NRF Cloud coap payload size limitation too small

[AdrianT]

Hi,

I am using NRF Cloud + coap comms to send data. However I have encountered payload size limitations that are not configurable via KConfig: sdk-nrf/subsys/net/lib/nrf_cloud/src/nrf_cloud_codec.c at main · nrfconnect/sdk-nrf

I have manually modified it to accomodate my payloads. I am also considering manually encoding my payload in cbor to avoid modifying SDK code. Is there a reason why this is limited to 64 bytes? There is no documentation pointing to the payload size limits for nrf cloud.

[Pete Skeggs]

Hello,

That particular function is not designed to handle CBOR objects which are not built-in to the nRF Cloud CoAP library. The 64 byte limitation is only on that function which only supports CBOR objects defined for struct nrf_cloud_obj_coap_cbor: https://github.com/nrfconnect/sdk-nrf/blob/19945a632cd9d772f5d094f22a97b5180b7be33b/include/net/nrf_cloud_codec.h#L70.

For your use case, I recommend that you encode your custom CBOR message yourself, and pass it to nrf_cloud_coap_obj_send(): https://github.com/nrfconnect/sdk-nrf/blob/19945a632cd9d772f5d094f22a97b5180b7be33b/include/net/nrf_cloud_coap.h#L455. Your object must have enc_src = NRF_CLOUD_ENC_SRC_PRE_ENCODED and type = NRF_CLOUD_OBJ_TYPE_COAP_CBOR.

[AdrianT]

Thanks. 

Do you have a code snippet for CBOR encoding a byte string? I assume NRF_CLOUD_OBJ_PRE_ENC_DEFINE macro should be enough to initialize the message with the correct enc_src and type.

 

Is using cbor_encode_message_out function (sdk-nrf/subsys/net/lib/nrf_cloud/coap/include/msg_encode.h at main · nrfconnect/sdk-nrf) correct for what I am trying to achieve?

[Pete Skeggs]

If all you are wanting to encode is a string, you can instead directly call nrf_cloud_coap_message_send():
https://github.com/nrfconnect/sdk-nrf/blob/a07b87c923420fdbfbdbff3698a3d30f5c1c33ee/include/net/nrf_cloud_coap.h#L218
which itself calls int coap_codec_message_encode() and eventually cbor_encode_message_out(), with the proper parameters to encode a string.

Be sure to set nrf_cloud_coap_message_send()'s json parameter to false.