MQTT - Message Queuing Telemetry Transport
What is MQTT?
MQTT stands for Message Queuing Telemetry Transport. It’s a lightweight, publish-subscribe network protocol used for messaging between devices, especially in low-bandwidth, high-latency, or unreliable networks. It’s widely used in IoT (Internet of Things) applications.
Why is MQTT useful?
MQTT is designed for efficient communication in constrained environments. It minimizes network bandwidth and device resource requirements, making it ideal for: * IoT devices (sensors, actuators) * Mobile applications * Remote monitoring systems
How it works?
Client connects to broker – A device (client) connects to an MQTT broker (server).
Client subscribes or publishes – The client either subscribes to a topic (to receive messages) or publishes a message to a topic.
Broker routes messages – The broker receives published messages and forwards them to all clients subscribed to the topic.
Clients receive messages – Subscribed clients get the message in real-time.
Where is MQTT used?
IoT ecosystems – Smart homes, industrial automation, agriculture.
Mobile messaging apps – Where low power and bandwidth are critical.
Remote monitoring – Oil rigs, pipelines, weather stations.
Healthcare – Wearable devices and patient monitoring systems.
Which OSI layer does this protocol belong to?
It provides messaging services to applications and devices.
Uses TCP (typically port 1883) for reliable delivery (or 8883 for secure TLS).
Implements publish/subscribe model, which is an application-level communication pattern.
What is a broker?
The broker is the central server that: * Accepts connections from clients (publishers/subscribers). * Routes messages from publishers to subscribers. * Ensures correct delivery based on topic matching and QoS (Quality of Service).
What ports does MQTT use?
Port 1883 – Default for MQTT over TCP (unencrypted).
Port 8883 – MQTT over TLS/SSL (secure).
What are MQTT topics?
Topics are string-based filters used by the broker to route messages. Example: “home/livingroom/temperature”.
What type of communication does MQTT support?
MQTT supports the Publish/Subscribe communication model, which enables asynchronous, decoupled messaging between devices via a central broker.
It allows one-way communication where publishers send messages and subscribers receive them.
It also supports two-way communication when clients both publish and subscribe on different topics, enabling bidirectional message exchange.
All communication flows through the broker — there is no direct client-to-client messaging.
What is a Publisher in MQTT?
A Publisher is a device or application that sends messages.
It decides what data to send and which topic to send it to.
It does not need to know who will receive the data.
What is a Subscriber in MQTT?
A Subscriber is a device or application that receives messages.
It subscribes to one or more topics of interest.
It only gets messages that match those topics.
What is the working flow of MQTT?
MQTT uses a publish/subscribe model involving three components: * Publisher: Sends messages to a specific topic. * Broker: Receives messages from publishers and routes them to subscribers. * Subscriber: Receives messages by subscribing to relevant topics.
Flow: * Publisher sends a message to a topic on the broker. * Broker receives the message and forwards it to all subscribers of that topic. * Subscribers receive the message asynchronously.
Does MQTT support multiple publishers and multiple subscribers?
Yes, MQTT fully supports multiple publishers and multiple subscribers.
Multiple publishers can publish messages to the same or different topics.
Multiple subscribers can subscribe to the same topic, and all will receive messages published to that topic.
The broker manages message routing, ensuring that each subscriber receives the messages they subscribed to.
This makes MQTT highly scalable and ideal for IoT and distributed messaging systems.
Topics in this section,
In this section, you are going to learn
Terminology
Version Info
MQTT Version |
RFC |
Year |
Core Idea / Contribution |
|---|---|---|---|
MQTT 3.1 |
|||
Proprietary (IBM) |
2010 |
First public version of MQTT, designed for lightweight M2M communication. |
|
MQTT 3.1.1 |
|||
OASIS Standard |
2014 |
Improved interoperability, clarified ambiguities, added UTF-8 support Widely adopted. |
|
MQTT 5.0 |
|||
OASIS Standard |
2019 |
Major upgrade: added properties, reason codes, shared subscriptions, session expiry, and enhanced error reporting. |
|
MQTT-SN 1.2 |
|||
Informational (OASIS) |
2020 |
MQTT for Sensor Networks optimized for non-TCP/IP networks like Zigbee. |
|
MQTT-TLS Profile (ACE) |
|||
RFC 9431 |
2023 |
Defines secure authentication and authorization using OAuth 2.0 and TLS for constrained environments |
Server (Machine A) — Broker Setup (IPv4)
Step-1 : Find the IP address of Ubuntu machine
test:~$ ifconfig docker0: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500 inet 172.17.0.1 netmask 255.255.0.0 broadcast 172.17.255.255 ether 02:42:c2:a4:22:08 txqueuelen 0 (Ethernet) RX packets 0 bytes 0 (0.0 B) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 0 bytes 0 (0.0 B) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 wlx503eaa8c24ae: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500 inet 192.168.0.31 netmask 255.255.255.0 broadcast 192.168.0.255 inet6 fd8d::7bbc:1295:8:db53:73a2:97c0:2dc2 prefixlen 64 scopeid 0x0<global> ether e4:54:e8:4e:e4:b9 txqueuelen 1000 (Ethernet) RX packets 10426884 bytes 7818062595 (7.8 GB) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 3605750 bytes 434038103 (434.0 MB) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 device interrupt 16 memory 0xdf000000-df020000 lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536 inet 127.0.0.1 netmask 255.0.0.0 inet6 ::1 prefixlen 128 scopeid 0x10<host> loop txqueuelen 1000 (Local Loopback) RX packets 1097677 bytes 88297298 (88.2 MB) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 1097677 bytes 88297298 (88.2 MB) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
Note
To find the IP address of your Ubuntu machine,use the ifconfig command.
From the output of ifconfig command,for example, 192.168.0.31 is used as the Server IPv4.
This server IPv4 is used when connecting MQTT server to a client.
Step-2 : Install Mosquitto Broker on Ubuntu
test:~$ pwd /home/test test:~$ sudo apt update test:~$ sudo apt install mosquitto mosquitto-clients
Step-3 : Edit the Mosquitto Configuration file
test:~$ sudo nano /etc/mosquitto/mosquitto.conf listener 1883 0.0.0.0 allow_anonymous true
Note
0.0.0.0 listens on all IPv4 interfaces
Step-4 : Start the mosquitto server
test:~$ sudo systemctl restart mosquitto test:~$ sudo systemctl enable mosquitto test:~$ sudo systemctl status mosquitto
Note
If server is not running,check logs at “sudo journalctl -u mosquitto” or “sudo systemctl status mosquitto” and fix the errors.
Step-5 : Allow connection from port 1883 (MQTT port)
test:~$ sudo ufw allow 1883 test:~$ sudo ufw reload
Step-6 : To check the server is listening on port
test:~$ sudo netstat -tuln | grep 1883 tcp 0 0 0.0.0.0:1883 0.0.0.0:* LISTEN
Step-7 : To view the logs
test:~$ sudo tail -f /var/log/mosquitto/mosquitto.log
Client (Machine B) — MQTT Client Setup (IPv4)(One-way Communication(Publisher->subscriber))
Step-1 : Install MQTT Client Tools
test:~$ sudo apt update test:~$ sudo apt install mosquitto-clients
Step-2 : Test Network Connectivity
test:~$ ping 192.168.0.31 PING 192.168.0.31 (192.168.0.31 56(84) bytes of data. 64 bytes from 192.168.0.31: icmp_seq=1 ttl=64 time=0.092 ms 64 bytes from 192.168.0.31: icmp_seq=2 ttl=64 time=0.036 ms 64 bytes from 192.168.0.31: icmp_seq=3 ttl=64 time=0.094 ms 64 bytes from 192.168.0.31: icmp_seq=4 ttl=64 time=0.105 ms 64 bytes from 192.168.0.31: icmp_seq=5 ttl=64 time=0.094 ms
Note
192.168.0.31 is the Server IPv4 address referenced in the “IPv4 Basic Setup on Ubuntu” section above.
Step-3 : Subscribe to Topic
test:~$ mosquitto_sub -h 192.168.0.31 -p 1883 -t test/topic
Keep this terminal open ,it waits for messages.
Step-4 : Publish Message
In a new terminal on client:
test:~$ mosquitto_pub -h 192.168.0.31 -p 1883 -t test/topic -m "Hello IPv4"
Expected output : You should see the message on the subscriber terminal.
test:~$mosquitto_sub -h 192.168.0.31 -p 1883 -t test/topic Hello IPv4
Step-5 : Wireshark Capture
Client (Machine B),Client (Machine C) — MQTT Client Setup (IPv4)(Two-way Communication(Publisher<->subscriber))
Step-1 : Install MQTT Client Tools in both Client Machines(B & C)
test:~$ sudo apt update test:~$ sudo apt install mosquitto-clients
Step-2 : Test Network Connectivity in both Client Machines(B & C)
test:~$ ping 192.168.0.31 PING 192.168.0.31 (192.168.0.31 56(84) bytes of data. 64 bytes from 192.168.0.31: icmp_seq=1 ttl=64 time=0.092 ms 64 bytes from 192.168.0.31: icmp_seq=2 ttl=64 time=0.036 ms 64 bytes from 192.168.0.31: icmp_seq=3 ttl=64 time=0.094 ms 64 bytes from 192.168.0.31: icmp_seq=4 ttl=64 time=0.105 ms 64 bytes from 192.168.0.31: icmp_seq=5 ttl=64 time=0.094 ms
Note
192.168.0.31 is the Server IPv4 address referenced in the “IPv4 Basic Setup on Ubuntu” section above.
