Fixed Header

MQTT runs over TCP port 1883 (or 8883 with TLS). Every packet starts with a fixed header of 2-5 bytes. For example, a CONNECT packet has type nibble 1 in the high bits of byte 0, plus a variable-length "remaining length" field encoding how many bytes follow.

Packet Types

Common control packet types:

CONNECT (1): client requests connection, carries protocol name "MQTT" and level 4 for v3.1.1
CONNACK (2): server acknowledges
PUBLISH (3): message to a topic like sensors/temp
SUBSCRIBE (8): client requests topic filters with QoS levels

The variable header adds fields like packet identifier, connect flags, and keep-alive interval (e.g. 60 seconds). The payload carries client ID, credentials, topic names, and message data.

Remaining Length Encoding

Each byte uses 7 data bits plus a continuation bit. Value 321 encodes as two bytes: 0xC1 0x02.

Why This Exercise

This task requires you to build MQTT CONNECT and PUBLISH packets. Mosquitto and AWS IoT Core parse this exact fixed-header format from millions of battery-powered sensors where minimal framing overhead matters on cellular links.

QoS levels add delivery guarantees: 0 is fire-and-forget, 1 is at-least-once with PUBACK, 2 is exactly-once with a four-step handshake.

Write a C program that builds MQTT CONNECT and PUBLISH packets. Show the packet bytes and a field breakdown.

Requirements:

Define the MQTT fixed header structure with packet type and remaining length
Build a CONNECT packet with protocol name (MQTT), protocol level (4), connect flags, and keep alive
Include a client ID in the CONNECT payload
Encode the remaining length using MQTT variable-length encoding
Build a PUBLISH packet with topic name and message payload
Print the raw bytes of both packets in hexadecimal
Print a breakdown of each field (type, flags, remaining length, variable header, payload)
Show the packet structure with clear printf labels

Fixed Header

Packet Types

Common control packet types:

CONNECT (1): client requests connection, carries protocol name "MQTT" and level 4 for v3.1.1
CONNACK (2): server acknowledges
PUBLISH (3): message to a topic like sensors/temp
SUBSCRIBE (8): client requests topic filters with QoS levels

The variable header adds fields like packet identifier, connect flags, and keep-alive interval (e.g. 60 seconds). The payload carries client ID, credentials, topic names, and message data.

Remaining Length Encoding

Each byte uses 7 data bits plus a continuation bit. Value 321 encodes as two bytes: 0xC1 0x02.

Why This Exercise

QoS levels add delivery guarantees: 0 is fire-and-forget, 1 is at-least-once with PUBACK, 2 is exactly-once with a four-step handshake.

Write a C program that builds MQTT CONNECT and PUBLISH packets. Show the packet bytes and a field breakdown.

Requirements:

Define the MQTT fixed header structure with packet type and remaining length
Build a CONNECT packet with protocol name (MQTT), protocol level (4), connect flags, and keep alive
Include a client ID in the CONNECT payload
Encode the remaining length using MQTT variable-length encoding
Build a PUBLISH packet with topic name and message payload
Print the raw bytes of both packets in hexadecimal
Print a breakdown of each field (type, flags, remaining length, variable header, payload)
Show the packet structure with clear printf labels

Implement the MQTT Protocol for IoT Messaging

Topic Overview

Fixed Header

Packet Types

Remaining Length Encoding

Why This Exercise

Implementation Task

Progressive Hints

Resources

Learn by building

Implement the MQTT Protocol for IoT Messaging

Topic Overview

Fixed Header

Packet Types

Remaining Length Encoding

Why This Exercise

Implementation Task

Progressive Hints

Resources

Learn by building