Chapter 25. PPP NCP: Point to Point Protocol Network Control Protocols

Protocol Description The Network Control Protocol (NCP), a protocol in the Point to Point Protocol (PPP) suite, provides services in the PPP link connection process to establish and configure 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 networklayer 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 network layer (layer 3) protocol negotiated is for the Internet Protocol (IP). The routers exchange IP Control Protocol (IPCP) messages negotiating options specific to the protocol. In other words, IPCP is the most commonly used Network Control 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 Network Control Protocols such as IPCP and IPv6CP use the same packet format as the Link Control Protocols.

Configuration Option format:

8	16	32bit
Type	Length	Configuration Option

Packet format:

8	16	32bit	Variable
Code	Identifier	Length	Data

• 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.