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)

MIME Composite Media Types: Multipart and Encapsulated Message Structures 1 2 3 MIME Extension for Non-ASCII Mail Message Headers

One of the main reasons why MIME was created was the significant restrictions that the RFC 822 standard places on how data in e-mail messages must be formatted. To follow the rules, messages must be encoded in US ASCII, a 7-bit data representation. This means that even though each byte can theoretically have any of 256 values, in ASCII only 128 values are valid. Furthermore, lines can be no longer than 1000 characters including the carriage return and line feed (“CRLF”) characters at the end, and those two characters cannot appear elsewhere.

For some types of data, such as text files, this is no big deal, but for others it is a serious problem. This is especially the case with binary data. If you look at the data in a video clip or MP3 file or executable program, it will appear to be “random gibberish”. In fact, such data is not random but is represented using specific rules, but the data is expressed in raw binary form, where any 8-bit byte can contain any value from 0 to 255, which is why it looks like “junk” to humans. More importantly, this means that this data does not follow the rules for RFC 822 files and cannot be sent directly in this form.

MIME Composite Media Types: Multipart and Encapsulated Message Structures 1 2 3 MIME Extension for Non-ASCII Mail Message Headers