What we know so far is that congestion control schemes must avoid congestion. In practice, this means that the bottleneck link (the link with the lowest bandwidth) cannot be overloaded. This means on average, the sum of the transmission rate allocated to all hosts at any given time should be less than or equal to the bottleneck link’s bandwidth.

Additionally, the congestion control scheme must be efficient. The bottleneck link is usually both a shared and an expensive resource. Usually, bottleneck links are wide-area links that are much more expensive to upgrade than the local area networks. The congestion control scheme should, therefore, ensure that such links are efficiently used. Mathematically, the control scheme should ensure that the sum of the transmission rate allocated to all hosts at any given time should be approximately equal to the bottleneck link’s bandwidth.