In the previous section, we showed how an application's perceived QoS depends on the allocation of resources to the application's traffic within network devices. Devices that provide QoS support do so by intelligently allocating resources to submitted traffic. For example, under congestion, a network device might choose to queue traffic of applications that are more latency tolerant instead of traffic of applications that are less latency tolerant. As a result, traffic of applications that are less latency tolerant can be forwarded immediately to the next network device. In this example interface capacity is a resource which is granted to the latency-intolerant traffic.
Device memory is a resource that has been granted to the latency-tolerant traffic.
In order to allot resources preferentially to certain traffic, it is necessary to identify different traffic and to associate it with the resources it requires. We do so by recognizing separate traffic flows within the network and by defining traffic handling parameters which apply to these flows. Devices identify packets as belonging to one flow or another via the process of classification. In order to invoke QoS mechanisms it is necessary to communicate to network devices the classification information necessary to classify packets to flows and a description of the handling that should apply to traffic on each flow. This is achieved through various signaling means and device configuration.