I want to connect two computer via serial but for each to see each other via a TCP/IP connection. Ie, create new network device on the computers that are in actual fact serial ports.
The reason for this is that I am actually testing the medium in which the serial connection is made (wireless), and part of the experiment will be to use TCP/IP.
The radio being tested is a telemetry radio for use in low power applications. It polls once a second, sending data out on the wireless channel every poll when something has been received via the serial port. It uses a Modbus RTU delimiter to determine the end of data coming in on the serial port.
slip and ppp are more suitable for use with actual serial modems from what I understand.
This is actually a very hard problem. TCP/IP is a very chatty protocol and you will have problems with the radio system you have described because of the pattern of packets and ACKs you will have. In the past for some similarly unsuited applications I worked on a system that fibbed about the TCP/IP connection by faking some packets while pushing the data over a link like you have.
It is a pain, but we were doing it to support sshing over a totally inappropriate channel (high loss and high latency with moving endpoints) but it worked.