SLIP
SLIP was designed to allow data to be transmitted via Transmission Control Protocol/Internet Protocol (TCP/IP) over serial connections in a UNIX environment. SLIP did an excellent job, but time proved to be its enemy. SLIP was developed in an atmosphere in which security was not an overriding concern; consequently, SLIP does not support encryption or authentication. It transmits all the data used to establish a connection (username and password) in clear text, which is, of course, dangerous in today's insecure world.
In addition to its inadequate security, SLIP also does not provide error checking or packet addressing, so it can be used only in serial communications. It supports only TCP/IP, and log in is accomplished through a terminal window.
Many operating systems still provide at least minimal SLIP support for backward capability to older environments, but SLIP has been replaced by a newer and more secure alternative: PPP. SLIP is still used by some government agencies and large corporations in UNIX remote access applications, so you might come across it from time to time.