unnel- ing. FastStart is shown in Figure 8.4, in which the Setup message contains the TerminalCapabilitySet information. This saves multiple messages and round trips. In H.245 tunneling, a separate H.245 control channel is not opened. Instead, H.245 messages are encapsulated in Q.931 messages in the call signaling channel. This saves overhead in opening and closing a second TCP connection. Versions 3 and 4 added more features to H.323 and additional annexes. Of interest to Internet devices is the support for H.323 URLs [2], full UDP sup- port instead of TCP, and also the standardization of the use of DNS by H.323 in Annex O. Version 5 is currently being worked on by the ITU-T and is sched- uled to be finished in late 2003. 8.4 Comparison SIP and H.323 were developed for different purposes by standards bodies with very different requirements. H.323 was developed by the ITU. Its design and implementation reflects its PSTN background and heritage, utilizing binary encoding and reusing parts of ISDN signaling. SIP, on the other hand, was SIP: Understanding the Session Initiation Protocol Calling Gatekeeper Called H.323 H.323 Terminal Terminal Setup (fastStart) Call Proceeding Alerting Connect (fastStart) OpenLogicalChanne 0penLogicalChannelAck OpenLogicalChannel OpenLogicalChannelAck RTP media session Figure 8.4 FastStart Connections with H.323. developed by the IETF with an Internet perspective, designed to be scalable over the Internet and work in an interdomain way utilizing the full set of Internet utilities and functions. While H.323 was deployed in early VoIP and IP videoconferencing appli- cations, SIP with its Internet architecture is gaining momentum and is emerging as the future signaling standard for IP communications, as IP telephony is some- times called. 8.4.1 Fundamental Differences The first key difference is in the encoding scheme used by the protocol. SIP is a text-based protocol like HTTP and SMTP, while H.323 uses binary-encoded ASN.1 messages. The binary encoding of H.323 may result in smaller me