Which QoS function modifies packet headers to influence how traffic is treated?

Marking is the QoS function that modifies packet headers by adding or changing QoS-specific fields (e.g., DSCP or CoS values). This modification signals to subsequent network devices how to prioritize and handle the marked traffic, influencing its treatment throughout the network. Policing drops or re-marks traffic that exceeds a configured rate, but it doesn't inherently modify headers to influence treatment; its primary action is enforcement. Classification identifies and categorizes traffic based on various criteria but doesn't alter the packet itself. Shaping buffers excess traffic and sends it out at a more consistent rate, which is a queuing mechanism, not a header modification.