What is the function of the Transport Layer of the OSI model?

The Transport Layer is responsible for flow control and ensuring messages are delivered error free.

On the sending side, messages are packaged for efficient transmission at the Transport Layer and assigned a tracking number so they can be reassembled in proper order. On the receiving side, the packets are reassembled, checked for errors and acknowledged. The Transport Layer performs error handling in that it ensures all data is received in the proper sequence and without errors. If there are errors, the data is retransmitted.

Common protocols that operate at the transport layer include TCP, UDP, SPX, and NetBEUI.