 are not trivial, and the multiple round trips needed to open a connection as shown in Figure 1.3 can add to message latency. Also, certificate verification can introduce process- ing delays. However, TLS transport has clear security advantages over UDP or TCP. Many operating systems already support TLS due to its wide use in secure Web browsers and servers. 1.4.3.4 The Stream Control Transport Protocol (SCTP) [12] is similar to TCP in that it provides reliable stream-based transport. However, it has some advantages over TCP transport for a message-based protocol. First, it has built-in message segmentation, SO that individual messages are separated at the transport layer. SIP: Understanding the Session Initiation Protocol Client Server ClientHello ServerHello Finished Finished Application data Figure 1.3 Opening of TLS connections. Another advantage is that SCTP avoids the so-called "head of line blocking" problem of TCP. This is a common TCP problem in which a dropped segment with a large window causes the entire window's worth of messages to wait in a buffer (i.e., be blocked) until the dropped segment is retransmitted. SCTP also supports multihoming, SO if one of a pair of load balancing servers fails, the other can immediately begin receiving the messages without even requiring a DNS or other database lookup. SCTP is a true layer 2 transport protocol that requires operating system level support to be used, which will initially delay its use in the Internet. Also, note that the advantages of SCTP over TCP only occur during packet loss. In a zero loss network, the performance of the two is identical. 1.4.4 Application Layer The top layer shown in Figure 1.1 is the application layer. This includes signal- ing protocols such as SIP and media transport protocols such as Real-time Transport Protocol (RTP), which is introduced in Section 7.2. Figure 1.1 includes H.323, introduced in Chapter 8, which is an alternative signaling pro- tocol to SIP developed by the International Telecommunication Union (ITU). Session D