AT Commands for JSON POST (Serial LTE Modem) Example

[EvanTaylor]

I'm running the Serial LTE Modem firmware and utilizing the reference docs in the LTE Link Monitor Terminal (My commands in yellow, blue/grey text is response)

NCS 1.4, SLM firmware, https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/nrf/applications/serial_lte_modem/doc/HTTPC_AT_commands.html

AT+CFUN=1

OK

AT+CFUN?

<bunch good things showing its connecting>

AT#XHTTPCCON=1,"test.domain.com",5000

#XHTTPCCON:1

OK

AT#XHTTPCREQ="POST","/post2","Host: test.domain.com:5000\nAccept: */*\nContent-Type: application/jsonl charset=utf-8\nContent-Length: 34",34

OK

#XHTTPCREQ:1

#XHTTPCREQ:1

OK

\n{"key1":"value1", "keyZ":"valueZ"}

OK

AT#XHTTPCCON=0

#XHTTPCCON=0

OK

Ready

Now my server does not see the JSON payload, I've tried multiple things on #XHTTPCREQ:1, but my server is just seeing this as an attempt at a form submission without a proper data payload (at least my little python-based Flask echo server does).  This makes no sense to me as I specifically sent the content-type to be application/json.

Im trying to evaluate these boards as a drop-in replacement option for Digi LTE modems (We're consistently getting 28-300+ second connect time with Digi, and 3-6 second connect time with the NRF9160 chips) on a custom MCU board we're using, so the preference is to get an AT Command protocol in place as those MCUs already interface over AT commands.

I just want to get a "hello, world" equivalent working with JSON payload over HTTP before I start looking at HTTPS/MQTT options going forward.  Any help here would be greatly appreciated.

~Evan

[Heidi]

Hi!

Sorry about the slow response time.

I just wanted to let you know I'm working on your case. I have not been able to reproduce this issue yet, but I'll let you know if I have any updates. 

Content-Type: application/jsonl charset

I'm pretty sure there's supposed to be a semi-colon after json, and before charset?

Best regards,

Heidi

[EvanTaylor]

Thanks Heidi.

I did not copy/paste this in correctly here but had tried so many combinations that I’m certain that’s not the core issue.  I’ll reattempt later today just incase to be sure, however.

~Evan

[Heidi]

Hi, I haven't been able to reproduce this issue, unfortunately. 

Please add CONFIG_AT_CMD_LOG_LEVEL_DBG=y to the prj.conf file and then provide me the resulting log file from running the application.

Also, if you're able to take a modem trace, that would be very helpful. 

Best regards,

Heidi

[EvanTaylor]

Can you please give me your list of AT commands in the LTE Link Monitor that successfully performs an HTTP POST with a JSON payload using the default Serial LTE Modem firmware?  This isn't a firmware issue, it's a documentation one.

The documentation states a JSON payload can be sent, but the example (https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/nrf/applications/serial_lte_modem/doc/HTTPC_AT_commands.html) is for application/x-www-form-urlencoded data:

AT#XHTTPCREQ="POST","/post","User-Agent: SLM/1.2.0
Accept: */*
Content-Type: application/x-www-form-urlencoded
Content-Length: 20
",20
OK
#XHTTPCREQ: 1
12345678901234567890
OK
#XHTTPCREQ: 0
#XHTTPCRSP: 576,1
HTTP/1.1 200 OK
Date: Wed, 09 Sep 2020 08:21:03 GMT
Content-Type: application/json; charset=utf-8
Content-Length: 405
Connection: keep-alive
ETag: W/"195-JTHehAiV7LQRCKihfzcZBX1rgGM"
Vary: Accept-Encoding
set-cookie: sails.sid=s%3AtApCs6p2Ja2on5dYO8QvhQSEEfnvkjOX.31HKOpZcip6MzzUoqPw2WZib0rPpimc5y10Mjczukoc; Path=/; HttpOnly
{"args":{},"data":"","files":{},"form":{"12345678901234567890":""},"headers":{"x-forwarded-proto":"https","x-forwarded-port":"443","host":"postman-echo.com","x-amzn-trace-id":"Root=1-5f589067-d61d0850c65f3568f9c9e050","content-length":"20",#XHTTPCRSP:165,0
"user-agent":"SLM/1.2.0","accept":"*/*","content-type":"application/x-www-form-urlencoded"},"json":{"12345678901234567890":""},"url":"">https://postman-echo.com/post"}

~Evan

[Heidi]

Hi,

I'm not able to reproduce your issue because I'm getting #XHTTPCREQ: -128 when sending the JSON string.

But I have contacted the developer who wrote the Serial LTE Modem application and I'll let you know when I hear back about the correct way to do this. 

Best regards,

Heidi

[Heidi]

Apparently, there is some issue with the current SLM HTTP client service to send JSON format that's being worked on right now.

I should have a working example for you tomorrow.