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. Mbed TLS integration with asynchronous network backends

The default Mbed TLS DTLS integration is not compatible with network backends where anj_net_recv() may return ANJ_NET_EINPROGRESS after starting a receive operation that remains active across multiple calls.

The Anjay Lite network API requires an operation that returns ANJ_NET_EINPROGRESS to be retried with the same parameters before another network operation is performed. Mbed TLS exposes non-blocking BIO callbacks through MBEDTLS_ERR_SSL_WANT_READ and MBEDTLS_ERR_SSL_WANT_WRITE, but its public API does not currently guarantee that an in-progress network operation will be resumed before another network operation is attempted.

In particular, during a DTLS handshake, expiry of the retransmission timer may cause Mbed TLS to attempt to send a retransmission while a receive operation is still in progress.

See Using Mbed TLS with Anjay Lite: configuration and troubleshooting for additional details and configuration considerations.

7.2. 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.

7.3. 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.

7.4. Notification payload may not reflect threshold crossing

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 goes back below (or above) that threshold before the notification message is constructed, the server will receive a notification containing the new value that itself would not have caused the notification to be triggered. This applies to all value-based attributes: lt, gt, st and edge.

7.5. 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.

7.6. 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).

7.7. 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.

7.8. Non valid hostname may appear in SNI extension

The DTLS Server Name Indication (SNI) extension is designed to communicate the expected server hostname during a DTLS handshake, particularly when it differs from the connection URI. According to RFC6066, the SNI extension must contain a valid hostname, not an IP address.

In Anjay Lite, if the LwM2M Server URI (/0/x/0) is configured with a raw IP address and the dedicated SNI field in the LwM2M Server Object (/0/x/14) is left unset, the default MbedTLS integration layer falls back to the URI host when populating SNI. As a result, the IP address may be sent in the SNI extension, which is non-compliant with RFC6066.

7.9. OSCORE limitations

Anjay Lite has some limitations related to OSCORE support:

  • When using OSCORE Appendix B2, Context ID negotiation may collide with an ongoing block-wise transfer. If the server attempts to start B2 in the middle of such a transfer, Anjay Lite rejects the B2 procedure and terminates the current exchange. For client-initiated exchanges, this may result in fallback to re-registration, potentially followed by a new B2 negotiation during Register. B2 is still allowed when it starts on the first block of the transfer, as this is required to support block-wise registration and other first-block exchanges.

  • On the first use of an OSCORE security context, the LwM2M specification requires using the Echo option and performing the procedure described in Appendix B.2. Performing this procedure derives a new security context, so using the Echo option after completing Appendix B.2 procedure seems redundant. Accordingly, Echo is not implemented in Anjay Lite.

7.10. Firmware Update cleanup after failed or timed-out Write

The Firmware Update process is not properly terminated and cleaned up when a FOTA Push transfer times out while a Write operation is in progress. A similar issue occurs when a Write operation fails in either Push or Pull mode.

In these cases, the Firmware Update cleanup procedure is not executed correctly, which may leave resources associated with the ongoing update allocated and the Firmware Update process in an inconsistent state.