MQTT 5.0 Features of Inflight Window & Message Queue

Introduction to Inflight Window and Message Queue

In order to improve message throughput efficiency and reduce the impact of network fluctuations, multiple unacknowledged QoS 1 and QoS 2 packets are allowed to exist on the network link at the same time by EMQ X broker. These packets that has been sent but unconfirmed will be stored in the inflight Window until the confirmation is completed.

Inflight Window and Receive Maximum

The CONNECT packet is added with a Receive Maximum attribute by the MQTT v5.0 protocol. The official interpretation of it is that the client uses this value to limit the maximum quantity of published messages with QoS of 1 and QoS of 2 that the client is willing to process simultaneously. There is no mechanism to limit the published messages with a QoS of 0 that the server is trying to send. That is to say, the server can send subsequent PUBLISH packets to the client with different packet identifiers while waiting for confirmation, until the number of unconfirmed packets reaches the Receive Maximum limit.

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EMQ is an open-source IoT data infrastructure software provider, delivering the world’s leading open-source MQTT message broker and stream processing database.