AH Protocol

The AH Protocol is a networking security measure.

The Authentication Header (AH) protocol is part of the Internet Protocol Security (IPSec) protocol. This protocol provides authentication for a data packet content and adds a validation code to the front of the packet in a "header."

Function

AH calculates a checksum, which is called an Integrity Check Value (ICV). Normally checksums are tagged to the end of the data. With AH, this value instead goes in a structure called a header at the beginning of the data. Both the sender and receiver of the data share a formula for calculating the ICV. If the data has been tampered with during transmission, the recalculated ICV will not match the value in the authentication header so the receiver discards the packet.

Features

The AH protocol operates in two modes. With Transport Mode, the AH header is inserted between the Internet Protocol (IP) header and the Transmission Control Protocol (TCP) header. In Tunnel Mode, the entire packet is encrypted, including its IP header, the AH header goes at the front of this and a new IP header is added in front of the AH header.

Purpose

The AH protocol does not provide privacy for the data in transit. Instead it makes the receiver aware of any tampering with the data packet contents during transmission.

References

• The TCP/IP Guide: IPSec Authentication Header (AH)