on the original requirement defined by the IETF and discussed new requirements taking the current IoT trends into account [6]. [PERSON] et al. provided a pioneering survey which reviews the history of research efforts in RPL from a variety of viewpoints [7]. They present a topic-oriented survey for each of 13 subtopics related to RPL, including load balancing. [PERSON] et al. performed a survey on RPL enhancements focusing on topology, security, and mobility [8]. [PERSON] et al. conducted another comprehensive survey [9] covering background knowledge on LLNs to RPL enhancements. Since [9] reviews RPL enhancements and extensions related to OFs, downward routing, and route maintenance, it includes congestion control and load balancing within its scope, and gives insights on the solutions to congestion via OF enhancements. However, [PERSON] et al. [9] classifies OF enhancements by routing metrics and multipath routing, not by the main purposes such as congestion control, which means that the readers interested in a specific purpose of RPL enhancement need to make additional efforts to understand the relevant literature.