 and authenticity services. However, they can be only employed for communication between SIP servers without providing any guarantee for true end-to-end security. The S/MIME is the only © 2009 by Taylor & Francis Group, LLC SIP Security: Threats, Vulnerabilities and Countermeasures TABLE 18.2 Common Security Mechanisms Supported in SIP digest IPSec S/MIME Confidentiality Hop-by-hop Hop-by-hop End-to-end (partial) Integrity Hop-by-hop Hop-by-hop End-to-end (partial) Authenticity One-way Hop-by-hop Hop-by-hop End-to-end Availability mechanism that could be employed for end-to-end security. Nevertheless until today it is not implemented by commercial SIP clients. Moreover, the need of intermediate nodes to modify some parts of the SIP message restricts S/MIME from providing only "partial" security services. This means that S/MIME can protect only certain parts of the SIP message; only these that remain unmod- ified along the whole path from the caller to the callee and vice versa. A final observation is that SIP specifications do not suggest any mechanism for mini- mizing the threat of service unavailability. Table 18.2 summarizes the common security mechanisms and protocols currently supported by SIP. The security mechanisms presented in this section can substantially restrict, or at best prevent, only some of the potential attacks and threats emerging in SIP architectures. In order to develop a truly resilient SIP architecture, additional security mechanisms, acting complementary to the existing ones, are necessary. During the last years this necessity has been identified and manifested by various researchers [26-37], who either propose the design of new security services or the enhancement of existing ones, in all cases aiming to satisfy the identified SIP security requirements. Ono and Tachimoto define a set of security requirements for providing both integrity and confidentiality for SIP signaling along the entire message path, ensuring that only trusted intermediate nodes can access the data [26]. is known as 