Understanding the MQTT Protocol Packet Structure

mqtt-protocolIn this tutorial we will take a more detailed look at the MQTT protocol, and how MQTT messages or packets are formatted.

We will be looking at:

  • The MQTT message format.
  • The MQTT message header
  • Message fields and coding
  • Control Message coding example


MQTT is a binary based protocol were the control elements are binary bytes and not text strings.

MQTT uses a command and command acknowledgement format.

That means each command has an associated acknowledgement.


Topic names, Client ID, User names and Passwords are encoded as  UTF-8 strings.

The Payload excluding MQTT protocol information like Client ID etc is binary data and the content and format is application specific.

The MQTT packet or message format consists of a 2 byte fixed header (always present) + Variable-header (not always present)+ payload (not always present).


Possible Packet formats are:

  • Fixed Header (Control field + Length) – Example CONNACK
  • Fixed Header (Control field + Length) + Variable Header -Example PUBACK
  • Fixed Header (Control field + Length)  + Variable Header + payload -Example CONNECT

The fixed header field consists of the control field and the variable length packet length field.

The minimum size of the packet length field is 1 byte which is for messages with a total length less than 127 bytes.(not including control and length fields).

The maximum packet size is 256MB. Small packets less than 127 bytes have a 1 byte packet length field.

Packets larger than 127 and less than 16383 will use 2 bytes. etc.

Note: 7 bits are used with the 8th bit being a continuation bit.

The minimum packet size is just 2 bytes with a single byte control field and a single byte packet length field. E.g the disconnect message is only 2 bytes.


The Control Field

The 8 bit control field is the first byte of the 2 byte fixed header. It is divided into two 4 bit fields,and contains all of the protocol commands and responses.

The first 4 Most significant bits are the command or message type field and the other 4 bits are used as control flags.


The table below is taken from the MQTT 3.1.1 specification and shows a sample of MQTT commands, and their associated codes.

Because they are the most significant part of an 8 bit byte field I have also shown their byte values in decimal as they would appear in the data packet.


Control Flags

Although there are 16 possible flags very few are actually used.

The publish message makes the most use of these flags as shown in the table below:


The duplicate flag is used when re-publishing a message with QOS or 1 or 2

QOS Flags are used when publishing and indicate the QOS level -0,1,2

Retain Message flag is also used on publishing.

Remaining Length

This is of variable length between 1 and 4 bytes. Each byte uses 7 bits for the length with the MSB used as a continuation flag.

The remaining length is the number of bytes following the length field, includes variable length header and payload as illustrated below:


The following illustrates the length field for a packet size of 64 and 321 bytes

Remaining Packet length 64 bytes of requires only 1 byte:


Packet length of 321 bytes requires a 2 byte remaining length field:


The following table taken from the specification shows packet sizes and packet length field.


Variable Length Header

As mentioned previously the variable length header field is not always present in an MQTT message.

Certain MQTT message types or commands require the use of this field to carry additional control information.

The variable length header field is similar, but not the same for all message types.

MQTT Connect and Disconnect Message Example

As an illustration we will now look at the packet details for a connect message.

Below is a real client connection and disconnect example showing the actual byte values for the sent and received data.

The CONNECT control code =0x10

The CONNACK control code =0x20

MQTT packet =control + length + protocol name + Protocol Level +Connect Flags + keep alive +Payload



  • Notice the connection (0x10) and connection acknowledge (0x20) control codes.
  • Notice the total length of hex17 or 23 bytes not including the control and length fields. The length field is only 1 byte.
  • You should also be able the see the client ID (python_test) in the sent packet.
  • When looking at the actual packet bytes Python prints the hex values unless it can match a ASCII character.In the example above the keep alive field is x00x3C but is displayed as x00<. Where Ascii < =0x3C




  • The client ID field is sent as the first part of the payload, and not as part of the header.
  • The Client ID is proceeded by a length field
  • The Connect flags indicate that a clean session is being requested.
  • Connection flags are part of the Variable Length header and used to indicate the presence or absence of username,password, and will message fields in the payload. It also contains the clean session flag and Will QOS.

