File Transfer Protocol (FTP) is a communication protocol developed to facilitate the transfer of files between two computer systems. It operates at the application layer of the TCP/IP protocol suite and is based on a client-server architecture. Its primary function is to enable uploading and downloading of files from one computer to another. FTP is one of the earliest Internet protocols, developed in the 1970s as part of the ARPANET project. Its foundational version is defined in RFC 959, published by the Internet Engineering Task Force (IETF).

The protocol is designed to allow file exchange between different file systems and platforms over a network. During file transfer, parameters such as filename, file type, transfer mode (ASCII or binary), and structure (block, stream, or record) are used to manage the transmission. In addition to file transfer operations, FTP also supports remote directory navigation, file deletion, renaming, and directory creation through specific commands.

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Operational Structure

FTP operates using two distinct TCP connections: a control connection and a data connection. The control connection, typically established on TCP port 21, remains open throughout the session and is used for the transmission of commands and responses between the client and the server. The data connection, on the other hand, is a temporary channel opened during file transfer to transmit the actual content.

There are two primary connection modes:

Active Mode: The server initiates the data connection to a client-specified port.
Passive Mode: The client initiates the data connection to a server-specified port; this mode is preferred for clients behind NAT or firewalls.

Each FTP session manages control streams and data streams separately, ensuring clear division between command exchange and file or directory transfers.

Access Modes

FTP servers typically support two primary access methods:

Local User Access: Authentication is performed using system-defined usernames and passwords. Users can be granted read and write permissions, enabling the implementation of individual or group-based access control on the FTP server.
Anonymous Access: No authentication is required. Users typically enter "anonymous" as the username and optionally provide an email address as the password. Most servers do not enforce strict password validation. For security reasons, anonymous access is usually restricted to read-only operations.

Command Set and Operations

FTP operates through a set of standardized, text-based commands exchanged between the client and the server. These commands allow users to log in, change directories, upload or download files, and perform administrative tasks. Common FTP commands include:

USER / PASS: User authentication
LIST: List directory contents
RETR: Retrieve (download) a file
STOR: Store (upload) a file
DELE: Delete a file
CWD: Change working directory
QUIT: Terminate the session

These commands and their associated reply codes are formally defined in RFC 959, which serves as the foundational specification for FTP communication.

Internationalization

Traditional FTP implementations are based on the ASCII character set, which can lead to issues when transferring files with names containing non-ASCII characters. To address this, RFC 2640 introduced internationalization extensions to FTP, including support for UTF-8 encoding, improved filename handling, and command behavior adjustments for multilingual environments.

Security Features

The original FTP protocol lacks built-in security mechanisms—user credentials and file contents are transmitted in plain text. To address these vulnerabilities, RFC 2228 introduced FTP Security Extensions, adding support for authentication, data integrity, confidentiality, and secure communication channels.

Key security mechanisms include:

AUTH: Initiates the type of secure connection (e.g., TLS or SSL)
ADAT: Transfers authentication data over secure channels
PROT: Defines the level of protection for the data channel (e.g., encryption or integrity)
PBSZ: Specifies the protection buffer size

These mechanisms are especially important when handling sensitive data and are recommended for modern FTP deployments.

Server Configuration and Management

FTP servers can be deployed using various software implementations. On GNU/Linux systems, vsftpd (Very Secure FTP Daemon) is a widely used option. FTP servers can enforce user-specific permissions, designate access directories, and restrict traversal to parent directories.

Anonymous users are typically limited to download-only access, while authenticated users may be granted full read/write privileges. Logging features are critical for auditing purposes and typically record file names, sizes, directions (upload/download), user accounts, and IP addresses.

Use Cases

FTP is commonly used for:

Uploading software updates to network servers
Transferring files to web hosting environments
Exchanging files with backup systems

Due to its simplicity and compatibility with legacy systems, FTP remains in use among network administrators. However, secure alternatives such as SFTP and FTPS are increasingly preferred in environments where data protection is a priority.

FTP is one of the oldest and most widely used file transfer protocols in Internet history. Its simplicity and broad compatibility allowed it to serve as a standard solution for decades. However, due to its lack of native security features, it is now recommended to use FTP in conjunction with secure extensions or to adopt more secure alternatives.

FTP (File Transfer Protocol)