Layer 4

The transport layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state and connection oriented. This means that the transport layer can keep track of the segments and retransmit those that fail.

Although it was not developed under the OSI Reference Model and does not strictly conform to the OSI definition of the Transport layer, the best known examples of a layer 4 protocol are the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)...