· "ONLY" If the virus is detected at the "START" of the connection, the IPS engine sends the block replacement message immediately

· When a virus is detected on a TCP session (FIRST TIME), but where "SOME PACKETS" have been already forwarded to the receiver, FortiGate "resets the connection" and does not send the last piece of the file. Although the receiver got most of the file content, the file has been truncated and therefore, can't be opened. The IPS engine also caches the URL of the infected file, so that if a "SECOND ATTEMPT" to transmit the file is made, the IPS engine will then send a block replacement message to the client instead of scanning the file again.

In flow mode, the FortiGate drops the last packet killing the file. But because of that the block replacement message cannot be displayed. If the file is attempted to download again the block message will be shown.