7. Known Issues and Limitations

There are a few known issues and edge cases that have been identified during development but have not yet been addressed. In addition, some features were deliberately not implemented due to their limited usefulness in the LwM2M context and the priority placed on keeping Anjay Lite’s footprint small.

7.1. Multi-Instance Bootstrap Write is not allowed

Bootstrap-Write requests that target only an Object and include multiple Object Instances in one payload aren’t supported. Use separate Bootstrap-Write requests for each instance instead. LwM2M 1.2 explicitly allows object-level Bootstrap-Write with multiple instances in one message.

7.2. Notification storing when offline

The Notification Storing When Disabled or Offline Resource (/1/x/6) is not supported. When values change while the client is offline or the server account is disabled, notifications are not queued and are lost. Changes become reportable again only after the client is online/enabled.

LwM2M 1.2 defines the /1/x/6 Resource (default: true) and the hqmax attribute for historical queuing, but neither has any effect in this client.

7.3. Notification payload may not reflect threshold breach

In certain cases, a notification may be triggered when the observed value crosses a threshold (for example, configured using the gt attribute), but the actual notification is not sent immediately. If the value later returns below (or above) that threshold before the notification message is constructed, the server will receive a notification containing a value that itself would not have caused the notification to be triggered. This applies to all value-based attributes: lt, gt, st and edge.

7.4. Default value of Multi-Instance Resource

Write-Replace semantics in LwM2M replace the entire array of instances of a Multiple-Instance Resource. Anjay Lite uses the inst_reset handler that clears all Resource Instances before applying the new set. As a result, any “default” instances or values not present in the Write-Replace payload are removed.

To retain default values, include them in the Write-Replace payload or use Partial Update or Write-Composite instead.

7.5. Silent Observations drop when observed path disappears

When anj_observe_data_model_changed() is called with ANJ_OBSERVE_CHANGE_TYPE_DELETED, the observation is dropped without sending a final Notify with 4.04 Not Found. According to RFC 7641, when a resource becomes unavailable, the notifier SHOULD send a 4.04 notification and MUST remove the observer entry. Anjay Lite removes the observation but currently skips sending the final 4.04 notification. Similarly, we do not inform the server about removing observations for other reasons (e.g., errors when building notifications).

7.6. Block-wise limitations

Anjay Lite does not support:

  • Early Block2 size negotiation for requests with Block1 option - in LwM2M terms, this means that the LwM2M Server cannot force Anjay Lite to change the block size of its responses when the request itself is block-wise. This can occur only with Read-Composite and Observe-Composite operations. Anjay Lite supports block-wise transfer in both directions, but only with the same block sizes.

  • Late Block2 size negotiation - it is not possible for the LwM2M Server to change the Block2 size of Anjay Lite responses in the middle of the transfer.

  • Block1 size negotiation - it is not possible for the LwM2M Server to change the Block1 size of Anjay Lite requests.

  • Handling block-wise server requests where Block2 starts from a block other than NUM=0 - the LwM2M Server cannot force Anjay Lite to start sending the response from a non-zero block offset or resume a block-wise response from the middle of the transfer.