Observe Notifications as CoAP Multicast Responses

The Constrained Application Protocol (CoAP) allows clients to "observe" resources at a server and to receive notifications as unicast responses upon changes of the resource state. In some use cases, such as those based on publish-subscribe, it would be convenient for the server to send a single notification addressed to all the clients observing the same target resource. This document updates RFC7252 and RFC7641, and it defines how a server sends observe notifications as response messages over multicast, synchronizing all the observers of the same resource on the same shared Token value. Besides, this document defines how the security protocol Group Object Security for Constrained RESTful Environments (Group OSCORE) can be used to protect multicast notifications end-to- end between the server and the observer clients.