What are the advantages of transport layer?

Answer 1

The transport layer is typically handled by processes in the host computer operational system, and not by routers and switches. The transport layer usually turns the unreliable and very basic service provided by the Network layer into a more powerful one. In the TCP/IP model, the transport layer is responsible for delivering data to the appropriate application process on the host computers. This involves statistical multiplexing of data from different application processes, i.e. forming data packets, and adding source and destination port numbers in the header of each transport layer data packet. Some transport layer protocols, for example TCP but not UDP, support virtual circuits, i.e. provide connection oriented communication over an underlying packet oriented datagram network. A byte-stream is delivered while hiding the packet mode communication for the application processes. Finally, some transport layer protocols, for example TCP but not UDP, provides end-to-end reliable communication, i.e. error recovery by means of error detecting code and automatic repeat request (ARQ) protocol. The ARQ protocol also provides flow control, which may be combined with congestion avoidance.