etwork (user-to-network interface), and X.75 defined interactions between nodes in the network (network-to-network interface). Networks implementing these specifications are colloquially known as X.25 networks. X.25 networks provide high functionality to terminals. Network nodes perform error detection by exchanging acknowledgment messages to ensure that the information transmitted through the link has not been cor- rupted. Terminals only have to release the data, and the network performs flow and error control until the data is safely delivered. As we observed in the telephone network, a network providing high functionality enables the implementation of simple, reliable terminals to make use of it. Packet Switching, IP, and the IETF IP and the Internet Paradigm X.25 networks present some weaknesses. Overloading the network with extraneous tasks such as flow control reduces its performance. Thus, packet-switched networks implemented using the PSTN paradigm did not exploit all the possibilities this technology offers. A paradigm shift was needed in order to make the best use of the newly developed technologies. This new paradigm, of course, is the Internet paradigm, and its main pro- tocol is Internet Protocol (IP). IP Connectivity The Internet differs from other networks in that its sole purpose is provid- ing connectivity. (The purpose of the PSTN, for instance, is providing tele- phone services and the purpose of the TV network is providing broadcasts.) A variety of services such as e-mail, the World Wide Web, videoconferencing, and file transfer are implemented based on end-to-end IP connectivity (Figure 2-5). In order to achieve true end-to-end connectivity, a common end-to-end protocol is implemented at the network layer-the Internet Protocol (IP) [RFC 791]. The implementation of IP by all systems within the network has two advantages: The network is independent of the underlying technology. Applications can make use of a common IP infrastructure. From a telecom perspective, IP infrastructure is quite a depa