Serial line multiplexing protocol

I need to simulate several independent serial lines using a single high-speed UART.

Clean solution would be, of course, to implement ppp and then use sockets.

Unfortunately one of the machine is a full-fledged ARM Linux, while the other is a rather limited embedded microcontroller where ppp/networking complexity would severely stress the hardware (id it will fit at all).

I have seen:

  • gsm-mux : it seems to have only one side of the protocol, relying on GSM modem implementation for the other side.
  • smodem : has such a protocol, but I didn't find sources for both ends.

Can someone suggest a similar protocol easily integrable in Linux and some (relatively) low end microcontroller sporting a simple RTK (probably FreeRTOS)?

Software should be unencumbered by heavy Licenses (BSD/Artistic/Apache and similar are OK).

Note: it seems against policy to ask for libs/protocols implementing a (very) specific requirement. I'm sorry, but I have no other idea about where to ask (and similar questions were answered); can somebody suggest where I can ask such question, please? I will then just delete the Question. TiA