Thus, the Authentication Header can be used to determine whether the message originated from the accepted sender and whether its contents are unchanged. The AH header, specified in RFC 2402, protects the entire message content by encrypting the IP packet. The HMAC algorithm, Hash-Based Message Authentication Code (HMAC), is used for encryption. An additional number field in the AH header defends against replay attacks. Data fields that must be modified by routers on the way to the destination are not secured.