MQTT background

MQTT (Message Queuing Telemetry Transport) is a publish/subscribe protocol in which clients exchange messages through a broker. It runs over TCP, and its fixed header can be as small as two bytes, which makes it a common choice for collecting equipment and sensor data.

How PDComm uses MQTT

PDComm uses MQTT as the broker transport of OPC UA PubSub. The OPC UA simulator programs (GUI, headless and MCP) publish UADP messages to an MQTT broker and subscribe to them. Read MQTT in PDComm first for where you use it and how it connects.

Standards

These pages explain concepts and cite the standards instead of reproducing them. Section numbers refer to MQTT 5.0 unless a page notes the MQTT 3.1.1 equivalent.

Standard	Document
MQTT 3.1.1	OASIS Standard, 29 October 2014 (published unchanged as ISO/IEC 20922:2016)
MQTT 5.0	OASIS Standard, 7 March 2019
MQTT mapping of OPC UA PubSub	OPC 10000-14 (OPC UA Part 14) §7.3.4
Mosquitto configuration	Eclipse Mosquitto mosquitto.conf(5) manual page

Reading order

Step	Page	What it covers
1	Publish/subscribe and topics	Brokers, clients, topic names and filters, wildcards, topic design
2	QoS and retained messages	Packet flows of QoS 0, 1 and 2, message order, retained messages
3	Sessions, keep alive and Will	Clean Session and Session Expiry, keeping a connection alive, abnormal disconnects
4	MQTT 5.0 features	Properties, reason codes, topic aliases, user properties, flow control
5	TLS and authentication	Server authentication, client certificates, user names and passwords, authorization
6	MQTT in PDComm	Where you use MQTT and how the simulator connects to a broker

Reference broker

The hands-on pages use Eclipse Mosquitto 2.1 as the reference broker. Mosquitto is an open-source broker that supports MQTT 3.1, 3.1.1 and 5.0. It ships the mosquitto_pub and mosquitto_sub command-line clients, so you can send and watch traffic yourself. Any other broker that supports MQTT 5.0 works the same way. Broker configuration lists what a broker needs.