Installing and Administering PPP

66 Chapter 4

Common pppd Options

Link Management

Idle Timer Link Control

The idle option allows you to limit the number of seconds that can pass

without receiving or transmitting the type of packets speciﬁed in the

ﬁlter ﬁle's keepup ﬁeld. The timer shuts down the link when the

speciﬁed number of seconds elapse. The idle option works on both the

calling and the answering pppd. If both have the idle option set, the end

that speciﬁes the shorter interval shuts down the line ﬁrst. The default

is to never shut down the link.

When to Invoke the Timer

Do not invoke the idle option when pppd is talking to a system that runs

PPP software that does not implement on-demand auto-dialing. The

session may not stay intact if the link is taken down by the idle timer.

These systems include those running most free PPP implementations, or

MS-DOS PCs running FTP Software’s PC/TCP.

Setting Idle Timer Values

The two criteria for determining idle timer values are:

• the cost of maintaining a connection

• the scarcity of resources

Limits for Outbound Calls

Set a relatively brief idle timeout for the system placing a call if the

connection costs per minute, for example, a long-distance telephone call.

On the other hand, if the telephone billing scheme is based on the

number of calls rather than duration, set a longer idle timer (or none at

all), on the calling system's pppd. Keep in mind that it can take 25

seconds or more to allocate a modem, dial out, complete the call, login to

the remote system, and negotiate PPP connectivity. The minimum idle

timer setting should be 30 seconds to accommodate the connection and

negotiations. All ABORT and TIMEOUT strings must be written with

the same thought in mind.

Limits for Inbound Calls

Set a relatively brief timeout for an answering system if it has too few

modems to accommodate a large number of incoming connections. The

answering system might also beneﬁt from a shorter idle timeout value if

it acts as a hub and provides its services via a toll-free WATS (800)