There are several products as MQTT Broker/Server.

Mosquitto is one of the most favored products by developers,

and this website will test Mosquitto as a base material.

MQTT is located in the application layer in the OSI 7 layer.

First, start by installing Mosquitto.

A brief overview of MQTT → § MQTT 개요, IOT 개발자사이트 | momoiot .