Which per-hop behavior modifies a packet header field to set the packet's class of service?

Marking is the process of modifying a packet header field, such as the Differentiated Services Code Point (DSCP) or Class of Service (CoS), to indicate its class of service. This allows subsequent network devices to apply appropriate QoS policies based on the marked value. Shaping and policing are traffic management techniques that control the rate of traffic, but they do not modify packet headers. Classification is the process of identifying and categorizing traffic flows, often as a precursor to marking or other QoS actions, but it does not directly modify the packet header.