Transport
Layer Sessions
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).