This is a very quick crash explanation of setting up a peer to peer IP link over serial. I researched this in order to use RF UART modules. This is what works for me. This is almost entirely ripped from this site (http://www.tldp.org/HOWTO/PPP-HOWTO/direct.html), but I wanted a local copy for myself.

• Two machines running linux with UART devices as /dev/ttyUSB0, configured at 115.2kbps.
• pppd installed
• Of my two RF modules, one has an FTDI UART↔USB chip onboard. The other has UART pins broken out. Connect RTS/CTS.
• Test the connection between the two units with minicom

pppd -detach crtscts noauth lock 11.0.0.1:11.0.0.2 /dev/ttyUSB0 115200 &

pppd -detach crtscts noauth lock 11.0.0.2:11.0.0.1 /dev/ttyUSB0 115200 &

These commands are nearly identical. The only difference is that the IP addresses (local:remote) are mirrored.