Lesson Five

FTP protocol

File Transfer Protocol (FTP) is a standard network protocol used to transfer files from one host to another host over a TCP-based network, such as the Internet. It is often used to upload web pages and other documents from a private development machine to a public web-hosting server. 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 hides (encrypts) the username and password, and encrypts the content, SSH File Transfer Protocol may be used.

The first FTP client applications were interactive command-line tools, implementing standard commands and syntax. Graphical user interfaces have since been developed for many of the popular desktop operating systems in use today, including general web design programs like Microsoft Expression Web, and specialist FTP clients such as CuteFTP.

FTP may run in active or passive mode, which determines how the data connection is established.In active mode, the client creates a TCP control connection to the server and sends the server the client's IP address and an arbitrary client port number, and then waits until the server initiates the data connection over TCP to that client IP address and client port number.In situations where the client is behind a firewall and unable to accept incoming TCP connections, passive mode may be used.

Data transfer can be done in any of three modes:

1. 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.
2. Block mode: FTP breaks the data into several blocks (block header, byte count, and data field) and then passes it on to TCP.
3. Compressed mode: Data is compressed using a single algorithm (usually run-length encoding).