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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)

Serial Line Internet Protocol (SLIP) 1 PPP Fundamentals and Operation

Even as SLIP was being documented as a “nonstandard” in RFC 1055, work was underway for a newer protocol to provide full-featured IP transmission over direct links between pairs of devices. The result is the Point-to-Point Protocol (PPP), which defines a complete method for robust data link connectivity between units using serial lines or other physical layers. It includes numerous capabilities and features, including error detection, compression, authentication, encryption and much more.

Even though PPP is called a “protocol”, and even though it is considered part of TCP/IP—depending on whom you ask—it is really more a protocol suite than a particular protocol. The operation of PPP is based on procedures defined in many individual protocols, as we will see in this section. Thus, PPP can be considered a “protocol suite within a protocol suite”. Alternately, its components can be viewed as “subprotocols” within PPP.

In this section I provide a comprehensive look at the operation of the Point-to-Point Protocol, in four subsections. The first describes the fundamentals of PPP, including its history, standards and an overview of its general operation. The second explains the “core” protocols that are responsible for setting up PPP links and basic operation. The third covers the protocols used to implement various special features in PPP, such as compression and encryption. The last subsection provides detailed information on the various frame formats used by PPP protocols.

Serial Line Internet Protocol (SLIP) 1 PPP Fundamentals and Operation