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Timeout. The timeout routine is started every 25.6 ms
(called a fast-tick timeout period) by a clock interrupt. The
routine has two sections: the fast timeout routine which
"wakes up 11 any Host or modem interrupt routine that has languished
(for example, when the Host input routine could not immed!ately
start a new input because of a shortage in buffer space); and
the slow timeout routine which marks lines as alive or dead,
updates the routing tables and does long term garbage collection
of queues and other data structures. (For example, it protects
the system from the c:11mulati ve effect of sueh fA.i lures as a
lost packet of a multiple packet message, where buffers are tied
up in message reassembly).
These two routines, Fast and Slow, are executed so that
fast timeout runs every clock tick (25.6 ms) and the slow timeout
runs every 25th clock tick (640 ms). Although they run off
a common interrupt, they are constructed to allow fast timeout
to interrupt slow timeout should slow timeout not complete
execution before the next timeout period. During garbage collection,
every table, most queues, and many states of the program are
t irned out. rrhus, if an entry remains in a table abnormally long
or if a routine remains in a particular state for abnormally
long, this entry or state is garbage-collected and the table
or routine is returned to its initial or nominal state. In
this way, abnormal conditions are not allowed to hang up the
system indefinitely.
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