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PPP Link Quality Monitoring/Reporting (LQM/LQR)
(Page 2 of 2)
Assuming that the negotiation is
successful, LQR will be enabled. A number of counters are set up that
keep track of various link statistics, and a timer used to regulate
the sending of quality reports over the link. Each time the timer expires
a special link quality report is generated and sent in a PPP frame over
the link. These are sent using the special PPP Protocol field
Each counter holds information about
a different statistic regarding the use of the link. Each of these counters
is reset to zero when LQR is set up and then incremented each time a
transmission is made or an event occurs that is relevant to the counter.
The statistics tracked include the following:
- The number of frames sent/received.
- The number of octets (bytes) in all frames sent/received.
- The number of errors that have occurred.
- The number of frames that had to be discarded.
- The number of link quality reports generated.
These counters are only reset at
the start of the link, so they contain figures kept cumulatively over
the life of the connection. The counters can be used in the absolute
sense, meaning that the counter value itself is reported. Alternately,
they can be expressed as relative (or delta) values, which represent
the change since the last report. This is done when a report is received
simply by subtracting the previous report's numbers from those in the
Using Link Quality Reports
LQR specifies the quality reporting
mechanism, but not specific standards for link quality, since these
are so implementation-dependent. Based on the numbers in these reports,
a device can decide for itself what conclusions to draw about link quality,
and in turn what action to take, if any. For example:
- Some devices might decide to shut down a link
if the absolute number of errors seen in any report reaches a certain
- Some might look at the trend in successive reporting
periods and take action if they detect certain trends, such as an increase
in the rate of discarded frames.
- Some devices might just log the information and
take no action at all.
Note that LQR aggregates its statistics
for all higher-layer protocols transmitted over a particular link. It
doesn't keep track of statistics for different higher-layer protocols
separately (which makes sense, since the quality of the link shouldn't
vary from one higher layer protocol to the next).
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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