IP Authentication Header

Protocol used to provide connectionless integrity and data origin authentication for IP datagrams (hereafter referred to as just integrity) and to provide protection against replays. (RFC 4302). Scope Note: AH ensures data integrity with a checksum that a message authentication code, such as MD5, generates. To ensure data origin authentication, AH includes a secret shared key in the algorithm that it uses for authentication. To ensure replay protection, AH uses a sequence number field within the IP authentication header.