SLIP (Serial line IP) is a link level protocol to carry IP over serial line (e.g. modem, RC232 interace). Each IP datagram is terminated by END character. The END character will be ESCAPEd if presents in the datagram. There is no checksum in SLIP and error detection and handling is assumed to be done in the upper protocol layer.
As serial interface speed is typically slow, CSLIP is used to optimize throughput by reducing the 20+20 TCP+IP header to 3 or 5 bytes. This is done by maintaining state information (fields that rarely changed) in the CSLIP protocol thus removing the need for such information to be present in the normal TCPIP header. CSLIP can maintain up to 16 connections. The smaller header size improve response time for interactive session on serial line.
PPP (Point-to-point Protocol) improves on SLIP by adding LCP (Link Control Protocol) and NCP (Network Control Protocol) capability. LCP allow the both ends to negotiate options (e.g. IP address negitiation for both ends). NCP allows PPP to support more than 1 network protocol (i.e. no just IP) on one serial line. Finally, PPP included checksum.
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