More on Principles of Congestion Control

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.

Max-min FairnessFurther

more, the congestion control scheme should be fair. Most congestion schemes aim at achieving max-min fairness. An allocation of transmission rates to sources is said to be max-min fair if:

1. No link in the network is congested
2. The rate allocated to a source $j$ cannot be increased without decreasing the rate allocated to another source $i$, whose allocation is smaller than the rate allocated to the source $j$.

In other words, this principle postulates that increasing the transmission rate of one end-system necessarily decreases the transmission rate allocated to another end-system with an equal or smaller allocation.

To visualize the different rate allocations, it’s useful to consider the graph shown below. Consider hosts A and B that share a bottleneck link.