rs, mainly humans, should employ technical and procedural means to defend against attackers that maliciously survey or spy on the Internet using network traffic analysis tools. This will protect the personal freedom and privacy, achieving digital dignity, and, moreover, defend confidentially in business, as well as in human relationships. That is why today we should say "On the Internet, nobody should know you are a dog" In this scope, privacy and anonymity when communicating over the Inter- net gained substantial consideration in the technical, procedural and legal domain. For every new service that is launched and massively adopted in the Internet, privacy issues arise immediately. The same applies for VoIP services, and especially for SIP, which currently prevails in this new market. There are various reasons why an end-user wishes to maintain its anonymity when * The New Yorker, July 5, 1993 issue (Vol. 69, no. 20) page 61. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security communicating using SIP. Firstly, a caller might wish to conceal his iden- tity from displaying in the receivers' phone, a feature that is usually used in mobile phone calls. On the other hand, a callee might want to be unlinkable from her/his personal preferences and direct marketing campaigns. In its original specification, SIP supports anonymity, since the originator of a call could remain "anonymous" to the callee, and for that reason default val- ues are used when the user agent initiates a call. This feature supports caller anonymity against the callee, but not to the entire set of SIP realms, since practically the user agent server of the serving domain requires strong authen- tication of the caller. Additionally, using tunnelling techniques, and especially end-to-end S/MIME encryption, selective anonymity can be supported. This option enables caller's privacy within the set of intermediate relays and the serving domains, if authentication is not required, but not against the callee. Finally, if