TCP-Illinois
TCP-Illinois is a variant of TCP congestion control protocol, developed at the University of Illinois at Urbana–Champaign. It is especially targeted at high-speed, long-distance networks. A sender side modification to the standard TCP congestion control algorithm, it achieves a higher average throughput than the standard TCP, allocates the network resource fairly as the standard TCP, is compatible with the standard TCP, and provides incentives for TCP users to switch. Principles of operation. TCP-Illinois is a loss-delay based algorithm, which uses packet loss as the "primary" congestion signal to determine the "direction" of  window size change, and uses queuing delay as the "secondary" congestion signal to adjust the "pace" of window size change. Similarly to the standard TCP, TCP-Illinois increases the window size W by formula_1 for each acknowledgment, and decreases formula_2 by formula_3 for each loss event. Unlike the standard TCP, formula_4 and formula_5 are not constants. Instead, they are functions of average queuing delay formula_6: formula_7, where formula_8 is decreasing and formula_9 is increasing. There are numerous choices of formula_8 and formula_9. One such class is: formula_12 formula_13 We let formula_8 and formula_9 be continuous functions and thus formula_16, formula_17  and formula_18. Suppose formula_19 is the maximum average queuing delay and we denote formula_20, then we also have formula_21. From these conditions, we have formula_22 This specific choice is demonstrated in Figure 1. Properties and Performance. TCP-Illinois increases the throughput much more quickly than TCP when congestion is far and increases the throughput very slowly when congestion is imminent. As a result, the window curve is concave and the average throughput achieved is much larger than the standard TCP, see Figure 2. It also has many other desirable features, like fairness, compatibility with the standard TCP, providing incentive for  TCP users to switch, robust against inaccurate delay measurement.