The mechanism for negotiating authentication and security with FTP was added under RFC 2228, which included the new FTP command AUTH. While this RFC does not explicitly define any required security mechanisms, e.g. SSL or TLS, it does require the FTPS client to challenge the FTPS server with a mutually known mechanism. If the FTPS client challenges the FTPS server with an unknown security mechanism, the FTPS server will respond to the AUTH command with error code "504 (not supported)". Clients may determine which mechanisms are supported by querying the FTPS server with the FEAT command, although servers are not necessarily required to be honest in disclosing what levels of security they support. Common methods of invoking FTPS security included AUTH TLS and AUTH SSL.