Installing and Administering PPP

72 Chapter 4

Common pppd Options

Link Quality Monitoring

The Link Quality Monitoring (LQM) option is deﬁned in the PPP protocol

speciﬁcation. When the PPP implementation supports LQM, PPP can

make policy decisions based on the quality of the link between peers.

When LQM is invoked, pppd requests that the other end of the

connection send Link Quality Report (LQR) packets back to pppd. If the

link goes down or is degraded, many or all of the LQR packets will be

lost. This also indicates that data packets have been lost and the

connection is too bad to warrant continued trafﬁc. pppd counts the

arriving LQRs, and if too many have been lost, pppd closes the

connection. LQM packets do not count against the idle line

disconnection timer, since they are part of the protocol rather than user

data.

Guides for Evaluation of Link Quality

pppd bases its evaluation of the link status on the arguments of two

associated options, lqrinterval and lqthreshold. The value of the

lqrinterval option tells pppd how often to request LQR packets from the

peer machine. The default value is every ten seconds, or ﬁve per minute,

allowing for timing slop. lqthreshold's argument is the minimum

acceptable link quality. The default lqthreshold is one out of ﬁve packets.

With the default values in place, the link will shut down if no LQR

packets arrived during the previous minute.

Adjusting LQM Behavior

The default LQM behavior, one successful packet out of ﬁve sent every

minute, is fairly permissive. pppd is more demanding about line quality

if you change the defaults. Either evaluation setting can be changed to

make LQM more stringent. For example, you can demand that at least

one LQR arrive per minute by specifying

lqrinterval 5 lqthreshold 1/12

Or you can demand that no more than one LQR be lost each minute by

specifying

lqrinterval 5 lqthreshold 11/12