A network session is an exchange of messages that occurs between two or more communication devices, lasting for some period of time. A session is established and is torn down when the session ends. Different types of sessions occur at three layers of the OSI model: the Transport layer, the Session layer, and the Application layer.
The Transport Layer operates at Layer 4 of the OSI model, providing reliable or unreliable, end-to-end delivery and flow control of data. Internet protocols that implement sessions at the Transport layer include Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).

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