Explanation/Reference:

Explanation
The correct one should be AH+ESP+ tunnel mode (inter-access between IPV6 sites), NAT-PT technology (inter-access between IPV4 and IPV6) IPSec sub-protocol header authentication protocol AH, which provides data integrity verification and data source identity for IP packets Authentication, using the HMAC algorithm,The HMAC algorithm is very similar to the Hash algorithm, which is generally evolved from the Hash algorithm, that is, the input message is combined with the symmetric key that has been shared by both parties in advance, and then the Hash algorithm is applied.
ESP. The principle of the protocol providing data integrity verification and data source identity authentication is the same as that of AH, but the verification scope of AH is smaller than that of ESP. The ESP protocol specifies the authentication algorithms that all IPSec systems must implement: HMAC-MD5, HMAC-SHA1, NULL. Compared with other rail technologies such as L2TP, GRE, AH, etc., ESP has a unique security mechanism - encryption, and can be used in combination with other tunneling protocols to provide stronger security support for users' remote communication.
Two modes of IPSec:
transfer mode (Transport Mode) is the default mode of IPSec, also known as end-to-end (End-to-End) mode, which is suitable for IPSec communication between two hosts.
Tunnel modeMode) is used for site-to-site communication between two gateways. The two gateways involved in the communication actually serve to provide secure communication services for the computers in the two networks bounded by them.