Step-3 : Subscribe to Topic BtoA in Client (Machine B)
test:~$ mosquitto_sub -h 192.168.0.31 -p 1883 -t topic/BtoA
Keep this terminal open ,it waits for messages.
Step-4 : Subscribe to Topic AtoB in Client (Machine C)
test:~$ mosquitto_sub -h 192.168.0.31 -p 1883 -t topic/AtoB
Keep this terminal open ,it waits for messages.
Step-5 : Publish Message BtoA in Client (Machine C)
In a new terminal on client (Machine C):
test:~$ mosquitto_pub -h 192.168.0.31 -p 1883 -t topic/BtoA -m "Hi from B"
Expected output : You should see the message on the subscriber terminal.
test:~$ mosquitto_sub -h 192.168.0.31 -p 1883 -t topic/BtoA Hi from B
Step-6 : Publish Message AtoB in Client (Machine B)
In a new terminal on client (Machine B):
test:~$ mosquitto_pub -h 192.168.0.31 -p 1883 -t topic/AtoB -m "Hello from A"
Expected output : You should see the message on the subscriber terminal.
test:~$ mosquitto_sub -h 192.168.0.31 -p 1883 -t topic/AtoB Hello from A
Step-7 : Wireshark Capture
Client (Machine B) — MQTT Client Setup (IPv4)(One-way Communication(Publisher->subscriber))
Step-1 : Install MQTT Client Tools
test:~$ sudo apt update test:~$ sudo apt install mosquitto-clients
Step-2 : Test Network Connectivity
test:~$ ping 192.168.0.31 PING 192.168.0.31 (192.168.0.31 56(84) bytes of data. 64 bytes from 192.168.0.31: icmp_seq=1 ttl=64 time=0.092 ms 64 bytes from 192.168.0.31: icmp_seq=2 ttl=64 time=0.036 ms 64 bytes from 192.168.0.31: icmp_seq=3 ttl=64 time=0.094 ms 64 bytes from 192.168.0.31: icmp_seq=4 ttl=64 time=0.105 ms 64 bytes from 192.168.0.31: icmp_seq=5 ttl=64 time=0.094 ms
Note
192.168.0.31 is the Server IPv4 address referenced in the “IPv4 Basic Setup on Ubuntu” section above.
Step-3 : Subscribe to Topic one
test:~$ mosquitto_sub -h 192.168.0.31 -p 1883 -t topic/one
Keep this terminal open ,it waits for messages.
Step-4 : Subscribe to Topic two
In a new terminal on client:
test:~$ mosquitto_sub -h 192.168.0.31 -p 1883 -t topic/two
Keep this terminal open ,it waits for messages.
Step-5 : Subscribe to Topic three
In a new terminal on client:
test:~$ mosquitto_sub -h 192.168.0.31 -p 1883 -t topic/three
Keep this terminal open ,it waits for messages.
Step-6 : Subscribe to Topic four
In a new terminal on client:
test:~$ mosquitto_sub -h 192.168.0.31 -p 1883 -t topic/four
Keep this terminal open ,it waits for messages.
Step-7 : Publish Message topic one
In a new terminal on client:
test:~$ mosquitto_pub -h 192.168.0.31 -p 1883 -t topic/one -m "Message from Pub1"
Expected output : You should see the message on the subscriber terminal.
test:~$ mosquitto_sub -h 192.168.0.31 -p 1883 -t topic/one Message from Pub1
Step-8 : Publish Message topic two
In a new terminal on client:
test:~$ mosquitto_pub -h 192.168.0.31 -p 1883 -t topic/one -m "Message from Pub2"
Expected output : You should see the message on the subscriber terminal.
test:~$ mosquitto_sub -h 192.168.0.31 -p 1883 -t topic/two Message from Pub2
Step-9 : Publish Message topic three
In a new terminal on client:
test:~$ mosquitto_pub -h 192.168.0.31 -p 1883 -t topic/one -m "Message from Pub3"
Expected output : You should see the message on the subscriber terminal.
test:~$ mosquitto_sub -h 192.168.0.31 -p 1883 -t topic/three Message from Pub3
Step-10 : Publish Message topic four
In a new terminal on client:
test:~$ mosquitto_pub -h 192.168.0.31 -p 1883 -t topic/one -m "Message from Pub4"
Expected output : You should see the message on the subscriber terminal.
test:~$ mosquitto_sub -h 192.168.0.31 -p 1883 -t topic/four Message from Pub4
Step-11 : Wireshark Capture
Server (Machine A) — Broker Setup (IPv6)
Step-1 : Find the IP address of Ubuntu machine
test:~$ ifconfig docker0: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500 inet 172.17.0.1 netmask 255.255.0.0 broadcast 172.17.255.255 ether 02:42:c2:a4:22:08 txqueuelen 0 (Ethernet) RX packets 0 bytes 0 (0.0 B) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 0 bytes 0 (0.0 B) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 wlx503eaa8c24ae: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500 inet 192.168.0.31 netmask 255.255.255.0 broadcast 192.168.0.255 inet6 fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 prefixlen 64 scopeid 0x0<global> ether e4:54:e8:4e:e4:b9 txqueuelen 1000 (Ethernet) RX packets 10426884 bytes 7818062595 (7.8 GB) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 3605750 bytes 434038103 (434.0 MB) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 device interrupt 16 memory 0xdf000000-df020000 lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536 inet 127.0.0.1 netmask 255.0.0.0 inet6 ::1 prefixlen 128 scopeid 0x10<host> loop txqueuelen 1000 (Local Loopback) RX packets 1097677 bytes 88297298 (88.2 MB) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 1097677 bytes 88297298 (88.2 MB) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
Note
To find the IP address of your Ubuntu machine,use the ifconfig command.
From the output of ifconfig command,for example, fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 is used as the Server IPv6.
This server IPv6 is used when connecting MQTT server to a client.
Step-2 : Install Mosquitto Broker on Ubuntu
test:~$ pwd /home/test test:~$ sudo apt update test:~$ sudo apt install mosquitto mosquitto-clients
Step-3 : Edit the Mosquitto Configuration file
test:~$ sudo nano /etc/mosquitto/mosquitto.conf listener 1883 :: allow_anonymous true
Note
:: listens on all IPv6 interfaces
Step-4 : Start the mosquitto server
test:~$ sudo systemctl restart mosquitto test:~$ sudo systemctl enable mosquitto test:~$ sudo systemctl status mosquitto
Note
If server is not running,check logs at “sudo journalctl -u mosquitto” or “sudo systemctl status mosquitto” and fix the errors.
Step-5 : Allow connection from port 1883 (MQTT port)
test:~$ sudo ufw allow 1883 test:~$ sudo ufw reload
Step-6 : To check the server is listening on port
test:~$ sudo netstat -tuln | grep 1883 tcp6 0 0 :::1883 :::* LISTEN
Step-7 : To view the logs
test:~$ sudo tail -f /var/log/mosquitto/mosquitto.log
Client (Machine B) — MQTT Client Setup (IPv6)(One-way Communication(Publisher->Subscriber))
Step-1 : Install MQTT Client Tools
test:~$ sudo apt update test:~$ sudo apt install mosquitto-clients
Step-2 : Test Network Connectivity
test:~$ ping6 fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 PING fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2(fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2) 56 data bytes 64 bytes from fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2: icmp_seq=1 ttl=64 time=342 ms 64 bytes from fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2: icmp_seq=2 ttl=64 time=46.2 ms 64 bytes from fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2: icmp_seq=3 ttl=64 time=63.4 ms 64 bytes from fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2: icmp_seq=4 ttl=64 time=8.95 ms 64 bytes from fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2: icmp_seq=5 ttl=64 time=12.6 ms 64 bytes from fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2: icmp_seq=6 ttl=64 time=14.0 ms
Note
fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 is the Server IPv6 address referenced in the “IPv6 Basic Setup on Ubuntu” section above.
Step-3 : Subscribe to Topic
test:~$ mosquitto_sub -h fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 -t test/topic
Keep this terminal open ,it waits for messages.
Step-4 : Publish Message
In a new terminal on client:
test:~$ mosquitto_pub -h fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 -p 1883 -t test/topic -m "Hello from client it is ipv6 address"
Expected output : You should see the message on the subscriber terminal.
test:~$ mosquitto_sub -h fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 -t test/topic Hello from client it is ipv6 address
Step-5 : Wireshark Capture
Client (Machine B),Client (Machine c) — MQTT Client Setup (IPv6)(Two-way Communication(Publisher<->Subscriber))
Step-1 : Install MQTT Client Tools in both Machines (B & C)
test:~$ sudo apt update test:~$ sudo apt install mosquitto-clients
Step-2 : Test Network Connectivity in both Machines (B & C)
test:~$ ping6 fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 PING fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2(fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2) 56 data bytes 64 bytes from fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2: icmp_seq=1 ttl=64 time=342 ms 64 bytes from fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2: icmp_seq=2 ttl=64 time=46.2 ms 64 bytes from fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2: icmp_seq=3 ttl=64 time=63.4 ms 64 bytes from fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2: icmp_seq=4 ttl=64 time=8.95 ms 64 bytes from fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2: icmp_seq=5 ttl=64 time=12.6 ms 64 bytes from fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2: icmp_seq=6 ttl=64 time=14.0 ms
Note
fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 is the Server IPv6 address referenced in the “IPv6 Basic Setup on Ubuntu” section above.
