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 Link Control Protocol (LCP) 1 2 3 PPP Authentication Protocols: Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP)

Operation of PPP Network Control Protocols

Each NCP operates very much like a “lite” version of LCP, as you can see by examining Figure 28 (and comparing it to Figure 26, which shows the messaging for LCP). Like LCP, each NCP performs functions for link setup, maintenance and termination—only it deals with its particular type of NCP link and not the “overall” LCP link. Each NCP uses a subset of seven of the message types defined in LCP, and uses them in very much the same way as the message type of the same name is used in LCP:

• Link Configuration: The process of setting up and negotiating the parameters of the particular NCP link (once an LCP link is established) is accomplished using Configure-Request, Configure-Ack, Configure-Nak and Configure-Reject messages just as described in the LCP topic, except these ones are particular to the NCP. The configuration options are of course different; they are the network layer protocol parameters being negotiated.
  
  
• Link Maintenance: Code-Reject messages can be sent to indicate invalid code values (NCP frame types).
  
  
• Link Termination: An NCP link can be terminated using Terminate-Request and Terminate-Ack. (Remember that NCP links are set up within an LCP link; there can be more than one NCP link open, and closing NCP links doesn't terminate the LCP link. Also, NCP links do not need to be closed when an LCP link is terminated.)
  

  Figure 28: PPP IP Control Protocol (IPCP) Message Exchanges

  The overall operation of the NCPs, such as IPCP, is very similar to that of LCP. After LCP configuration (including authentication) is complete, IPCP Configure-Request and Configure-Ack messages are used to establish an IPCP link. IP Data can then be sent over the link. If the IPCP connection is no longer needed it may be terminated, after which the LCP link remains open for other types of data to be transmitted. It is not necessary, however, to explicitly terminate the IPCP link before terminating the LCP connection.

  
  

PPP Link Control Protocol (LCP) 1 2 3 PPP Authentication Protocols: Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP)