o-end mechanism involves the caller and/or callee SIP user agents and is realized by features of the SIP protocol specifically designed for this purpose (e.g., SIP authentication and SIP message body encryption). Hop-by- hop mechanisms secure the communication between two successive SIP enti- ties in the path of signaling messages. SIP does not provide specific features for the hop-by-hop protection and relies on network-level or transport-level security. Hop-by-hop mechanisms are needed because intermediate elements may play active roles in SIP processing by reading and/or writing some parts of the SIP messages. End-to-end security cannot apply to these parts of mes- sages that are read and written by intermediate SIP entities. 22.3.4.2 Security Mechanisms Two main security mechanisms are used with SIP: authentication and data encryption. Data authentication is used to authenticate the sender of the mes- sage and to ensure that critical message information is not modified in transit. This is to prevent an attacker from modifying and/or replaying SIP requests C 2009 by Taylor & Francis Group, LLC SIP Security and Quality of Service Performance and responses. Data encryption is used to ensure confidentiality of SIP com- munications, SO that only the intended recipient decrypt and read the data. This is usually done using encryption algorithms such as Data Encryption Standard (DES) and Advanced Encryption Standard (AES). Four fundamental security services are required for the SIP protocol: preserving the confidentiality and integrity of messaging, preventing replay attacks or message spoofing, providing for the authentication and privacy of the participants in a session, and preventing denial-of-service attacks. Entities within SIP messages separately require the security services of confidentiality, integrity, and authentication. Rather than defining new security mechanisms specific to SIP, SIP re-uses wherever possible existing security models derived from the HTTP and SMTP space. Full encryption of messages prov