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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  TCP/IP Key Applications and Application Protocols
           9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)
                9  TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)
                     9  TCP/IP Electronic Mail Message Formats and Message Processing: RFC 822 and MIME

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TCP/IP Electronic Mail RFC 822 Standard Message Format Processing and Interpretation
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MIME Message Format Overview, Motivation, History and Standards
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TCP/IP Enhanced Electronic Mail Message Format: Multipurpose Internet Mail Extensions (MIME)

The RFC 822 e-mail message format is the standard for the exchange of electronic mail in TCP/IP internetworks. Its use of simple ASCII text makes it easy to create, process and read e-mail messages, which has contributed to the success of e-mail as a worldwide communication method.

Unfortunately, while ASCII text is great for writing simple memorandums and other short messages, it provides no flexibility to support other types of communication. To allow e-mail to carry multimedia information, arbitrary files, and messages in languages using character sets other than ASCII, the Multipurpose Internet Mail Extensions (MIME) standard was created.

In this section I describe MIME and how it is used for modern e-mail messaging. I begin with an overview of MIME and discussion of its history and the standards that define it. I describe the two overall MIME message structures and provide a summary of the important MIME-specific headers. I then explain the important MIME Content-Type header in more detail, and discuss MIME discrete media types, subtypes and parameters. I discuss the more complex MIME multipart and encapsulated message structures, and then the different methods by which data can be encoded into MIME message bodies. I conclude with the special MIME extension to allow support for non-ASCII characters in ordinary e-mail headers.

Background Information: MIME is a message format that augments the basic RFC 822 message format, rather than replacing it. This section assumes that you have basic familiarity with the RFC 822 format and the more important e-mail message headers.


Note: While MIME was developed specifically for mail, its encoding and data representation methods have proven so useful that it has been adopted by other application protocols as well. One of the best known of these is the HyperText Transfer Protocol (HTTP), which uses MIME headers for indicating the characteristics of data being transferred. Some elements of MIME were in fact developed not for e-mail but for use by HTTP or other protocols, and I indicate this where appropriate. Be aware, however, that HTTP only uses elements of MIME; there are important differences, and HTTP messages are not MIME-compliant.


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TCP/IP Electronic Mail RFC 822 Standard Message Format Processing and Interpretation
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MIME Message Format Overview, Motivation, History and Standards
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