Let's have a brief review at how the applications can use the physical networks.
Applications cannot make use of the physical network directly, but do so through several layers of network protocols.
The transport protocols provide data transfer functionalities directly to applications. One typical example of transport functionalities is whether the data transfer is reliable, or how reliable it is.
Meanwhile, some mechanics of the transport protocols are transparent to applications. Because the Internet is such a large and loosely coupled system, it is impossible to have a central node to manage all connections. Each connection will have to tune the sending rate by itself without knowing the existence of other connections. This is the Internet congestion control problem, and a good congestion control algorithm should realize relatively good efficiency and fairness throughout the whole system, no matter what the initial state is.