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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

PPP Core Protocols: Link Control, Network Control and Authentication 1 2 3 PPP Network Control Protocols (IPCP, IPXCP, NBFCP and others)

LCP Link Maintenance

Once the link has been negotiated, LCP passes control to the appropriate authentication and/or NCP protocols as discussed in the PPP Link Setup and Phases topic. Eventually the link setup will complete and go into the open state. LCP messages can then be used by either device to manage or debug the link:

• Code-Reject and Protocol-Reject: These frame types are used to provide feedback when one device receives an invalid frame due to either an unrecognized LCP Code (LCP frame type) or a bad Protocol identifier.
  
  
• Echo-Request, Echo-Reply and Discard-Request: These frames can be used for testing the link.

Finally, when the link is ready to be shut down, LCP terminates it. The device initiating the shutdown (which may not be the one that initiated the link in the first place) sends a Terminate-Request message. The other device replies back with a Terminate-Ack. A termination request indicates that the device sending it needs to close the link. Like a four-year-old who tells you he “needs to go now, bad!”, this is a request that cannot be denied. J

The standard RFC 1570, PPP LCP Extensions, also defines two new LCP message types. The Identification message is used to allow a device to identify itself to its peer on the link. The Time-Remaining message lets one device tell the other how much time remains in the current session.

Note that many of the other protocols used in PPP are modeled after LCP. They use the same basic techniques for establishing protocol connections, and send and receive a subset of LCP message types. They also exchange configuration options in a similar manner. The next topic shows how the Network Control Protocols (NCPs) are based on LCP. You will see the same thing in looking at feature protocols such as CCP, ECP and others.

PPP Core Protocols: Link Control, Network Control and Authentication 1 2 3 PPP Network Control Protocols (IPCP, IPXCP, NBFCP and others)