Use MQTT 5 flow control with Receive Maximum to prevent device buffer overflow

Steps

1. In the MQTT 5 CONNECT packet, set the Receive Maximum property to the maximum number of QoS 1 and QoS 2 publishes the client can handle concurrently (e.g., 10 for a constrained device).
2. The broker honours this limit and does not send more than Receive Maximum unacknowledged QoS 1/2 PUBLISH packets to the client at any time.
3. The client tracks in-flight messages and only sends PUBACK or PUBCOMP to release a slot; do not send DISCONNECT without draining or the broker may resend on reconnect.
4. For device-to-cloud publishing, set the broker-side Receive Maximum in the CONNACK to cap the device's own outgoing QoS 1/2 in-flight window.
5. Combine Receive Maximum with the Session Expiry Interval property on CONNECT to control how long the broker retains undelivered QoS 1/2 messages across disconnections.
6. Monitor the Topic Alias Maximum property from the CONNACK to know how many topic aliases the broker supports before using aliases to reduce per-message overhead.