File Transfer: FTP Computer Networking

File Transfer: FTP

FTP (File Transfer Protocol) is a protocol for transferring a file from one host to another host. The protocol dates back to 1971 (when the Internet was still an experiment), but remains enormously popular. FTP is described in [RFC 959]. Figure 2.3-1 provides an overview of the services provided by FTP.

FTP moves files between local and remote file systems.

In a typical FTP session, the user is sitting in front of one host (the local host) and wants to transfer files to or from a remote host. In order for the user to access the remote account, the user must provide a user identification and a password. After providing this authorization information, the user can transfer files from the local file system to the remote file system and vice versa. As shown in Figure 2.3-1, the user interacts with FTP through an FTP user agent. The user first provides the hostname of the remote host, which causes the FTP client process in the local host to establish a TCP connection with the FTP server process in the remote host. The user then provides the user identification and password, which get sent over the TCP connection as part of FTP commands. Once the server has authorized the user, the user copies one or more files stored in the local file system into the remote file system (or vice versa).

HTTP and FTP are both file transfer protocols and have many common characteristics; for example, they both run on top of TCP, the Internet's connection-oriented, transport-layer, reliable data transfer protocol. However, the two application-layer protocols have some important differences. The most striking difference is that FTP uses two parallel TCP connections to transfer a file, a control connection and a data connection. The control connection is used for sending control information between the two hosts -- information such as user identification, password, commands to change remote directory, and commands to "put" and "get" files. The data connection is used to actually send a file. Because FTP uses a separate control connection, FTP is said to send its control information out-of-band. In Chapter 6 we shall see that the RTSP protocol, which is used for controlling the transfer of continuous media such as audio and video, also sends its control information out-ofband.

HTTP, as you recall, sends request and response header lines into the same TCP connection that carries the transferred file itself. For this reason, HTTP is said to send its control information in-band. In the next section we shall see that SMTP, the main protocol for electronic mail, also sends control information in-band.