Probabilistic Model Checking of CAPTCHA Admission Control for DoS Resistant Anti-SPIT Protection

Voice over IP (VoIP) service is expected to play a key role to new ways of communication. It takes advantage of Internet Protocols by using packet networks to transmit voice and multimedia data, thus providing extreme cost savings. On the other hand, this technology has inherited drawbacks, like SPAM over Internet Telephony (SPIT). A well-established method to tackle SPIT is the use of CAPTCHAs. CAPTCHAs are vulnerable to Denial of Service (DoS) attacks, due to their excessive demands for bandwidth. We suggest that anti-SPIT protection should be combined with appropriate admission control policies, for mitigating the effects of DoS attacks. In order to identify how effective is this technique, we quantify the costs and the benefits in bandwidth usage through probabilistic model checking four different admission control policies. We conclude with comments on how appropriate is each policy in tackling DoS attacks.