[TUHS] Happy birthday Morris worm

Doug McIlroy doug at cs.dartmouth.edu
Wed Nov 13 23:47:54 AEST 2019

Most of this post is off topic; the conclusion is not.

On the afternoon of Martin Luther King Day, 1990, AT&T's
backbone network slowed to a crawl. The cause: a patch intended
to save time when a switch that had taken itself off line (a
rare, but routine and almost imperceptible event) rejoined the
network. The patch was flawed; a lock should have been taken
one instruction sooner.

Bell Labs had tested the daylights out of the patch by
subjecting a real switch in the lab to tortuously heavy, but
necessarily artificial loads. It may also have been tested on
a switch in the wild before the patch was deployed throughout
the network, but that would not have helped.

The trouble was that a certain sequence of events happening
within milliseconds on calls both ways between two heavily
loaded switches could evoke a ping-pong of the switches leaving
and rejoining the network.

The phenomenon was contagious because of the enhanced odds of a
third switch experiencing the bad sequence with a switch that
was repeatedly taking itself off line. The basic problem (and
a fortiori the contagion) had not been seen in the lab because
the lab had only one of the multimillion-dollar switches to
play with.

The meltdown was embarrassing, to say the least. Yet nobody
ever accused AT&T of idiocy for not first testing on a private
network this feature that was inadvertently "designed to
compromise" switches.


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