MQTT Publish Payload Size Limitation in Serial LTE Modem App 2.0.0

[Kimberly]

Hello, 

Our tests and the MQTT publish documentation indicate a payload size limitation of 256 bytes for MQTT publish, which we see is set with an enum in slm_at_mqtt.c. Our understanding is that this limitation is  smaller than in previous versions of SLM, though documentation suggests the 9160 can support transmissions of up to 2kb.

Could you please provide clarity on the size limitation - why it was restricted and if there is a preferred method which supports the publishing of longer messages? 

Thank you. 

[Hakon]

Hello,

Our tests and the MQTT publish documentation indicate a payload size limitation of 256 bytes for MQTT publish, which we see is set with an enum in slm_at_mqtt.c.

This is the limitation when Publish in command mode.

When publish in data mode, it's close to 4K.

Used to be NET_IPV4_MTU(572) in command mode.

[Kimberly]

Great, thanks. I tested the above and was able to send a >2kb string in datamode using the lte ink monitor. Around >2300 bytes, mosquitto received it as smaller packets of varying sizes.