Hacker Crackdown, page 50 by Bruce Sterling
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he new and unprecedently sophisticated System 7 network.
The stations with System 7 were programmed to switch over to a backup net in case of any problems. In mid-December 1989, however, a new high-velocity, high-security software patch was distributed to each of the 4ESS switches that would enable them to switch over even more quickly, making the System 7 network that much more secure.
Unfortunately, every one of these 4ESS switches was now in possession of a small but deadly flaw.
In order to maintain the network, switches must monitor the condition of other switches--whether they are up and running, whether they have temporarily shut down, whether they are overloaded and in need of assistance, and so forth. The new software helped control this bookkeeping function by monitoring the status calls from other switches.
It only takes four to six seconds for a troubled 4ESS switch to rid itself of all its calls, drop everything temporarily, and re-boot its software from scratch. Starting over from scratch will generally rid the switch of any software problems that may have developed in the course of running the system. Bugs that arise will be simply wiped out by this process. It is a clever idea. This process of automatically re- booting from scratch is known as the "normal fault recovery routine." Since AT&T's software is in fact exceptionally stable, systems rarely have to go into "fault recovery" in the first place; but AT&T has always boasted of its "real world" reliability, and this tactic is a belt-and-suspenders routine.
The 4ESS switch used its new software to monitor its fellow switches as they recovered from faults. As other switches came back on line after recovery, they would send their "OK" signals to the switch. The switch would make a little note to that effect in its "status map," recognizing that the fellow switch was back and ready to go, and should be sent some calls and put back to regular work.
Unfortunately, while it was busy bookkeeping with th