Step-3 : Subscribe to Topic BtoA in Client (Machine B)
test:~$ mosquitto_sub -fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2c2 -p 1883 -t topic/BtoA
Keep this terminal open ,it waits for messages.
Step-4 : Subscribe to Topic AtoB in Client (Machine C)
test:~$ mosquitto_sub -fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2c2 -p 1883 -t topic/AtoB
Keep this terminal open ,it waits for messages.
Step-5 : Publish Message BtoA in Client (Machine C)
In a new terminal on client (Machine C):
test:~$ mosquitto_pub -h fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 -p 1883 -t topic/BtoA -m "Hi from B"
Expected output : You should see the message on the subscriber terminal.
test:~$ mosquitto_sub -h fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 -p 1883 -t topic/BtoA Hi from B
Step-6 : Publish Message AtoB in Client (Machine B)
In a new terminal on client (Machine B):
test:~$ mosquitto_pub -h fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 -p 1883 -t topic/AtoB -m "Hello from A"
Expected output : You should see the message on the subscriber terminal.
test:~$ mosquitto_sub -h fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 -p 1883 -t topic/AtoB Hello from A
Step-7 : Wireshark Capture
Client (Machine B) — MQTT Client Setup (IPv6)(one-way Communication(Publisher->Subscriber))
Step-1 : Install MQTT Client Tools
test:~$ sudo apt update test:~$ sudo apt install mosquitto-clients
Step-2 : Test Network Connectivity
test:~$ ping6 fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 PING fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2(fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2) 56 data bytes 64 bytes from fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2: icmp_seq=1 ttl=64 time=342 ms 64 bytes from fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2: icmp_seq=2 ttl=64 time=46.2 ms 64 bytes from fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2: icmp_seq=3 ttl=64 time=63.4 ms 64 bytes from fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2: icmp_seq=4 ttl=64 time=8.95 ms 64 bytes from fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2: icmp_seq=5 ttl=64 time=12.6 ms 64 bytes from fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2: icmp_seq=6 ttl=64 time=14.0 ms
Note
fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 is the Server IPv6 address referenced in the “IPv6 Basic Setup on Ubuntu” section above.
Step-3 : Subscribe to Topic one
test:~$ mosquitto_sub -h fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 -p 1883 -t topic/one
Keep this terminal open ,it waits for messages.
Step-4 : Subscribe to Topic two
In a new terminal on client:
test:~$ mosquitto_sub -h fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 -p 1883 -t topic/two
Keep this terminal open ,it waits for messages.
Step-5 : Subscribe to Topic three
In a new terminal on client:
test:~$ mosquitto_sub -h fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 -p 1883 -t topic/three
Keep this terminal open ,it waits for messages.
Step-6 : Subscribe to Topic four
In a new terminal on client:
test:~$ mosquitto_sub -h fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 -p 1883 -t topic/four
Keep this terminal open ,it waits for messages.
Step-7 : Publish Message topic one
In a new terminal on client:
test:~$ mosquitto_pub -h fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 -p 1883 -t topic/one -m "Message from Pub1"
Expected output : You should see the message on the subscriber terminal.
test:~$ mosquitto_sub -h fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 -p 1883 -t topic/one Message from Pub1
Step-8 : Publish Message topic two
In a new terminal on client:
test:~$ mosquitto_pub -h fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 -p 1883 -t topic/one -m "Message from Pub2"
Expected output : You should see the message on the subscriber terminal.
test:~$ mosquitto_sub -h fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 -p 1883 -t topic/two Message from Pub2
Step-9 : Publish Message topic three
In a new terminal on client:
test:~$ mosquitto_pub -h fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 -p 1883 -t topic/one -m "Message from Pub3"
Expected output : You should see the message on the subscriber terminal.
test:~$ mosquitto_sub -h fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 -p 1883 -t topic/three Message from Pub3
Step-10 : Publish Message topic four
In a new terminal on client:
test:~$ mosquitto_pub -h fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 -p 1883 -t topic/one -m "Message from Pub4"
Expected output : You should see the message on the subscriber terminal
test:~$ mosquitto_sub -h fd8d:7bbc:1295:8:db53:73a2:97c0:2dc2 -p 1883 -t topic/four Message from Pub4
Step-11 : Wireshark Capture
Apply filter in wireshark as tcp.port == 1883
MQTT CONNECT Packet
S.No |
Protocol Packets |
Description |
Size(bytes) |
|---|---|---|---|
1 |
MQTT CONNECT Packet |
Client request to connect to the MQTT broker |
Varies (minimum ~12 bytes + payload) |
Fixed Header |
Packet Type (1 for CONNECT), Flags, Remaining Length |
2+ |
|
Protocol Name |
MQTT (length-prefixed UTF-8 string) |
6 |
|
Protocol Level |
MQTT version (4 for 3.1.1, 5 for 5.0) |
1 |
|
Connect Flags |
Flags for Clean Session, Will, Username, Password |
1 |
|
Keep Alive |
Time interval in seconds |
2 |
|
Properties (MQTT 5.0 only) |
Optional metadata |
Variable |
|
Client Identifier |
Unique ID for client |
Variable |
|
Will Topic (optional) |
Topic for last will message |
Variable |
|
Will Message (optional) |
Message content |
Variable |
|
Username (optional) |
Username for authentication |
Variable |
|
Password (optional) |
Password for authentication |
Variable |
|
MQTT CONNACK Packet
S.No |
Protocol Packets |
Description |
Size(bytes) |
|---|---|---|---|
2 |
MQTT CONNACK Packet |
Acknowledgment from broker to clients CONNECT request |
4 (MQTT 3.1.1)or more(MQTT 5.0) |
Fixed Header |
Packet Type (2 for CONNACK), Remaining Length |
2 |
|
Connect Acknowledge Flags |
Session Present flag |
1 |
|
Connect Return Code / Reason Code |
Connection status (0 = success) |
1 |
|
Properties (MQTT 5.0 only) |
Optional metadata |
Variable |
|
PUBLISH Packet
S.No |
Protocol Packets |
Description |
Size(bytes) |
|---|---|---|---|
3 |
PUBLISH Packet |
Publish message to a topic |
Varies |
Fixed Header |
Packet type + QoS + DUP + Retain + Remaining Length |
2+ |
|
Topic Name |
UTF-8 encoded topic |
Variable |
|
Packet Identifier |
Required for QoS 1 and 2 |
2 |
|
Properties (MQTT 5.0 only) |
Optional metadata |
Variable |
|
Payload |
Actual message content |
Variable |
|
SUBSCRIBE Packet
S.No |
Protocol Packets |
Description |
Size(bytes) |
|---|---|---|---|
4 |
SUBSCRIBE Packet |
Client subscribes to topics |
Varies |
Fixed Header |
Packet type + Remaining Length |
2+ |
|
Packet Identifier |
Unique ID for this packet |
2 |
|
Properties (MQTT 5.0 only) |
Optional metadata |
Variable |
|
Topic Filters + QoS |
List of topics and QoS levels |
Variable |
|
SUBACK Packet
S.No |
Protocol Packets |
Description |
Size(bytes) |
|---|---|---|---|
5 |
SUBACK Packet |
Acknowledgment of SUBSCRIBE |
Varies |
Fixed Header |
Packet type + Remaining Length |
2+ |
|
Packet Identifier |
Matches SUBSCRIBE packet |
2 |
|
Properties (MQTT 5.0 only) |
Optional metadata |
Variable |
|
Return Codes |
One per topic filter |
Variable |
|
PUBACK Packet
S.No |
Protocol Packets |
Description |
Size(bytes) |
|---|---|---|---|
6 |
PUBACK Packet |
Acknowledgment for QoS 1 PUBLISH |
4+ |
Fixed Header |
Packet Type (4), Remaining Length |
2 |
|
Packet Identifier |
ID of acknowledged PUBLISH |
2 |
|
Reason Code |
Success or failure |
1 |
|
Properties |
Optional (MQTT 5.0 only) |
Variable |
|
PUBREC Packet
S.No |
Protocol Packets |
Description |
Size(bytes) |
|---|---|---|---|
7 |
PUBREC Packet |
Received acknowledgment for QoS 2 PUBLISH |
4+ |
Fixed Header |
Packet Type (5), Remaining Length |
2 |
|
Packet Identifier |
ID of received PUBLISH |
2 |
|
Reason Code |
Success or failure |
1 |
|
Properties |
Optional (MQTT 5.0 only) |
Variable |
|
PUBREL Packet
S.No |
Protocol Packets |
Description |
Size(bytes) |
|---|---|---|---|
8 |
PUBREL Packet |
Release message for QoS 2 flow |
4+ |
Fixed Header |
Packet Type (6), Remaining Length |
2 |
|
Packet Identifier |
ID of released message |
2 |
|
Reason Code |
Success or failure |
1 |
|
Properties |
Optional (MQTT 5.0 only) |
Variable |
|
S.no |
Use Case |
Description |
|---|---|---|
1 |
Smart Home Automation |
MQTT enables communication between smart devices like lights, thermostats, and sensors for real-time control and automation. |
2 |
Industrial IoT (IIoT) |
Used in factories for monitoring machinery, predictive maintenance, and real-time analytics with low latency. |
3 |
Connected Vehicles |
Facilitates vehicle-to-cloud communication for telemetry, diagnostics, and infotainment systems. |
4 |
