问题：

[多选] 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.