4.2 PPP Session Establishment
4.2.1 The four phases through which PPP establishes a point-to-point connection
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:
  1. Link establishment and configuration negotiation-An originating PPP node sends LCP frames to configure and establish the data link.
  2. 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.
  3. 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.
  4. 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.