age link utilization of AUXL (Mbps) 69.03 75.96 Average end-to-end fax data packets delay (sec) 0.2023 0.2017 0.2017 Average end-to-end signaling delay (sec) 0.2014 0.2015 0.2009 Average SIP call setup times (sec) 3.4023 3.4042 3.4020 Average fax call setup time (sec) 4.2086 4.2106 4.2060 Source: From Choudhary, U., Perl, E., and Sidhu, D. Using T.38 and SIP for real- time fax transmission over IP networks. 26th Annual IEEE Conference on Local Computer Networks (LCN'01), 74, IEEE November 2001. With permission. SIP appears to be a powerful and useful signaling protocol supporting mobility for wireless IP networks but it has inherent weaknesses and dan- gers. These weaknesses and dangers are discussed in Section 22.2.4. 22.2.4 Effects of Security Protocols on Performance of SIP SIP has been in use for setting up secure calls in VoIP. For this, SIP employs several security protocols such as TLS, DTLS or IPSec, combined with TCP, UDP or SCTP, as a security protocol in VoIP (Figure 22.13). These secu- rity mechanisms introduce additional overheads into SIP performance. From a privacy point of view VoIP delivered over an IP network is vulnerable to message spoofing and eavesdropping. Impersonation of a legal user could also result to false accounting and avoidance of payment. Hence many vendors resort to encrypting VoIP messages. SIP therefore assumes an additional role of authenticating users who send and receive VoIP messages with the poten- tial to negatively affect the performance of SIP. An obvious effect of a security protocol on the performance of SIP is the call setup delays. Traditionally SIP was not developed with its own security features but the standard recommends that existing security mechanisms could be used instead. Figure 22.13 illus- trates how SIP uses some of the security mechanisms in the protocol stack. IPSec FIGURE 22.13 Protocol stack for SIP security. © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security Two factors are responsible for the increased call set