Which statements about MPLS Label Distribution Protocol（LDP） are valid？（）

• A、LDP hello packets are sent to UDP port 646
• B、LDP hello packets are sent to TCP port 711
• C、LDP sessions are TCP sessions to port 646
• D、LDP sessions are TCP sessions to port 711
• E、LDP establishes a peer relationship with another router that must be directly attached
• F、LDP can establishes a peer relationship with another router that is not directly attached