What Is an FTP Server?
In the last 10 years, the Internet has evolved quickly. Throughout this evolution, the File Transfer Protocol (FTP) has remained a current and useful technology. This article looks at what an FTP server is, why and how such a technology can be leveraged for Internet communications.
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History
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The first published standard for the File Transfer Protocol predates the Transmission Control Protocol and Internet Protocol (TCP/IP) in use today. Published in 1971, FTP ran over the Network Control Protocol (NCP), the Internet's predecessor. It was designed to facilitate indirect exchanges across the network--those exchanges in which data or programs were transmitted to a different system to be executed or worked on.
Uses
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FTP was originally designed for large bulk transfers of data, so it's ideally suited for file downloads. This is particularly true across the Internet, where FTP is used commonly as the mechanism for downloading patches, songs and purchased programs.
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Function
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At a technical level, FTP uses an out-of-band control mechanism. This means the data moves between client and server using one communication path. Control messages that govern the flow of that data move between the systems on a different communication channel. Because FTP was designed for file transfers, it leverages the TCP as an underlying protocol, which includes more robust packet delivery assurance.
Identification
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Much like most other client/server relationships, the FTP client is the system that makes a request for data, and the FTP server provides that data. It is a common Internet practice to reserve a specific machine or even a set (or "farm") of machines to function as FTP servers for an enterprise. Their purpose is to listen for client requests and (assuming security checks allow for it) quickly and accurately transmit large files to those clients. If your web browser suddenly reads "ftp://<server>" you'll know you're leveraging the FTP protocol in your download.
Benefits
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Web pages, which commonly download through the HyperText Transfer Protocol (HTTP), are usually comprised of many small files that must be simultaneously downloaded to a wide variety of clients in small degrees. Any given web page may make dozens or even hundreds of information requests just to populate a single page. This is a fundamentally different style of work than one request that results in several dozen megabytes of continuous data download. It is precisely because of this difference that FTP servers have continued to have their place amid the many web servers on the Internet.
Design
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Because FTP servers are designed for large file transfers, they are best used as a part of an overall Internet server strategy. It's common, for instance, to have general informational pages hosted by web (HTTP) servers and include links in these pages to separate servers used to host large files for download--such as music, videos, large PDF documents, computer code and patches. Because security can be an issue with FTP (originally designed to send usernames and passwords in clear text), it is typical to see FTP servers host files that do not require specific permission for download--or to use a federated security model that leaves authentication and authorization to other systems.
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