Denial-of-service attacks consume the resources of a remote host or
network, thereby denying or degrading service to legitimate users.
Such attacks are among the hardest security problems to address
because they are simple to implement, difficult to prevent, and
very difficult to trace. In the last several years, Internet denialof-
service attacks have increased in frequency, severity and sophistication.
Howard reports that between the years of 1989 and 1995,
the number of such attacks reported to the Computer Emergency
Response Team (CERT) increased by 50 percent per year [25].
More recently, a 1999 CSI/FBI survey reports that 32 percent of respondents
detected denial-of-service attacks directed against their
sites [16]. Even more worrying, recent reports indicate that attackers
have developed tools to coordinate distributed attacks from
many separate sites