AH, Authentication Header

Often referred to as AH protocol, the Authentication Header ensures data integrity with the checksum of a message generated by an authentication code. To ensure replay protection, it uses sequences in the number field within the Authentication Header. More simply stated, AH makes sure that sensitive information is not compromised while in route to the final destination. Although AH will authenticate as much of an IP datagram as it can, values in certain fields of an IP header can’t always be determined by the receivers. The Authentication Header can’t protect these fields but does have the ability to protect individual payloads of the IP packet. AH is often used as an alternative to ESP (Encapsulating Security Payload) protocol. While ESP can perform authentication, it often has an adverse effect on system performance where AH does not. It also goes one step further by authenticating the entire datagram. ESP protocol will not authenticate the leading IP header or any data that precedes it. The Authentication Header is also preferred for it’s open source nature and ease of use.