rifying Authorities (AVA) The so-called AVA system [11] illustrates how the prevention of voice spam may be achieved by extending the call setup procedure. This method is founded C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security upon a "call me back scheme" and the use of two new entities in the IP infras- tructure, namely: (a) the mediator, and (b) the AVA. Through the exchange of information between these two entities, SPIT is mitigated by anonymously blocking unwanted calls. This way the caller is not aware of the existence of the callee, provided that the call failed to be established. 19.5.1.2 SPIT Mitigation through a Network Layer Anti-SPIT Entity The anti-SPIT entity [12] is based on an approach which detects and miti- gates SPIT by using a sniffing-oriented network-level entity. This entity filters and analyzes the network traffic SO as to detect a SPIT call by means of edu- cated guesses. These guesses are based on a list of criteria, which contribute to different extent toward the final decision. The mechanism only takes the SIP packets into account and ignores the rest of the network traffic. Finally, handling actions are enforced based on the results of the SPIT analysis. These actions rely on the policies adopted by the domain to which the specific user belongs, and the end-user's preferences. 19.5.1.3 SPIT Detection Based on Reputation and Charging Techniques This approach proposes a SPIT detection mechanism [13] based on two dif- ferent techniques, namely reputation (i.e., trust networks) and payments at risk. The reputation-based technique is based on the notion of trust, in other words, SPIT detection relies solely on the callee's trust toward the caller. On the other hand, the charging based technique aims at reducing SPIT by increasing its cost. This is achieved by imposing a charge to each message sent by the caller. The specific technique functions alongside simultaneously with other anti-SPIT frameworks such as authentication modules and white lists. 1