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A simple and yet effective denial of service (DoS) attack on voice over IP (VoIP) proxies that based on session initiation protocol (SIP), is to flood the server with requests that their addresses do not exist and are irresolvable. Different security schemes and possible attacks have studied from the qualitative view on the most of network security researches and these researches are less considered the quantitative analysis. In this paper, a queueing model has suggested for evaluation of these attacks and the server under attack is modeled by a two-dimensional Markov chain. We can develop a memory effective algorithm for the calculating of stationary probability distribution by the help of this model, which we can use it for finding other interesting performance metrics like the buffer occupancy percentages for regular requests and attack requests. This paper proposes a more general analytical approach for the study of security measures of a computer network under this kind of attacks and we hope that this method shows a new way for the quantitative evaluation of more complicated security cases in computer networks.