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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

Additional MIME Headers

In addition to the five basic headers above, the MIME standard allows additional ones to be defined. The only restriction is that they all must start with the word “Content-”, which again, clearly labels them as describing content of a MIME entity (message or body part). Obviously, both sender and recipient must support a custom header for it to be useful.

Several new MIME headers have in fact been created and documented in various Internet RFCs. Some are actually designed not specifically for use by e-mail messages, but for use by other protocols that make use of MIME technology, such as the HyperText Transfer Protocol (HTTP). Here are three notable ones.

In multipart MIME messages, this header may be given to MIME body parts to control how information is presented to the user. The two most common values are “inline”, which says the content is intended to be displayed automatically along with other body parts, and “attachment”, which indicates that the content is separate from the main document. This header is defined in RFC 2183.

Allows the location of a MIME body part to be identified using a uniform resource identifier (URI). This is sometimes used when encoding HTML and other multimedia-enabled document formats into e-mail using MIME multipart messages. It is defined in RFC 2557.

Specifies the length of a MIME entity in bytes. This header is not commonly used in e-mail applications of MIME, but is an important header in HTTP. It is described in the HTTP standards, first appearing in RFC 1945.

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