|  | 12.1 | The Transport Layer
| 12.1.1 | Purpose of the transport layer

The phrase "quality of service" is often used to describe the purpose of Layer 4 - the transport layer. Its primary duties are to transport and regulate the flow of information from source to destination, reliably and accurately. The end-to-end control, provided by sliding windows, and reliability in sequencing numbers and acknowledgments are primary duties of Layer 4.

To understand reliability and flow control, think of a student who studies a foreign language for one year. Now imagine he/she visits the country where the language is used. In conversation he/she must ask everyone to repeat their words (for reliability) and to speak slowly, so he/she can catch the words (flow control).