Control Packet Table Summary

Control Packet Variable Header Payload
CONNECT Required Required
PUBLISH Required Optional
PUBACK Required None
PUBREC Required None
PUBREL Required None
PUBCOMP Required None
SUBSCRIBE Required Required
SUBACK Required Required
UNSUBSCRIBE Required Required
UNSUBACK Required Required


Wireshark Network Analysis

In response to a reader question regarding TCP protocol I created this screen shot taken from wireshark.


It shows an MQTT client connecting and publishing (QOS 1). You can clearly see the ACK packets which have a total packet length of 58 bytes.

We know that ACK packets are 2 bytes.

Therefore the TCP packet without MQTT is around 56 bytes.

What is also interesting to note, and something I hadn’t thought of until I did the packet capture, is that each MQTT command or response will get a TCP ACK and maybe also an MQTT ACK.

If you look at the screenshot you can the MQTT connection get a TCP ACK response and an MQTT Connect ACK response.

The MQTT Connect ACK response gets a TCP ACK response.


MQTT V3.1.1 Specification pdf

Modified Client

To produce the screen shots showing the bytes I modified the original client and placed it in the same folder as the normal client.

modified python client

To use it I changed the import from

import paho.mqtt.client as mqtt


import paho.mqtt.mclient as mqtt

Here is the download

Video -MQTT Protocol Packet Structure

Resources and related Tutorials

Please rate? And use Comments to let me know more
[Total: 18   Average: 3.8/5]


  1. Hi, I am Ehsan who is a beginner in MQTT.
    I am trying to set reserved message types for doing specific tasks in MQTT but I cannot find the message types in source code of MQTT.

    I would appreciate it if you guide me.


    1. They are in the client.py file under lib/site-packages/paho/ (windows)
      Here is what they look like
      # Message types
      CONNECT = 0x10
      CONNACK = 0x20
      PUBLISH = 0x30
      PUBACK = 0x40
      PUBREC = 0x50
      PUBREL = 0x60
      PUBCOMP = 0x70
      SUBSCRIBE = 0x80
      SUBACK = 0x90
      UNSUBSCRIBE = 0xA0
      UNSUBACK = 0xB0
      PINGREQ = 0xC0
      PINGRESP = 0xD0
      DISCONNECT = 0xE0

  2. Hi Steve,
    Just to clarify for the remaining length, each byte uses 7 bits of length and MSB as the continuation bit.

    I noticed in Encoding Remain Length Field Example 1 has 9 bits (1 continuation bit, 8 length bits) instead of 8 bits (1 continuation bit, 7 length bits).
    In Encoding Remain Length Field Example 2, the MSB has 9 bits too.

    Can you please clarify and clear my doubts.
    Thank you.

  3. Hello, can you send the codes of example 1. How do I capture the message that the message reaches the recipient? Is there sample code?

    1. Hi
      The modified client script is on the web page. I assume you are looking for a script that displays the header information in detail.
      I did think of creating one but not really sure of the demand.
      Can you tell Why do you need such a script.

  4. Hi Steve,

    We are using IBM MQ 8 and the MQXR service and have seen instances where messages reside in the MQTT transmit queue and are flagged as in-doubt out for a client. From reading what the docs say, it sounds like the server has sent the message to the client but the client has not acknowledged the receipt. Am I correct here? The client then seems to get hung up trying to read messages and requires us to ‘clear’ the stuck messages. The client seems to get a nullPointerException. Is there a good tutorial I can point them to so they can connect and clear or acknowledge the previously delivered messages?

    1. Sorry but I’m not familiar with the client or server. Is the the transmit queue on the client?I would check that the messages do get delivered.Does it happen on all QOS settings.
      I would try a google search on
      BM MQ 8 nullPointerException
      if you haven’t done so already

      1. I was able to capture a tcpdump. Looks like what is happening is this:

        MQTT client connects (already has a durable subscription in place)
        MQTT client sends ‘Connect Command’
        MQTT Broker sends a message that is waiting in the MQTT transmit queue and marks message as ‘in doubt out’
        MQTT client sends the subscription request
        MQTT Broker ACKs it
        MQTT Broker sends a message to MQTT client
        MQTT client ACKs it

        So it looks like the MQTT client is just dropping messages until it has sent the subscribe packet. If the Broker has messages waiting, is it correct to send them once the client connects or should it be waiting until the subscribe comes in? Granted this is a durable subscription so messages are already waiting on it. My guess is the client must be ready to accept messages once the ‘Connect Command’ has been sent.

