PPP provides a method of
establishing, configuring, maintaining, and terminating a
point-to-point connection. In order to establish communications over a
point-to-point link, PPP goes through four distinct phases:
- Link establishment and configuration negotiation-An originating
PPP node sends LCP frames to configure and establish the data link.
- Link-quality determination-The link is
tested to determine whether the link quality is sufficient to
bring up network-layer protocols. Note that this is an optional phase.
- Network-layer protocol configuration negotiation-The originating
PPP node sends NCP frames to choose and configure network-layer
protocols. The chosen network-layer protocols-such as IP, Novell
IPX, and AppleTalk-are configured, and packets from each network-layer
protocol can be sent.
- Link termination-The link remains configured for communications
until LCP or NCP frames close the link or until some external event
occurs (for example, an inactivity timer expires or a user
intervenes).
There are three classes of LCP frames:
- Link establishment frames-Used to establish and configure a
link.
- Link termination frames-Used to terminate a link.
- Link
maintenance frames-Used to manage and debug a link.
LCP frames are used to accomplish the work of each of the LCP
phases:
(1) Link establishment; (2) Link quality;
(3) Network-layer protocol; (4) Link termination. These phases
will be described in the following sections.
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