IETF RFC 2404 PDF

This memo describes the use of the HMAC algorithm [RFC-2104] in conjunction with the SHA-1 algorithm [FIPS-180-1] as an authentication mechanism within the revised IPSEC Encapsulating Security Payload [ESP] and the revised IPSEC Authentication Header [AH]. HMAC with SHA-1 provides data origin authentication and integrity protection.

Further information on the other components necessary for ESP and AH implementations is provided by [Thayer97a].