The File Transfer Protocol
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.
Reply Codes
The first three characters of every response from the server are a three-digit reply code. Reply codes are interpreted as follows:
- 1xx: positive preliminary reply
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.
- 2xx: positive completion reply
The command operation has successfully completed. An FTP server sends a positive completion reply when data transfer completes.
- 3xx: positive intermediate reply
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.
- 4xx: transfer negative reply
The command operation has failed, but can be retried later.
- 5xx: permanent negative completion reply
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.
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