the exchange of user credentials (based on biometric data). 19.5.1.7 RFC 4474 This proposal [17] focuses on identity-management and aims at resolving the problem of user authentication by using PKI and certificate authorities. So as to enable the functioning of this RFC, two new sip-header fields are required: (a) identity, containing a signature that is used to verify the caller's identity (b) identity-info, for transmitting a reference to the signer's certificate. Despite the fact that this infrastructure had not been initially proposed for SPIT, it can nevertheless be easily implemented to SPIT. 19.5.1.8 SIP SAML The Security Assertion Markup Language (SAML) [18] is used for the expres- sion of security assertions, such as authentication, role membership, or per- missions. A SIP-authentication service is proposed that authenticates the user via certain asserted features. Each VoIP message includes caller identity infor- mation along with a reference to a SAML assertion which has various features of the caller and the caller's domain certificate. This infrastructure tries to avoid the frequent change of SPITter addresses through identity control. 19.5.1.9 DSIP Differentiated SIP [19] is an extension to the SIP protocol which classifies users into three distinct categories. These categories are deduced from the e-mail context: the white list, made up legitimate callers, the black list comprises the SPITters and the grey-list contains the callers who have yet to be categorised. DSIP usually employs human verification test for the uncategorised callers. © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security Once the grey-list callers have succeeded the test, the communication with the callee is established. 19.5.1.10 Voice Spam Detector The voice spam detector system [20] is a combination of anti-SPIT techniques that is founded upon reputation and trust. The basic characteristics of a VSD system are the following: (a) Presence filtering that depends on the current status of the ca