How MQTT broker communicate to client (display device or any output device)

Question

MQTT protocol works on pub-sub concept. Client can subscribe and publish topic to the Broker. Clients who are sensor kind of thing can publish respective topics (temperature, humidity, switch on/off request etc) to Broker. Broker will send the respective data to the subscribed clients (display device, any device which can on and off). Broker usually hosted on open internet.

I'm not getting the point how Broker will send the information to client device as the wifi connected device usually couldn't be reachable from open Internet as there will be no public IP assigned to such IoT devices. (I mean to say IP communication from Open internet to IoT device should not be successful)

Answer 1

The connection to the broker is established from the client to the broker, as this is an outbound connection as far as any home router/NAT device is concerned it will work just fine (in the same way a web browser would open a connection to a HTTP server).

The connection is also persistent, meaning the client opens it and keeps it open for the lifetime of the client. This means that the broker always has an open connection to forward messages to the client.

At no point does the broker try to connect to the client.