tocol (UDP); Process or application TELNET protocol Transport layer, host-to- host protocol Network layer IP, OSPF, RIP, BGP, ICMP protocol ARP, RARP Bearer network, data link and Ethernet, SLIP/PPP, physical layer PSTN, ISDN, xDSL, Cable modem, ATM, frame relay, leased lines protocol Figure 6.36 TCP/IP protocols and their interrelationship with the OSI reference model. Introduction to Telecommunications Network Engineering Simple Mail Transport Protocol (SMTP); File Transfer Protocol (FTP); Hypertext Transfer Protocol (HTTP); Telnet, Virtual Terminal Protocol; Domain Name System (DNS); Simple Network Management Protocol (SNMP); Trivial File Transfer Protocol (TFTP); Dynamic Host Configuration Protocol (DHCP); Real-Time Transport Protocol (RTP). We introduce these protocols later in this section. Figure 6.37 shows an overview of an Internet connection in which messages are routed independ- ently by a connectionless IP layer from host to host with the help of the IP address. Each router has interfaces to many networks and based on the IP address it decides which way to choose. In the receiving hosts a message is given to either TCP or UDP depending on the protocol identification in the IP packet header. If connection-oriented TCP is used, a virtual connection is first established between hosts end to end. Then IP packets are transmitted, retransmitted if required, and reordered by the TCP layer SO that the applica- tion feels as if there were a wire connection to the far-end hosts. The port number in the TCP or UDP packet identifies to which process or application the message is to be directed from TCP or UDP. Host A Host B Router 1 Router 2 Router 3 LAN A LAN B Network 1 Network 2 Figure 6.37 Example Internet connection. Data Communications Host A in LAN A in Figure 6.37 uses MAC addresses to communicate with router 1 as discussed in Section 6.5. At router 1, the data link layer pro- tocol (MAC and LLC) is disassembled and only network layer data are for- warded to network 1, which may use point-to-point channels