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MQTT is a messaging protocol for the Internet of Things
Basic details and TerminolyMQTT 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 RepresentationsBits | 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 ServiceAt Most Once | 0 | At Least Once | 1 | Exactly Once | 2 |
Topic NamesMust be at least 1 character long | Case sensitive | Can include spaces | Name structured divided by slashes |
Example: /news/sports/europe WildcardsMulti 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 StructureThe structure is formed by the aggregation of 3 sub-structures: fixed header, variable length header and payload. |
MQTT Control Packet Structure: Fixed HeaderField | 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 LengthField | 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 PacketsPacket | 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 |
| | CONNECTImportant 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. |
CONNACKSent 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 CodesCode | 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 |
PUBLISHThis 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 1Client/Server protocol interaction:
1. Client PUBLISH Server
2. Client PUBACK Server |
Protocol Exchange: QoS 2Client/Server protocol interaction:
1. Client PUBLISH Server
2. Client PUBREC Server
3. Client PUBREL Server
4. Client PUBCOMP Server |
SecurityTLS is the recommended cryptographic protocol to be used with MQTT. Implementations should use port 8883. |
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