RFC 2392 (rfc2392) - Page 2 of 6

Content-ID and Message-ID Uniform Resource Locators

RFC 2392               Message- & Content-ID URLs            August 1998


   The "mid" (Message-ID) and "cid" (Content-ID) URL schemes provide
   identifiers for messages and their body parts.  The "mid" scheme uses
   (a part of) the message-id of an email message to refer to a specific
   message.  The "cid" scheme refers to a specific body part of a
   message; its use is generally limited to references to other body
   parts in the same message as the referring body part.  The "mid"
   scheme may also refer to a specific body part within a designated
   message, by including the content-ID's address.

   A note on terminology.  The terms "body part" and "MIME entity" are
   used interchangeably.  They refer to the headers and body of a MIME
   message, either the message itself or one of the body parts contained
   in a Multipart message.

2. The MID and CID URL Schemes

   RFC 1738 [URL] reserves the "mid" and "cid" schemes for Message-ID
   and Content-ID respectively.  This memorandum defines the syntax for
   those URLs.  Because they use the same syntactic elements they are
   presented together.


   The URLs take the form

     content-id    = url-addr-spec

     message-id    = url-addr-spec

     url-addr-spec = addr-spec  ; URL encoding of RFC 822 addr-spec

     cid-url       = "cid" ":" content-id

     mid-url       = "mid" ":" message-id [ "/" content-id ]

     Notes: In Internet mail messages, the addr-spec in a Content-ID
     [MIME] or Message-ID [822] header is enclosed in angle brackets
     ().  Since addr-spec in a Message-ID or Content-ID might contain
     characters not allowed within a URL; any such character (including
     "/", which is reserved within the "mid" scheme) must be hex-encoded
     using the %hh escape mechanism in [URL].

   A "mid" URL with only a "message-id" refers to an entire message.
   With the appended "content-id", it refers to a body part within a
   message, as does a "cid" URL.  The Content-ID of a MIME body part is
   required to be globally unique.  However, in many systems that store
   messages, body parts are not indexed independently their context
   (message).  The "mid" URL long form was designed to supply the
   context needed to support interoperability with such systems.



Levinson                    Standards Track