File Transfer Protocol (FTP)

The File Transfer Protocol (FTP) is a standard network protocol used to transfer computer files from one host to another host over a TCP-based network, such as the Internet.

FTP is built on a client-server architecture and uses separate control and data connections between the client and the server. FTP users may authenticate themselves using a clear-text sign-in protocol, normally in the form of a username and password, but can connect anonymously if the server is configured to allow it. For secure transmission that protects the username and password, and encrypts the content, FTP is often secured with SSL/TLS (FTPS). SSH File Transfer Protocol (SFTP) is sometimes also used instead, but is technologically different.

The original specification for the File Transfer Protocol was written by Abhay Bhushan and published as RFC 114 on 16 April 1971.

Data transfer can be done in any of three modes:

Stream mode: Data is sent as a continuous stream, relieving FTP from doing any processing. Rather, all processing is left up to TCP. No End-of-file indicator is needed, unless the data is divided into records.

Block mode:  FTP breaks the data into several blocks (block header, byte count, and data field) and then passes it on to TCP.

Compressed mode: Data is compressed using a single algorithm.

Login:

FTP login utilizes a normal username and password scheme for granting access. The username is sent to the server using the USER command, and the password is sent using the PASS command. If the information provided by the client is accepted by the server, the server will send a greeting to the client and the session will commence. If the server supports it, users may log in without providing login credentials, but the same server may authorize only limited access for such sessions.

Anonymous FTP:

A host that provides an FTP service may provide anonymous FTP access. Users typically log into the service with an 'anonymous'  account when prompted for user name.

Secure FTP: Securing FTP transfers may be accomplished by several methods.

FTPS: FTPS is an extension to the FTP standard that allows clients to request that the FTP session be encrypted. This is done by sending the "AUTH TLS" command. The server has the option of allowing or denying connections that do not request TLS. This protocol extension is defined in the proposed standard: RFC 4217. Implicit FTPS is a deprecated standard for FTP that required the use of a SSL or TLS connection. It was specified to use different ports than plain FTP.

SFTP: The SSH file transfer protocol or secure FTP (SFTP), also transfers files and has a similar command set for users, but is built on different software technology. SFTP uses the Secure Shell protocol (SSH) to transfer files. Unlike FTP, it encrypts both commands and data, preventing passwords and sensitive information from being transmitted openly over the network. It cannot interoperate with FTP software.

 

FTP works on port number 21.