internal schema of interactive communication. To improve quality, several protocols are developed and evolving. Data Communications One protocol introduced in Section 6.6.10 is the Real-Time Transport Protocol, which operates on the top of UDP. It cannot guarantee high QoS because it has no control on the lower layer protocols where, for example, network layer congestion can occur. Another protocol designed for control of lower layers is the Resource Reservation Protocol. An endpoint uses RSVP to request a simplex flow through an IP network with specified QoS bounds, for example, delay and throughput. If routers along the path agree to honor the request, they approve it; otherwise they deny it. Every router on the way has to support RSVP and the QoS requirements given. Both ends have to use RSVP to request QoS if it is needed in both directions [3]. Another issue is signaling, that is, how a telephone call is established over an IP network. Quite similar signaling phases that we illustrated in Chapter 2 are needed between endpoints or gateways. The main two proto- cols are the H.323 recommendation of ITU-T and the Session Initiation Pro- tocol (SIP) of IETF. Both support signaling for multimedia sessions including video in addition to ordinary telephone calls. They define signaling messages, which are exchanged for call establishment, maintenance, and clearing. IP networks can carry voice and video already and we may expect that these applications will expand as standards become mature and widely imple- mented. The Internet will also be used for facsimile calls and videoconferenc- ing as standards evolve. Internet and private intranets will carry a larger and larger part of PSTN traffic but a lot of work remains to be done before IP technology can replace PSTN the infrastructure. The main issues to be solved are QoS and reliability. 6.6.13 Summary Our main intention in this Internet section was to get a clear view of how most popular Internet services, such as Web and e-mail, are really imple- mented from the dis