Application usage models vary widely. Two other categories are of interest. Widespread deployment of best-effort Internet protocols has led to significant advances in the technology of adaptive applications. These applications share the property that they are designed to operate as effectively as possible over a wide range of network performance levels. A simple example of adaptivity is an FTP application based on TCP that moves data exactly as fast as the network allows.
Adaptive applications are valuable even in the presence of network QoS control.
The combination of adaptive applications and QoS control puts performance choices in the hands of the user rather than the application designer. A single application, operating with different QoS control parameters, can meet the varying needs of a wide range of users, or of the same user in different circumstances. Users are able to trade off application performance and the cost of network resources according to local priorities. Adaptive applications combined with correctly designed QoS control mechanisms can assure a minimum level of application performance, while offering better performance when network resources are not constrained. Users receive the benefits of controlled performance without losing the incentive to shift workload to less utilized resources.