The PPP protocol is documented in a slew of RFC's. Some of these are listed below.
http://www.ietf.org/rfc/rfc1332.txt
http://www.ietf.org/rfc/rfc1334.txt
http://www.ietf.org/rfc/rfc1570.txt
http://www.ietf.org/rfc/rfc1661.txt
http://www.ietf.org/rfc/rfc1662.txt
http://www.ietf.org/rfc/rfc1877.txt
http://www.ietf.org/rfc/rfc1962.txt
http://www.ietf.org/rfc/rfc1968.txt
http://www.ietf.org/rfc/rfc1969.txt
http://www.ietf.org/rfc/rfc1973.txt
http://www.ietf.org/rfc/rfc1978.txt
http://www.ietf.org/rfc/rfc1979.txt
http://www.ietf.org/rfc/rfc1990.txt
http://www.ietf.org/rfc/rfc1994.txt
http://www.ietf.org/rfc/rfc2118.txt
http://www.ietf.org/rfc/rfc2284.txt
http://www.ietf.org/rfc/rfc2419.txt
http://www.ietf.org/rfc/rfc2427.txt
http://www.ietf.org/rfc/rfc2433.txt
http://www.ietf.org/rfc/rfc2516.txt
http://www.ietf.org/rfc/rfc2548.txt
http://www.ietf.org/rfc/rfc2637.txt
http://www.ietf.org/rfc/rfc2661.txt
http://www.ietf.org/rfc/rfc2865.txt
http://www.ietf.org/rfc/rfc2866.txt
http://www.ietf.org/rfc/rfc2868.txt
http://www.ietf.org/rfc/rfc2869.txt
http://www.ietf.org/rfc/rfc3576.txt
http://www.ietf.org/rfc/rfc3579.txt