All in all, a very nice piece of work. I'm not reviewing for other than IoT issues. An issue in Section 2.2.1 (you decide if it's minor or major or really just a question): I do not understand why the Echo option requires opaque data, and why this should not be standardized, as it seems that the behavior you are seeking is standardized. As you give two example methods in the draft, why not reserve a few extra bits to specify which method is in use? This would also allow you to drop the necessary callback routines in libraries. Nit: The last sentence in 2.2: is this meant to be limited to OSCORE or all uses of DTLS? I found the inner/outer text confusing, and that a diagram might actually help.