This memo describes the use of the Advanced Encryption Standard (AES) Galois Message Authentication Code (GMAC) as a mechanism to provide data origin authentication@ but not confidentiality@ within the IPsec Encapsulating Security Payload (ESP) and Authentication Header (AH). GMAC is based on the Galois/Counter Mode (GCM) of operation@ and can be efficiently implemented in hardware for speeds of 10 gigabits per second and above@ and is also well-suited to software implementations.