RFC 2111 (rfc2111) - Page 2 of 5

Content-ID and Message-ID Uniform Resource Locators

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RFC 2111                    CID and MID URLs                  March 1997


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 ]

      Note: in Internet mail messages, the addr-spec in a Content-ID
      [MIME] or Message-ID [822] header are 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.

   A implementation conforming to this specification is required to
   support the "mid" URL long form (message-id/content-id).  Conforming
   implementations can choose to, but are not required to, take
   advantage of the content-id's uniqueness and interpret a "cid" URL to
   refer to any body part within the message store.

   In limited circumstances (e.g., within multipart/alternate), a single
   message may contain several body parts that have the same Content-ID.
   That occurs, for example, when identical data can be accessed through
   different methods [MIME, sect. 7.2.3].  In those cases, conforming
   implementations are required to use the rules of the containing MIME
   entity (e.g., multi-part/alternate) to select the body part to which
   the Content-ID refers.




Levinson                    Standards Track
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