Describe the Characteristics of the FTP Protocol
The File Transfer Protocol, or FTP, is one of the oldest applications on the Internet. In fact, it is older than the Internet. The FTP specification was first published in 1971, whereas the structure of the Internet was defined in 1974 with the publication of the TCP/IP protocol. The File Transfer Protocol was adapted for use over the Internet in 1980.
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Transfer Protocol
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FTP is an application, but it is also a transfer protocol, like HTTP (the Hypertext Transfer Protocol). Web surfers will see that addresses in the address bar of their browser usually starts with "http://," which means that the HTTP protocol is in effect. When FTP is transporting files, the address reads "ftp://" instead.
Access
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FTP has three forms of access. The original implementations of the application were as a command line utility. When a computer user typed in FTP at the system prompt, FTP presented its own environment with its own command line prompt, indicated by "ftp>." Access to ftp is rare nowadays because there are now many graphical interfaces for the application. Most computer users know FTP through a browser/directory-type window that enables "drag and drop" actions to pass transfer instructions. The third access type is a library of programming utilities that enables FTP functions to be integrated into other programs.
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Transport Characteristics
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Network communications happen in one of two modes. One is "connection-oriented," the other "connectionless." In the connection-oriented model, the two sides establish a session and send messages and data back and forth in the context of the session. In connectionless communication, each transaction is a request from the client followed by a response from the server and nothing more. FTP is connection-oriented. It establishes not one, but two sessions. The first connection carries commands and responses to those commands, while the second connection is a channel for passing files.
Packets
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The file transfer connection does not operate as a stream. That is, the contents of the file are not transferred in a continual unit. As with any Internet application, the data is broken up into smaller segments and transferred in a structure called a packet. Each packet's receipt is acknowledged by the receiver. The header of the packet includes information on the position of the current segment in the overall stream, enabling the receiver to request that a packet get resent in the case of missing data. The receiving FTP program is also able to use this information to re-sequence data that arrives out of order.
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References
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