Healthcare Monitoring |
Supports wearable devices and remote patient monitoring systems by transmitting health data securely and efficiently. |
5 |
Agriculture & Farming |
Enables smart irrigation, soil monitoring, and livestock tracking using sensors and MQTT-based gateways. |
6 |
Energy Management |
Used in smart grids and solar systems to monitor energy usage, battery levels, and grid status. |
7 |
Retail & Supply Chain |
Tracks inventory, monitors refrigeration units, and manages logistics in real-time. |
8 |
Smart Cities |
Powers applications like traffic monitoring, waste management, and environmental sensors. |
9 |
Asset Tracking |
Provides real-time location and condition updates for logistics and fleet management. |
10 |
Remote Monitoring & Control |
Used in oil rigs, pipelines, and remote stations to monitor and control equipment over unreliable networks. |
S.no |
Feature |
Description |
|---|---|---|
1 |
Lightweight Protocol |
Designed for low-bandwidth, high-latency, or unreliable networks, making it ideal for IoT. |
2 |
Publish/Subscribe Model |
Decouples message producers (publishers) and consumers (subscribers) for scalable communication. |
3 |
Quality of Service (QoS) |
Offers three levels (0, 1, 2) to ensure message delivery reliability based on application needs. |
4 |
Retained Messages |
Broker stores the last message on a topic and delivers it to new subscribers immediately. |
5 |
Persistent Sessions |
Maintains client subscriptions and undelivered messages across reconnects. |
6 |
Small Code Footprint |
Suitable for embedded systems and constrained devices. |
7 |
Bi-directional Communication |
Supports both client-to-server and server-to-client messaging. |
8 |
Security via TLS |
Supports secure communication using TLS/SSL encryption. |
9 |
Extensible with MQTT 5.0 |
Adds features like user properties, reason codes, and enhanced error reporting. |
Lightweight Protocol - Testcases
# |
Test Case |
Description |
Expected Result |
|---|---|---|---|
1 |
Connect with Low Bandwidth |
Connect client over a low-bandwidth network |
Connection succeeds |
2 |
Publish with Low Bandwidth |
Publish message over low-bandwidth |
Message is delivered |
3 |
Subscribe with Low Bandwidth |
Subscribe to topic over low-bandwidth |
Subscription succeeds |
4 |
Connect with High Latency |
Connect client with simulated high latency |
Connection succeeds |
5 |
Publish with High Latency |
Publish message with high latency |
Message is delivered |
6 |
Subscribe with High Latency |
Subscribe with high latency |
Subscription succeeds |
7 |
Connect with Packet Loss |
Connect with simulated packet loss |
Connection retries and succeeds |
8 |
Publish with Packet Loss |
Publish with packet loss |
Message is retried or dropped based on QoS |
9 |
Subscribe with Packet Loss |
Subscribe with packet loss |
Subscription succeeds |
10 |
Connect with Intermittent Network |
Connect with unstable network |
Connection is re-established |
11 |
Publish with Intermittent Network |
Publish during network drop |
Message is queued or retried |
12 |
Subscribe with Intermittent Network |
Subscribe during network drop |
Subscription resumes |
13 |
Connect with Cellular Network |
Connect over 2G/3G/4G |
Connection succeeds |
14 |
Publish with Cellular Network |
Publish over mobile network |
Message is delivered |
15 |
Subscribe with Cellular Network |
Subscribe over mobile network |
Subscription succeeds |
16 |
Connect with Satellite Link |
Connect over satellite internet |
Connection succeeds |
17 |
Publish with Satellite Link |
Publish over satellite |
Message is delivered |
18 |
Subscribe with Satellite Link |
Subscribe over satellite |
Subscription succeeds |
19 |
Connect with Wi-Fi |
Connect over Wi-Fi |
Connection succeeds |
20 |
Publish with Wi-Fi |
Publish over Wi-Fi |
Message is delivered |
21 |
Subscribe with Wi-Fi |
Subscribe over Wi-Fi |
Subscription succeeds |
22 |
Connect with Ethernet |
Connect over wired network |
Connection succeeds |
23 |
Publish with Ethernet |
Publish over wired network |
Message is delivered |
24 |
Subscribe with Ethernet |
Subscribe over wired network |
Subscription succeeds |
25 |
Connect with VPN |
Connect over VPN |
Connection succeeds |
26 |
Publish with VPN |
Publish over VPN |
Message is delivered |
27 |
Subscribe with VPN |
Subscribe over VPN |
Subscription succeeds |
28 |
Connect with NAT |
Connect behind NAT |
Connection succeeds |
29 |
Publish with NAT |
Publish behind NAT |
Message is delivered |
30 |
Subscribe with NAT |
Subscribe behind NAT |
Subscription succeeds |
31 |
Connect with IPv6 |
Connect using IPv6 |
Connection succeeds |
32 |
Publish with IPv6 |
Publish using IPv6 |
Message is delivered |
33 |
Subscribe with IPv6 |
Subscribe using IPv6 |
Subscription succeeds |
34 |
Connect with IPv4 |
Connect using IPv4 |
Connection succeeds |
35 |
Publish with IPv4 |
Publish using IPv4 |
Message is delivered |
36 |
Subscribe with IPv4 |
Subscribe using IPv4 |
Subscription succeeds |
37 |
Connect with TLS |
Secure connection with TLS |
Connection succeeds |
38 |
Publish with TLS |
Publish over TLS |
Message is encrypted and delivered |
39 |
Subscribe with TLS |
Subscribe over TLS |
Subscription succeeds |
40 |
Connect with Minimal Payload |
Connect with small message size |
Message is delivered |
41 |
Publish with Minimal Payload |
Publish 1-byte message |
Message is delivered |
42 |
Subscribe with Minimal Payload |
Subscribe to topic with small payload |
Subscription succeeds |
43 |
Connect with QoS 0 |
Connect using QoS 0 |
Message is delivered once, no retry |
44 |
Publish with QoS 0 |
Publish using QoS 0 |
Message is delivered best-effort |
45 |
Publish with QoS 1 |
Publish using QoS 1 |
Message is delivered at least once |
46 |
Publish with QoS 2 |
Publish using QoS 2 |
Message is delivered exactly once |
47 |
Connect with Clean Session |
Connect with clean session flag |
No session state retained |
48 |
Connect with Persistent Session |
Connect with persistent session |
Session state is retained |
49 |
Connect with Keep Alive |
Use keep-alive interval |
Connection is maintained |
50 |
Connect with Will Message |
Use last will message |
Will message is sent on disconnect |
Publish/Subscribe Model - Testcases
# |
Test Case |
Description |
Expected Result |
|---|---|---|---|
1 |
Basic Publish |
Publish message to a topic |
Message is accepted by broker |
2 |
Basic Subscribe |
Subscribe to a topic |
Subscription is acknowledged |
3 |
Publish and Receive |
Publish and receive message |
Subscriber receives message |
4 |
Multiple Subscribers |
Multiple clients subscribe to same topic |
All receive published message |
5 |
Multiple Publishers |
Multiple clients publish to same topic |
All messages are delivered |
6 |
Unsubscribe from Topic |
Unsubscribe from topic |
No further messages received |
7 |
Publish Without Subscriber |
Publish to topic with no subscribers |
Message is discarded (unless retained) |
8 |
Subscribe Without Publisher |
Subscribe to topic with no publisher |
No messages received until published |
9 |
Wildcard Subscription (+) |
Subscribe using single-level wildcard |
Messages from matching topics received |
10 |
Wildcard Subscription (#) |
Subscribe using multi-level wildcard |
Messages from all subtopics received |
11 |
Topic Hierarchy |
Use hierarchical topic structure |
Messages routed correctly |
12 |
Retained Message |
Publish retained message |
New subscribers receive last retained message |
13 |
QoS 0 Publish/Subscribe |
Use QoS 0 for pub/sub |
Message delivered best-effort |
14 |
QoS 1 Publish/Subscribe |
Use QoS 1 for pub/sub |
Message delivered at least once |
15 |
QoS 2 Publish/Subscribe |
Use QoS 2 for pub/sub |
Message delivered exactly once |
16 |
Clean Session |
Connect with clean session |
No previous subscriptions retained |
17 |
Persistent Session |
Connect with persistent session |
Subscriptions retained |
18 |
Offline Subscriber |
Subscriber offline during publish |
Message delivered on reconnect (if QoS > 0 and persistent session) |
19 |
Offline Publisher |
Publisher offline |
No messages published |
20 |
Broker Restart |
Restart broker during pub/sub |
Messages resume after reconnect |
21 |
Client Disconnect |
Disconnect client during pub/sub |
Messages resume after reconnect |
22 |
Publish Large Payload |
Publish large message |
Message delivered if within broker limits |
23 |
Subscribe to Nonexistent Topic |
Subscribe to topic with no publisher |
No messages received until published |
24 |
Publish to Nonexistent Topic |
Publish to topic with no subscribers |
Message discarded unless retained |
25 |
Subscribe with Shared Subscription |
Use shared subscription |
Load balanced among subscribers |
26 |
Publish with Delay |
Delay between publish and subscribe |
Subscriber receives message if retained or persistent |
27 |
Subscribe with Delay |
Delay between subscribe and publish |
