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 Content-Type Header and Discrete Media: Types, Subtypes and Parameters 1 2 3 4 5 6 MIME Content-Transfer-Encoding Header and Encoding Methods

MIME Multipart Message Type

The multipart media type is the more common of the two, and for good reason: it is an incredibly powerful mechanism. It allows one message to contain many different kinds of information that can be used in different ways. Each piece of data is encoded separately as a MIME body part and the parts are combined into a single e-mail message. How these parts are used depends on the semantics of the message, which is indicated by the MIME subtype. RFC 2046 describes several of these, and a few new ones have also been defined by the IANA registration scheme described in the previous topic.

The following are the most common multipart media subtypes, and how they are used. The first four are defined in RFC 2046, and the others as indicated:

• multipart/mixed: Indicates that the body parts are not really related, but have just been bundled for transport in a single message for convenience. For example, this might be used by someone to send an office memo along with a vacation snapshot just for fun. This subtype is also used sometimes when the parts are related but the relationship is communicated to the recipient in some other way (such as a description in a distinct body part).
  
  
• multipart/alternative: Specifies that the body parts are alternative representations of the same information. The recipient decodes the parts and chooses the one that is best suited to his or her needs. A common use of this is in sending HTML-encoded e-mail. Some e-mail clients can't display HTML, so it is courteous to send a multipart/alternative message containing the message in both HTML and plain text forms.
  
  The alternatives should be placed in the message in increasing order of preference, meaning that the preferred format goes last. In the case of a document that includes plain text and rich text alternatives—such as the example above with plain text and HTML versions of a document—the plainest format should go first and the “fanciest” last.
  
  
• multipart/parallel: Tells the recipient that the body parts should all be displayed at the same time (“in parallel”). One example of how this might be used would be if someone sent an audio clip along with explanatory text to be displayed alongside it as it played.
  
  
• multipart/digest: This is used to allow a message to carry a digest, such as a collection of other e-mail messages.
  
  
• multipart/related: Indicates specifically that the body parts are related to each other. Special parameters are used to provide more information on how they are to be interpreted. This subtype was defined in RFC 2387.
  
  
• multipart/encrypted: Used for encrypted data. The first body part contains information on how the data is to be decrypted, and the second contains the data itself. This subtype was defined in RFC 1847.

MIME Content-Type Header and Discrete Media: Types, Subtypes and Parameters 1 2 3 4 5 6 MIME Content-Transfer-Encoding Header and Encoding Methods