问题：

[多选] Which three statements about classification marking of traffic at Layer 2 are true？ （）

A . a Frame Relay header includes a 1-bit discard eligible （DE） bit to provide the class of service （CoS）
B . an ATM header includes a 1-bit DE field to provide the CoS
C . an MPLS EXP field is inserted in the Layer 3 IP precedence field to identify the CoS
D . the CoS field only exists inside Ethernet frames when 802.1Q or Inter-Switch Link （ISL） trunking is used
E . in the IEEE 802.1p standard， three bits are used to identify the user priority bits for the CoS
F . in the IEEE 802.1q standard， six bits are used to identify the user priority bits for the CoS