RFC 1331: The Point-to-Point Protocol (PPP) for the Transmission of Multi-protocol Datagrams over Point-to-Point Links
RFC 1171: Point-to-Point Protocol for the transmission of multi-protocol datagrams over Point-to-Point links
RFC 1172: Point-to-Point Protocol (PPP) initial configuration options
RFC 1548: The Point-to-Point Protocol (PPP)
The Point-to-Point Protocol (PPP) for the Transmission of Multi-protocol Datagrams over Point-to-Point Links