MQTT

Clients keep a connection to a broker, publish messages to named topics, and subscribe with topic filters. QoS 0 delivers at most once, QoS 1 at least once, and QoS 2 exactly once, at the cost of more round trips.

For clients that disconnect, a persistent session (clean session off in 3.1.1, a session expiry interval in 5.0) makes the broker keep the client's subscriptions and queue QoS 1 and 2 messages for it until it reconnects. A retained message on a topic is stored and sent to new subscribers.

Small headers and long-lived connections suit constrained devices on unreliable networks, which is why MQTT is a default for sensor data, fleet telemetry, and industrial monitoring.

It assumes a broker is reachable. What a device publishes while it cannot reach the broker depends on the client library's own buffering, and devices cannot exchange messages with each other without the broker.

Offline Protocol's telemetry is opt-in runtime observability that the SDK batches and uploads itself, and messages between devices travel over the mesh with store-and-forward, so devices do not need a broker in reach to talk to each other.

The two can work together: an app can publish to an MQTT broker when it has a connection and use the mesh between devices when it does not. See delay-tolerant networking.