NCP Explained

When a PPP link has been initiated, the LCP passes control to the appropriate Network Control Protocol (NCP). NCP is a set of protocols forming a part of PPP. PPP is a data link layer protocol that can transmit multiprotocol data between two point-to-point. 

PPP’s modular model allows LCP to set up the link and then transfer the details of a network protocol to a particular NCP. Each network protocol has a corresponding NCP and each NCP has a corresponding RFC. For example, IPv4, IP6, IPX, and many others have corresponding NCPs. These NCPs uses the same packet as the LCPs. 

After the LCP configuration and authentication in the basic link, the inappropriate Network Control Protocol is calling on to complete the particular configuration of the network layer protocol being used. After the successful Network Control Protocol configuration of the network layer protocol, the network protocol is in the open state on the established LCP link. From there, PPP can carry the corresponding network-layer protocol packets.

Internet Protocol Control Protocol (IPCP) 

When LCP has established the link, the routers exchange IPCP messages for option negotiation. IPCP is responsible for configuring, enabling, and disabling the IPv4 modules on both ends of the PPP link. The same responsibilities are allocated to IPV6CP for IPv6 address. IPCP negotiates two options:

  • Compression – Compression protocol allows devices to negotiate an algorithm to compress TCP and IP headers and save bandwidth. For compression, the Van Jacobson TCP/IP header compression is used. The Van Jacobson TCP/IP header compression compresses the size of the TCP/IP headers to 3 bytes. Which is the important improvement on slow serial lines, particularly for interactive traffic? This compression method is similar in concept to the Protocol-Field-Compression and Address-and-Control-Field-Compression (ACFC) options in LCP.
  • IP-Address- IP address allows the initiating device to send the Configure-Request. To specify an IP address it needs 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 before the advent of broadband technologies such as DSL and cable modem services. 

After the Network Control Protocol process completion, the link goes into the open state and LCP gets the link in the maintenance phase. Link traffic contains possible combinations of LCP, NCP, and network-layer protocol packets. After completion of data transfer, Network Control Protocol terminates the protocol link and LCP terminates the PPP connection. The different NCPs are; 

  • Internet Protocol Control Protocol (IPCP) 
  • OSI Network Layer Control Protocol (OSINLCP) 
  • Internetwork Packet Exchange Control Protocol (IPXCP) 
  • DECnet Phase IV Control Protocol (DNCP) 
  • NetBIOS Frames Control Protocol (NBFCP)
  • IPv6 Control Protocol (IPV6CP)