A.1 MQTT features

MQTT is a light weight publish/subscribe messaging transport protocol, particularly well-suited to event-oriented interactions. It was specifically designed for constrained environments such as those found in Machine to Machine (M2M) and Internet of Things (IoT) contexts where a small code footprint is required and/or network bandwidth is at a premium.

MQTT includes reliability features which allow recovery from loss of network connectivity without requiring explicit involvement of the applications that are using it, however it does require an underlying network protocol that provides ordered, lossless, bi-directional connections.

The features of MQTT include:

• The use of the publish/subscribe message pattern which provides one-to-many message distribution and decoupling of applications. This is described further in clause A.3.1.
• Bidirectional communications. An entity can subscribe to receive messages without having a reliable IP address. This could be used to allow unsolicited requests to be sent to a Receiver, or an asynchronous response to be sent to an Originator, where the Originator or Receiver does not have an externally accessible IP address. It thus eliminates the need for long polling and can reduce the need for triggering.
• A messaging transport that is agnostic to the content of the payload. The message payload can be text or binary.
• A Session concept that can survive loss of network connectivity and can persist across multiple consecutive network connections. Messages can be stored and subsequently forwarded when connectivity is restored.
• Three levels of reliability (referred to as "qualities of service") for message delivery within a Session:
  • "At most once", where messages are delivered according to the best efforts of the operating environment. Message loss can occur. This level could be used, for example, with ambient sensor data where it does not matter if an individual reading is lost as the next one will be published soon after.
  • "At least once", where messages are assured to arrive but duplicates might occur. This is best suited to messages which have idempotent semantics.
  • "Exactly once", where message are assured to arrive exactly once. This level could be used, for example, with billing systems where duplicate or lost messages could lead to incorrect charges being applied.
• A small transport overhead and protocol exchanges designed to minimize network traffic, with consequent additional savings on battery power when compared to HTTP.
• A Retained Message option, allowing new subscribers to get the last message to have been published on a topic prior to their subscription.
• A mechanism to notify interested parties when an abnormal disconnection occurs.