Subscriber receives future messages |
28 |
Publish with Authentication |
Publish with valid credentials |
Message accepted |
29 |
Subscribe with Authentication |
Subscribe with valid credentials |
Subscription accepted |
30 |
Publish with Invalid Credentials |
Publish with invalid credentials |
Access denied |
31 |
Subscribe with Invalid Credentials |
Subscribe with invalid credentials |
Access denied |
32 |
Publish with TLS |
Publish over secure connection |
Message encrypted and delivered |
33 |
Subscribe with TLS |
Subscribe over secure connection |
Subscription encrypted and acknowledged |
34 |
Publish from IoT Device |
Publish from constrained device |
Message delivered |
35 |
Subscribe from IoT Device |
Subscribe from constrained device |
Messages received |
36 |
Publish from Mobile App |
Publish from mobile client |
Message delivered |
37 |
Subscribe from Mobile App |
Subscribe from mobile client |
Messages received |
38 |
Publish from Web App |
Publish from browser |
Message delivered |
39 |
Subscribe from Web App |
Subscribe from browser |
Messages received |
40 |
Publish with JSON Payload |
Publish structured data |
Payload received intact |
41 |
Subscribe with JSON Parsing |
Parse JSON message |
Data parsed successfully |
42 |
Publish with Binary Payload |
Publish binary data |
Payload received intact |
43 |
Subscribe with Binary Handling |
Handle binary message |
Data processed correctly |
44 |
Publish to Multiple Topics |
Publish to different topics |
Messages routed correctly |
45 |
Subscribe to Multiple Topics |
Subscribe to multiple topics |
Messages from all topics received |
46 |
Publish with Topic Alias |
Use topic aliasing |
Message delivered using alias |
47 |
Subscribe with Topic Alias |
Receive message via alias |
Message routed correctly |
48 |
Publish with Message Expiry |
Set message expiry interval |
Message discarded after expiry |
49 |
Subscribe After Expiry |
Subscribe after message expiry |
Message not received |
50 |
Publish/Subscribe Performance Test |
Measure latency and throughput |
Low latency and high throughput observed |
Quality of Service(QoS) - Testcases
# |
Test Case |
Description |
Expected Result |
|---|---|---|---|
1 |
Publish with QoS 0 |
Publish message with QoS 0 |
Message delivered best-effort |
2 |
Publish with QoS 1 |
Publish message with QoS 1 |
Message delivered at least once |
3 |
Publish with QoS 2 |
Publish message with QoS 2 |
Message delivered exactly once |
4 |
Subscribe with QoS 0 |
Subscribe to topic with QoS 0 |
Messages received with best-effort |
5 |
Subscribe with QoS 1 |
Subscribe to topic with QoS 1 |
Messages received at least once |
6 |
Subscribe with QoS 2 |
Subscribe to topic with QoS 2 |
Messages received exactly once |
7 |
QoS 0 with Packet Loss |
Simulate packet loss during QoS 0 publish |
Message may be lost |
8 |
QoS 1 with Packet Loss |
Simulate packet loss during QoS 1 publish |
Message is retried |
9 |
QoS 2 with Packet Loss |
Simulate packet loss during QoS 2 publish |
Message is retransmitted and deduplicated |
10 |
QoS 0 with Network Delay |
Publish with high latency |
Message may be delayed or lost |
11 |
QoS 1 with Network Delay |
Publish with high latency |
Message is eventually delivered |
12 |
QoS 2 with Network Delay |
Publish with high latency |
Message is delivered once |
13 |
QoS 0 with Broker Restart |
Restart broker during publish |
Message may be lost |
14 |
QoS 1 with Broker Restart |
Restart broker during publish |
Message is redelivered |
15 |
QoS 2 with Broker Restart |
Restart broker during publish |
Message is delivered once after recovery |
16 |
QoS 0 with Client Disconnect |
Disconnect client after publish |
Message may be lost |
17 |
QoS 1 with Client Disconnect |
Disconnect client after publish |
Message is retried after reconnect |
18 |
QoS 2 with Client Disconnect |
Disconnect client after publish |
Message is delivered once after reconnect |
19 |
QoS 0 with Retained Message |
Publish retained message |
Delivered once to new subscribers |
20 |
QoS 1 with Retained Message |
Publish retained message |
Delivered at least once to new subscribers |
21 |
QoS 2 with Retained Message |
Publish retained message |
Delivered exactly once to new subscribers |
22 |
QoS 0 with Clean Session |
Connect with clean session |
No message persistence |
23 |
QoS 1 with Clean Session |
Connect with clean session |
Messages may be redelivered |
24 |
QoS 2 with Clean Session |
Connect with clean session |
Messages delivered once |
25 |
QoS 0 with Persistent Session |
Connect with persistent session |
No message persistence |
26 |
QoS 1 with Persistent Session |
Connect with persistent session |
Messages redelivered after reconnect |
27 |
QoS 2 with Persistent Session |
Connect with persistent session |
Messages delivered once after reconnect |
28 |
QoS 0 with Shared Subscription |
Use shared subscription |
Message delivered to one subscriber |
29 |
QoS 1 with Shared Subscription |
Use shared subscription |
Message delivered at least once to one subscriber |
30 |
QoS 2 with Shared Subscription |
Use shared subscription |
Message delivered exactly once to one subscriber |
31 |
QoS 0 with Wildcard Topic |
Subscribe using wildcard |
Messages received with best-effort |
32 |
QoS 1 with Wildcard Topic |
Subscribe using wildcard |
Messages received at least once |
33 |
QoS 2 with Wildcard Topic |
Subscribe using wildcard |
Messages received exactly once |
34 |
QoS 0 with Mobile Client |
Publish from mobile device |
Message may be lost |
35 |
QoS 1 with Mobile Client |
Publish from mobile device |
Message is retried |
36 |
QoS 2 with Mobile Client |
Publish from mobile device |
Message delivered once |
37 |
QoS 0 with IoT Device |
Publish from constrained device |
Message may be lost |
38 |
QoS 1 with IoT Device |
Publish from constrained device |
Message is retried |
39 |
QoS 2 with IoT Device |
Publish from constrained device |
Message delivered once |
40 |
QoS 0 with TLS |
Publish over TLS |
Message may be lost |
41 |
QoS 1 with TLS |
Publish over TLS |
Message is retried |
42 |
QoS 2 with TLS |
Publish over TLS |
Message delivered once |
43 |
QoS 0 with Broker Cluster |
Publish to clustered broker |
Message may be lost |
44 |
QoS 1 with Broker Cluster |
Publish to clustered broker |
Message is retried |
45 |
QoS 2 with Broker Cluster |
Publish to clustered broker |
Message delivered once |
46 |
QoS 0 with Message Expiry |
Set expiry interval |
Message may expire before delivery |
47 |
QoS 1 with Message Expiry |
Set expiry interval |
Message may be retried before expiry |
48 |
QoS 2 with Message Expiry |
Set expiry interval |
Message delivered once before expiry |
49 |
QoS 1 with Duplicate Detection |
Detect duplicate messages |
Duplicates handled by client |
50 |
QoS 2 with Duplicate Detection |
Detect duplicate messages |
No duplicates received |
Retained Messages - Testcases
# |
Test Case |
Description |
Expected Result |
|---|---|---|---|
1 |
Publish Retained Message |
Publish message with retain flag |
Broker stores the message |
2 |
Subscribe After Retained Publish |
Subscribe after retained message is published |
Subscriber receives retained message |
3 |
Subscribe Before Retained Publish |
Subscribe before retained message is published |
Subscriber does not receive retained message |
4 |
Update Retained Message |
Publish new retained message |
Broker replaces old retained message |
5 |
Clear Retained Message |
Publish empty message with retain flag |
Broker clears retained message |
6 |
Retained Message with QoS 0 |
Publish retained message with QoS 0 |
Message stored and delivered best-effort |
7 |
Retained Message with QoS 1 |
Publish retained message with QoS 1 |
Message stored and delivered at least once |
8 |
Retained Message with QoS 2 |
Publish retained message with QoS 2 |
Message stored and delivered exactly once |
9 |
Retained Message with Wildcard Topic |
Subscribe using wildcard |
Retained messages from matching topics received |
10 |
Retained Message with Shared Subscription |
Use shared subscription |
One subscriber receives retained message |
11 |
Retained Message with Clean Session |
Connect with clean session |
Retained message delivered |
12 |
Retained Message with Persistent Session |
Connect with persistent session |
Retained message delivered |
13 |
Retained Message with Offline Subscriber |
Subscriber offline during publish |
Retained message delivered on reconnect |
14 |
Retained Message with Broker Restart |
Restart broker after publish |
Retained message persists |
15 |
Retained Message with Client Reconnect |
Reconnect client |
Retained message delivered again |
16 |
Retained Message with Multiple Topics |
Publish retained messages to multiple topics |
Each topic stores its own retained message |
17 |
Retained Message with JSON Payload |
Publish retained JSON message |
JSON message delivered intact |
18 |
Retained Message with Binary Payload |
