Serial Line Internet Protocol (SLIP) is a layer two encapsulation for use over dial-up or other point-to-point links. SLIP is very simple and lacking in security and other controls. SLIP requires that IP addressing be configured on each end prior to use.
SLIP is documented in RFC 1055. However, as the RFC points out, SLIP is not intended to become an IETF standard. SLIP is largely deprecated for most data communications, but it is still useful in situations where minimum overhead is required. For very slow links, there is a compressed version of SLIP (CSLIP) that employs TCP/IP header compression.
See: RFC 1144.
References:
http://www.tcpipguide.com/free/t_SerialLineInternetProtocolSLIP.htm