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.