ARM Limited As Internet of Things (IoT) deployments increase steadily the need for a better user experience for handling the authentication and authorization tasks in constrained environments increases. While several technologies have been developed already that allow federated access to protected resource the nature of IoT deployments requires care with the limited resources available on many of these devices. This document defines a new OAuth 2.0 authorization grant for the interaction between constrained clients and resource servers to obtain access tokens for access to protected resources. It does so by leveraging prior work on OAuth 2.0, CoAP, and DTLS.