Chapter 7 Firewall for FortiOS 5.0 : Network defense : Blocking external probes : Evasion techniques
  
Evasion techniques
Attackers employ a wide range of tactics to try to disguise their techniques. If an attacker disguises a known attack in such a way that it is not recognized, the attack will evade your security and possibly succeed. FortiGate security recognizes a wide variety of evasion techniques and normalizes data traffic before inspecting it.