Chapter 7 Firewall for FortiOS 5.0 : Network defense : Blocking external probes : Evasion techniques : Negotiation codes
  
Negotiation codes
Telnet and FTP servers and clients support the use of negotiation information to allow the server to report what features it supports. This information has been used to exploit vulnerable servers. To avoid this problem, the FortiGate unit removes negotiation codes before IPS inspection.