File Transfer Protocol (FTP) | Computer Networks - Computer Science Engineering (CSE) PDF Download

What is FTP?

FTP stands for File Transfer Protocol.

FTP is a standard internet protocol provided by TCP/IP used for transmitting files from one host to another. It is mainly used for transferring web page files from their creator to the computer that acts as a server for other computers on the internet. It is also commonly used for downloading files from servers to a local computer.

Objectives of FTP

• It provides for the sharing of files.
• It is used to encourage the use of remote computers.
• It transfers data more reliably and efficiently for large files.

Why FTP?

Although transferring files from one system to another is very simple and straightforward, it can sometimes cause problems. For example, two systems may have different file conventions (Windows uses "\\" in file paths, while Linux uses "/"). They may have different ways to represent text and data (like different line ending characters). Two systems may also have different directory structures.

FTP protocol overcomes these problems by establishing two separate connections between hosts. One connection is used for commands and responses (the control connection), and another is used for the actual file data (the data connection).

The Mechanism of FTP

The FTP process involves a client and a server. The client initiates the connection and sends commands, while the server responds and provides the requested files.

The FTP client has three components: the user interface, the client control process, and the client data transfer process. The server has two components: the server control process and the server data transfer process.

Types of Connections in FTP

• Control Connection: This connection is created to send commands and receive responses. It uses port 21 on the server. The control connection remains active during the entire FTP session. Simple commands like USER (username), PASS (password), and LIST (list files) are sent over this connection. 
• Data Connection: This connection is used only for transferring the actual file data. It opens whenever a file transfer is requested and closes once the transfer is complete. The port used for this connection depends on the connection mode (explained below). 

Connection Modes

How the data connection is established is a critical part of FTP. There are two modes:

• Active Mode: In Active Mode, the client tells the server which port it is listening on. The server then initiates the data connection by connecting back to the client on that specified port, typically from its own port 20. This can be a problem for clients behind firewalls, as the firewall may block the incoming connection from the server.
• Passive Mode: To solve the firewall problem, Passive Mode was created. In this mode, the client asks the server to "listen" on a port. The server opens a random high-numbered port and sends that port number to the client. The client then initiates the data connection to the server on that new port. Most modern FTP clients use Passive Mode by default.

Data Transfer Modes

FTP can transfer files in two primary modes:

• ASCII Mode: This mode is used for transferring plain text files. It automatically converts line-ending characters between different systems (for example, from Windows format (CRLF) to Unix format (LF)), ensuring the text file is readable on the destination computer.
• Binary Mode (or Image Mode): This mode is used for all other file types, like images, videos, and executable programs (.exe). It transfers the file byte-for-byte, making an exact copy with no modifications. Using the wrong mode (e.g., transferring an image in ASCII mode) will corrupt the file.

FTP Clients

An FTP client is a program that implements the FTP protocol, allowing you to connect to an FTP server to upload or download files. It provides a set of commands to manage the connection and transfer files. Some basic commands include:

• LIST: Lists the files in the current directory on the server.
• RETR: Retrieves (downloads) a file.
• STOR: Stores (uploads) a file.
• QUIT: Closes the connection.

• While you can use command-line FTP clients, many are now built into web browsers or exist as graphical user interface (GUI) applications (like FileZilla), which makes file transfer as easy as dragging and dropping.

  FTP Security

  Standard FTP is Insecure: By default, standard FTP is not secure. All data, including usernames, passwords, and file content, is sent in clear text. This means anyone "eavesdropping" on the network can easily steal this information.

  Secure Alternatives: To address this major security flaw, secure versions of FTP were developed:

  • FTPS (FTP over SSL/TLS): This adds an SSL/TLS encryption layer to standard FTP, securing the control and data connections. It uses the same commands but protects them from being read.
  • SFTP (SSH File Transfer Protocol): This is not technically FTP. It is a completely different protocol that runs over the Secure Shell (SSH) protocol. It provides robust authentication and encrypted data transfer, and is often preferred for its security and simpler firewall configuration (it uses only one port).

• Advantages of FTP

  • Speed: FTP is one of the fastest ways to transfer large files from one computer to another, as its protocol is optimized for bulk data transfer.
  • Efficient: You can resume a transfer if the connection is lost, meaning you don't have to start from the beginning again.
  • Directory Listing: FTP has a standardized way to list directories and navigate remote file systems.
  • Back & forth movement: FTP easily allows for both uploading (sending) and downloading (receiving) files between a client and a server.

• Disadvantages of FTP

  • Lack of Security: The biggest disadvantage of the standard protocol is that it sends credentials and data in clear text, making it vulnerable to interception. This is why FTPS or SFTP should always be used instead.
  • Firewall Complexity: The need for two separate connections and two different modes (Active/Passive) can make configuring FTP through firewalls complicated.
  • Inefficient for Small Files: FTP establishes new data connections for transfers, which can be inefficient when transferring thousands of very small files due to the overhead of each connection.