File transfer is one of the most common uses of the Internet. The File Transfer Protocol (FTP) is command- and response-based. The client sends a command; the server processes the command and sends back a response. Generally, these commands are hidden behind graphical user interfaces and web browsers.
The first three characters of every response from the server are a three-digit reply code. Reply codes are interpreted as follows:
The command operation has successfully started. An FTP server sends a positive preliminary reply during a data transfer when the data socket is connected, but no data has been transferred. We discuss data transfers in Chapter 11.
The command operation has successfully completed. An FTP server sends a positive completion reply when data transfer completes.
The command operation has successfully completed, but the server is waiting for another command from the client. An FTP server sends a positive intermediate reply in response to the client sending the username during authentication.
The command operation has failed, but can be retried later.
The command operation has failed, and should not be retried later. An FTP server sends a permanent negative completion reply when a user attempts to access a directory to which he does not have rights.
The client ...