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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 Message Format Overview, Motivation, History and Standards 1 2 3 4 MIME Content-Type Header and Discrete Media: Types, Subtypes and Parameters

Basic Structure Types

The exact method by which data is encoded in the message body and MIME headers are included depends on the overall structure of the MIME message. There are two basic structure types, which are described based on the kind of media the message carries:

• Simple Structure (Discrete Media): MIME messages carrying a single discrete media type, such as a text message or a graphical image, use a simple structure. Only one encoding of information is present in the body of the message.
  
  
• Complex (Composite Media) Structure: Some MIME messages carry a composite media type, which allows multiple different media to be contained in a single message, such as a text message and a graphical image, or to encapsulate another e-mail message in its entirety. Many of these use a more complex structure where the body of the message contains several MIME body parts.

Collectively, both whole MIME messages and individual body parts are called MIME entities. Each set of MIME headers provides information about either type of MIME entity: a MIME message as a whole, or a body part in a composite message. When a MIME message is received, the recipient first examines the headers in the message as a whole (the RFC 822 headers) to determine the overall message type. This then indicates if the message uses a simple or complex structure. If the latter, the body of the message is parsed and each individual body part is individually interpreted, including its individualized headers. The topic on composite media types has more details on how these body parts are formatted.

MIME Message Format Overview, Motivation, History and Standards 1 2 3 4 MIME Content-Type Header and Discrete Media: Types, Subtypes and Parameters