File Transfer Protocol

The File Transfer Protocol (FTP) is a software standard for transferring computer files between machines with widely different operating systems. It belongs to the application layer of the Internet protocol suite. It is an 8-bit client-server procedure, capable of handling any type of resource without further processing, such as MIME or Uuencode. However, this has extremely high latency; that is, the time between beginning the request and starting to receive the required data can be quite long, and a sometimes-lengthy login procedure is required.

Overview

File Transfer Protocol is commonly run on two ports, 20 and 21. Port 20 is a data stream which transfers the data between the client and the server. Port 21 is the control stream and is the port where commands are passed to the server. While data is being transferred via the data stream, the control stream sits idle. This can cause problems with large data transfers through firewalls which time out sessions after lengthy periods of idleness. While the resource may well be successfully transferred, the control session can be disconnected by the firewall, causing an error to be generated.

The objectives of FTP, as outlined by its RFC, are:

• To promote sharing of resources (computer programs and/or data)
• To encourage indirect or implicit use of remote computers
• To shield a user from variations in resource storage systems among different hosts
• To transfer data reliably and efficiently.
  

Disadvantages are:

• Passwords and resource contents are sent in clear text, which can be intercepted by eavesdroppers
• Multiple TCP/IP connections are used, one for the control connection, and one for each download, upload, or directory listing. Firewall software needs additional logic to account for these connections
• It is hard to filter active mode traffic on the client side by using a firewall, since the client must open an arbitrary port in order to receive the connection. This problem is largely resolved by using passive mode
• It is possible to abuse the protocol's built-in proxy features to tell a server to send data to an arbitrary port of a third computer
• Is an extremely high latency protocol due to the number of commands needed to initiate a transfer
• Though usable directly by a user at a terminal, is designed mainly for use by client programs.

Many sites that run FTP servers enable so-called "anonymous ftp". Under this arrangement, users do not need an account on the server. By default, the account name for the anonymous access is 'anonymous'. This account does not need a password. Although users are commonly asked to send their email addresses as their passwords for authentication, usually there is trivial or no verification, depending on the server and its configuration.

FTP and Web Browsers

Most recent web browsers and resource managers can connect to servers. This allows manipulation of remote files over Web through an interface similar to that used for local files. This is done via an URL, which takes the form ftp://<ftpserveraddress> (e.g., ftp://ftp.gimp.org/). A password can optionally be given in the URL, e.g.: ftp://<login>:<password>@<ftpserveraddress>. Most web-browsers require the use of passive mode, which not all servers are capable of handling.

Each resource located on the remote server has a unique Universal Resource Locator (URL) address e.g. if you select the thumbnail image below, (image opens in new window), it brings up a picture of the author having just completed a recent helicopter lesson.  The image was taken with a Fujifilm FinePix E550 digital camera, the image was uploaded to my PC via a Universal Serial Bus (USB) interface and from there uploaded to a web hosting account  provided by 1and1.co.uk.