## 1 Introduction  
Internet protocol technology is rapidly spreading to new domains where constrained embedded devices such as sensors and actuators play a prominent role. This expansion of the Internet is comparable in scale to the spread of the Internet in the '90s and the resulting Internet is now commonly referred to as the Internet of Things (IoT). The integration of embedded devices into the Internet introduces several new challenges, since many of the existing Internet technologies and protocols were not designed for this class of devices. These embedded devices are typically designed for low cost and power consumption and thus have very limited power, memory and processing resources and are often disabled for long-times (sleep periods) to save energy. The networks formed by these embedded devices also have different characteristics than those typical in today's Internet. These constrained networks have different traffic patterns, high packet loss, low throughput, frequent topology changes and small useful payload sizes. In the past few years, several innovations were developed to enable the extension of Internet technologies to constrained devices, moving away from proprietary architectures and protocols. Most of these efforts focused on the networking layer: _IPv6 over Low-Power Wireless Personal Area Networks_ (RFC 4919) [1], _Transmission of IPv6 Packets over IEEE 802.15.4 Networks_ (RFC 4944) [2], IETF _routing over low-power and lossy networks_[3] or the ZigBee adoption of _Internet Protocol Version 6_ (IPv6) [4]. These new standards enable the realization of an Internet of Things, where end-to-end IP-based network connectivity with tiny objects such as sensors and actuators becomes possible. However, it was not global connectivity that was at the basis of the great success of the current Internet, but the World Wide Web and the resulting web service technologies. Today, an embedded counterpart of web service technology is needed in order to exploit all great opportunities offered by the Internet of Things and turn it into a Web of Things. Recently, standardization work has started within the IETF _Constrained RESTful Environments_ (CoRE) working group [5] to allow the integration of constrained devices with the Internet at the service level. With these technologies, it has now become possible to deploy a self-organizing sensor network, to interconnect it with IPv6 Internet and to build applications that interact with these networks using embedded web service technology. Application developers using high-level programming languages can count on these standardized technologies in the realization of a Semantic Web of Things or Sensor Web, which enables data producers and users to publish and access sensor information via web- and standards based interfaces. In this paper, we will briefly review the history of integrating constrained devices into the Internet, with a prime focus on the IETF standardization work. The key realizations of the related IETF working groups will be discussed, complemented with an extensive overview of related research results that illustrate how these novel technologies can be extended or used to tackle other problems and with a discussion on open issues and challenges. The remainder of this paper is organized as follows. In Section 2, we discuss the evolution from proprietary solutions towards IP-based integration of constrained devices using standardized protocols, introducing the most relevant standardization bodies and groups. In the following sections we will extensively discuss these standardization efforts, related work and open issues and challenges. Section 3 introduces IEEE 802.15.4 as it is the most widely used physical layer and MAC layer in constrained networks, providing the foundations for the networking protocols at the higher layers. From that point on, the focus is shifted to the IETF standardization, discussing IETF 6 LoWPAN, IETF RoLL and IETF CoRE extensively in Sections 4, 5 and 6 respectively.