Kaa is a good example of OpenSource IoT Platforms, it provides a data collection, notification and device to device communication for you.

Language	Connectivity Protocols
Java	Kaa Protocol (KP) over MQTT, CoAP and TCP

Kaa device to device communication is based on Events and you can send events even into server instead of devices. as its documentation says:

The Kaa Events subsystem is designed to generate endpoint events in real time, send them to other endpoints of the same owner and to Kaa server for processing.