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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.
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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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