The most common transport protocol is TCP, or transmission control protocol, which was developed in the early 70s. TCP provides reliable data streaming services in a unicast way. But here we are more interested in how TCP performs congestion control, in order to understand why it is not efficient in high bandwidth wide area networks.
TCP's congestion control mechanism is called AIMD, or additive increase multiplicative decrease, because it increases the congestion window size about one full-sized packet per RTT, and halves the size once a packet loss is detected.
Longer RTT means TCP needs a longer time to reach the maximum throughput, or the available network bandwidth. If there is a packet loss during this period, then TCP will never realize the optimal throughput.
Because TCP's performance relies on RTT, different concurrent flows with different RTTs will have different throughput. This is another problem of TCP, also known as RTT bias.