document: Flash crowds and denial of service attacks : characterization and implications for CDNs and web sites The paper studies two types of events that often overload Web sites to a point when their services are degraded or disrupted entirely - flash events ( FEs ) and denial of service attacks ( DoS ) . The former are created by legitimate requests and the latter contain malicious requests whose goal is to subvert the normal operation of the site . We study the properties of both types of events with a special attention to characteristics that distinguish the two . Identifying these characteristics allows a formulation of a strategy for Web sites to quickly discard malicious requests . We also show that some content distribution networks ( CDNs ) may not provide the desired level of protection to Web sites against flash events . We therefore propose an enhancement to CDNs that offers better protection and use trace-driven simulations to study the effect of our enhancement on CDNs and Web sites .
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