August Betzler

CoAP congestion control for the internet of things

CoAP is a lightweight RESTful application layer protocol devised for the IoT. Operating on top of UDP, CoAP must handle congestion control by itself. The core CoAP specification defines a basic congestion control mechanism, but it is not capable of adapting to network conditions. However, IoT scenarios exhibit significant resource constraints, which pose new challenges on the design of congestion control mechanisms. In this article we present CoCoA, an advanced congestion control mechanism for CoAP being standardized by the Internet Engineering Task Force CoRE working group. CoCoA introduces a novel round-trip time estimation technique, together with a variable backoff factor and aging mechanisms in order to provide dynamic and controlled retransmission timeout adaptation suitable for the peculiarities of IoT communications. We conduct a comparative performance analysis of CoCoA and a variety of alternative algorithms including state-of-the-art mechanisms developed for TCP. The study is based on experiments carried out in real testbeds. Results show that, in contrast to the alternative methods considered, CoCoA consistently outperforms the default CoAP congestion control mechanism in all evaluated scenarios.

Congestion control for CoAP cloud services

The Constrained Application Protocol (CoAP) is a new Web protocol for the Internet of Things that allows to connect IoT devices directly to services hosted in the cloud. CoAP is based on UDP to better fit the requirements of constrained environments with resource-constrained nodes and low-power communication links. Being an Internet protocol, CoAP must still adhere to congestion control, primarily to keep the backbone network stable. Thus, the base specification uses conservative parameter values for the number of open requests, the retransmission timers, and the overall message rate. More powerful CoAP nodes, however, can use metrics to optimize these parameters to achieve a better quality of service. For this, the IETF CoRE working group is designing an advanced congestion control mechanism for CoAP called CoCoA. This paper presents first evaluation results for a mechanism that improves the communication between cloud services and resource-constrained IoT devices. We implement CoCoA for the Californium (Cf) CoAP framework and evaluate its performance on a wireless sensor network testbed that runs IPv6. Our results show that CoCoA can better utilize the available network capacity and can increase throughput by 19-112%.

CoCoA+: an advanced congestion control mechanism for CoAP

The Constrained Application Protocol (CoAP) has been designed by the Internet Engineering Task Force (IETF) for Internet of Things (IoT) devices. Due to the limited radio channel capacities and hardware resources of such devices, congestion can be a serious problem. CoAP addresses this important issue with a basic congestion control mechanism. CoCoA, an Internet-Draft proposal, introduced alternative congestion control mechanisms for CoAP. Yet, there has been limited evaluation of these congestion control mechanisms in the literature. In this paper, we assess the methods applied in CoCoA in detail and propose improvements to address the shortcomings observed in the congestion control mechanisms. We carry out simulations to compare the congestion control performance for default CoAP, CoCoA, and our new proposal, CoCoA+, in a variety of network topologies and use cases. The results show that CoCoA+ outperforms default CoAP and achieves better results than CoCoA in the majority of considered cases.

Congestion control in reliable CoAP communication

The development of IPv6 stacks for wireless constrained devices that have limited hardware resources has paved the way for many new areas of applications and protocols. The Constrained Application Protocol (CoAP) has been designed by the IETF to enable the manipulation of resources for constrained devices that are capable of connecting to the Internet. Due to the limited radio channel capacities and hardware resources, congestion is a common phenomenon in networks of constrained devices. CoAP implements a basic congestion control mechanism for the transmission of reliable messages. Alternative CoAP congestion control approaches are a recent topic of interest in the IETF CoRE Working Group. New Internet-Drafts discuss the limitations of the default congestion control mechanisms and propose alternative ones, yet, there have been no studies in the literature that compare the original approach to the alternative ones. In this paper, we target this crucial study and perform evaluations that show how the default and alternative congestion control mechanisms compare to each other. We use the Cooja simulation environment, which is part of the Contiki development toolset, to simulate CoAP within a complete protocol stack that uses IETF protocols for constrained networks. Through simulations of different network topologies and varying traffic loads, we demonstrate how the advanced mechanisms proposed in the drafts perform relative to the basic congestion control mechanism.

