http://www.rfc-editor.org/rfc/rfc1963.txt
RFC 1963: PPP Serial Data Transport Protocol (SDTP), K. Schneider, S. Venters1996-08-01T23:00:00-00:00This document describes a new Network level protocol (from the PPP point of view), PPP Serial Data Transport Protocol, that provides encapsulation and an associated control protocol for transporting serial data streams over a PPP link. This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind.