File and Message Transfer Overview and Application Categories
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I said in the introduction to this section that file and message transfer applications are one of the most important types used in TCP/IP. Transferring files between networked computers is the most basic type of network communication. In fact, it wouldn't even be an exaggeration to say that this may be the most important class of internetworking applications. Some of the members of this group are so common that many people use them every day without even thinking about it.
To understand these applications, let's first take a quick step back to look at the fundamental concept of a file. Simply put, a file is just a collection of information that is treated as a single unit by a computer system. Files are stored in directories or folders in a file system. In modern computers, files are normally expressed as a sequence of bytes or characters, and each file is read, written, copied or otherwise manipulated as an independent object. In addition to the data it contains, each file has associated with it file attributes that describe it.
For our purposes, the critical characteristic of a file is that it is a self-contained object carrying arbitrary information. Since files are the building blocks of information in computer systems, it's no surprise that the transfer of information in networking was originally defined in terms of the movement of these files. Some of the protocols describing how to transfer files predate all of the modern protocols in the lower levels of TCP/IP, including IP version 4, TCP and UDP. It's not the case that file transfer was an early application of internetworking, but that internetworking was invented in large part to permit file transfer!
Files in modern computing systems are inherently designed to be generic; they can contain any type of information. The significance of the contents of a file depends entirely on the user or software program that examines it. The TCP/IP file and message transfer protocols have in common the notion of moving files from one computer to another. Where they differ is in how the files are handled and processed. There are two basic approaches: general file transfer, and message transfer.
General transfer applications normally treat the file as a black box, moving them from place to place and paying little or no attention to what the files contain. The TCP/IP File Transfer Protocol (FTP) and Trivial File Transfer Protocol (TFTP) fall into this category. FTP has been around in one form or another for over 30 years now and is still widely used.
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