9.5.1.4 DAPES The DAPES system [14] determines in real time depending on whether a call is identified as being SPIT or not. The main characteristics of DAPES are the following: firstly, all messages are sent through proxies that serve as authenti- cators, secondly, all outbound proxies have certificates granted by Trusted CA, thirdly, all communication between proxies has to be encrypted and fourthly, the sender has to be authenticated by its domain proxy. In order for all of the above to be effective, certain characteristics have to be supported. Current systems are therefore unable to effectively implement the abovemen- tioned infrastructure. 19.5.1.5 Progressive Multi Grey-Leveling Progressive multi grey-leveling (PMG) [15] determines and allocates a grey- level for each caller that enables it to determine whether a call qualifies as C 2009 by Taylor & Francis Group, LLC SIP Vulnerabilities for SPIT, SPIT Identification Criteria SPIT or not. This mechanism is not based on the feedback of other users with regards to a specific caller but on the last calls made by that caller itself. In practice, the grey level of each user is calculated by means of the addition of a long-term and short-term level. If a summary is greater than a threshold than the user is considered as being a SPITter and all his calls are consequently blocked. However, a caller who is classified as a SPITter is bound to loose such status since the grey-level of the caller is not constant. 19.5.1.6 Biometric Framework for SPIT Prevention This technique [16] offers an approach based on identity-management since a SPITter changes his identity frequently. The specific technique proposes that personal detail (such as biometric data) of all users be recorded, SO as to create a unique link between every user's identity and its biometric data. So as to enable the functioning of this infrastructure, each user must be regis- tered to authenticated servers when using VoIP. This technique requires an effective communication between servers that can enable 