11.1.1 This page will describe the functions of the transport layer.
The primary duties of the transport layer are to transport and regulate the flow of information from a source to a 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 visits the country where that language is used. In conversation, words must be repeated for reliability. People must also speak slowly so that the conversation is understood, which relates to flow control.
The transport layer establishes a logical connection between two endpoints of a network. Protocols in the transport layer segment and reassemble data sent by upper-layer applications into the same transport layer data stream. This transport layer data stream provides end-to-end transport services.
The two primary duties of the transport layer are to provide flow control and reliability. The transport layer defines end-to-end connectivity between host applications. Some basic transport services are as follows:
• Segmentation of upper-layer application data
• Establishment of end-to-end operations
• Transportation of segments from one end host to another
• Flow control provided by sliding windows
• Reliability provided by sequence numbers and acknowledgments
TCP/IP is a combination of two individual protocols. IP operates at Layer 3 of the OSI model and is a connectionless protocol that provides best-effort delivery across a network. TCP operates at the transport layer and is a connection-oriented service that provides flow control and reliability. When these protocols are combined they provide a wider range of services. The combined protocols are the basis for the TCP/IP protocol suite. The Internet is built upon this TCP/IP protocol suite.
The next page will explain how the transport layer controls the flow of data.
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