Queue Mode Operation
The LWM2M Server MUST support Queue Mode and the LWM2M Client SHOULD support Queue Mode.
When the LWM2M Client has registered with Current Transport Binding and Mode parameter including “Q” (see chapter 5.3), The LWM2M Server does not immediately send downlink requests on the transport layer used in Queue mode, but instead waits until the LWM2M Client is online on the transport layer.
The LWM2M Client lets the LWM2M Server know it is awake by sending a registration update message as a Confirmable message. The LWM2M Server then makes any queued requests to the LWM2M Client in a serial fashion. The LWM2M Client MUST wait at least ACK_TIMEOUT [COAP] seconds from the last CoAP message it sent to the LWM2M Server before intentionally going offline. If the LWM2M Server is not successful in sending a request, then it stops emptying the queue and keeps the request for the next time the LWM2M Client is online.
A typical Queue Mode sequence follows the following steps:
- The LWM2M Client registers to the LWM2M Server and requests the LWM2M Server to run in Queue mode by using the correct Binding value in the registration.
- The LWM2M Client uses the CoAP ACK_TIMEOUT parameter to set a timer for how long it shall stay awake since last sent message to the LWM2M Server. After ACK_TIMEOUT without any messages from the LWM2M Server, the LWM2M Client SHOULD sleep until sending next periodic “Update” operation.
- When the LWM2M Server receives a message from the Client (e.g. a notification or a registration update), it checks its request queue for the LWM2M Client and performs the needed CoAP operation(s) (e.g. GET, PUT, and POST). Note: There could be several requests in the queue). Each request is sent serially to the LWM2M Client, waiting for request to be Acknowledged before sending the next request. If a request is unsuccessful then it is returned to the queue. The LWM2M Client may have pending Observer notifications.
Below is an example flow for Queue Mode in relation to Device Management & Service Enablement Interface.
Figure 23: Example of Device Management & Service Enablement interface exchanges for Queue Mode.
Below is an example flow for Queue Mode in relation to Information Reporting Interface
Figure 24: Example of an Information Reporting exchange for Queue Mode.