Sustained, large queue buildups generally indicate congestion in the network and can affect application performance through increased queueing delays and frame loss. Queues can begin filling up for a number of reasons, such as over-subscription of a link or back pressure from a downstream device. When queues begin filling up and all buffering is exhausted, frames are dropped. This has a detrimental effect on application throughput. Congestion control techniques are used to reduce the risk of queue overruns without adversely affecting network throughput.
Congestion control covers features that define how the system responds when congestion occurs or active measures taken to prevent the network from entering a congested state. These features include link level flow control (LLFC), Weighted random early detection (WRED), transient buffer congestion detection.