RFC 2392 (rfc2392) - Page 2 of 6
Content-ID and Message-ID Uniform Resource Locators
Alternative Format: Original Text Document
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