What is the maximum message length for a MQTT broker?

MQTT (Message Queuing Telemetry Transport) is a lightweight publish/subscribe messaging protocol widely used in the Internet of Things (IoT) for communication in low-bandwidth environments. Regarding the maximum message size for MQTT brokers, the MQTT protocol itself does not explicitly define a maximum message size in version 3.1; however, in practical applications, many MQTT brokers have their own limitations. These limitations are influenced not only by the design of the MQTT broker software but also by the operating system and network environment. For example, common MQTT brokers like Mosquitto have a default message payload size limit of 256 MB. However, this value can be adjusted via the configuration file. In Mosquitto's configuration file, the maximum message size can be set using the message_size_limit configuration option. If set to 0, it indicates no size restriction. Additionally, the network environment between MQTT clients and servers must be considered, such as the Maximum Transmission Unit (MTU) in TCP/IP protocols, which may affect the actual maximum transmittable message size. In summary, although the MQTT protocol itself does not strictly specify a maximum message size in version 3.1, in practical applications, the message size for MQTT brokers is typically determined by the broker software settings and network environment. When designing systems, these parameters should be configured appropriately based on actual needs to ensure stable operation and efficient communication.