Message Transfer

Other TCP/IP applications work with particular types of files, processing and interpreting them in various ways. These files are usually designed for the specific purpose of communication, and are thus called messages; these applications allow users to construct, send and receive messages that fit a particular message format. There are several prominent TCP/IP messaging applications we'll examine in this Guide:

• Electronic Mail (E-Mail): A system that allows users to exchange “letters” (in fact any type of document) in a manner equivalent to the conventional postal system, but with the advantages of great speed and simplicity. Electronic mail has not replaced regular mail entirely, but many people now use it for the vast majority of their correspondence.
  
  
• Network News (Usenet): An application that is like electronic mail in that it allows users to send messages. However, while e-mail is normally used to allow a message to be sent to one user or a small number of recipients, network news is a way for thousands of users to share messages on various topics. Any user can contribute a message that can be seen by others, any of whom can respond. Unlike the case with e-mail, recipients do not need to be explicitly identified, which makes network news far more suitable to communication amongst large groups of people who may not even know each other. This was one of the first TCP/IP applications to create something like an “electronic bulletin board”: an online community.
  
  
• Hypertext (World Wide Web): You probably don't even need me to explain what the World Wide Web is, such is its great significance in modern internetworking. Hypertext moves the idea of messaging beyond the simple exchange of text messages or plain files, to the notion of rich messages that can contain a variety of types of information. This includes text, graphics, multimedia and embedded files. Most importantly, hypertext allows one document to be linked to another, forming the “web” of related documents that led to the name “World Wide Web”. The Web is almost certainly the single most important TCP/IP application, used daily by millions of people.

Each of these applications was at one point somewhat distinct, but in recent years a number of developments have caused the lines between them to become greatly blurred. Electronic mail is no longer limited to simple text messages; it can now be used to carry general files by encoding them into text form using special methods, and even to carry hypertext documents. World Wide Web clients (browsers) continue to be enhanced to let them access other types of servers and files, and can also be used for general file transfer. These developments all mean even more functionality and flexibility for the TCP/IP user—and a bit more care required on the part of the TCP/IP learner.