Explanation
Note: Asynchronous mode: is the main mode of operation of BFD. In this mode, after the BFD session is established, the two systems periodically send BFD control messages to each other. If the packet sent from the peer is not received, the BFD session is considered to be in the Down query mode. It is the second mode of BFD. When a BFD session exists in a system, the BFD control packet is periodically sent. The overhead affects the normal operation of the system, and the query mode can be adopted.

Explanation
Note: TCP reverse source detection principle, when the device receives a SYN message, it will detect the existence of the source IP. The TCP reverse source detects the SYN-ACK sequence number that is sent incorrectly. If the source exists, it will send an RST message to request a three-way handshake. Because the reverse source detection mechanism is not affected by the successful establishment of the session table, it is recommended to use the reverse source detection technology to defend against SYN flood attacks. If you use TCP proxy attack defense, you must enable the state detection mechanism.