We discuss congestion control algorithms, using network awareness as a criterion to categorize different approaches. The first category ("the box is black") consists of a group of algorithms that consider the network as a black box, assuming no knowledge of its state, other than the binary feedback upon congestion. The second category ("the box is grey") groups approaches that use measurements to estimate available bandwidth, level of contention or even the temporary characteristics of congestion. Due to the possibility of wrong estimations and measurements, the network is considered a grey box. The third category ("the box is green") contains the bimodal congestion control, which calculates explicitly the fair-share, as well as the network-assisted control, where the network communicates its state to the transport layer; the box now is becoming green.
We go beyond a description of the different approaches to discuss the tradeoffs of network parameters, the accuracy of congestion control models and the impact of network and application heterogeneity on congestion itself.