This parameter specifies the time that a virtual circuit is kept active
before it is closed because of inactivity.
The timeout period restarts when a packet is sent or received on the
virtual circuit.
The default timeout starts at 120 seconds and ranges between 5 and 240
seconds.  
With decreasing activity, the timeout inches up toward 240; with
increasing activity (as manifested by a shortage of file descriptors),
the timeout decrements in 20-second chunks.  It never falls below
5 seconds.
If you change the default, the timeout period you specify remains
fixed. 