Publish retained binary message |
Binary message delivered intact |
19 |
Retained Message with Large Payload |
Publish large retained message |
Message stored and delivered |
20 |
Retained Message with Small Payload |
Publish small retained message |
Message stored and delivered |
21 |
Retained Message with Mobile Client |
Publish from mobile device |
Message retained and delivered |
22 |
Retained Message with IoT Device |
Publish from constrained device |
Message retained and delivered |
23 |
Retained Message with Web Client |
Publish from browser |
Message retained and delivered |
24 |
Retained Message with TLS |
Publish over secure connection |
Message retained securely |
25 |
Retained Message with Authentication |
Publish with valid credentials |
Message retained |
26 |
Retained Message with Invalid Auth |
Publish with invalid credentials |
Message rejected |
27 |
Retained Message with Topic Alias |
Publish using topic alias |
Retained message stored correctly |
28 |
Retained Message with Expiry Interval |
Set expiry interval |
Message expires after interval |
29 |
Retained Message with QoS Downgrade |
Subscribe with lower QoS |
Message delivered at subscribers QoS |
30 |
Retained Message with QoS Upgrade |
Subscribe with higher QoS |
Message delivered at publishers QoS |
31 |
Retained Message with Broker Cluster |
Publish to clustered broker |
Retained message replicated |
32 |
Retained Message with NAT |
Publish behind NAT |
Message retained and delivered |
33 |
Retained Message with VPN |
Publish over VPN |
Message retained and delivered |
34 |
Retained Message with IPv4 |
Publish using IPv4 |
Message retained and delivered |
35 |
Retained Message with IPv6 |
Publish using IPv6 |
Message retained and delivered |
36 |
Retained Message with Topic Hierarchy |
Publish to hierarchical topic |
Retained message stored at each level |
37 |
Retained Message with Broker Limits |
Publish near brokers retained limit |
Message retained if within limits |
38 |
Retained Message with Broker Policy |
Publish with broker policy restrictions |
Message retained or rejected based on policy |
39 |
Retained Message with Logging Enabled |
Enable broker logging |
Retained message logged |
40 |
Retained Message with Audit Trail |
Review audit logs |
Retained message history visible |
41 |
Retained Message with Monitoring Tool |
Monitor retained message delivery |
Message delivery confirmed |
42 |
Retained Message with GUI Client |
Publish from GUI |
Message retained and visible |
43 |
Retained Message with CLI Client |
Publish from CLI |
Message retained and visible |
44 |
Retained Message with Broker Crash |
Crash broker after publish |
Retained message persists |
45 |
Retained Message with Client Crash |
Crash client after publish |
Message retained on broker |
46 |
Retained Message with Duplicate Topic |
Publish to same topic repeatedly |
Last message retained |
47 |
Retained Message with QoS Mismatch |
QoS mismatch between publisher and subscriber |
Message delivered at subscribers QoS |
48 |
Retained Message with Topic Filter |
Subscribe with topic filter |
Retained message delivered if matched |
49 |
Retained Message with Message History |
Check previous retained messages |
Only last message retained |
50 |
Retained Message Performance Test |
Measure delivery time to new subscriber |
Immediate delivery observed |
Persistent Sessions - Testcases
# |
Test Case |
Description |
Expected Result |
|---|---|---|---|
1 |
Connect with Persistent Session |
Connect with cleanSession=false |
Session is maintained |
2 |
Subscribe with Persistent Session |
Subscribe to topic |
Subscription is stored |
3 |
Disconnect and Reconnect |
Disconnect and reconnect |
Subscription is restored |
4 |
Publish During Disconnect |
Publish while client is disconnected |
Message is queued |
5 |
Receive Queued Messages |
Reconnect and receive queued messages |
Messages are delivered |
6 |
QoS 1 Message Queuing |
Publish QoS 1 message during disconnect |
Message is queued and delivered at least once |
7 |
QoS 2 Message Queuing |
Publish QoS 2 message during disconnect |
Message is queued and delivered exactly once |
8 |
QoS 0 Message During Disconnect |
Publish QoS 0 message during disconnect |
Message is discarded |
9 |
Multiple Subscriptions |
Subscribe to multiple topics |
All subscriptions are retained |
10 |
Session Expiry |
Set session expiry interval |
Session expires after timeout |
11 |
Session Expiry Cancellation |
Reconnect before expiry |
Session remains active |
12 |
Session with Retained Messages |
Subscribe to topic with retained message |
Retained message is delivered |
13 |
Session with Will Message |
Set LWT and disconnect unexpectedly |
Will message is delivered |
14 |
Session with Clean Start |
Connect with clean start |
Previous session is cleared |
15 |
Session with Authentication |
Connect with valid credentials |
Session is maintained |
16 |
Session with Invalid Credentials |
Connect with invalid credentials |
Connection denied |
17 |
Session with TLS |
Connect securely |
Session is maintained |
18 |
Session with Shared Subscription |
Use shared subscription |
Load-balanced messages retained |
19 |
Session with Wildcard Subscription |
Subscribe using wildcard |
Subscription retained |
20 |
Session with Topic Alias |
Use topic alias |
Alias retained across reconnect |
21 |
Session with Message Expiry |
Publish with expiry interval |
Message expires if not delivered in time |
22 |
Session with Offline Publisher |
Publisher offline |
Messages queued for subscribers |
23 |
Session with Offline Subscriber |
Subscriber offline |
Messages queued |
24 |
Session with Broker Restart |
Restart broker |
Session state is preserved |
25 |
Session with Client Crash |
Simulate client crash |
Session is preserved |
26 |
Session with Network Drop |
Simulate network failure |
Session is preserved |
27 |
Session with Mobile Client |
Use mobile client |
Session is preserved across reconnects |
28 |
Session with IoT Device |
Use constrained device |
Session is preserved |
29 |
Session with Web Client |
Use browser-based client |
Session is preserved |
30 |
Session with NAT |
Client behind NAT |
Session is preserved |
31 |
Session with VPN |
Client over VPN |
Session is preserved |
32 |
Session with IPv4 |
Use IPv4 connection |
Session is preserved |
33 |
Session with IPv6 |
Use IPv6 connection |
Session is preserved |
34 |
Session with Broker Cluster |
Use clustered broker |
Session state is synchronized |
35 |
Session with Logging Enabled |
Enable broker logging |
Session events are logged |
36 |
Session with Audit Trail |
Review audit logs |
Session activity is recorded |
37 |
Session with Monitoring Tool |
Monitor session state |
Session status visible |
38 |
Session with QoS Downgrade |
Subscriber requests lower QoS |
Messages delivered at requested QoS |
39 |
Session with Duplicate Client ID |
Connect with same client ID |
Previous session ends, new session starts |
40 |
Session with Message Ordering |
Publish multiple messages |
Messages delivered in order |
41 |
Session with Message Delay |
Delay message delivery |
Messages delivered after reconnect |
42 |
Session with Message Compression |
Compress messages |
Messages delivered correctly |
43 |
Session with JSON Payload |
Publish structured data |
Payload delivered intact |
44 |
Session with Binary Payload |
Publish binary data |
Payload delivered intact |
45 |
Session with Large Payload |
Publish large message |
Message delivered if within limits |
46 |
Session with Small Payload |
Publish small message |
Message delivered |
47 |
Session with Will Delay Interval |
Set will delay |
Will message delayed |
48 |
Session with Will Properties |
Set custom properties |
Properties delivered with message |
49 |
Session with Clean Disconnect |
Disconnect gracefully |
Session remains active |
50 |
Session with Broker Throttling |
Broker under load |
Session state preserved |
Small Code Footprint - Testcases
# |
Test Case |
Description |
Expected Result |
|---|---|---|---|
1 |
Compile MQTT Client on Microcontroller |
Build MQTT client on embedded board |
Compilation succeeds |
2 |
Connect from Embedded Device |
Connect to broker from constrained device |
Connection succeeds |
3 |
Publish from Embedded Device |
Publish message from microcontroller |
Message is delivered |
4 |
Subscribe from Embedded Device |
Subscribe to topic from microcontroller |
Messages are received |
5 |
Use Minimal MQTT Library |
Use lightweight MQTT library |
Library fits within memory limits |
6 |
Flash Size Check |
Check firmware size with MQTT client |
Size is within device limits |
7 |
RAM Usage Check |
Measure runtime memory usage |
Memory usage is minimal |
8 |
CPU Load Check |
Measure CPU usage during MQTT operations |
CPU usage is low |
9 |
Power Consumption Test |
Measure power usage during MQTT session |
Power usage is minimal |
