up delay. These are authentication of messages and security handshake. A security channel is established during the handshake. During this process a key is generated and messages are exchanged. This results in additional setup delay. Also during authentication the SIP messages are encrypted resulting in increased packet size or payload, causing additional delay. There are two types of handshakes, full handshake and abbreviated hand- shake. During full handshake for normal session initiation, the user proxy or servers authenticate themselves and also create an encryption session key. During abbreviated handshake (Phase 2 rekey) a previous security channel could be reused for session resumption. This is called abbreviated security handshake because the information of the previous security channel is still available. Fewer messages are exchanged during the abbreviated handshake since the authentication process may be skipped and consequently the delay involved is smaller as well. Furthermore, additional setup delays are introduced by the processes required for establishing confidentiality and integrity of messages. These pro- cesses include encryption and adding of message authentication code (MAC) to the message. For example SHA-1 MAC process results in 25 bytes of extra data on top of the original message. In addition every encryption step intro- duces its own processing delay which worsens if multiple levels of encryption and decryption are involved. Different security mechanisms have different effects on SIP particularly in terms of delays and overheads. This section summarizes the performance of SIP when combined with IPSec, TLS and DTLS within the domains of trans- port protocols such as TCP, UDP and streaming control transport proto- col (SCTP). Noticeable performance degradation was observed during SIP call setup up [12]. A relatively large call setup delay is observed with the combination of TLS and SCTP than in other combinations (e.g., TLS/TCP or TLS/UDP). The best performance is achieved with the combina