Transport Layer (4)
Handles the reliablity of the connection and logical separation of applications. For example, if a computer is surfing the internet with a web browser and at the same time listening to internet-radio, this layer ensures the correct data arrives to the correct application.
In addition, this layer handles flow-control (ensuring one side does not send information faster than the other can receive) and data integrity (ensuring the data is not corrupt).
The most common Transport layer protocol is TCP.