PPP Link Setup and Phases
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Before data can be exchanged on a PPP connection, a link must be set up between the two devices. As part of this setup task, a configuration process is undertaken whereby the two configure the link and agree on the parameters for how data should be passed between them. Only after this is completed can frames actually pass over the link.
The PPP Link Configuration Protocol (LCP) is generally in charge of setting up and maintaining PPP links. LCP may invoke an authentication protocol (PAP or CHAP) when PPP is configured to use authentication. After an LCP link has been opened, PPP invokes one or more Network Control Protocols (NCPs) for the layer three protocol being carried on the link. These perform any network-layer-specific configuration needed before the link can carry that particular network layer protocol.
The operation of a PPP link can be described as having a life of sort. Just as humans are born, grow, have an adult life span and then die, a PPP link is established, configured, used and eventually terminated. The process of setting up, using and closing a PPP link is described in the PPP standard as a series of phases or states. This is a type of finite state machine (FSM), a tool used to explain the operation of protocols.
When we talk about a PPP link overall, we are talking about the status of the LCP connection between the two devices; again, LCP governs the overall state of PPP as a whole. Once an LCP link has been opened, each of the NCPs used on the link can be opened or closed independently of the overall PPP (LCP) link. We'll see how this works momentarily.
An excellent way of understanding how PPP works is to look at these phases, and the process by which transition is made from one to the next during the lifetime of the link. For purposes of clarity, this description is based on an example where device A is a PC performing a dial-up networking connection to a remote host B (see Figure 25).
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