of Things protocol.
We used the latest version of MIMIC MQTT Simulator to test several MQTT
brokers with MQTT 5 support, and in a short time have discovered some
differences in protocol behavior between them.
Test 1 - CONNECT with Maximum Packet Size set to low valueIn the case of 0, one broker disconnects immediately, and the other sends a
CONNACK with Reason Code. In this case, the spec clearly says
It is a Protocol Error to include the Maximum Packet Size more than once, or for the value to be set to zero.Both behaviors are legal, since protocol errors must result in disconnect, but
at the discretion of the server it may send a CONNACK with reason code, as
Curiously, if the value is 10, again one broker disconnects immediately, but
the other does not send anything, presumably because the CONNACK would
have violated the limit. This results in a timeout at the client.
Test 2 - PUBLISH with empty topic name and no topic aliasIn this case, one broker disconnects and the other sends PUBACK with
Reason Code. In this case, the spec clearly says
It is a Protocol Error if the Topic Name is zero length and there is no Topic Alias.Protocol errors must result in disconnect as detailed here.
Test 3 - PUBLISH to wildcard topicFor example, to handle the error of a PUBLISH with QOS = 1 to an illegal
wildcard topic, one broker returns a PUBACK with the Reason Code of 0x90,
whereas another returns a DISCONNECT with that Reason Code. This
behavior is expected with QOS 0, but since it is not defined as a protocol
error, for QOS 1 and 2 the PUBACK and PUBREC response should be used.
This is what our logs show:
INFO 02/06.10:58:25 - MQTT[AGT=1] - sent PUBLISH (110 bytes)
INFO 02/06.10:58:25 - MQTT[AGT=1] - rcvd PUBREC rc=0x90 Reason String The topic is not valid: ...
INFO 02/06.11:01:25 - MQTT[AGT=1] - sent PUBLISH (110 bytes)
INFO 02/06.11:01:25 - MQTT[AGT=1] - rcvd DISCONNECT reason 0x90 (Reason String PUBLISH with wildcard character (#/+) was sent.)
Test 4 - PUBLISH with Payload Format propertyFor a PUBLISH with property Payload Format set, the spec says
A Server MUST send the Payload Format Indicator unaltered to allOne of the brokers discards the Payload Format Indicator property if it is 0.
subscribers receiving the Application Message [MQTT-3.3.2-4]
If this property is missing, it can be assumed to be 0. While not breaking
anything, this is technically incorrect.
ConclusionSince MQTT 5 is so recent, you need to test many features of your selected
MQTT brokers and IoT applications with different scenarios to mitigate
costly surprises before deployment. MIMIC MQTT Simulator can really help