Which four statements about an IP service level agreement （SLA） are true？ （）

• A、SLA responder knows type of operation， the port used， and duration.
• B、SLA responder inserts in and out timestamps in the packet payload.
• C、SLA sender and responder use the IP SLA Control Protocol to communicate with each other before
• D、SLA sender and responder communication can be encrypted.
• E、SLA sender and responder communication can be authenticated with MD5.