Protocols such as MQTT, the Constrained Application Protocol (CoAP), the Data Distribution Protocol (DDP), the Advanced Message Queuing Protocol (AMQP), and the Extensible Messaging and Presence Protocol (XMPP) run on top of lower-layer communication protocols and provide the ability for both clients and servers to efficiently agree on data to exchange. REST communications can also be run very effectively within many IoT systems. As of the time of writing, REST and MQTT are popular choices for IoT systems.