Layer 4 - The transport layer

The transport layer provides the functional and procedural means of transferring variable-length data sequences from a source to a destination host via one or more networks. This layer has end-to-end significance and provides a connectionless or connection-oriented service to the session layer. This layer is responsible for connection establishment, management, and release.

The transport layer controls the reliability of a given link through end-to-end flow control, segmentation/de-segmentation, and error control. This layer also provides multiplexing functions of multiplexing various data connections over a single network layer.

Some protocols operating at the transport layer are TCP, UDP, SCTP, NBF, and so on.