n transport layer may be the first point where pieces belonging to the same message meet again. The destination transport layer then reproduces the original message from the received data segments. The transport layer acts as an interface layer between network connection-oriented lower layers and application service-oriented upper lay- ers. Its responsibility is (typically) to check that end-to-end transmission is error free, that packets are not lost on the way, and that data were delivered in their original order to the upper layer. For this it may include end-to-end acknowledgment and retransmission procedures. TCP/IP Layer 7 Application Telnet, FTP Presentation Application SMTP, DNS SNMP, HTTP Session Transport Transport TCP, UDP Network Internet Data link Ethernet Physical network Figure 6.11 The TCP/IP stack and OSI reference model. Data Communications The transport layer usually provides two basic service classes to the ses- sion layer: Transport of isolated datagrams through the network: Transmitted mes- sages may arrive in different order and errors may occur. Examples: UDP of the Internet (actually this does not belong to OSI proto- cols), and the Transport Protocol, class one (TP1), of OSI (IS 9072). 2. Error-free point-to-point channel: Such a channel delivers messages in the same order in which they were sent. Examples of these are the Transmission Control Protocol (TCP) of the Internet (not included in OSI protocol standards) and TP4 of OSI (IS 8072/8073). UDP and TCP protocols are explained in Section 6.6. 6.3.3.6 Session Layer The transport layer ensures that end-to-end transmission between computers is successful. Actually, the task of communications is accomplished by the four layers below the session layer. The three uppermost layers are not needed for data transmission but they help make applications compatible SO that the application programs running in computers understand each other. The session layer allows users on different machines to establish ses- sions between them. It can be used, f