SLIP is an asynchronous, serial line protocol developed for running TCP/IP
over serial communications lines in a point-to-point configuration. SLIP
was developed to transmit IP packets over low-speed, sometimes noisy,
asynchronous communications lines where error recovery and an efficient line
protocol are needed. The SLIP protocol is now being replaced with a new
serial line protocol named "PPP", which uses a more efficient means of
establishing a point-to-point IP connection.
MacTCP 1.0.2 includes hooks that let third-party developers write different
link-layer modules. This makes possible the development of interfaces to
SLIP, PPP, and to any other link layer that someone may need, like
broadband, X.25, FDDI, and so on. Apple does not provide support in MacTCP
for SLIP or any other serial line protocol; these have to come from
third-party developers.