MQTT 128kB packet size

[JWizard]

Hello,

I am using NRF9160 running the Serial LTE Modem application in NCS 1.5.0.

When connecting with AWS IoT over a secure MQTT connection the maximum MQTT packet size allowed by AWS IoT is 128kB. When I publish a message of this size from an external device my nrf9160 device outputs the error -128. What is this error? 128 does not appear in errno.h.

How can I allow the maximum packet size of 128kB from AWS IoT to be received by the nrf9160? If this size is not reachable what is the maximum limit and how do I enable that limit?

What is the maximum outgoing limit and how do I enable that limit?

[JWizard]

I was able to get past the handshake by increasing the CONFIG_MBEDTLS_HEAP_SIZE. In my case to 81920.

Now the error is POLLHUP. From what I can tell at this moment this occurs immediately, the first time the socket is polled.

[Didrik Rokhaug]

Hi, and again, sorry for this taking so long.

But, I have some good news this time: I have found a solution/workaround for the POLLHUP problem.

After some digging, it  turned out that the connection to AWS is fine. While I still don't understand 100% why the POLLHUP flag is set, I believe that either the POLLHUP flag itself is a bug, or the application's handling of the POLLHUP is wrong. I'll have to discuss this more with our developers before I can be sure which one it is.

Regardless, you can simply ignore the POLLHUP, and everthing will work fine.

What I did was to add a 'continue' inside the if that checks if fds.revents has the POLLIN flag set, after the call to mqtt_input (and checking the return value) in mqtt_thread_fn in slm_at_mqtt.c.

In an unmodified SLM, it would be here: https://github.com/nrfconnect/sdk-nrf/blob/master/applications/serial_lte_modem/src/mqtt_c/slm_at_mqtt.c#L237

I'll try to keep you updated as we open PRs to make AWS connections with native TLS supported.

Best regards,

Didrik

[Didrik Rokhaug]

Actually, it turns out the POLLHUP issue has already been fixed in upstream Zephyr.

It will be included in the NCS fork in the next upmerge, and be included in NCS 1.8.0.

You can find the fix here: https://github.com/zephyrproject-rtos/zephyr/pull/37794

It should apply cleanly to NCS v1.7.0.