Serial lte modem mqtt connect timeout problem

[brave_niuniu]

Hi Nordic，

    I am using the nrf52840 as the host and the nRF9160 as the modem with the serial_lte_modem library.

    Normally, when the nrf52840 sends an MQTT connection request (AT#XMQTTCON=1), if incorrect broker parameters are configured, modem_cmd_send() will return the result after the timeout period. However, through my testing, I found that if the timeout is set to more than 170 seconds, modem_cmd_send() will only return after 170 seconds.

    I read the document and it didn't mention the specific timeout duration, which confuses me. I hope someone can answer my question. 

[Simon D-M]

Hi,

This is strange, because modem_cmd_send() uses default Zephyr semaphores, which should be working fine. So it doesn't make sense that it returns after 170 seconds.

What I believe is most likely to be happening, is that maybe after 170 seconds there is something sent to the UART. Can you maybe monitor the UART between the 2 chips to see if there is something happening on it while the function is waiting for an answer?

Best regards,
Simon D-M

[brave_niuniu]

Hi simon,

    I monitored the logs of nrf9160, and it really has nothing to do with semaphores; it's an error returned by mqtt_connect();

    

    I want to know the timeout period returned by mqtt_connect() when the MQTT broker parameters are configured incorrectly, because I need to set a time limit in my application.

    Best regards,

    Niuniu

[Simon D-M]

Hi,

Sorry, I misunderstood your problem.

I wasn't able to reproduce your delay, what version of the SDK / SLM are you using ? I believe some changes to MQTT were made in the latest versions.

Best regards,

Simon D-M

[brave_niuniu]

Hi simonm

    I'm sorry for my late reply. The version of SLM is built based on ncs2.9.2.

    I connected to the correct broker IP and port, but added an incorrect SSL certificate, and mqtt_connect() waited for a while before returning the result. I just want to know if Zephyr has a connection timeout handling for the interior of mqtt_connect, and if so, what the timeout period is.

Best regards,

Niuniu

    

[Simon D-M]

Hi Niuniu,

in NCS, our MQTT implementation is working with BSD socket under the hood.

So the timeout delay should be able to be modified with the "CONFIG_NET_SOCKETS_CONNECT_TIMEOUT" Kconfig (link). By default, this value is at 3000ms, and has a range from 0ms up to 60000ms.

Best regards,

Simon D-M