Usenet Message Format and Special Headers
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Usenet is designed to permit users to exchange information in the form of messages that are sent from one computer to another. Like any message-based networking application, it is necessary that all Usenet client and server software agree to use a common message format. This ensures that all devices and programs are able to interpret all Usenet articles in a consistent manner.
While Usenet was created as an alternative to e-mail, and there are obviously differences in how each treats messages, there are also many similarities. Both are text-oriented messaging systems with similar needs for communicating both content and control information. The creators of Usenet realized that there would be many advantages to basing the Usenet message format on the one used for e-mail, rather than creating a new format from scratch. For one thing, the e-mail message format was already widely used, and adopting it for Usenet would save implementation time and effort. It would also enhance compatibility between e-mail and Usenet messages, allowing software designed to process or display e-mail to also work with Usenet articles.
For this reason, the Usenet message format was defined based on the RFC 822 standard for e-mail messages, discussed thoroughly in its own subsection of the section on e-mail in this Guide. RFC 822 messages begin with a set of headers that contain control and descriptive information about the message, followed by a blank line and then the message body, which contains the actual content.
One important attribute of the RFC 822 standard is the ability to define custom headers that add to the regular set of headers defined in the standard itself. Usenet articles require some types of information not needed by e-mail, and these can be included in specially-defined headers while still adhering to the basic RFC 822 format. At the same time, headers specific to e-mail that are not needed for Usenet can be omitted.
Thus, there is no structural difference at all between a Usenet article and an e-mail message; they differ only in the kinds of headers they contain, and the values for those headers. For example, a Usenet message will always contain a header specifying the newsgroup(s) to which the article is being posted, but will not carry a To: line as an e-mail message would.
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