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MQTT is a messaging protocol for the 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
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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 |
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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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