MQTT management packets are the smallest unit of knowledge switch in MQTT. MQTT shoppers and servers trade management packets for performing their work, corresponding to subscribing to subjects and publishing messages.
At the moment, MQTT defines 15 forms of management packets. If we classify them primarily based on their performance, we will categorize these packets into three classes: connection, publishing, and subscribing.
Amongst them, the CONNECT packet is utilized by the consumer to provoke a connection to the server, and the CONNACK packet is shipped as a response to point the results of the connection. If one needs to terminate the communication or encounters an error that requires terminating the connection, the consumer and server can ship a DISCONNECT packet after which shut the community connection.
The AUTH packet is a brand new sort 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 consumer periodically sends a PINGREQ packet to the server to point that it’s nonetheless energetic, then judges whether or not the server is energetic in line with 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 pair of messages.
The SUBSCRIBE packet is utilized by the consumer to subscribe to subjects, 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 at all times exists in all management packets. The existence and content material of the Variable Header and Payload depend upon the particular packet sort.
For instance, the PINGREQ packet used for holding alive solely contains the Fastened Header, whereas the PUBLISH packet used for transmitting software messages contains all three elements.
The Fastened Header consists of three fields: MQTT Management Packet Sort, Flags, and Remaining Size.
The MQTT Management Packet Sort is situated 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 could be discovered within the MQTT 5.0 specification – MQTT Management Packet Sorts.
In reality, aside from the MQTT Management Packet Sort and Remaining Size fields, the content material of the remaining a part of the MQTT packet will depend on the particular packet sort. So, this subject 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 sort. 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 degree utilized by the present PUBLISH packet.
- Bit 0: Retain, signifies whether or not the present PUBLISH packet is retained.
In all different packet varieties, these 4 bits stay reserved, which means they’ve a set worth that can not be arbitrarily modified.
The ultimate Remaining Size subject 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 complete size is the same as the Fastened Header’s size plus the Remaining Size.
Variable Byte Integer
Nevertheless, the size of the Fastened Header just isn’t mounted. With a purpose to decrease the packet dimension as a lot as potential, MQTT makes use of the Remaining Size subject as a variable byte integer.
In MQTT, there are numerous fields of variable size. For instance, the Payload half within the PUBLISH packet is used to hold the precise software message, and the size of the applying message just isn’t mounted. So, we want a further subject to point the size of those variable-length contents in order that the receiving finish can parse them accurately.
For a 2 MB software message, which is a complete of two,097,152 bytes, we would wish a 4-byte integer to point its size. Nevertheless, not all software messages are that enormous; in lots of instances, they’re just a few KB and even just some 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 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 knowledge.
The contents of the Variable Header in MQTT depend upon the particular packet sort. For instance, the Variable Header of the CONNECT packet contains the Protocol Title, Protocol Stage, Join Flags, Maintain Alive, and Properties in that order.
The Variable Header of a PUBLISH packet contains the Subject title, Packet Identifier (if QoS just isn’t 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 not omit any subject except the protocol explicitly requires or permits it.
For instance, within the Variable Header of the CONNECT packet, if the Join Flags are positioned straight after the Protocol Title, it might lead to a parsing failure. Equally, within the Variable Header of the PUBLISH packet, the packet identifier is simply current when QoS just isn’t 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 subject adopted by a set of properties. The Property Size signifies the whole size of all of the properties that observe.
All properties are optionally available, as they normally have a default worth. If there isn’t a property, then the worth of the Property Size is 0.
Every property consists of an identifier that defines the aim and information sort of the property and a particular worth. Completely different properties might have completely different information varieties.
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 line with the info sort declared by their identifiers.
The order of properties could be arbitrary as a result of we will know which property it’s and its size primarily based on the Identifier.
Properties are usually 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 just isn’t 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 record of MQTT properties, together with their identifiers, property names, information varieties, and utilization scopes, please seek advice from MQTT 5.0 Specification – Properties.
Payload
Lastly, we’ve got the Payload. The Variable Header of the packet could be seen as its supplementary data, 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 software message, which is the first operate of the PUBLISH packet. The QoS, Retain, and different fields within the Variable Header of the PUBLISH packet present extra capabilities associated to the applying message.
The SUBSCRIBE packet follows an analogous sample. The Payload comprises the subjects to subscribe to and their corresponding subscription choices, which is the first activity of the SUBSCRIBE packet.