T1: A Survival Guide

Chapter 9. PPP

Everything secret degenerates; nothing is safe that does not bear discussion and publicity.

—Lord Acton

HDLC, as specified by ISO, falls short of the Platonic ideal of a generic link layer protocol. It cannot multiplex higher-layer network protocols; cannot detect faults, slowly degrading links, or looped-back links; and is not easily extensible or dynamically configurable. Even after Cisco’s extensions to HDLC, many of these problems remained unsolved.

To address these shortcomings, the IETF developed the Point-to-Point Protocol (PPP). PPP framing is based on HDLC, but it was designed from the outset to be a multiprotocol link layer. A system of configurable option negotiation allows both ends of a link to choose a mutually compatible set of configuration parameters, even if the configuration differs between the two ends of the link. PPP’s negotiation mechanism allows for easy extensibility, a feature that has been exploited by several minor revisions to the protocol after its most recent major standardization in 1994. Well-designed protocols can provide a framework for generations, and PPP has adapted nearly effortlessly to the sea of changes in the Internet since 1994.

A great deal of PPP’s success stems from the solid foundation of HDLC on which it was built, but its innovations add tremendous flexibility. PPP can be used on everything from the high-speed SONET links at the core of the Internet to the lowly dial-up links connecting subscribers over common ...