PPP Network Control Protocols (IPCP, IPXCP, NBFCP and others)
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One of the reasons why PPP is such a powerful technology is that it is flexible and expandable. Even though it was originally created with the idea of carrying IP datagrams, PPP's designers recognized that it would be short-sighted to think so narrowly. PPP could easily carry data from many types of network layer protocols, and on some networks, it might even be advantageous to let it carry datagrams from different layer three protocols simultaneously.
Allowing PPP to support multiple network layer protocols would require it to have knowledge of each one's idiosyncrasies. If we used only LCP for link configuration, it would need to know all the unique requirements of each layer three protocol. This would also require that LCP be constantly updated as new layer three protocols were defined and as new parameters were defined for existing ones.
Instead of this inflexible design, PPP takes a modular approach to link establishment. LCP performs the basic link setup, and after (optional) authentication, invokes a Network Control Protocol (NCP) that is specific to each layer three protocol that is to be carried over the link. The NCP conducts a negotiation of any parameters that are unique to the particular network layer protocol. More than one NCP can be run for each LCP link; the process of how this is done and the relationship between the NCP links and LCP link can be seen in the topic on PPP link setup and phases.
Each of the common network layer technologies has a PPP NCP defined for it in a separate RFC. These documents are usually named in this pattern: The PPP <layer three protocol name> Control Protocol. The most common ones are The PPP Internet Protocol Control Protocol (IPCP), The PPP Internetworking Packet Exchange Control Protocol (IPXCP), and The PPP NetBIOS Frames Control Protocol (NBFCP). These are the NCPs for IP, IPX and NBF (also called NetBEUI), respectively. A separate NCP is also defined for IP version 6, the PPP IP Version 6 Control Protocol (IPv6CP).
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