From FileZilla Wiki
The Transport Layer Security (TLS) protocol gets used to cryptographically protect a socket connection. It is the successor of the Secure Sockets Layer (SSL) protocol. TLS gets used for the FTP over TLS/SSL (FTPS) protocol.
Points of interest
- Section 7.2.1. of RFC 2246: Closure alerts
The client and the server must share knowledge that the connection is ending in order to avoid a truncation attack. Either party may initiate the exchange of closing messages. [...] Each party is required to send a close_notify alert before closing the write side of the connection.
- Lots of FTP servers did not follow the specifications and were vulnerable to truncation attacks. FileZilla will properly fail transfers on those servers.