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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 Core Protocols: Link Control, Network Control and Authentication

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PPP Network Control Protocols (IPCP, IPXCP, NBFCP and others)
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PPP Link Control Protocol (LCP)
(Page 1 of 3)

Of all the different PPP suite protocols, the single most important protocol is the PPP Link Control Protocol (LCP). LCP is the “boss” of PPP; it is responsible for its overall successful operation, and for “supervising” (in a way) the actions of other protocols.

PPP is about links, and LCP is about controlling those links. As I discussed in the PPP fundamentals section, the operation of a PPP link can be thought of as proceeding through various “life stages” just as a biological organism does. There are three main stages of “link life” and LCP plays a key role in each one:

  • Link Configuration: The process of setting up and negotiating the parameters of a link.

  • Link Maintenance: The process of managing an opened link.

  • Link Termination: The process of closing an existing link when it is no longer needed (or when the underlying physical layer connection closes).

Each of these functions corresponds to one of the “life phases” of a PPP link. Link configuration is performed during the initial Link Establishment phase of a link; link maintenance occurs while the link is open, and of course, link termination happens in the Link Termination phase. Figure 26 represents a summary of the LCP link, showing the different message exchanges performed by LCP during these different life phases of a PPP connection.

Figure 26: PPP Link Control Protocol (LCP) Message Exchanges

This diagram provides an overview of many of the message exchanges performed by LCP during different phases of a PPP connection. Link Configuration is here shown as a simple exchange of a Configure-Request and Configure-Ack. After subsequent exchanges using other PPP protocols to authenticate and configure one or more NCPs, the link enters the Link Open phase. In this example, an Echo-Request and Echo-Reply message are first used to test the link, followed by the sending and receiving of data by both devices. One Data message is shown being rejected due to an invalid Code field. Finally, the link is terminated using Terminate-Request and Terminate-Ack messages.


LCP Frames

Devices use LCP to control the PPP link by sending special LCP messages across the physical link between them. These messages are called both LCP packets and LCP frames; while the standard uses "packet", the term "frame" is preferred because layer two messages are normally called frames. There are eleven different LCP frame types defined in the main PPP document, which are divided into three groups that correspond to the three link “life stages” above. Four LCP frame types are used for link configuration, five for maintenance and two for termination. The frame formats themselves are described in the topic on LCP frames. Below I will discuss each of the three major functions of LCP and how the frames are used in each.

Key Concept: The PPP Link Control Protocol (LCP) is the most important protocol in the PPP suite. It is responsible for configuring, maintaining and terminating the overall PPP link. The two devices using PPP employ a set of LCP frames to conduct LCP operations.

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Version 3.0 - Version Date: September 20, 2005

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