Adaptive quality of service marking

Technologies for adaptively classifying network packets for quality of service (QoS) are provided herein. Adaptive classifiers track network packets transferred in data flows and network sessions, and apply various QoS classifications to the packets. An adaptive classifier can apply different QoS markings to packets in a data flow based on data flow metadata, such as the age of the data flow and/or the amount of data transmitted in the data flow. An adaptive classifier can assign various priority levels to network packets during different phases of a network session. Routers that receive the packets can prioritize transmission of the packets based on the applied QoS markings or assigned priority levels.