PIT when a SPIT call is actually under transit to the callee, or under pro- cessing to the callee's proxy server or SIP-phone. The issue here is to mitigate or avoid any annoyance, and if possible, to minimize the overheads and to con- serve resources. Finally, handling means any reaction to the SPIT. Table 19.5 illustrates this classification. C 2009 by Taylor & Francis Group, LLC SIP Vulnerabilities for SPIT, SPIT Identification Criteria TABLE 19.5 Classification of the Anti-SPIT Mechanisms Mechanism Prevent Detect Handle AVA [5] Anti-SPIT Entity [6] Reputation Charging [7] DAPES [8] PGM [9] Biometrics [10] RFC 4474 [11] SIP SAML [12] DSIP [13] VoIP SEAL [14] VSD [15] 19.6 Anti-SPIT Mechanisms Evaluations 19.6.1 Assessment Criteria An assessment of the proposed anti-SPIT mechanisms requires the definition of various qualitative and quantitative criteria. We have based our research on the following criteria: Percentage of SPIT calls avoided. How many SPIT call attempts have been identified and handled by the anti-SPIT mechanism? Reliability. The precision of making the right adjustments about SPIT calls and callers, in terms of false positive and negative rates. Latency. Due to the real-time nature of VoIP, quick decisions regarding SPIT detection are a major requirement, especially when legitimate calls are ana- lyzed. In such a case, trustworthy users should not tolerate large delays. Human interference. This metric represents the transparency of the anti- SPIT mechanisms to the end-user. Resource overhead for the SIP provider. SIP providers should estimate the required resources for the implementation of the mechanism. This quan- titative criterion seems essential for providers, since the number of calls that should be analyzed per unit time might be enormous, when the number of registered users increases. Vulnerabilities. This parameter refers to the capability of a SPITter to bypass any of the anti-SPIT countermeasures Privacy risk. This criterion is associated with the collection, manipulation, and dissemi