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MQTT - Lightweight Messaging Protocol Cheat Sheet by

MQTT is a messaging protocol for the Internet of Things
iot     messaging     mqtt     internet-of-things

Basic details and Terminoly

MQTT Control Packet
A packet of inform­ation sent across the network
Applic­ation Message
The data carried by the protocol
Topic Name
A label attached to an Applic­ation Message that can be subscribed by clients
Topic Filter
An expression used to express interest in one or more topics (can use wildcards)
Session
Stateful intera­ction between a client and a server
Note: TCP ports 1883 (default) and 8883 (TLS) are commonly used for MQTT.

Data Repres­ent­ations

Bits
Bits in a byte, from 7 (MSB) to 0 (LSB).
Integers
Big-endian ordered, 16-bits
Strings
UTF-8 strings, prefixed by its length

Quality of Service

At Most Once
0
At Least Once
1
Exactly Once
2

Topic Names

Must be at least 1 character long
Case sensitive
Can include spaces
Name structured divided by slashes
Exam­ple: /news­/sp­ort­s/e­urope

Wildcards

Multi level
#
Used to match any number of levels within a topic tree, including the parent level itself.
Single level
+
Used to match a single level within a topic tree.
Reserved
$
Topics starting with the dollar sign ('$') are reserved for server purposes and should not be used by clients.
Notes:
Multi-­level wildcard must always be the last symbol on the filter. Either on its own or preceded by the topic level separator.

Single­-level: can be used in conjun­ction with the multi-­level one.

Exam­ples:
+/spo­rts/# - valid
sports+ - not valid
 

MQTT Control Packet Structure

The structure is formed by the aggreg­ation of 3 sub-st­ruc­tures: fixed header, variable length header and payload.

MQTT Control Packet Structure: Fixed Header

Field
Desc­rip­tion
Control Packet Type
4 bit repres­ent­ation of the packet type.
Flags
4 bit flags specific to each packet type.
Remaining Length
Number of bytes remaining in the packet.
Note #1: Control Packet Type and the Flags are stored in a single byte.
Note #2: The Remaining Length does not include the bytes used to encode itself

MQTT Control Packet Structure: Variable Length

Field
Desc­rip­tion
Packet Identifier
Used to establish a relati­onship between different MQTT Control Packets
Payload
A payload associated with the MQTT Control Packet
Note: for the PUBLISH control packet, the payload is the applic­ation message.

MQTT Control Packets

Packet
Name
CONNECT
Client request to connect
CONNACK
Connect ACK
PUBLISH
Publish message
PUBACK
Publish ACK
PUBREC
Publish received
PUBREL
Publish release
PUBCOMP
Publish complete
SUBSCRIBE
Client subscribe request
SUBACK
Subscribe ACK
UNSUBS­CRIBE
Unsubs­cribe request
UNSUBACK
Unsubs­cribe ACK
PINGREQ
Ping request
PINGRESP
Ping response
DISCONNECT
Client discon­necting
 

CONNECT

Impo­rtant elemen­ts:
- Connect Flags: to specify the behavior of the connec­tion.
- Keep Alive: maximum time interval, in seconds, that can elapse between client transm­ission of control packets.

Connect Flags:
- Clean Session: controls the lifetime of the session state (0 to resume state, 1 to discard previous state).
- Will Flag: indicates that a will message is to be sent upon dirty client discon­nec­tion.
- Will QoS: indicates QoS level for the will message.
- Will Retain: indicates the retain policy for the will message.
- Password: indicates whether (1) or not (0) a password must be present in the payload.
- User Name: indicates whether (1) or not (0) a user name must be present in the payload.

Payl­oad:
Length prefixed fields whose presence is determined according to the value of flags in the variable header. The fields are, in order: client identi­fier, will topic, will message, user name, password.

CONNACK

Sent by the server in response to a connection request sent by a client. The important elements on the packet structure are: connect acknow­led­gement flags and the connect return code.
Note: if a session is already present and the connection request does not have the clean session, the server must set the session present flag to 1.

CONNACK: Error Codes

Code
Desc­rip­tion
0x00
Connection accepted
0x01
Connection refused (protocol version)
0x02
Connection refused (ident­ifier rejected)
0x03
Connection refused (server unavai­lable)
0x04
Connection refused (bad user/p­ass­word)
0x05
Connection refused (Unaut­hor­ized)
6-255
Reserved

PUBLISH

This packet is used to transport applic­ation messages for a client to a server or from a server to a client.
The important elements on the packet structure are:
- DUP Flag: indicates whether this is the first time the message is being sent (0) or whether it might be a re-del­ivery attempt (1) of a previous message.
- QoS Level: quality of service for an applic­ation message.
- Retain Flag: indicates that the applic­ation message and its QoS must be stored in the server and delivered to future subscr­ibers of that topic.
Note: The QoS flag affect how many messages can be stored on the server and sent to the client.

Protocol Exchange: QoS 1

Client­/Server protocol intera­ction:

1. Client PUBLISH Server
2. Client PUBACK Server

Protocol Exchange: QoS 2

Client­/Server protocol intera­ction:

1. Client PUBLISH Server
2. Client PUBREC Server
3. Client PUBREL Server
4. Client PUBCOMP Server

Security

TLS is the recomm­ended crypto­graphic protocol to be used with MQTT. Implem­ent­ations should use port 8883.

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