The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Network Interface Layer (OSI Data Link Layer) Protocols            9  TCP/IP Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP)                 9  Point-to-Point Protocol (PPP)                      9  PPP Core Protocols: Link Control, Network Control and Authentication

PPP Network Control Protocols (IPCP, IPXCP, NBFCP and others) 1 2 3 PPP Feature Protocols

PPP was designed to provide layer two connectivity over a variety of serial links and other physical layer technologies, some of which have much more of a concern about security than others. For example, suppose you hook two machines in your computer lab together with a serial cable and want to run PPP between them. When one of these initiates a PPP link with the other, you don't really need to worry about “who's calling”. On the other hand, consider an Internet Service Provider using PPP for remote dial-in users. They generally want to allow only their customers to connect, not just anyone.

The PPP protocol suite was designed to include the use of an optional authentication protocol for links where authentication is important. During basic link setup by LCP, devices can negotiate the use of an authentication protocol. If they agree, after the LCP link is set up a series of authentication messages are sent to verify the identity of the device initiating the link. Only if authentication is successful can the link configuration proceed.

The PPP suite initially defined two different authentication protocols: the Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP).

PPP Network Control Protocols (IPCP, IPXCP, NBFCP and others) 1 2 3 PPP Feature Protocols