orks are attached to the backbones and LANs at universities, companies, and ISPs. Figure 6.46 shows an example in which one European residential cus- tomer uses his ISP to access the Internet to view the Web page of a company in the United States. The glue that holds the Internet together is the Internet Protocol. For communications, the IP address from the ISP's address range is provided to the customer who wants to access the service. If the customer knows the uniform service locator of the Web page he wants to view, it is translated into the destination IP address and the exchange of IP packets can start. In the following sections, we introduce higher layer protocols that are needed to interpret information in the payload of IP packets properly. Data Communications Leased transpacific lines to Asia U.S. backbone Leased transatlantic European backbone lines National Regional network network IP Ethernet IP Ethernet LAN LAN of an ISP of a company Access server providing of ISP Web service A residential customer of the ISP PSTN, ADSL, or other access system Figure 6.46 Interconnection of IP networks via IP switching network. 6.6.9 Host-to-Host Protocols As shown in Figure 6.37, the protocols on the top of the network layer oper- ate end to end. The two available options for end-to-end transmission service are TCP for reliable connection-oriented communications and UDP for connectionless datagram communication. Which one is used depends on the needs of the application protocol on the top of them as shown in Figure 6.36. 6.6.9.1 TCP IP provides connectionless datagram transmission through the network. This means that it routes each packet of data independently by using the IP address in each packet. Most applications, such as file transfer, require that the packets arrive in the original order and if one of them is in error it must be transmitted again. The procedures required for these functions are imple- mented in the TCP. The TCP that runs only in the data source and the des- tination machines provides connectio