3. MIME Header Fields

MIME defines a number of new RFC 822 header fields that are used to describe the content of a MIME entity. These header fields occur in at least two contexts:

1. As part of a regular RFC 822 message header
2. In a MIME body part header within a multipart construct

The formal definition of these header fields is as follows:

entity-headers := [ content CRLF ]
                  [ encoding CRLF ]
                  [ id CRLF ]
                  [ description CRLF ]
                  *( MIME-extension-field CRLF )

MIME-message-headers := entity-headers
                        fields
                        version CRLF
                        ; The ordering of the header
                        ; fields implied by this BNF
                        ; definition should be ignored.

MIME-part-headers := entity-headers
                     [ fields ]
                     ; Any field not beginning with
                     ; "content-" can have no defined
                     ; meaning and may be ignored.
                     ; The ordering of the header
                     ; fields implied by this BNF
                     ; definition should be ignored.

The syntax of the various specific MIME header fields will be described in the following sections.

---

MIME Header Field List:

• MIME-Version: Declares MIME version
• Content-Type: Specifies media type of content
• Content-Transfer-Encoding: Specifies encoding method
• Content-ID: Unique identifier for content
• Content-Description: Text description of content
• MIME-extension-field: Extension fields

Key Points:

• Entity headers can appear in message headers or body part headers
• Fields not beginning with "content-" may have no defined meaning in body parts
• The order of header fields should be ignored