RFC 2111 (rfc2111) - Page 3 of 5

Content-ID and Message-ID Uniform Resource Locators

RFC 2111                    CID and MID URLs                  March 1997


   A "cid" URL is converted to the corresponding Content-ID message
   header [MIME] by removing the "cid:" prefix, converting %hh hex-
   escaped characters to their ASCII equivalents and enclosing the
   remaining parts with an angle bracket pair, "".  For
   example, "mid:foo4%" corresponds to

        Message-ID: [email protected]>

   A "mid" URL is converted to a Message-ID or Message-ID/Content-ID
   pair in a similar fashion.

   Both message-id and content-id are required to be globally unique.
   That is, no two different messages will ever have the same Message-ID
   addr-spec; no different body parts will ever have the same Content-ID
   addr-spec.  A common technique used by many message systems is to use
   a time and date stamp along with the local host's domain name, e.g.,
   .

Some Examples

   The following message contains an HTML body part that refers to an
   image contained in another body part.  Both body parts are contained
   in a Multipart/Related MIME entity.  The HTML IMG tag contains a
   cidurl which points to the image.

     From: 
     To: 
     Subject: A simple example
     Mime-Version: 1.0
     Content-Type: multipart/related; boundary="boundary-example-1";
                   type=Text/HTML

     --boundary-example 1
     Content-Type: Text/HTML; charset=US-ASCII

     ... text of the HTML document, which might contain a hyperlink
     to the other body part, for example through a statement such as:
     [email protected]" ALT="IETF logo">

     --boundary-example-1
     Content-ID: foo4*
     Content-Type: IMAGE/GIF
     Content-Transfer-Encoding: BASE64








Levinson                    Standards Track