Which per-hop behavior modifies the IPv4 ToS field value?

Marking is the process of modifying the ToS (Type of Service) field in an IPv4 header or the Traffic Class field in an IPv6 header to indicate a specific QoS (Quality of Service) treatment. This allows network devices to prioritize or differentiate traffic based on these markings. Shaping and Policing are mechanisms that enforce traffic rates based on existing markings or classifications, but they do not modify the ToS field themselves. Classification is the process of identifying and categorizing traffic, which often precedes marking, but it doesn't change the packet header.