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Understanding the network.pdf - Back to Home

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• LCP link connection and negotiation<br />

• PPP au<strong>the</strong>ntication and link quality management<br />

• Network control pro<strong>to</strong>col negotiation<br />

Link Establishment<br />

Before any PPP link can be established, <strong>the</strong> physical layer transport link must be<br />

established. When used with dedicated links, <strong>the</strong> process is invoked when <strong>the</strong><br />

interface is enabled. With a dial-on-demand link configuration, <strong>the</strong> link is ei<strong>the</strong>r<br />

established manually or by a ULP request. When a physical link state has been<br />

established and verified, PPP sends LCP Configure-Request messages. The link<br />

source and destination peers <strong>the</strong>n negotiate <strong>the</strong> link configuration parameters (Is<br />

<strong>the</strong> link single or multipoint? What kind of compression, if any, will be used with this<br />

link? and so on). After all <strong>the</strong> configuration parameters have been determined, each<br />

peer sends a Configuration-Acknowledgment LCP message.<br />

PPP Au<strong>the</strong>ntication<br />

After <strong>the</strong> link has been established, <strong>the</strong> LCP notifies <strong>the</strong> au<strong>the</strong>ntication layer that<br />

PPP link has been established. If au<strong>the</strong>ntication is enabled, <strong>the</strong> au<strong>the</strong>ntication<br />

process is started. PPP au<strong>the</strong>ntication will make several attempts <strong>to</strong> au<strong>the</strong>nticate<br />

<strong>the</strong> link. If <strong>the</strong>se fail, <strong>the</strong> LCP is notified and <strong>the</strong> link is <strong>to</strong>rn down. If au<strong>the</strong>ntication<br />

is successful or if no au<strong>the</strong>ntication option is configured, <strong>the</strong> NCP layer is notified.<br />

PPP also supports link quality verification. This is an optional and additional step<br />

before NCP negotiation is invoked. Its purpose is <strong>to</strong> verify that <strong>the</strong> link is of a<br />

suitable transport quality <strong>to</strong> sustain <strong>network</strong> layer pro<strong>to</strong>col traffic. If <strong>the</strong> link quality<br />

tests fail, <strong>the</strong> PPP link is terminated and re-established.<br />

NCP Negotiation<br />

When au<strong>the</strong>ntication and link quality testing are complete, NCP negotiation can<br />

begin. A separate NCP is used for each Layer 3 pro<strong>to</strong>col being transported across <strong>the</strong><br />

link. NCPs can be brought up and taken down at any time during <strong>the</strong> links duration.<br />

When <strong>the</strong> link is brought down, ei<strong>the</strong>r by an adverse transport event or through a<br />

user-initiated termination, <strong>the</strong> LCP will notify each of <strong>the</strong> NCPs in operation, so <strong>the</strong>y<br />

can close open events and notify dependent applications of <strong>the</strong> connectivity loss.

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