Large-Scale Performance Evaluation of the IETF Internet of Things Protocol Suite for Smart City Solutions

The Internet of Things (IoT) intends to interconnect massive amount of heterogeneous, smart devices, with the goal of interweaving the virtual world with the physical world. Smart Cities are typical IoT application domains, comprising networks with large number of sensors that survey environmental data in order to provide different services, such as City Mobility solutions that facilitate on-street parking and traffic flow monitoring. Standards and specifications defined by the Internet Engineering Task Force (IETF), like IPv6, the IPv6 over Low power Wireless Personal Area Networks (6LoWPAN) adaptation layer, and the IPv6 Routing Protocol for Low-Power and Lossy Networks (RPL) are cornerstones of the IoT. In this paper we carry out large-scale experimental evaluations in an IoT testbed of 251 nodes to analyze the performance of the IETF IoT protocol suite in such a large-scale network. We define a City Mobility Solution application, using traces from a commercial Smart City deployment and the commonly employed Contiki implementation of IETF IoT protocol suite. The results show that the out of the box Contiki IoT protocol stack is not capable of delivering a satisfying performance. However, after a thorough analysis of the initial results, a set of improved parameter configurations is derived that allows the network to achieve much higher performance. Among others, improvements of 60.39% in PDR and 63.67% in delay are achieved. Furthermore, the paper presents and discusses the technical solutions and best-practice guidelines for a specific City Mobility solution being developed by Urbiotica, a company with ample expertise in Smart City deployments.

A Holistic Approach to ZigBee Performance Enhancement for Home Automation Networks

Wireless home automation networks are gaining importance for smart homes. In this ambit, ZigBee networks play an important role. The ZigBee specification defines a default set of protocol stack parameters and mechanisms that is further refined by the ZigBee Home Automation application profile. In a holistic approach, we analyze how the network performance is affected with the tuning of parameters and mechanisms across multiple layers of the ZigBee protocol stack and investigate possible performance gains by implementing and testing alternative settings. The evaluations are carried out in a testbed of 57 TelosB motes. The results show that considerable performance improvements can be achieved by using alternative protocol stack configurations. From these results, we derive two improved protocol stack configurations for ZigBee wireless home automation networks that are validated in various network scenarios. In our experiments, these improved configurations yield a relative packet delivery ratio increase of up to 33.6%, a delay decrease of up to 66.6% and an improvement of the energy efficiency for battery powered devices of up to 48.7%, obtainable without incurring any overhead to the network.

Evaluation of Advanced Congestion Control Mechanisms for Unreliable CoAP Communications

Networks of constrained devices play an important role in the Internet of Things (IoT). In such networks congestion may lead to significant performance decrease and is a recurring phenomenon given the restricted hardware capacities of constrained devices and the limitations of low-power radios. The Constrained Application Protocol (CoAP) designed for IoT communications defines a basic congestion control mechanism for the exchange of messages with end-to-end reliability between two endpoints. Yet, for transmissions without end-to-end reliability the CoAP base specification does not determine any congestion control mechanism. Proposals for congestion control mechanisms for unreliable CoAP communications are made in the CoAP observe and CoAP Simple Congestion Control/Advanced (CoCoA) Internet drafts, which introduce a static and a dynamic rate limitation for outgoing messages, respectively. In this paper experimental evaluations are carried out to determine the performance of the different proposals. We find out that, in contrast with the other analyzed approaches, CoCoA is able to maintain high performance in all the analyzed scenarios, thanks to its adaptive nature.

Experimental evaluation of congestion control for CoAP communications without end-to-end reliability

The Constrained Application Protocol (CoAP) has been designed by the Internet Engineering Task Force (IETF) for Internet of Things (IoT) communications. CoAP is a lightweight, request/response-based RESTful protocol that has been tailored to fulfill the requisites of IoT environments, such as severely limited device hardware and link capacities. In IoT networks, congestion is a major issue that causes performance losses or may even render the network useless. Thus, the use of a congestion control mechanism is essential for the performance of such networks. CoAP defines a very basic congestion control mechanism for the reliable exchange of messages between endpoints, however it does not specify congestion control for communications without end-to-end reliability, even though the latter represent a relevant share of CoAP communications. Two extensions to CoAP, Observe and Simple CoAP Congestion Control/Advanced (CoCoA), introduce rate control mechanisms for such communications yet these extensions have not yet been compared or evaluated. In this paper, we empirically evaluate these rate control mechanisms for unreliable CoAP communications between devices over emulated GPRS/UMTS links and in a real IEEE 802.15.4 multihop testbed of constrained devices. The results show that in contrast to Observe, CoCoA performs better than, or at least similarly to, default CoAP in terms of both packet delivery ratio and delay in all analyzed scenarios.