In constructing a high-level user agent, it may be desirable to allow one body to make reference to another. Accordingly, bodies may be labeled using the "Content-ID" header field, which is syntactically identical to the "Message-ID" header field:

       id :=  "Content-ID" ":" msg-id

Like the Message-ID values, Content-ID values must be generated to be world-unique.

The Content-ID value may be used for uniquely identifying MIME entities in several contexts, particularly for cacheing data referenced by the message/external-body mechanism. Although the Content-ID header is generally optional, its use is mandatory in implementations which generate data of the optional MIME Content-type "message/external-body". That is, each message/external-body entity must have a Content-ID field to permit cacheing of such data.

It is also worth noting that the Content-ID value has special semantics in the case of the multipart/alternative content-type. This is explained in the section of this document dealing with multipart/alternative.

6.2. Optional Content-Description Header Field

The ability to associate some descriptive information with a given body is often desirable. For example, it may be useful to mark an "image" body as "a picture of the Space Shuttle Endeavor." Such text may be placed in the Content-Description header field.

       description := "Content-Description" ":" *text

The description is presumed to be given in the US-ASCII character set, although the mechanism specified in [RFC-1522] may be used for non-US-ASCII Content-Description values.

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Next: 7. The Predefined Content-Type Values

6. Additional Content-Header Fields

6.1. Optional Content-ID Header Field

6.2. Optional Content-Description Header Field