 the data in transit [21]. In the case of SIP, this definition must be further elaborated due to the fact that various SIP entities may require access to the signaling data in order to process and deliver them to its final destination. Consequently, an integrity service in SIP should guarantee that SIP signaling data can be manipulated only by SIP-authorized entities across the entire signaling path. Note that a signaling path may include several intermediate entities (servers) and end-users. Therefore, integrity services should be pro- vided in both an end-to-end and a hop-by-hop fashion. 18.4.3 SIP Security Requirement 3: Authenticity The requirement of authenticity can guarantee data originality [21]. For the SIP case, data originality means that the incoming data are indeed the data sent by some other entity. In other words, authenticity supports the genuine- ness of the SIP entity that generates the corresponding signaling data. 18.4.4 SIP Security Requirement 4: Availability Last but not least, availability provides assurance that whenever a resource is required for the completion of a task the system can instantly provide it [21]. Specifically for SIP, availability could be interpreted as the user's ability to invoke SIP-based VoIP services at any given moment. 18.4.5 SIP-specific Security Restrictions Apart from the security requirements listed above, during the design phase of a service it is also important to take into account the following SIP-specific security restrictions: 1. Intermediates nodes must (at least) have access to specific SIP message headers, in order to process and route the messages to the proper des- tination. Therefore, it is not generally acceptable to encrypt the entire SIP message. 2. SIP requests like CANCEL and ACK and naturally SIP responses must not be challenged for authentication since they cannot be resubmitted. 18.5 SIP Security Mechanisms and Services According to RFC 3261 [7], SIP security basically relies on well-known secu- rity mechanisms that have been successfully a