Understanding TCP/IP

13.6 Return Codes

The server replies to individual FTP protocol commands set by the client with a message with a three-digit return code followed by text clarifying the return code. The three-digit return code has the form xyz where:

x takes any of these following values:

• 1: A positive preliminary reply when starting some action. Before the client can send another command, one can expect a message about the termination of the action.
• 2: A positive completion reply (the requested action has been successfully completed). The client can send commands.
• 3: A positive immediate reply after which the client has to perform a concrete action. For instance, after entering a username, a password is required.
• 4: A transient negative completion reply. This means ...