http://www.rfc-editor.org/rfc/rfc1333.txt
RFC 1333: PPP Link Quality Monitoring, W. Simpson1992-05-01T23:00:00-00:00The Point-to-Point Protocol (PPP) [1] provides a standard method of encapsulating Network Layer protocol information over point-to-point links. PPP also defines an extensible Link Control Protocol, which allows negotiation of a Quality Protocol for continuous monitoring of the viability of the link. [STANDARDS-TRACK]