This paper is a review on the problem of denial-of-service (DoS) attacks and proposed ways to deal with it. Broadcast authentication is an important application in sensor networks.Public Key Cryptography (PKC) is desirable for this application,but due to the resource constraints on sensor nodes, these operations are expensive, which means sensor networks using PKC are susceptible to Denial of Service (DoS) attacks: attackers keep broadcasting bogus messages, which will incur extra costs, thus

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... haust the energy of the honest nodes. In addition, the long time to verify each message using PKC increases the response time of the nodes; it is impractical for the nodes to validate each incoming message before forwarding it.We describe the nature of the problem and look for its root causes,further presenting brief insights and suggested approaches for defending against DoS.