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Availability in a system means that users can access their expected services at expected times. Attacks on availability are called denial of service attacks. In this paper, we propose a novel categorization of these attacks. Based on the proposed categorization, we also propose and analyze some necessary principles for the design of protocols that are resilient against such attacks. We also show that some existing protocols in the TCP/IP protocol suite are not consistent with the proposed principles and, therefore, they are vulnerable. We then analyze some instances of such attacks and propose modifications in those protocols to make them compatible with the proposed principles which improve their resilience