The PPP protocol is documented in a slew of RFC's. Some of these are listed below.
RFC 1332 The PPP Internet Protocol Control Protocol (IPCP)
RFC 1334 PPP Authentication Protocols
RFC 1661 The Point-to-Point Protocol (PPP)
RFC 1662 PPP in HDLC-like Framing
RFC 1877 PPP Internet Protocol Control Protocol Extensions for Name Server Addresses
RFC 1962 The PPP Compression Control Protocol (CCP)
RFC 1968 The PPP Encryption Control Protocol (ECP)
RFC 1969 The PPP DES Encryption Protocol (DESE)
RFC 1974 PPP Stac LZS Compression Protocol
RFC 1978 PPP Predictor Compression Protocol
RFC 1990 The PPP Multilink Protocol (MP)
RFC 1994 PPP Challenge Handshake Authentication Protocol (CHAP)
RFC 2419 The PPP DES Encryption Protocol, Version 2 (DESE-bis)
RFC 2427 Multiprotocol Interconnect over Frame Relay
RFC 2433 Microsoft PPP CHAP Extensions
RFC 2516 A Method for Transmitting PPP Over Ethernet (PPPoE)
RFC 2548 Microsoft Vendor-specific RADIUS Attributes
RFC 2637 Point-to-Point Tunneling Protocol (PPTP)
RFC 2661 Layer Two Tunneling Protocol (L2TP)
RFC 2865 Remote Authentication Dial In User Service (RADIUS)