The Internet of Things (IoT) can be considered as a modern manifestation of 
Mark Weiser's classic vision of ubiquitous computing where tiny networked 
computers become part of everyday objects interweaving the virtual world and 
the physical world. The concept of the IoT originated some 15 years ago from 
linking real-world artifacts to virtual counterparts through radio-frequency 
identification (RFID) tags. More recently, environments have become 
‘smart' by augmenting physical objects with sensing or actuation 
capabilities and networking them with digital services. The ongoing 
standardization of Internet protocols for such IoT devices enables the 
seamless integration of smart things into the Internet. This trend is expected 
to eventually result in hundreds of billions of connected devices that need to 
be programmed, managed, and maintained. It has been shown that Web technology 
can significantly ease this process by providing well-known patterns and tools 
for developers and users. The existing solutions are, however, often too 
heavyweight for highly resource-constrained IoT devices. Indeed, most 
connected devices are expected to remain resource-constrained, as progress in 
technology witnessed by Moore's Law is primarily leveraged to minimize 
dimensions, power consumption, and unit costs. 

This dissertation presents a comprehensive solution for the seamless 
integration of highly resource-constrained IoT systems into the World Wide 
Web. Our thesis is that existing protocols and programming models do not 
effectually meet the needs of the IoT. We identify two key challenges for the 
vision to succeed: application-layer interoperability and improved usability 
for both developers and users. Both requirements can be met by an approach 
that amalgamates results from the field of Wireless Sensor Networks and the 
World Wide Web. This leads to the research questions (i) how to scale Web 
technology down to resource-constrained devices, (ii) how to scale it up to 
hundreds of billions of devices, and (iii) how to use it to improve the 
usability of the tiny networked computers. Our work addresses the resulting 
challenges with the following contributions: Being actively involved in the 
design and standardization of the Constrained Application Protocol (CoAP) 
within the Internet Engineering Task Force (IETF), we (i) evaluate the new Web 
protocol in the different components of IoT systems, namely 
resource-constrained devices, Cloud-based services, and user interaction. 
Based on this, we (ii) propose system architectures and guidelines for an 
optimal implementation and utilization of CoAP. Furthermore, we (iii) present 
concepts and tools for Web-like software development for the IoT. To support 
our thesis, we also (iv) provide working open source implementations of our 
concepts, which build the basis for several IoT projects in academia and 
industry. 

More concretely, we show in this dissertation that the CoAP protocol suite 
closes the technological gap between low-power IoT devices and the well-known 
patterns of the Web. We first consider resource-constrained environments, 
where efficient Web technology can relieve application developers from the 
burdens of embedded programming while maintaining the performance of classic 
approaches. In addition to a proof of concept and system evaluation, we give 
guidelines that allow for significantly smaller memory footprints of CoAP 
implementations. Next, we show that the low overhead of the new protocol also 
improves performance in unconstrained environments, such as IoT cloud services 
that have to manage the myriad of IoT devices. We present a system 
architecture for scalable back-end services that outperforms classic 
high-performance Hypertext Transfer Protocol (HTTP) Web servers as well as 
other state-of-the-art CoAP implementations. Finally, our work evaluates 
usability aspects of the Web programming model for IoT applications. We show 
that Web mashups, that is, the linking of different services through 
lightweight scripting, are also directly applicable to our concepts for highly 
resource-constrained systems. Complementary, we study Web browser support for 
CoAP to fully close the gap between IoT devices and the Web. Based on these 
findings, we motivate the design decisions behind CoAP, in particular for our 
contributions, and explain how developers can improve their protocol 
implementations accordingly. 

Along with this dissertation, we deliver open source implementations of our 
approach that go beyond prototypes. Our Erbium (Er) REST Engine is an 
optimized CoAP implementation for constrained environments. It provides 
application developers with resource handler abstractions like regular Web 
frameworks while maintaining a small memory footprint. Erbium became the 
default CoAP implementation for Contiki, a popular embedded operating system 
for the IoT that is used in many industry products. Our Java-based Californium 
(Cf) CoAP framework enables high-performance CoAP services. The project also 
contains a DTLS 1.2 implementation called Scandium (Sc) and the Actinium (Ac) 
application server, a RESTful runtime system for Web-like IoT mashups. 
Representing the state of the art for RESTful IoT services, the Californium 
project was adopted by the Eclipse Foundation within its IoT Industry Working 
Group.