Layer 4 - 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).
TCP/IP is a combination of two individual protocols - TCP and IP. IP is a Layer 3 protocol - a connectionless service that provides best-effort delivery across a network. TCP is a Layer 4 protocol - a connection-oriented service that provides flow control as well as reliability. Pairing the protocols enables them to provide a wider range of services. Together, they represent the entire suite. TCP/IP is the Layer 3 and Layer 4 protocol on which the Internet is based.
Both TCP and UDP use port (or socket) numbers to pass information to the upper layers. Port numbers are used to keep track of different conversations that cross the network at the same time. Application software developers have agreed to use the well-known port numbers that are defined in RFC1700. Any conversation bound for the FTP application uses the standard port number 21. Conversations, that do not involve applications with well-known port numbers, are assigned port numbers that have been randomly selected from within a specific range. These port numbers are used as source and destination addresses in the TCP segment.
Some ports are reserved in both TCP and UDP, although applications might not be written to support them.
Port numbers have the following assigned ranges:
End systems use port numbers to select proper applications. Originating source port numbers are dynamically assigned by the source host; usually, it is a number larger than 1023.
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Version 1.0 - Version Date: March 29, 2005
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