MQTT Control Packet Utility

MQTT Packet Decoder

Decode raw MQTT control packet bytes and inspect packet type, fixed-header flags, Remaining Length, QoS, DUP, RETAIN, topic name, packet identifier, payload and common CONNECT, PUBLISH and subscription fields.

✓ CONNECT ✓ PUBLISH ✓ SUBSCRIBE ✓ ACK Packets ✓ Remaining Length ✓ UTF-8 Payload

MQTT

MQTT Control Packet Decode

● Ready

MQTT Packet Hex Bytes Enter one complete MQTT control packet beginning with the fixed-header byte. Spaces, commas, colons, hyphens, line breaks and continuous hexadecimal are accepted.

Packet Length Validation Remaining Length determines the number of bytes following the MQTT fixed header.

Payload Display

Decoder scope: this page decodes one MQTT control packet from its fixed header through its declared Remaining Length. It focuses on the standard packet structure used by MQTT 3.1.1-style packets. MQTT 5 properties introduce additional fields that require version-specific interpretation.

MQTT Packet Decode Result Packet Decoded

Decoded MQTT Packet —

Packet Type —

Type Value —

DUP —

QoS —

RETAIN —

Remaining Length —

Header Bytes —

Total Packet Size —

Topic —

Packet Identifier —

Protocol —

Client ID —

Packet-Specific Fields —

Payload —

Fixed Header Breakdown —

Validation —

What Is an MQTT Control Packet?

MQTT communication is carried in control packets. Every packet begins with a fixed header containing a four-bit packet type, four flag bits and a variable-byte Remaining Length field.

Depending on the packet type, a variable header and payload can follow the fixed header.

MQTT Fixed Header

First Byte:

Bits 7:4
Control Packet Type

Bits 3:0
Packet-specific flags

Following Bytes:
Remaining Length

MQTT Control Packet Types

Value	Packet	Purpose
1	CONNECT	Client connection request
2	CONNACK	Connection acknowledgement
3	PUBLISH	Publish application message
4	PUBACK	QoS 1 publish acknowledgement
5	PUBREC	QoS 2 publish received
6	PUBREL	QoS 2 publish release
7	PUBCOMP	QoS 2 publish complete
8	SUBSCRIBE	Subscribe to topic filters
9	SUBACK	Subscribe acknowledgement
10	UNSUBSCRIBE	Remove subscriptions
11	UNSUBACK	Unsubscribe acknowledgement
12	PINGREQ	Keep-alive request
13	PINGRESP	Keep-alive response
14	DISCONNECT	Disconnect notification

MQTT Remaining Length

Remaining Length uses a variable-byte encoding. Each byte contributes seven data bits, and bit 7 indicates whether another length byte follows.

Example:

0x7F
= 127 bytes

0x80 0x01

= (0 & 127)
+ (1 × 128)

= 128 bytes

MQTT PUBLISH Packet

A PUBLISH packet carries an application message. Its fixed-header flags include DUP, QoS and RETAIN.

PUBLISH First Byte:

Bit 3:
DUP

Bits 2:1:
QoS

Bit 0:
RETAIN

The variable header begins with a UTF-8 Topic Name. A Packet Identifier follows the topic when QoS is greater than zero.

MQTT Topic Name Encoding

MQTT UTF-8 strings are preceded by a two-byte big-endian length field.

00 04 74 65 73 74

00 04:
Length = 4

74 65 73 74:
"test"

MQTT CONNECT Packet

CONNECT is the first packet sent by an MQTT client after the network connection has been established. Its variable header includes protocol name, protocol level, connect flags and keep-alive value.

The payload can contain Client Identifier, Will Topic, Will Payload, User Name and Password depending on the Connect Flags.

MQTT CONNECT Flags

Bit	Meaning
7	User Name Flag
6	Password Flag
5	Will Retain
4:3	Will QoS
2	Will Flag
1	Clean Session in MQTT 3.1.1
0	Reserved

MQTT SUBSCRIBE Packet

SUBSCRIBE contains a Packet Identifier followed by one or more topic-filter entries. In MQTT 3.1.1 each entry contains a UTF-8 topic filter and a requested QoS byte.

MQTT QoS Levels

QoS	Meaning
0	At most once
1	At least once
2	Exactly once

MQTT Packet Decoder FAQs

What does MQTT packet byte 0x30 mean?

The upper nibble is 3, identifying PUBLISH. The lower four bits are zero, so DUP=0, QoS=0 and RETAIN=0.

How is MQTT Remaining Length encoded?

It uses one to four bytes with seven value bits per byte and bit 7 as the continuation flag.

Where is the MQTT topic in a PUBLISH packet?

The PUBLISH variable header starts with a two-byte UTF-8 string length followed by the Topic Name bytes.

Does QoS 0 PUBLISH contain a Packet Identifier?

No. A PUBLISH Packet Identifier is present only when the QoS level is greater than zero.

What is MQTT packet type 12?

Packet type 12 is PINGREQ.

Can this decoder parse MQTT 5 packets?

It can decode the common fixed-header structure, but MQTT 5 adds property lengths and property fields that require version-specific parsing. This page focuses on the classic MQTT 3.1.1 packet layout.