10 |
Sleep Mode Compatibility |
Enter sleep mode with MQTT client |
Device wakes and reconnects |
11 |
Wake on MQTT Event |
Wake device on incoming MQTT message |
Device wakes and processes message |
12 |
Publish with QoS 0 |
Publish with minimal overhead |
Message delivered best-effort |
13 |
Subscribe with QoS 0 |
Subscribe with minimal overhead |
Messages received best-effort |
14 |
Use Static Memory Allocation |
Avoid dynamic memory allocation |
No memory fragmentation |
15 |
Use MQTT-SN |
Use MQTT for sensor networks |
Protocol works with minimal stack |
16 |
Use MQTT over UDP |
Use MQTT-SN over UDP |
Message delivered with low overhead |
17 |
Use MQTT over Serial |
Use MQTT over UART |
Message delivered |
18 |
Use MQTT over Bluetooth |
Use MQTT over BLE |
Message delivered |
19 |
Use MQTT over LoRaWAN |
Use MQTT with LoRa gateway |
Message delivered |
20 |
Use MQTT over NB-IoT |
Use MQTT over narrowband IoT |
Message delivered |
21 |
Use MQTT on RTOS |
Run MQTT client on real-time OS |
Client operates correctly |
22 |
Use MQTT on Bare Metal |
Run MQTT client without OS |
Client operates correctly |
23 |
Use MQTT on FreeRTOS |
Integrate MQTT with FreeRTOS |
Client operates correctly |
24 |
Use MQTT on Zephyr |
Integrate MQTT with Zephyr OS |
Client operates correctly |
25 |
Use MQTT on TinyOS |
Integrate MQTT with TinyOS |
Client operates correctly |
26 |
Use MQTT on RIOT OS |
Integrate MQTT with RIOT OS |
Client operates correctly |
27 |
Use MQTT on Mbed OS |
Integrate MQTT with Mbed OS |
Client operates correctly |
28 |
Use MQTT on ESP32 |
Run MQTT on ESP32 |
Client connects and communicates |
29 |
Use MQTT on STM32 |
Run MQTT on STM32 |
Client connects and communicates |
30 |
Use MQTT on Raspberry Pi Pico |
Run MQTT on RP2040 |
Client connects and communicates |
31 |
Use MQTT on Arduino |
Run MQTT on Arduino board |
Client connects and communicates |
32 |
Use MQTT on NRF52 |
Run MQTT on Nordic chip |
Client connects and communicates |
33 |
Use MQTT on TI CC3200 |
Run MQTT on TI chip |
Client connects and communicates |
34 |
Use MQTT on Particle Photon |
Run MQTT on Particle device |
Client connects and communicates |
35 |
Use MQTT on BeagleBone |
Run MQTT on BeagleBone |
Client connects and communicates |
36 |
Use MQTT on Linux SBC |
Run MQTT on single-board computer |
Client connects and communicates |
37 |
Use MQTT with Minimal Stack |
Use MQTT with reduced stack size |
Client operates correctly |
38 |
Use MQTT with Minimal Heap |
Use MQTT with reduced heap size |
Client operates correctly |
39 |
Use MQTT with Static Topics |
Use fixed topic strings |
Reduces memory usage |
40 |
Use MQTT with Static Buffers |
Use fixed-size buffers |
Reduces memory fragmentation |
41 |
Use MQTT with No TLS |
Use plain MQTT for minimal footprint |
Client connects and communicates |
42 |
Use MQTT with Lightweight TLS |
Use minimal TLS stack (e.g., mbedTLS) |
Secure connection established |
43 |
Use MQTT with Watchdog Timer |
Integrate with watchdog |
Client resets on failure |
44 |
Use MQTT with OTA Updates |
Update firmware over MQTT |
Update succeeds |
45 |
Use MQTT with Sensor Data |
Publish sensor data |
Message delivered |
46 |
Use MQTT with Actuator Control |
Receive control messages |
Actuator responds |
47 |
Use MQTT with Sleep Scheduling |
Schedule sleep/wake cycles |
MQTT session resumes |
48 |
Use MQTT with Battery Monitoring |
Monitor battery while using MQTT |
Battery usage is efficient |
49 |
Use MQTT with Event Triggering |
Trigger events via MQTT |
Events executed |
50 |
Use MQTT with Minimal Dependencies |
Build with no external libraries |
Client compiles and runs |
Bi-directional Communication - Testcases
# |
Test Case |
Description |
Expected Result |
|---|---|---|---|
1 |
Client Publishes to Broker |
Client sends message to broker |
Broker receives message |
2 |
Broker Delivers to Subscriber |
Broker sends message to subscribed client |
Client receives message |
3 |
Client Publishes and Subscribes |
Same client publishes and subscribes |
Client receives messages from broker |
4 |
Multiple Clients Communicate |
Clients publish and subscribe to each other |
Messages exchanged via broker |
5 |
Client Publishes to Topic A |
Client A publishes to topic |
Client B subscribed to topic receives message |
6 |
Client Subscribes to Topic B |
Client subscribes to topic |
Receives messages from broker |
7 |
Client Sends Command |
Client sends control command |
Target device receives and executes command |
8 |
Device Sends Status Update |
Device publishes status |
Monitoring client receives update |
9 |
Client Publishes JSON Payload |
Send structured data |
Subscriber receives and parses JSON |
10 |
Client Publishes Binary Payload |
Send binary data |
Subscriber receives binary intact |
11 |
Client Publishes with QoS 0 |
Send message with QoS 0 |
Message delivered best-effort |
12 |
Client Publishes with QoS 1 |
Send message with QoS 1 |
Message delivered at least once |
13 |
Client Publishes with QoS 2 |
Send message with QoS 2 |
Message delivered exactly once |
14 |
Client Subscribes with QoS 0 |
Subscribe with QoS 0 |
Messages received best-effort |
15 |
Client Subscribes with QoS 1 |
Subscribe with QoS 1 |
Messages received at least once |
16 |
Client Subscribes with QoS 2 |
Subscribe with QoS 2 |
Messages received exactly once |
17 |
Client Sends Command to IoT Device |
Client publishes command |
IoT device receives and acts |
18 |
IoT Device Sends Sensor Data |
Device publishes sensor reading |
Client receives data |
19 |
Client Publishes via Web App |
Web client sends message |
Subscriber receives message |
20 |
Client Subscribes via Web App |
Web client subscribes |
Receives messages from broker |
21 |
Client Publishes via Mobile App |
Mobile client sends message |
Subscriber receives message |
22 |
Client Subscribes via Mobile App |
Mobile client subscribes |
Receives messages from broker |
23 |
Client Publishes via CLI |
Command-line client sends message |
Subscriber receives message |
24 |
Client Subscribes via CLI |
Command-line client subscribes |
Receives messages from broker |
25 |
Client Publishes via Script |
Script sends message |
Subscriber receives message |
26 |
Client Subscribes via Script |
Script subscribes |
Receives messages from broker |
27 |
Client Publishes with TLS |
Secure publish |
Message encrypted and delivered |
28 |
Client Subscribes with TLS |
Secure subscribe |
Message encrypted and received |
29 |
Client Publishes with Authentication |
Authenticated publish |
Message accepted |
30 |
Client Subscribes with Authentication |
Authenticated subscribe |
Subscription accepted |
31 |
Client Publishes with Retain Flag |
Publish retained message |
New subscribers receive last message |
32 |
Client Subscribes After Retain |
Subscribe after retained message |
Receives retained message |
33 |
Client Publishes with Topic Alias |
Use topic alias |
Message routed correctly |
34 |
Client Subscribes with Topic Alias |
Receive message via alias |
Message received |
35 |
Client Publishes with Delay |
Delay between messages |
Messages still delivered |
36 |
Client Subscribes with Delay |
Delay between subscription and publish |
Messages received if retained |
37 |
Client Publishes with Expiry |
Set message expiry |
Message discarded after expiry |
38 |
Client Subscribes After Expiry |
Subscribe after expiry |
Message not received |
39 |
Client Publishes to Multiple Topics |
Publish to multiple topics |
Subscribers to each topic receive messages |
40 |
Client Subscribes to Multiple Topics |
Subscribe to multiple topics |
Messages from all topics received |
41 |
Client Publishes to Wildcard Topic |
Publish to topic matching wildcard |
Subscribers receive message |
42 |
Client Subscribes with Wildcard |
Subscribe using wildcard |
Messages from matching topics received |
43 |
Client Publishes with Will Message |
Set LWT and disconnect |
Will message delivered to subscribers |
44 |
Client Subscribes to LWT Topic |
Subscribe to LWT topic |
Receives will message on disconnect |
45 |
Client Publishes with Session |
Persistent session publish |
Messages queued if subscriber offline |
46 |
Client Subscribes with Session |
Persistent session subscribe |
Messages received after reconnect |
47 |
Client Publishes with Clean Session |
Clean session publish |
No messages retained |
48 |
Client Subscribes with Clean Session |
Clean session subscribe |
No subscriptions retained |
49 |
Client Publishes with Shared Topic |
Publish to shared topic |
Message delivered to one subscriber |
50 |
Client Subscribes with Shared Topic |
Subscribe to shared topic |
Load-balanced message delivery |
Security via TLS - Testcases
# |
Test Case |
Description |
Expected Result |
|---|---|---|---|
1 |
TLS Connection Establishment |
Connect client with TLS enabled |
Secure connection established |
2 |
TLS Handshake Success |
Perform TLS handshake |
