Why dont we finish writing the IP and TCP protocols first. And then its up to the user to write his own lowlevel interface code, for example if the IP wants to send a packet to the network it calls a function something like this:
send_packet(data_pointer, size)

The send_packet function is specific for each type of low level interface.

I have decided to use SLIP because it would be "overkill" to use a ethernet NIC with an 8051 and SLIP has better performance than PPP and its very easy to use with linux