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The TCP/IP Guide

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Table Of Contents  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 Fundamentals and Operation

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PPP Overview, History and Benefits
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PPP Link Setup and Phases
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PPP Components and General Operation
(Page 1 of 2)

PPP stands for the Point-to-Point Protocol, but even the standard that defines PPP immediately starts describing protocols that comprise it, which is why I consider it a protocol suite. At the highest level, the functions of PPP can be broken down into several components. Each of these encompasses a general class of PPP functionality, and is represented by either one protocol in the suite or a set of protocols.

Main PPP Components

The PPP standard itself describes three “main” components of PPP:

  • PPP Encapsulation Method: The primary job of PPP is to take higher-layer messages such as IP datagrams and encapsulate them for transmission over the underlying physical layer link. To this end, PPP defines a special frame format for encapsulating data for transmission, based on the framing used in the HDLC protocol. The PPP frame has been specially designed to be small in size and contain only simple fields, to maximize bandwidth efficiency and speed in processing.

  • Link Control Protocol (LCP): The PPP Link Control Protocol (LCP) is responsible for setting up, maintaining and terminating the link between devices. It is a flexible, extensible protocol that allows many configuration parameters to be exchanged to ensure that both devices agree on how the link will be used.

  • Network Control Protocols (NCPs): PPP supports the encapsulation of many different layer three datagram types. Some of these require additional setup before the link can be activated. After the general link setup is completed with LCP, control is passed to the PPP Network Control Protocol (NCP) specific to the layer three protocol being carried on the PPP link. For example, when IP is carried over PPP the NCP used is the PPP Internet Protocol Control Protocol (IPCP). Other NCPs are defined for supporting the IPX protocol, the NetBIOS Frames (NBF) protocol, and so forth.

The PPP encapsulation method and LCP are defined in the main PPP standard and some support standards; the NCPs are described in separate standard documents, one per NCP.

Additional PPP Functional Groups

While the three components above do constitute much of the total package of PPP, I would add to the list of components in the standard two additional functional groups. These represent some of the many extra protocols that have been added over time to the suite to support or enhance the basic operation of PPP:

  • LCP Support Protocols: Several protocols are included in the PPP suite that are used during the link negotiation process, either to manage it or to configure options. Examples include the authentication protocols CHAP and PAP, which are used by LCP during the optional authentication phase.

  • LCP Optional Feature Protocols: A number of protocols have been added to the basic PPP suite over the years to enhance its operation after a link has been set up and datagrams are being passed between devices. For example, the PPP Compression Control Protocol (CCP) allows compression of PPP data, the PPP Encryption Control Protocol (ECP) enables datagrams to be encrypted for security, and the PPP Multilink Protocol (PPP MP) allows a single PPP link to be operated over multiple physical links. The use of these features often also requires additional setup during link negotiation, so several define extensions (such as extra configuration options) that are negotiated as part of LCP.

Each of these additional protocols is generally defined by a different standards document. You can find a list of some of these in the topic on PPP standards.

Previous Topic/Section
PPP Overview, History and Benefits
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PPP Link Setup and Phases
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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