COAP_CLIENT_LIB request issue

[Christian77]

Hi.

I'm using nrf Connect SDK v3.0.1 and Zephyr v4.0.99 to implement a IoT device which connects to a Thingsboard server with CoAP. With the COAP_CLIENT Lib I want to download data in 945 chunks of 512 byte size.

I've used the coap_download example as a template for my implementation. I create the request and send it to the library with coap_client_req(). When the server responds, the callback function gets called in which I process the data (copy it to a queue in my case) and give a sem to tell the coap thread to cancel the request (coap_client_cancel_request()). This works as many times as I have configured in CONFIG_COAP_CLIENT_MAX_REQUESTS. After that the lib reports an error "No more free requests".

It looks for me that the cancel of the request didn't work. But maybe I didn't understand the usage of the COAP_CLIENT lib.
Are there any hints to use it correctly?

Thanks
BR
Christian

[Achim Kraus]

Maybe this is either a limitation or a bug in zephyr's "coap_client" implementation (NCS 2.9.1 / zephyr v3.7.99).

The "get_free_request" uses "exchange_lifetime_exceeded" and that extends the "usage" of the internal request for "3 * ack_timeout", even after the call of "coap_client_cancel_request"!

Not sure, if using "pending.params.ack_timeout" and not "pending.timeout" is really well, but that's more a question to the developers of that code and their considerations.

This limits the number of possible requests per time. Assuming an "ack_timeout" of 2s, each internal request may only be used every 6s. With 6 CONFIG_COAP_CLIENT_MAX_REQUESTS that will result in 1 request per second.    

I guess, it requires an ticket in the zephyr project to fix it or the get aware, how to handle that case proper.