RISE AB RISE AB Ericsson AB The Constrained Application Protocol (CoAP) allows clients to "observe" resources at a server, and receive notifications as unicast responses upon changes of the resource state. In some use cases, such as based on publish-subscribe, it would be convenient for the server to send a single notification to all the clients observing a same target resource. This document defines how a CoAP server sends observe notifications as response messages over multicast, by synchronizing all the observers of a same resource on a same shared Token value. Besides, this document defines how Group OSCORE can be used to protect multicast notifications end-to-end from the CoAP server to the multiple CoAP clients registered as observers.