Which statements about MPLS Label Distribution Protocol（LDP） are valid？（）
- ALDP hello packets are sent to UDP port 646
- BLDP hello packets are sent to TCP port 711
- CLDP sessions are TCP sessions to port 646
- DLDP sessions are TCP sessions to port 711
- ELDP establishes a peer relationship with another router that must be directly attached
- FLDP can establishes a peer relationship with another router that is not directly attached