Chapter 8 Hardware Acceleration : NP4 Acceleration : Configuring NP4 traffic offloading : Packet fast path requirements
  
Packet fast path requirements
Packets within the session must then also meet packet requirements.
Incoming packets must not be fragmented.
Outgoing packets must not require fragmentation to a size less than 385 bytes. Because of this requirement, the configured MTU (Maximum Transmission Unit) for network processors’ network interfaces must also meet or exceed the network processors’ supported minimum MTU of 385 bytes.
If packet requirements are not met, an individual packet will use FortiGate unit main processing resources, regardless of whether other packets in the session are offloaded to the specialized network processor(s).
In some cases, due to these requirements, a protocol’s session(s) may receive a mixture of offloaded and non-offloaded processing.
For example, FTP uses two connections: a control connection and a data connection. The control connection requires a session helper, and cannot be offloaded, but the data connection does not require a session helper, and can be offloaded. Within the offloadable data session, fragmented packets will not be offloaded, but other packets will be offloaded.
Some traffic types differ from general offloading requirements, but still utilize some of the network processors’ encryption and other capabilities. Exceptions include IPsec traffic and active-active high availability (HA) load balanced traffic.