PPP Link Quality Monitoring/Reporting (LQM/LQR)
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PPP includes optional authentication in recognition of the varying security needs of the many different kinds of links over which PPP may operate. These links also differ greatly in terms of their quality. Just as we don't need to worry about authentication much when two machines are linked with a short cable, we also can feel pretty confident that data sent between them is going to arrive intact. Now, contrast that to a PPP session established over a long-distance telephone call. For that matter, how about PPP over a dial-up call using an analog cellular phone?
PPP includes in its basic package a provision for detecting errors in sent frames (a CRC field), and higher-layer protocols like TCP also include methods of providing robustness on noisy lines. These techniques allow a link to tolerate problems, but provide little in the way of useful information about what the status of the link is. In some situations, devices may want to be able to keep track of how well the link is working, and perhaps take action on it. For example, a device experiencing too many errors on a dial-up connection might want to cut off and retry a new call. In some cases a device might want to try an alternate method of attachment if the current physical link is not working well.
Recognizing this need, the PPP suite includes a feature that allows devices to analyze the quality of the link between them. This is called PPP Link Quality Monitoring or LQM. PPP is set up generically to allow any number of different monitoring functions to be used, but at present there is only one, called Link Quality Reporting (LQR). LQR works by having a device request that its peer (the other device on the link) keep track of statistics about the link and send them in reports on a regular basis.
Before LQR can be used it must be set up, which is done by LCP as part of the negotiation of basic link parameters in the Link Establishment phase. The device opening the link requests link monitoring by including the Quality-Protocol configuration option in its Configure-Request frame. Again, LQR is the only quality protocol presently defined. The configuration option also specifies a reporting period that indicates the longest period of time the requesting device wants to go between receiving reports.
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