Observing Resources in CoAP

CoAP is a RESTful application protocol for constrained nodes and networks. The state of a resource on a CoAP server can change over time. This document specifies a simple protocol extension for CoAP that enables CoAP clients to "observe" resources, i.e., to retrieve a representation of a resource and keep this representation updated by the server over a period of time. The protocol follows a best- effort approach for sending new representations to clients, and provides eventual consistency between the state observed by each client and the actual resource state at the server.