A.QoS preclassification is only supported on generic routing encapsulation （GRE） and IPsec VPNs
B.QoS preclassification is not required in Layer 2 Tunneling Protocol （L2TP）， Layer2 Forwarding （L2F）， and Point-to-Point Tunneling Protocol （PPTP） VPNs
C.QoS preclassification is supported on IPsec AH VPNs， but not on IPsec ESP VPNs
D.the QoS-for-VPNs feature （QoS preclassification） is designed for VPN transport interfaces
E.with IPsec tunnel mode， the type of service （ToS） byte value is copied automatically from the original IP header to the tunnel header

A.timing
B.queuing
C.differential
D.serialization
E.packetization
F.propagation

A.latency must be kept below 150 ms
B.latency is not a concern as long as jitter is kept below 30 ms
C.voice packets do not require a specific type of queuing
D.voice packets are rather small
E.voice packets require TCP for rapid retransmission of dropped packets
F.voice packets require a fairly constant bandwidth reserved for voice control traffic as well as forthe voice payload

A.header compression
B.payload compression
C.low latency queuing （LLQ）
D.weighted random early detection （WRED）
E.traffic policing
F.traffic shaping

A.a drawback of implementing RSVP is the requirement to migrate to a supporting routing protocol
B.RSVP uses DSCP to signal QoS requirements to routers 
C.RSVP is a routing protocol
D.RSVP is an IP protocol that uses IP protocol ID 46， and TCP/UDP ports 3455
E.RSVP specifically provides a level of service for rate-sensitive and delay-sensitive traffic
F.the flow-based approach of RSVP is ideal for large， scalable implementations such as the public internet