The transport layer is responsible for establishing a temporary communication session between two applications and delivering data between them.
TCP/IP uses two protocols to achieve this:
Transmission Control Protocol (TCP)
User Datagram Protocol (UDP)
Primary Responsibilities of Transport Layer Protocols
Tracking the individual communication between applications on the source and destination hosts
Segmenting data for manageability and reassembling segmented data into streams of application data at the destination
Identifying the proper application for each communication stream