MQTT protocol steps

the MQTT protocol steps in a clean way. Think of MQTT as a conversation between a client (device) and a broker (server).

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1. Connect

• Client establishes a TCP connection with the broker (default port 1883, or 8883 if TLS/SSL is used).

• Then sends an MQTT CONNECT packet with:

  • Client ID (unique name, e.g., device123).
  • Optional username/password.
  • Keep Alive time (heartbeat interval).
  • Optional Will Message (sent if client disconnects unexpectedly).

Broker replies with CONNACK (Connection Acknowledgment). If success → you’re “online”.

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2. Publish / Subscribe Setup

• Publish (PUB): Client sends a message to a topic (like a channel name). Example: Topic: "sensor/temperature" Message: "25.5 C"
• Subscribe (SUB): Client tells broker what topics it wants to receive. Example: subscribe to "device/123/cmd" → broker forwards messages from that topic.

Broker replies with SUBACK (subscription acknowledgment).

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3. Message Exchange

Once subscribed:

• Other clients publish messages → broker forwards to subscribers.
• Client can also publish its own data.

Messages have QoS (Quality of Service) levels:

1. QoS 0 – At most once (fire and forget).
2. QoS 1 – At least once (broker confirms receipt).
3. QoS 2 – Exactly once (more handshake, rare in small devices).

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4. Keep Alive / Ping

• Client must send PINGREQ at intervals < keep-alive time (set during CONNECT).
• Broker replies with PINGRESP.
• If broker doesn’t receive ping or messages → assumes client is dead → may send Will Message.

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5. Disconnect

• Client can send a DISCONNECT packet before closing TCP.
• If device dies suddenly (no DISCONNECT), broker uses keep-alive timeout to detect.

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Sequence in Practice

Here’s a normal MQTT session:

1. TCP connect → Broker.
2. CONNECT → Broker.
3. CONNACK ← Broker.
4. SUBSCRIBE → "device/123/cmd".
5. SUBACK ← Broker.
6. PUBLISH → "device/123/data" : "Temp=25.3".
7. PINGREQ ↔ PINGRESP (every keep-alive).
8. DISCONNECT → Close connection cleanly.

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Visual Example (Sensor Device)

[Device] --CONNECT--> [Broker]
[Broker] <--CONNACK-- [Device]

[Device] --SUBSCRIBE "cmd"--> [Broker]
[Broker] <--SUBACK------------- [Device]

[Device] --PUBLISH "data:25C"--> [Broker]
[Broker] --FORWARD "data:25C"--> [Subscribers]

[Device] --PINGREQ--> [Broker]
[Broker] <--PINGRESP-- [Device]

[Device] --DISCONNECT--> [Broker]

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Bro, if you understand this flow, you can explain MQTT protocol step by step in interviews.