# Experimental Evaluation of Unicast and Multicast CoAP Group Communication
## 1 Introduction  
The number of devices connected to the Internet is rapidly increasing. On the one hand, within traditionally-connected domains, the number of devices and the variety of device types are gradually increasing. On the other hand, traditionally unconnected domains are discovering the benefits of the Internet and are joining it continuously. The resulting Internet is now referred to as the Internet of Things (IoT) to stress the fact that it is connecting all sorts of things, not just people, computers and smart phones. It is expected that the IoT will soon contain more embedded devices than the traditionally-connected devices. These embedded devices are often constrained in their resources since they are optimized for low cost and/or battery operation. Most commonly, these devices have constraints in terms of the available amount of RAM, ROM and CPU power. They are often required to consume very little power, since they are often battery powered or rely on energy harvesting. Ideally, these devices should run for years without the need for costly battery replacements. These device constraints impose constraints on the networks in which these devices operate. These networks typically have lower bandwidths and higher error rates than common Internet networks. These networks are usually referred to as Low-power and Lossy Networks (LLNs). The early integration of constrained devices into the Internet resulted in a situation that was similar to what computer networks looked like about two decades ago: islands of computers communicating with their own protocol interconnected by complex multi-protocol gateways, which are inherently complex to design, manage and deploy. The resulting end-to-end Internet Protocol (IP) architecture has tackled those issues and has been widely accepted as the only alternative to design scalable and efficient networks of large numbers of communicating devices. For the future IoT, scalability, stability and efficiency are even more important than ever. IP therefore is the future proof choice for the IoT [1]. On top of that, past efforts have shown the feasibility of implementing a minimal footprint IPv6 stack on constrained devices (100 kB ROM, 10 kB RAM). A logical consequence of this evolution was the investigation of also adopting web service technologies on top of such end-to-end IP connectivity to avoid isolated islands in terms of APIs and semantics. Web services are at the basis of the success of today's Internet. They are ideal for building distributed applications through communication using well-defined message sequences and data formats in a system-independent way. Web-style applications and IoT applications share many of these basic communication properties; they are composed of separate systems that exchange data. Different forms of web paradigms exist, but research has shown that the REST paradigm is most suitable for constrained devices [2]. As a result of device and LLN constraints, the use of standard Internet protocols becomes often unfeasible, since these protocols were not designed to accommodate those constraints. In the past few years, there were many efforts to enable the extension of the Internet technologies to constrained devices. Most of these efforts were focusing on the networking layer, e.g., IPv6 over Low power Wireless Personal Area Networks (6 LoWPAN) [3] and IPv6 Routing Protocol for Low-Power and Lossy Networks (RPL) [4]. More recently, work has started to allow the integration of constrained devices in the Internet at the service level. The Internet Engineering Task Force (IETF) Constrained RESTful Environments (CoRE) working group [5] has realized the Representational State Transfer (REST) architecture in a suitable form for the most constrained nodes and networks. As a result, the Constrained Application Protocol (CoAP) [6] was introduced, a specialized RESTful web transfer protocol for use with constrained networks and nodes.