Chapter 18 Troubleshooting : Life of a Packet : Packet flow
  
Packet flow
After the FortiGate unit’s external interface receives a packet, the packet proceeds through a number of steps on its way to the internal interface, traversing each of the inspection types, depending on the security policy and security profile configuration. The diagram in Figure 319 is a high level view of the packet’s journey.
The description following is a high-level description of these steps as a packet enters the FortiGate unit towards its destination on the internal network. Similar steps occur for outbound traffic.