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                           9  TCP/IP Enhanced Electronic Mail Message Format: Multipurpose Internet Mail Extensions (MIME)

TCP/IP Enhanced Electronic Mail Message Format: Multipurpose Internet Mail Extensions (MIME) 1 2 3 MIME Basic Structures and Headers

MIME Capabilities

The idea behind MIME is both clever and elegant—which means I like it! RFC 822 restricts e-mail messages to be ASCII text, but that doesn't mean that we cannot define a more specific structure for how that ASCII text is created. Instead of just letting the user type an ASCII text message, we can use ASCII text characters to actually encode non-text information (commonly called attachments). Using this technique, MIME allows regular RFC 822 e-mail messages to carry the following:

• Non-text information, including graphic files, multimedia clips and all the other non-text data examples I listed earlier;
  
  
• Arbitrary binary files, including executable programs and files stored in proprietary formats (for example, AutoCAD files, Adobe Acrobat PDF files and so forth);
  
  
• Text messages that use character sets other than ASCII. This even includes the ability to use non-ASCII characters in the headers of RFC 822 e-mail messages.

MIME even goes one step beyond this, by actually defining a structure that allows multiple files to be encoded into a single e-mail message, including files of different types. For example, someone working on a budget analysis could send one e-mail message that includes a text message, a Powerpoint presentation, and a spreadsheet containing the budget figures. This capability has greatly expanded e-mail’s usefulness in TCP/IP.

All of this is accomplished through special encoding rules that transform non-ASCII files and information into an ASCII form. Headers are added to the message to indicate how the information is encoded. The encoded message can then be sent through the system like any other message. SMTP and the other protocols that handle mail pay no attention to the message body, so they don't even know MIME has been used.

The only changes required to the e-mail software is adding support for MIME to e-mail client programs: both the sender and receiver must support MIME to encode and decode the messages. Support for MIME was not widespread when MIME was first developed, but the value of the technique is so significant that it is present in nearly all e-mail client software today. Furthermore, most clients today can also use the information in MIME headers to not only decode non-text information but pass it to the appropriate application for presentation to the user.

TCP/IP Enhanced Electronic Mail Message Format: Multipurpose Internet Mail Extensions (MIME) 1 2 3 MIME Basic Structures and Headers