Packet Identifier Management

SUBSCRIBE, UNSUBSCRIBE, and PUBLISH (QoS 1, QoS 2) packets require a Packet Identifier. The Packet Identifier is used to distinguish the response packets. Users need to set a Packet Identifier for the following request packets.

Packet Identifier is managed by endpoint.

To set a Packet Identifier for the request packet, users need to get a Packet Identifier from the endpoint.

Here are member functions to get a Packet Identifier. There are two methods: acquire and register. For ordinary use cases, acquire is the preferred choice. If users want to manage the Packet Identifier themselves, they can use register. When you get the Packet Identifier, the ownership of the Packet Identifier is transferred to the user.

When the user calls a packet sending function with the packet that has the Packet Identifier set, the ownership of the Packet Identifier is transferred to the library (async_mqtt endpoint). Once the ownership is transferred, the Packet Identifier will automatically become reusable when the response packet is received, an error occurs, or the send process is canceled. Therefore, the user should not call the release APIs.

If the user wants to release the obtained Packet Identifier before the ownership is transferred to the endpoint, they can use the following release APIs.