Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 222
File Transfer Protocol (FTP)

TELNET provides users with the ability to act as a local terminal even though users are not directly attached to the host. One other TCP/IP application that provides network services for users on a network is a file transfer protocol. With TCP/IP, there are three popular types of file access protocols in use: FTP, Trivial File Transfer Protocol (TFTP), and Network File System (NFS). This FTP protocol provides for files to be transferred reliably across the network under the complete control of the user. FTP is transaction based. Every command is replied to using a number scheme similar to the SMTP protocol (discussed in a moment).

FTP is very robust. Remember the previous discussion on ports and sockets and how they are established and used. The FTP protocol actually uses two port assignments (and therefore two connections): 20 and 21. Remember that most connections between two network stations are made via one source port and one destination port. A network station wanting a connection to a remote network station must connect to two ports on the destination station in order for FTP to work.

Port 20 is used for the initial setup of the connection and as the control connection. No data passes over this circuit except for control information. Port 21 is used for user data (the file to be transferred) to pass over the connection.

Similar to the TELNET arguments, simply typing FTP <domain name or IP address> will establish the connection. The command line should then read FTP> (this depends on your application). With the advent of Windows and Windows-like operating systems, FTP now has a GUI interface in order to take some of the harshness out of the protocol. After the connection is established, the server process awaits a command from the client. To transfer a file from the server to the client, the user types in get <a name of a file>, which is transmitted over to the remote network station. With this, a second connection is established between the server and client FTP process. It is known as the data connection. Now we have two connections, but only during the file transfer process. Once the file is transferred, the data connection port is closed.

This is the well-known (or assigned) FTP data port. From a user’s standpoint, to establish a connection between itself and a remote station, the command is similar to TELNET: FTP <domain name or IP address>. A user could also type in FTP and wait for the FTP prompt. At the prompt, the user would use the OPEN command to establish the connection.

File Transfer Protocol (FTP)