  5. Hi Steve,

    Can you share the code for the output shown in “MQTT Message Example – 1” fig? I am trying to write python code to work on the packet data but I do not know how to do it. Any examples or guidance you can provide on that would be helpful.

    thank you.

  6. Hi Steve,

    Am using MQTT to develop a chat system, I need to send some additional information during publish and I dont want to include it in Payload( for some reason). Is it right way to go with Variable header field for the same??

  7. I enjoyed reading through, it is very crisp and detailed article.

    Could you also please add a section how the data gets transitioned through MQTT over Google Protocol Buffer and significance of various QoS ?

  8. Thank you for your explanation, .. I can now connect and publish message to the brooker. But i still fail to publish with retain message. My first byte is 0x31 which is 0x30 for publish and 0001 for retain flag. The message succesfully sent to brooker but not retained.

  9. Hello,
    I am using “mosquitto” implementation for MQTT installed on OpenWrt and Ubuntu.
    I am sending the content of a file using mosquitto_pub and receiving it via mosquitto_sub. I did packet analysis of this transmission and found that,
    1. If the file size 1434 Bytes then the content of the file gets split into TCP packet and MQTT packet.

    Though the received content is intact, I am curious to know that why is this happening.

    1. That is the way TCP/IP works. An MQTT packet needs to be inserted into a TCP/IP packet.
      THe maximum size of the IP Packet is limited by the transmission media. See wiki MTU https://en.wikipedia.org/wiki/Maximum_transmission_unit
      This means that a large MQTT packet will need to be split over several TCP/IP packets.
      AT the other end of the link the packet is reassembled by the MQTT client and appears as a single MQTT packet. Take a look at this tutorial it may help
      section on frames

  10. Thanks, reading this explanation makes me understand, why some people think MQTT protocol is more like practise assignment for “Network programming 101” course than universally adopted protocol for IoT. Since it is exactly like that. Sigh.

  11. Thanks for the reply and the code links, but I guess what I’m asking is more a philosophical question rather than practical (as I’m writing my own code on an arduino for memory reasons and the “difficult bit” is all done).

    It just occurred to me that since all I’m doing is posting a temperature and as I got my code up and running I discovered that works using the last will and testament method on two broker services I have tried; is is there any reason I shouldn’t do it that way as by the very virtue of having a last will and testament the protocol is designed for the broker to detect the dropped TCP connection.

    Just a thought….!

    For good practice I am implementing the Publish and disconnect as well.

    1. I think the idea is good and a pity that the spec didn’t have it i.e
      connect with small message and disconnect.
      However as it is implemented I would prefer the connect+ pub+ disconnect.
      There are also practical implications of getting a dropped connections.

  12. Just started using MQTT for a thermometer project. At first sight It’s not clear to me why I should use the Publish packets. Isn’t it enough to simply put the reading in the last will and testament?

    so: CONNECT (receive CONNACK) and then disconnect the TCP

    The alternative would seem to be:
    CONNECT (receive CONNACK)
    PUBLISH (receive PUBACK)
    DISCONNECT and then disconnect the TCP
    which seems wasteful for a lightweight protocol ?

    1. The last will and testament is only sent if the client din’t disconnect properly i.e. a network failure. In your situation you might want to look at the publish helper module which just lets you publidh and it takes care of the connection.
      Take a look here
      It’s at the bottom of the page.
      The publish module is part of the mqtt client so you don’t need to install anything else.

  13. Indeed. Many thanks for a great illustration and explanation.
    I was looking to calculate the Overall Packet Size from the Network side. My understanding is that the MQTT rides over the IP Packet, so How much shall I add to calculate the Minimum Size of Nerwork packaet carrying MQTT Port Traffic?

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