The transport layer

Provides reliable, transparent transfer of data over networks.
Segments, data stream, datagram
Connection oriented and connectionless.
End-to-end flow control.
Error detection and recovery.
Segmentation & reassembly.

The basic roles of the Transport Layer are to establish end-to-end connections from one computer to another on the network and provide reliable "transport" of data between devices. 

Basic Transport Layer Services:

  Resource Utilization (multiplexing)  Connection Management (establishing)  Flow Control (Buffering / Windowing)  Reliable Transport (positive acknowledgement / error checking) 

Flow Control  Once the connection has occurred and transfer is in progress, congestion of the data flow can occur at a destination for a variety of reasons. Possible options include:  The destination can become overwhelmed if multiple devices are trying to send it data at the same time.

    It may become overwhelmed if the source is sending faster than it can physically receive.