Installing and Administering PPP

Chapter 4 73
Common pppd Options
Link Quality Monitoring
Weighing the Costs of LQM
pppd discovers line failures more quickly if you decrease the lqrinterval
because LQR's will arrive more frequently. However, sending more
LQR's increases monitoring traffic, slowing down the transfer of user
data. So you should remember to also consider raising the lqthreshold.
If the lqthreshold is ’lqthreshold 5/6’, no more than one LQR can be
dropped per minute. If two LQRs in a row are dropped, pppd shuts down
the line.
LQM Response to Link Failures
LQM is particularly useful for detecting and responding to total link
failures. The response can include failover protection which switches the
connection to an available dial-up modem. Most total failures, for
example, a backhoe digging through the telephone company’s cable,
cause the connection's CSU/DSU or modem to deassert the Carrier
Detect signal. pppd observes this as a hangup event.
Failure Without Disconnection
But some failure modes, like misconfigured flow control and over-reliance
on in-band XON/XOFF flow control, leave modems connected even
though the PPP peers are unable to communicate. In this situation, the
peers observe an LQM failure and take appropriate action, usually
disconnecting and redialing. LQM is also useful if an intercontinental
telephone connection is of such poor quality that significant numbers of
packets are damaged in transit. Again, the PPP implementations can
decide, based on LQM measurements, to hang up and redial in hopes of
getting a better connection. This is a very rare occurrence if your
modems provide V. 42 or MNP 4 error correction, but it does occasionally
happen. Whatever the cause, the disconnection and redial operation can
happen without user intervention or application awareness. This is
because even if PPP frames are damaged or lost, the upper protocol
layers arrange for retransmission as needed to provide the user with a
complete data stream. The user simply experiences a pause while the
modems reestablish the connection.