Which two are characteristics of RSVP？ （）
- A RSVP itself provides bandwidth and delay guarantees.
- B For RSVP to be end-to-end， all devices must support RSVP.
- C RSVP reservations are maintained by a centralized reservations server.
- D An RSVP compatible QoS mechanism must be used to implement guarantees according to RSVP reservations.