TCP/IP can be used on a wide range of carrier, NetBSD/amiga supports Ethernet, Arcnet and serial lines. There are three reasons for using a serial line protocol in preference to the others:
The disadvantage of a serial connection is that it's slower than Ether- and Arcnet, NetBSD can use at most 57,6kBd making it a lot slower than Ethernet's 10MBd and Arcnet's 4(?)MBd.
There are two possible protocols to connect a host running NetBSD/amiga to another host using a serial line (possibly over a phone-line):
The choice here depends on whether you use a dial-up connection through a modem or if you use a static connection (null-modem or leased line). If you dial up for your IP connection, it's wise to use PPP as it offers some possibilities to auto-negotiate ip-addresses and routes, which can be quite painful to do by hand. If you want to connect to another machine which is directly connected, use SLIP, as this is supported by about every operating system and more easy to set up with fixed addresses and routes.
PPP on a direct connection is a bit difficult to setup, as it's easy to timeout the initial handshake; with SLIP, there's no such initial handshake, i.e. you start up one side, and when the othersite has its first packet, it will send it over the line.
RFCs 1331 and 1332 describe PPP and TCP/IP over PPP. SLIP is defined in RFC 1055.