Which three practices are important to the implementation of a DiffServ QoS architecture？（）

A. Because services are allocated throughout the network before the transmission of data begins， traffic classes can be guaranteed QoS services.
B. Traffic classes are marked with Layer 2 markings to allow for more granularity in identifying traffic classes.
C. Traffic classes are marked with Layer 3 markings to allow them to traverse different network types without loss of QoS information.
D. Traffic classes allow networks to provide proper QoS treatment of packets without applications having to request services.
E. Based upon the network capabilities， QoS markings are placed on packets as needed throughout the network to allow for proper QoS treatment of packets.
F. QoS markings are placed on packets as close to the network edge as possible to allow distribution and core devices to determine QoS actions as quickly as possible.