An M2M/IoT resource directory entity may discover and register to other resource directory entities by including criteria attributes in discovery broadcasts and then processing link and metadata that may be included in responses. The resource directory with registrations to other resource directories may service a client request by selecting other resources directories to query based on the request or a query criteria contained therein, and then process link and metadata in responses from the other resource directory entities to aggregate a response to the client. Directory-to-directory registrations may be facilitated via using hierarchical parent-child resource directory identifiers, a peer-to-peer identifier regime, or a combination of the two. Observe notifications may include indicia registrations states, e.g., via change and delete response codes.