PPP Link Setup and Phases
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Link Dead Phase
By design, the PPP link always begins and ends in this phase. This phase represents the situation where there is no physical layer link established between the two devices. It remains here until the physical layer link is set up, at which point the link proceeds to the Link Establishment phase.
In our example, when A is first turned on, there is no physical layer connection (modem connection) between it and B. Once the connection is made, the link can proceed to phase 2. (Note that in a direct connection, such as a serial cable linking two PCs, the link may only stay in the Link Dead phase for a fraction of a second, until the physical layer connection is detected.)
The physical layer is now connected and LCP performs the basic setup of the link. Device A sends an LCP configuration request message to device B over the physical link, specifying the parameters it wishes to use. If Device B agrees, it replies with an acknowledgement. If B doesn't agree, it sends back a negative acknowledgment or rejection, telling device A what it won't accept. Device A can then try a different configuration request with new parameters that device B will hopefully accept. This process is described in more detail in the topic covering LCP.
Hopefully, A and B will eventually come to agreement. If so, the status of the link is considered LCP open, and will proceed to the Authentication phase. If they cannot come to an agreement, the physical link is terminated, and we go back to the Link Dead phase.
In many cases, a device may require authentication before it will permit connection of another device. (This is certainly usually the case when PPP is used for dial-up.) Authentication is not considered mandatory in PPP, however. When it is used, the appropriate authentication protocol (CHAP or PAP) is employed.
After successful authentication, the link proceeds to the Network-Layer Protocol phase. If authentication is not successful, the link fails and transitions to the Link Termination phase.
Once the basic link has been configured and authentication completed, the general setup of the LCP link is complete. Now, the specific configuration of the appropriate network layer protocol is performed by invoking the appropriate NCP, such as IPCP, IPXCP and so forth.
Each particular network layer protocol whose NCP is successfully configured is considered to be open on the LCP link. More than one NCP can be open on a particular PPP link, and each can be closed independently when it is no longer needed. After all necessary NCPs have been invoked, the link proceeds to the Link Open state, even if none were successfully opened.
In this state, the LCP link and one or more NCP links are open and operational. Data can be passed for each NCP that has been successfully set up.
The link can be terminated at any time by either device for a variety of reasons. These may include user request (you hit the disconnect button when you want to log off your dial-up session); link quality problems (the modem hangs up on you due to line noise); or some other cause (you spend too much time in the bathroom and your ISP's idle timer logs you out J). When any of these occur the LCP link is broken and the link transitions to the Link Termination phase.
The device terminating the link sends a special LCP termination frame, and the other device acknowledges it. The link then goes back to the Link Dead phase. In the case where the termination was by request and the physical layer connection is still active, the PPP implementation is supposed to specifically signal the physical layer to terminate the layer one connection.
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