Monday, 1 October 2012

The Purpose of NCP in PPP

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.
There are a large number of proposed standard protocols, which specify the operation of PPP over different kinds of point-to-point links. Each has a status of elective. Point-to-point circuits in the form of asynchronous and synchronous lines have long been the mainstay for data communications. In the TCP/IP world, the de facto standard SLIP protocol has served admirably in this area, and is still in widespread use for dial-up TCP/IP connections. However, SLIP has a number of drawbacks that are addressed by the Point-to-Point Protocol. PPP has three main components:
• A method for encapsulating datagrams over serial links.
• A Link Control Protocol (LCP) for establishing, configuring, and testing the  data-link connection.
• A family of Network Control Protocols (NCPs) for establishing and configuring different network-layer protocols.

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