Connecting the web with the web of things: lessons learned from implementing a CoAP-HTTP proxy

The Constrained Application Protocol (CoAP) is a lightweight application layer protocol for the Internet of Things. CoAP is based on HTTP mechanisms to build RESTful Web services. In contrast to HTTP, CoAP was designed for machine-to-machine (M2M) communication and uses a binary representation. This allows efficient transport and processing in resource-constrained networks such as Wireless Sensor Networks (WSN). Due to the analogy to REST, a mapping between CoAP and HTTP is possible. In this paper one of the first translating CoAP-HTTP proxies is presented, that provides HTTP clients transparent access to CoAP resources and vice versa. Furthermore, caching relieves CoAP servers, which solves a key challenge for the Internet of Things: to allow a constant/permanent availability of resources from a network of constrained devices, which are required to minimize data transmissions due to their noticeable restrictions in power consumption. We describe, which issues of the translation we found during the time of implementation and testing, and explain how the proxy handles these issues. Finally, an evaluation, using real WSN hardware, is given, and an approximation scheme on how much transmissions can be saved by caching resources is provided.