Why classify traffic in QoS?

Classifying traffic in Quality of Service (QoS) is the foundational step that allows network devices to identify specific types of traffic. This identification is crucial because it enables the network to apply different QoS policies, such as prioritizing critical applications (e.g., voice or video) over less time-sensitive data (e.g., file transfers). Without classification, all traffic would be treated equally, making it impossible to guarantee performance for essential services. While configuring traffic-matching rules and providing services based on class are subsequent steps in the QoS process, they depend entirely on the initial classification. Writing the class identifier into a packet header field (marking) is also a follow-up action that uses the classification information to signal how the packet should be treated by downstream devices. Therefore, the primary reason for classifying traffic is to identify which traffic needs special handling.