Authentication Header (AH) Protocol
The AH Protocol is designed to authenticate the source and to check the integrity of the payload carried by the IP packet but it doesn’t provide privacy. Using hashing function and a symmetric key calculates the message digest and inserts it in the authentication header. The AH is put in the appropriate location of the packet depending on the mode of operation of IPSec.
When an IP datagram carries an authentication header, the protocol value of the IP header is changed to 51. The next header field in the AH carries the original value of the protocol field of the IP header.