Handshake completes successfully |
3 |
TLS Handshake Failure |
Use invalid certificate |
Connection fails |
4 |
TLS with Valid Certificate |
Use valid CA-signed certificate |
Connection succeeds |
5 |
TLS with Self-Signed Certificate |
Use self-signed certificate |
Connection may succeed with proper trust settings |
6 |
TLS with Expired Certificate |
Use expired certificate |
Connection fails |
7 |
TLS with Revoked Certificate |
Use revoked certificate |
Connection fails |
8 |
TLS with Mismatched Hostname |
Certificate hostname mismatch |
Connection fails |
9 |
TLS with Mutual Authentication |
Use client and server certificates |
Connection succeeds with mutual auth |
10 |
TLS with Cipher Suite Negotiation |
Negotiate cipher suite |
Secure cipher selected |
11 |
TLS with TLS 1.2 |
Use TLS version 1.2 |
Connection succeeds |
12 |
TLS with TLS 1.3 |
Use TLS version 1.3 |
Connection succeeds |
13 |
TLS with SSL |
Use deprecated SSL version |
Connection fails |
14 |
TLS with Certificate Pinning |
Pin server certificate |
Connection succeeds only with pinned cert |
15 |
TLS with CA Chain |
Use full CA chain |
Connection succeeds |
16 |
TLS with Missing CA Chain |
Omit CA chain |
Connection fails |
17 |
TLS with Encrypted Payload |
Send encrypted message |
Payload is encrypted |
18 |
TLS with Large Payload |
Send large encrypted message |
Message delivered securely |
19 |
TLS with Small Payload |
Send small encrypted message |
Message delivered securely |
20 |
TLS with JSON Payload |
Send structured data securely |
JSON delivered securely |
21 |
TLS with Binary Payload |
Send binary data securely |
Binary delivered securely |
22 |
TLS with QoS 0 |
Publish with QoS 0 over TLS |
Message delivered securely |
23 |
TLS with QoS 1 |
Publish with QoS 1 over TLS |
Message delivered securely |
24 |
TLS with QoS 2 |
Publish with QoS 2 over TLS |
Message delivered securely |
25 |
TLS with Retained Message |
Publish retained message over TLS |
Message retained securely |
26 |
TLS with Will Message |
Configure LWT over TLS |
Will message delivered securely |
27 |
TLS with Topic Alias |
Use topic alias over TLS |
Alias resolved securely |
28 |
TLS with Message Expiry |
Set expiry over TLS |
Message expires securely |
29 |
TLS with Clean Session |
Connect with clean session over TLS |
Session cleared securely |
30 |
TLS with Persistent Session |
Connect with persistent session over TLS |
Session maintained securely |
31 |
TLS with Shared Subscription |
Use shared subscription over TLS |
Message delivered securely |
32 |
TLS with Wildcard Subscription |
Subscribe using wildcard over TLS |
Messages received securely |
33 |
TLS with Broker Restart |
Restart broker with TLS enabled |
Secure reconnection succeeds |
34 |
TLS with Client Reconnect |
Reconnect client with TLS |
Secure session resumed |
35 |
TLS with Network Drop |
Simulate network failure |
Secure reconnection succeeds |
36 |
TLS with Mobile Client |
Connect mobile client securely |
TLS connection succeeds |
37 |
TLS with IoT Device |
Connect constrained device securely |
TLS connection succeeds |
38 |
TLS with Web Client |
Connect browser-based client securely |
TLS connection succeeds |
39 |
TLS with VPN |
Connect over VPN with TLS |
Double encryption maintained |
40 |
TLS with NAT |
Connect behind NAT with TLS |
TLS connection succeeds |
41 |
TLS with IPv4 |
Connect using IPv4 with TLS |
TLS connection succeeds |
42 |
TLS with IPv6 |
Connect using IPv6 with TLS |
TLS connection succeeds |
43 |
TLS with Broker Cluster |
Connect to clustered broker with TLS |
Secure connection maintained |
44 |
TLS with Logging Enabled |
Enable TLS logging |
Secure events logged |
45 |
TLS with Audit Trail |
Review audit logs |
TLS activity recorded |
46 |
TLS with Monitoring Tool |
Monitor TLS traffic |
Encrypted traffic visible |
47 |
TLS with Performance Benchmark |
Measure TLS overhead |
Acceptable latency observed |
48 |
TLS with Certificate Rotation |
Rotate server certificate |
Connection resumes securely |
49 |
TLS with Certificate Renewal |
Renew certificate |
Connection resumes securely |
50 |
TLS with Certificate Revocation Check |
Enable revocation check |
Revoked certs rejected |
Extensible with MQTT5.0 - Testcases
# |
Test Case |
Description |
Expected Result |
|---|---|---|---|
1 |
Connect with MQTT 5.0 |
Connect using MQTT version 5.0 |
Connection succeeds |
2 |
Connect with MQTT 3.1.1 |
Connect using older version |
Connection succeeds with limited features |
3 |
Publish with User Property |
Add custom user property to publish message |
Property is delivered to subscriber |
4 |
Subscribe with User Property |
Add user property to subscribe request |
Property is acknowledged |
5 |
Receive User Property |
Receive message with user property |
Property is visible to subscriber |
6 |
Publish with Multiple User Properties |
Add multiple user properties |
All properties are delivered |
7 |
Publish with Reason Code |
Send message with reason code |
Code is processed by broker |
8 |
Connect with Reason Code |
Connect and receive reason code |
Code indicates success or failure |
9 |
Disconnect with Reason Code |
Disconnect with reason code |
Broker logs reason |
10 |
Subscribe with Reason Code |
Subscribe and receive reason code |
Code indicates subscription status |
11 |
Enhanced Error Reporting |
Trigger error and check reason string |
Descriptive error message received |
12 |
Invalid Topic Alias |
Use invalid alias |
Error reason code returned |
13 |
Valid Topic Alias |
Use valid alias |
Message routed correctly |
14 |
Topic Alias Mapping |
Map alias to topic |
Alias resolved |
15 |
Topic Alias Reuse |
Reuse alias for multiple messages |
Messages routed correctly |
16 |
Session Expiry Interval |
Set session expiry |
Session expires after interval |
17 |
Message Expiry Interval |
Set message expiry |
Message discarded after expiry |
18 |
Subscription Identifier |
Use subscription identifier |
Identifier is returned with messages |
19 |
Flow Control |
Use receive maximum |
Broker limits message inflow |
20 |
Maximum Packet Size |
Set max packet size |
Larger packets rejected |
21 |
Request Response Information |
Request broker response info |
Info returned in CONNACK |
22 |
Request Problem Information |
Request detailed error info |
Problem info returned |
23 |
Server Disconnect |
Broker disconnects client with reason |
Client receives disconnect reason |
24 |
Client Disconnect |
Client disconnects with reason |
Broker logs reason |
25 |
Retain Available |
Check if broker supports retained messages |
Capability confirmed |
26 |
Wildcard Subscription Available |
Check wildcard support |
Capability confirmed |
27 |
Shared Subscription Available |
Check shared subscription support |
Capability confirmed |
28 |
Subscription Options |
Use no local, retain as published, etc. |
Options applied |
29 |
No Local Option |
Use no local option |
Client does not receive its own messages |
30 |
Retain As Published Option |
Use retain as published |
Retain flag preserved |
31 |
Retain Handling Option |
Set retain handling |
Retained messages handled accordingly |
32 |
Assigned Client Identifier |
Let broker assign client ID |
ID returned in CONNACK |
33 |
Server Keep Alive |
Use server-defined keep-alive |
Interval applied |
34 |
Authentication Method |
Use custom auth method |
Auth exchange initiated |
35 |
Authentication Data |
Send auth data |
Broker processes data |
36 |
Enhanced Auth Exchange |
Perform multi-step auth |
Auth completes successfully |
37 |
Reason String in CONNACK |
Receive reason string on connect |
String explains result |
38 |
Reason String in PUBACK |
Receive reason string on publish ack |
String explains result |
39 |
Reason String in SUBACK |
Receive reason string on subscribe ack |
String explains result |
40 |
Reason String in DISCONNECT |
Receive reason string on disconnect |
String explains result |
41 |
Connect with Clean Start |
Use clean start flag |
Session starts fresh |
42 |
Connect with Session Expiry |
Use session expiry with clean start |
Session expires after interval |
43 |
Publish with Payload Format Indicator |
Set payload format (e.g., JSON) |
Format indicated to subscriber |
44 |
Publish with Content Type |
Set content type (e.g., application/json) |
Type visible to subscriber |
45 |
Publish with Response Topic |
Set response topic |
Subscriber replies to specified topic |
46 |
Publish with Correlation Data |
Include correlation data |
Data used to match responses |
47 |
Subscribe with Subscription ID |
Use subscription ID |
ID included in messages |
48 |
Broker Capability Advertisement |
Broker advertises supported features |
Client adapts behavior |
49 |
Client Capability Declaration |
Client declares limits (e.g., max packet size) |
Broker respects limits |
50 |
MQTT 5.0 Interoperability |
Test with MQTT 3.1.1 client |
Limited compatibility, no 5.0 features |
Reference links