Content Overview As its name implies, the TCP/IP transport layer does the work of transporting data between applications on source and destination devices. A thorough understanding of the operation of the transport layer is essential to understanding modern data networking. This module will describe the functions and services of this critical layer of the TCP/IP network model. Many of the network applications that are found at the TCP/IP application layer are familiar to even casual network users. HTTP, FTP and SMTP, for example, are acronyms that are commonly seen by users of Web browsers and e-mail clients. This module also describes the function of these and other applications from the TCP/IP networking model. Students completing this module should be able to:
Content 11.1 TCP/IP Transport Layer 11.1.1 Introduction to transport layer The primary duties of the transport layer, Layer 4 of the OSI model, are to transport and regulate the flow of information from the source to the destination, reliably and accurately. End-to-end control and reliability are provided by sliding windows, sequencing numbers, and acknowledgments. To understand reliability and flow control, think of someone who studies a foreign language for one year and then they visit the country where that language is used. In conversation, words must be repeated for reliability and to speak slowly so that the meaning of the conversation is not lost, this is flow control. The transport layer provides transport services from the source host to the destination host. It establishes a logical connection between the endpoints of the network. Transport services segment and reassemble several upper-layer applications onto the same transport layer data stream. This transport layer data stream provides end-to-end transport services. The transport layer data stream is a logical connection between the endpoints of a network. Its primary duties are to transport and regulate the flow of information from source to destination reliably and accurately. The primary duty of Layer 4 is to provide end-to-end control using sliding windows and to provide reliability in sequencing numbers and acknowledgments. The transport layer defines end-to-end connectivity between host applications. Transport services include the following basic services: TCP/IP is a combination of two individual protocols. IP operates at Layer 3, and is a connectionless protocol that provides best-effort delivery across a network. TCP operates at Layer 4, and is a connection-oriented service that provides flow control as well as reliability. By pairing these protocols, a wider range of services is provided. Together, they are the basis for an entire suite of protocols called the TCP/IP protocol suite. The Internet is built upon this TCP/IP protocol suite. Web Links Transport Layer Protocols http://www.mcpprep.com/WebHelp/ccna/ ccna_obj_35.htm
Content 11.1 TCP/IP Transport Layer 11.1.2 Flow control As the transport layer sends data segments, it tries to ensure that data is not lost. A receiving host that is unable to process data as quickly as it arrives could be a cause of data loss. The receiving host is then forced to discard it. Flow control avoids the problem of a transmitting host overflowing the buffers in the receiving host. TCP provides the mechanism for flow control by allowing the sending and receiving host to communicate. The two hosts then establish a data-transfer rate that is agreeable to both. Web Links Network Flow Control http://netlab.caltech.edu/netlab-pub/ EoTwiley.pd
Content 11.1 TCP/IP Transport Layer 11.1.3 Session establishment, maintenance, and termination overview Multiple applications can share the same transport connection in the OSI reference model. Transport functionality is accomplished on a segment-by-segment basis. In other words, different applications can send data segments on a first-come, first-served basis. The segments that arrive first will be taken care of first. These segments can be routed to the same or different destinations. This is referred to as the multiplexing of upper-layer conversations. One function of the transport layer is to establish a connection-oriented session between similar devices at the application layer. For data transfer to begin, both the sending and receiving applications inform the respective operating systems that a connection will be initiated. One node initiates a connection that must be accepted by the other. Protocol software modules in the two operating systems communicate with each other by sending messages across the network to verify that the transfer is authorized and that both sides are ready. The connection is established and the transfer of data begins after all synchronization has occurred. During transfer, the two machines continue to communicate with their protocol software to verify that data is received correctly. Figure shows a typical connection between the sending and receiving systems. The first handshake requests synchronization. The second and third handshakes acknowledge the initial synchronization request, as well as synchronizing connection parameters in the opposite direction. The final handshake segment is an acknowledgment used to inform the destination that both sides agree that a connection has been established. After the connection has been established, data transfer begins. Congestion can occur during data transfer for two reasons. First, a high-speed computer might be capable of generating traffic faster than a network can transfer it. Second, if many computers simultaneously need to send datagrams to a single destination, that destination can experience congestion, although no single source caused the problem. When datagrams arrive too quickly for a host or gateway to process, they are temporarily stored in memory. If the traffic continues, the host or gateway eventually exhausts its memory and must discard additional datagrams that arrive. Instead of allowing data to be lost, the transport function can issue a “not ready” indicator to the sender. Acting like a stop sign, this indicator signals the sender to stop sending data. When the receiver can handle additional data, the receiver sends a “ready” transport indicator. When this indicator is received, the sender can resume the segment transmission. At the end of data transfer, the sending host sends a signal that indicates the end of the transmission. The receiving host at the end of the data sequence acknowledges the end of transmission and the connection is terminated. Web Links Layer 3 - Transport Layer http://www.internet-tips.net/Networks/ osi_transport.ht
Content 11.1 TCP/IP Transport Layer 11.1.4 Three-way handshake TCP is a connection-oriented protocol. TCP requires connection establishment before data transfer begins. For a connection to be established or initialized, the two hosts must synchronize their Initial Sequence Numbers (ISNs). Synchronization is done through an exchange of connection establishing segments that carry a control bit called SYN, for synchronize, and the ISNs. Segments that carry the SYN bit are also called “SYNs".