Authentication Header is the segment of internet protocol security (IPsec). It is used for IPsec packet authentication and it gives data integrity and security. To guarantee information origin authentication, AH includes a secret shared key in the algorithm that it uses for authentication. To guarantee replay security, AH utilizes the number field within the AH header. AH ensures that your information has not been messed with in route to its last objective. There are two ways to use AH : first is transport mode, in this mode the data contained in the datagram is transported in the clear. It does not give more security. Second is, Tunnel mode that makes a new IP header and utilizes it as the outermost IP header of the datagram.