Overview of the PPP protocol with a comparison with the old SLIP protocol.
Serial and dial-up lines do not provide a framing. Instead they offer only a raw bit transport service.
Therefore a framing protocol is needed so that the receiver can detect frame (packet) boundaries.
SLIP (Serial Line IP) was an early and very simplistic protocol that provided this framing.
PPP (Point to Point Protocol) is a successor to SLIP. PPP is a collection of protocols each providing a specific function like authentication, link layer negotiation or compression.