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)

An Example NCP: The Internet Protocol Control Protocol (IPCP)

Let's look at the NCP for IP, IPCP. When PPP is set up to carry IP datagrams, IPCP is invoked in the Network-Layer Protocol phase (one of the PPP phases) to set up an IP NCP link between the two devices. The setup is carried out using the four “Configure-” messages. For IP, there are two configuration options that can be specified in an IPCP Configure-Request:

• IP-Compression-Protocol: Allows devices to negotiate the use of something called “Van Jacobson TCP/IP header compression”. This compresses the size of TCP and IP headers to save bandwidth. Thus, this is similar in concept to the Protocol-Field-Compression and Address-and-Control-Field-Compression (ACFC) options in LCP.
  
  
• IP-Address: Allows the device sending the Configure-Request to either specify an IP address it wants to use for routing IP over the PPP link, or to request that the other device supply it with one. This is most commonly used for dial-up networking links.

Again, the receiving device can send back an IPCP Configure-Ack, an IPCP Configure-Nak, or an IPCP Configure-Reject, just as they work in LCP. The other NCPs are similar, but use different configuration options. After configuration is complete, data can be sent for the layer three protocol corresponding to the NCP negotiated. This is indicated by using the appropriate value for the Protocol field in PPP data frames containing that layer three data.

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