The law of Conservation of Flow, which states that an input must either be absorbed or sent on as an output (possibly with modification), is an attractive tool with which to analyze network protocols for security properties. One of its uses is to detect disruptive network elements that launch Denial of Service attacks by absorbing or discarding packets. Its use requires several assumptions about the protocols being analyzed. In this paper, the authors examine the WATCHERS algorithm to detect misbehaving routers. They show that it uses Conservation of Flow without sufficient verification of its assumptions, and can consequently be defeated. They suggest improvements to make the use of Conservation of Flow valid.
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