Protocols
Eight protocols carry most machine-to-machine traffic. They are not rivals so much as answers to different questions: who starts the conversation, what happens when the link drops, and how much a device must understand about the thing it is talking to.
| Protocol | Since | Pattern |
|---|---|---|
| MQTT | 1999 | Publish / subscribe through a broker |
| CoAP | 2014 | Request / response, REST-style |
| LwM2M | 2017 | Device management and data, object model |
| AMQP 1.0 | 2012 | Brokered queues, reliable transfer |
| OPC UA | 2008 | Client / server with an information model; PubSub since 2018 |
| DDS | 2004 | Data-centric publish / subscribe, no broker |
| Modbus | 1979 | Poll / response, registers |
| Matter | 2022 | Clusters and attributes; local control |
MQTT
Written in 1999 by Andy Stanford-Clark of IBM and Arlen Nipper of Arcom to monitor an oil pipeline over a satellite link. Three delivery guarantees (QoS 0, 1, 2), retained messages and a “last will” that tells subscribers when a device drops off. Version 3.1.1 became an OASIS standard in 2014; version 5.0 followed in 2019.
CoAP
HTTP’s verbs and URIs squeezed into a four-byte header for microcontrollers. “Observe” (RFC 7641) lets a client subscribe to a resource instead of polling it; OSCORE (RFC 8613) protects the message itself, so security survives a proxy.
LwM2M
Lightweight M2M defines what a managed device looks like — standard objects for connectivity, location, firmware and so on — and how a server bootstraps, reads, writes and updates it. Version 1.0 was published in 2017, 1.1 in 2018 and 1.2 in 2020.
AMQP 1.0
Heavier than MQTT and built for back-end integration: flow control, transactions and settlement of each transfer. Common on the cloud side of a machine fleet rather than on the device.
OPC UA
The lingua franca of factory automation. Its strength is the information model: a machine describes its own structure, so a client can discover what a press or a robot exposes rather than being told in advance.
DDS
Peers discover each other and share typed data with quality-of-service contracts — deadlines, history, reliability. It is the middleware underneath ROS 2, and turns up in vehicles, avionics and medical devices.
Modbus
The ancestor still in service. No security and no discovery, but almost every industrial gateway speaks it, which is why new systems are so often built on top of it.
Matter
Version 1.0 shipped in October 2022. It standardises how home devices from different makers are commissioned and controlled locally, with device attestation certificates issued at manufacture.
Choosing
A battery sensor on a lossy network: CoAP, or LwM2M if it also needs managing. Many devices feeding many consumers through one place: MQTT. A factory cell where a client must discover what a machine offers: OPC UA. Robots and vehicles sharing state with deadlines and no central server: DDS. Legacy equipment: Modbus at the edge, translated by a gateway into one of the others.