MQTT management packets are the smallest unit of information switch in MQTT. MQTT shoppers and servers alternate management packets for performing their work, akin to subscribing to matters and publishing messages.
At present, MQTT defines 15 kinds of management packets. If we classify them primarily based on their performance, we are able to categorize these packets into three classes: connection, publishing, and subscribing.
Amongst them, the CONNECT packet is utilized by the shopper to provoke a connection to the server, and the CONNACK packet is distributed as a response to point the results of the connection. If one desires to terminate the communication or encounters an error that requires terminating the connection, the shopper and server can ship a DISCONNECT packet after which shut the community connection.
The AUTH packet is a brand new kind of packet launched in MQTT 5.0, and it’s used solely for enhanced authentication, offering safer authentication for shoppers and servers.
The PINGREQ and PINGRESP packets are used for connection keep-alive and probing. The shopper periodically sends a PINGREQ packet to the server to point that it’s nonetheless lively, then judges whether or not the server is lively in response to whether or not the PINGRESP packet is returned in time.
The PUBLISH packet is used to publish messages, and the remaining 4 packets are used to acknowledge QoS 1 and a couple of messages.
The SUBSCRIBE packet is utilized by the shopper to subscribe to matters, whereas the UNSUBSCRIBE packet serves the other objective. The SUBACK and UNSUBACK packets are used to return the outcomes of subscription and unsubscription, respectively.
MQTT Packet Format
In MQTT, no matter the kind of management packet, all of them encompass three elements: Fastened Header, Variable Header, and Payload.
The Fastened Header all the time exists in all management packets. The existence and content material of the Variable Header and Payload rely upon the particular packet kind.
For instance, the PINGREQ packet used for holding alive solely consists of the Fastened Header, whereas the PUBLISH packet used for transmitting utility messages consists of all three elements.
The Fastened Header consists of three fields: MQTT Management Packet Kind, Flags, and Remaining Size.
The MQTT Management Packet Kind is positioned within the excessive 4 bits of the primary byte of the Fastened Header. It’s an unsigned integer that represents the kind of the present packet.
For instance, 1 signifies a CONNECT packet, 2 signifies a CONNACK packet, and so forth. The detailed mapping will be discovered within the MQTT 5.0 specification – MQTT Management Packet Varieties.
Actually, apart from the MQTT Management Packet Kind and Remaining Size fields, the content material of the remaining a part of the MQTT packet is determined by the particular packet kind. So, this area determines how the receiver ought to parse the next content material of the packet.
The remaining low 4 bits within the first byte of the Fastened Header include flags decided by the management packet kind. Nevertheless, as of MQTT 5.0, solely the 4 bits within the PUBLISH packet have been assigned particular meanings:
- Bit 3: DUP, signifies whether or not the present PUBLISH packet is a retransmitted packet.
- Bit 2,1QoS, signifies the quality-of-service stage utilized by the present PUBLISH packet.
- Bit 0: Retain, signifies whether or not the present PUBLISH packet is retained.
In all different packet sorts, these 4 bits stay reserved, which means they’ve a set worth that can’t be arbitrarily modified.
The ultimate Remaining Size area signifies the variety of bytes within the remaining a part of the management packet, which incorporates the Variable Header and the Payload. Subsequently, an MQTT management packet’s whole size is the same as the Fastened Header’s size plus the Remaining Size.
Variable Byte Integer
Nevertheless, the size of the Fastened Header will not be mounted. With the intention to reduce the packet measurement as a lot as potential, MQTT makes use of the Remaining Size area as a variable byte integer.
In MQTT, there are lots of fields of variable size. For instance, the Payload half within the PUBLISH packet is used to hold the precise utility message, and the size of the appliance message will not be mounted. So, we’d like a further area to point the size of those variable-length contents in order that the receiving finish can parse them appropriately.
For a 2 MB utility message, which is a complete of two,097,152 bytes, we would wish a 4-byte integer to point its size. Nevertheless, not all utility messages are that giant; in lots of circumstances, they’re just a few KB and even only a few bytes. Utilizing a 4-byte integer to point a message size of solely 2 bytes can be extreme.
Subsequently, MQTT introduces variable byte integers, which make the most of the decrease 7 bits of every byte to encode information, whereas the very best bit signifies whether or not there are extra bytes to observe.
This fashion, when the packet size is lower than 128 bytes, the variable byte integer solely wants one byte to point. The utmost size of a variable byte integer is 4 bytes, permitting it to point a size of as much as (2^28 – 1) bytes, which is 256 MB of information.
The contents of the Variable Header in MQTT rely upon the particular packet kind. For instance, the Variable Header of the CONNECT packet consists of the Protocol Identify, Protocol Stage, Join Flags, Preserve Alive, and Properties in that order.
The Variable Header of a PUBLISH packet consists of the Matter identify, Packet Identifier (if QoS will not be 0), and Properties in that order.
The fields within the Variable Header should strictly observe the protocol specification as a result of the receiver will solely parse them within the specified order. We can’t omit any area except the protocol explicitly requires or permits it.
For instance, within the Variable Header of the CONNECT packet, if the Join Flags are positioned immediately after the Protocol Identify, it might lead to a parsing failure. Equally, within the Variable Header of the PUBLISH packet, the packet identifier is simply current when QoS will not be 0.
Properties
Properties are an idea launched in MQTT 5.0. They’re the final a part of the Variable Header. The properties encompass the Property Size area adopted by a set of properties. The Property Size signifies the overall size of all of the properties that observe.
All properties are optionally available, as they normally have a default worth. If there is no such thing as a property, then the worth of the Property Size is 0.
Every property consists of an identifier that defines the aim and information kind of the property and a selected worth. Totally different properties might have completely different information sorts.
For instance, one is a two-byte integer, and one other is a UTF-8 encoded string, so we have to parse the properties in response to the information kind declared by their identifiers.
The order of properties will be arbitrary as a result of we are able to know which property it’s and its size primarily based on the Identifier.
Properties are sometimes designed for particular functions. For instance, the CONNECT packet has a Session Expiry Interval property to set the session’s expiration time.
Nevertheless, this property will not be wanted in a PUBLISH packet. Subsequently, MQTT strictly defines the utilization scope of properties, and a legitimate MQTT management packet shouldn’t include properties that don’t belong to it.
For a complete checklist of MQTT properties, together with their identifiers, property names, information sorts, and utilization scopes, please seek advice from MQTT 5.0 Specification – Properties.
Payload
Lastly, now we have the Payload. The Variable Header of the packet will be seen as its supplementary info, whereas the Payload is used to realize the core objective of the packet.
For instance, within the PUBLISH packet, the Payload is used to hold the precise utility message, which is the first perform of the PUBLISH packet. The QoS, Retain, and different fields within the Variable Header of the PUBLISH packet present extra capabilities associated to the appliance message.
The SUBSCRIBE packet follows the same sample. The Payload incorporates the matters to subscribe to and their corresponding subscription choices, which is the first process of the SUBSCRIBE packet.
