Example: MQTT connect
This example provides insight into the structure of packets in MQTT as well as the interaction between the client and broker. MQTT uses different packets to accomplish tasks such as connecting, subscribing, and publishing. You can use XCTU to perform a basic example of sending a broker a connect packet and receiving the response from the server, without requiring any coding. This is a good way to see how the client interacts with the broker and what a packet looks like. The following table is an example connect packet:
| Description | Hex value | |
|---|---|---|
| CONNECT packet fixed header | ||
| byte 1 | Control packet type | 0x10 |
| byte 2 | Remaining length | 0x10 |
| CONNECT packet variable header | ||
| Protocol name | ||
| byte 1 | Length MSB (0) | 0x00 |
| byte 2 | Length LSB (4) | 0x04 |
| byte 3 | (M) | 0x4D |
| byte 4 | (Q) | 0x51 |
| byte 5 | (T) | 0x54 |
| byte 6 | (T) | 0x54 |
| Protocol level | ||
| byte 7 | Level (4) | 0x04 |
| Connect flags | ||
| byte 8 |
CONNECT flags byte, see the table below for the bits. |
0x02 |
| Keep alive | ||
| byte 9 | Keep Alive MSB (0) | 0x00 |
| byte 10 | Keep Alive LSB (60) | 0x3C |
| Client ID | ||
| byte 11 | Length MSB (0) | 0x00 |
| byte 12 | Length LSB (4) | 0x04 |
| byte 13 | (D) | 0x44 |
| byte 14 | (I) | 0x49 |
| byte 15 | (G) | 0x47 |
| byte 16 | (I) | 0x49 |
The following table describes the fields in the packet:
|
Field name |
Description |
|---|---|
|
Protocol Name |
The connect packet starts with the protocol name, which is MQTT. The length of the protocol name (in bytes) is immediately before the name itself. |
|
Protocol Level |
Refers to the version of MQTT in use, in this case a value of 4 indicates MQTT version 3.1.1. |
|
Connect Flags |
Indicate certain aspects of the packet. For simplicity, this example only sets the Clean Session flag, which indicates to the client and broker to discard any previous session and start a new one. |
|
Keep Alive |
How often the client pings the broker to keep the connection alive; in this example it is set to 60 seconds. |
|
Client ID |
The length of the ID (in bytes) precedes the ID itself. Each client connecting to a broker must have a unique client ID. In the example, the ID is DIGI. When using the Paho MQTT Python libraries, a random alphanumeric ID is generated if you do not specify an ID. |
The following table provides the CONNECT flag bits from byte 8, the CONNECT flags byte.
| CONNECT Flag Bit(s) | Bit 7 | Bit 6 | Bit 5 | Bit 4 | Bit 3 | Bit 2 | Bit 1 | Bit 0 |
|---|---|---|---|---|---|---|---|---|
| User name flag | 0 | |||||||
| Password flag | 0 | |||||||
| Will retain | 0 | |||||||
| Will QoS | 0 | 0 | ||||||
| Will flag | 0 | |||||||
| Clean session | 1 | |||||||
| Reserved | 0 |
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