Comparison of MQTT implementations

MQTT (MQ Telemetry Transport or Message Queuing Telemetry Transport) is an ISO standard (ISO/IEC PRF 20922) publish-subscribe-based messaging protocol. It works on top of the TCP/IP protocol. It is designed for connections with remote locations where a "small code footprint" is required or the network bandwidth is limited. The publish-subscribe messaging pattern requires a message broker.
All comparison categories use the stable version of each implementation listed in the overview section. The comparison is limited to features that relate to the MQTT protocol.
Overview.
The following table lists MQTT both libraries and implementations, along with general information about each.
A more complete list of MQTT implementations can be found on GitHub.
Protocol support.
There are several versions of the MQTT protocol currently standardized. Below is a list containing the more recent versions of the MQTT protocol, with the organization that standardized them.
The following table lists the versions of MQTT that each implementation supports, and also lists their support for SSL/TLS and TCP. The security provided by SSL/TLS may be desirable depending on the type traffic being sent between devices, as MQTT transmits messages in the clear.
Quality of service levels offered.
From the MQTT page, quality of service (QoS) is described as,Quality of service refers to traffic prioritization and resource reservation control mechanisms rather than the achieved service quality. Quality of service is the ability to provide different priority to different applications, users, or data flows, or to guarantee a certain level of performance to a data flow.A description of each QoS level is found below.
The following table lists each implementation's support of the QoS levels.
Portability concerns.
Portability concerns in this section refers to technical details that may be deciding factors in selecting an implementation to use. In general, this table should be used by those with more knowledge about the device they will be using.
General requirements.
The following table shows various requirements that may be useful when deciding on which implementation to use for a device.