Chapter 11 IPsec VPN for FortiOS 5.0 : Hardware offloading and acceleration : Overview : HMAC check offloading
  
HMAC check offloading
The Hash-based Message Authentication Code (HMAC) check can also be offloaded to hardware. SHA-256, SHA-384, or SHA-512 cannot be off-loaded to hardware, and must be processed using only software resources.
To enable HMAC check offloading
configure system global
set ipsec-hmac-offload (enable|disable)
end