Hello Hein!
The least I want to do is to implement a new PPP library by myself… mostly because I was unable to find a comprehensive description on PPP. It’s described in many dozens of RFC documents and didn’t have the time (yet) to locate the most appropriate RFCs or read them through. I wasn’t in the mood to do so, and had no time either.
I have done some investigation in the topic of “open/free PPP implementations for embedded OSes” and the consequences are the following: either I pick the linux pppd package and strip down to our needs (and resource limitations), or I consider lwip’s PPP stack (which is also a derivative of the linux pppd) and make it run with FreeRTOS properly. Either way, it’s not gonna be an easy ride.
(The third solution is that I find out how to fix our current PPP stack to work with the LTE modem. That PPP stack works perfectly with an older 3G modem using CHAP authentication, but with the new modem the service provider wants PAP authentication and it fails for some reason - most probably our PPP stack is buggy. My idea now is that somehow we make the LTE modem work in 3G mode and perhaps the service provider will ask for CHAP with the new modem too. By the way, the network speed is not that important, because the modem is connected to the MCU via UART @ 115200 bps.)