PPP NCP: Point to Point Protocol Network Control Protocols

The Network Control Protocol (NCP) phase in the PPP link connection process is used for establishing and configuring different network-layer protocols such as IP, IPX or AppleTalk.

After a NCP has reached the Opened state, PPP will carry the corresponding network-layer protocol packets. Any supported network-layer protocol packets received when the corresponding NCP is not in the Opened state MUST be silently discarded.

During this phase, link traffic consists of any possible combination of LCP, NCP, and network-layer protocol packets.

The most common layer 3 protocol negotiated is IP. The routers exchange IP Control Protocol (IPCP) messages negotiating options specific to the protocol. The corresponding network control protocol for IPv6 is IPv6CP.

IPCP negotiates two options: compression and IP address assignments. However, IPCP is also used to pass network related information such as primary and backup Windows Name Service (WINS) and Domain Name System (DNS) servers.
Protocol Structure - PPP NCP: Point to Point Protocol Network Control Protocols

Network Control Protocols such as IPCP and IPv6CP use the same packet format as the Link Control Protocols.

Configuration Option format:

• Code - The Code field is one octet and identifies the type of the packet. 
• Identifier - The Identifier field is one octet and aids in matching requests and replies.
• Length - The Length field is two octets and indicates the length of the packet .
• Data - The Data field is zero or more octets. The format of the Data field is determined by the Code field.

PPP Network Control Protocols are defined by IETF (http://www.ietf.org ).