 as billing and network management have not been associated with the Internet, although today these mecha- nisms are in place. But, in general, the Internet emphasizes flexibility-the capability to route packets around congested or failed points. As discussed in Chapter 4, "The PSTN," the PSTN uses circuit switching, SO a dedicated circuit is set up and taken down for each call. This allows charging based on minutes and circuits used, which, in turn, allows chain- of-supply dealings. The major emphasis of the PSTN is on reliability. Although the Internet and the PSTN have different models and different ways of managing or routing traffic through the network, they share the same physical foundation in terms of the transport infrastructure, or the types of communication links they use. (Chapter 3, "Establishing Communications Channels," discusses packet switching and circuit switching in detail.) Internet Protocols The Internet is a collection of networks that are interconnected logically as a single large, virtual network. Messages between computers are exchanged by using packet switching. Networks can communicate with one another because they all use an internetworking protocol. Protocols are formal descriptions of messages to be exchanged and of rules to be followed in order for two or more systems to exchange information in a manner that the parties will understand. The following sections examine the Internet's protocols: IP (Internet Protocol), TCP (Transmission Control Protocol), User Datagram Protocol (UDP), Stream Control Transmission Protocol (SCTP) and Datagram Congestion Control Protocol (DCCP), Internet Control Message Protocol (ICMP), Internet Group Management Protocol (IGMP), Address Resolution Protocol (ARP) and Reverse Address Resolu- tion Protocol (RARP), routing protocols, and network access protocols. This col- lection of protocols is often referred to as the TCP/IP suite, although it contains much more than just TCP and IP. The IETF has technical responsibility for the suite, which is the most p