Some useful Internet RFCs about PPP include the following:
1661 and 1662, which describe the major features of the PPP protocol
1334, which describes authentication protocols, such as Password Authentication Protocol (PAP) and Challenge-Handshake Authentication Protocol (CHAP)
2516, an informational RFC that describes PPP over Ethernet (PPPoE)
To obtain copies of PPP RFCs, specify the number of the RFC on the IETF RFC web page at http://www.ietf.org/rfc.html .