Point-to-Point Protocol (PPP)

Point-to-Point Protocol, specified in RFC 1661, carries network-layer packets over a direct connection between two nodes, such as a dial-up line, a serial leased line or, in its PPP over Ethernet form, a broadband connection. It has three parts: a framing method, the Link Control Protocol that establishes, configures and tests the link, and a family of Network Control Protocols that set up each network-layer protocol to be carried, such as IP. It replaced the older Serial Line Internet Protocol, which had no negotiation, no authentication and no error detection.

Authentication is an optional phase between link establishment and the start of network traffic. The methods most often named are PAP, which sends the password in clear text, CHAP, which uses a challenge and a hash, and the Extensible Authentication Protocol, which was first defined for PPP and later reused by IEEE 802.1X. Core PPP does not encrypt the traffic it carries; encryption came from optional extensions or from other layers. Its frames are also what several remote access tunnels wrap: PPTP and L2TP both carry PPP sessions across an IP network, relying on other mechanisms for confidentiality.

Exam relevance: questions in this area tend to link PPP with remote access and its authentication options. Candidates are expected to rank PAP, CHAP and EAP correctly and to remember that core PPP provides no confidentiality.