cid and mid URLs: Content-ID and Message-ID Uniform Resource Locators
How to reference parts of email messages using cid: and mid: web addresses.
Levinson, E., "Content-ID and Message-ID Uniform Resource Locators", RFC 2111, DOI 10.17487/RFC2111, March 1997, <https://www.rfc-editor.org/info/rfc2111>.
@misc{rfc2111,
series = {Request for Comments},
number = 2111,
howpublished = {RFC 2111},
publisher = {RFC Editor},
doi = {10.17487/RFC2111},
url = {https://www.rfc-editor.org/info/rfc2111},
author = {E. Levinson},
title = {{Content-ID and Message-ID Uniform Resource Locators}},
pagetotal = 5,
year = 1997,
month = mar,
} RFC text
Contents
Network Working Group E. Levinson
Request for Comments: 2111 XIson, Inc.
Category: Standards Track March 1997
Content-ID and Message-ID Uniform Resource Locators
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
The Uniform Resource Locator (URL) schemes, "cid:" and "mid:" allow
references to messages and the body parts of messages. For example,
within a single multipart message, one HTML body part might include
embedded references to other parts of the same message.
1. Introduction
The use of [MIME] within email to convey Web pages and their
associated images requires a URL scheme to permit the HTML to refer
to the images or other data included in the message. The Content-ID
Uniform Resource Locator, "cid:", serves that purpose.
Similarly Net News readers use Message-IDs to link related messages
together. The Message-ID URL provides a scheme, "mid:", to refer to
such messages as a "resource".
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
RFC1738 [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.
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, "<" and ">". For
example, "mid:foo4%25foo1@bar.net" corresponds to
Message-ID: <foo4%foo1@bar.net>
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.,
950124.162336@XIson.com.
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: foo1@bar.net
To: foo2@bar.net
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:
<IMG SRC="cid:foo4*foo1@bar.net" ALT="IETF logo">
--boundary-example-1
Content-ID: foo4*foo1@bar.net
Content-Type: IMAGE/GIF
Content-Transfer-Encoding: BASE64
R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk5
NSBJRVRGLiBVbmF1dGhvcml6ZWQgZHVwbGljYXRpb24gcHJvaGliaXRlZC4A
etc...
--boundary-example-1--
The following message points to another message (hopefully still in
the recipient's message store).
From: bar@none.com
To: phooey@all.com
Subject: Here's how to do it
Content-type: text/html; charset=usascii
... The items in my
<A HREF= "mid:960830.1639@XIson.com/partA.960830.1639@XIson.com">
previous message</A>, shows how the approach you propose can be
used to accomplish ...
3. Security Considerations
The URLs defined here provide an addressing or referencing mechanism.
The values of these URLs disclose no more about the originators
environment than the corresponding Message-ID and Content-ID values.
Where concern exists about such disclosures the originator of a
message using mid and cid URLs must take precautions to insure that
confidential information is not disclosed. Those precautions should
already be in place to handle existing mail use of the Message-ID and
Content-ID.
4. References
[822] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages," August 1982, University of Delaware, STD 11, RFC
822.
[MIME] N. Borenstein, N. Freed, "MIME (Multipurpose Internet Mail
Extensions) Part One: Mechanisms for Specifying and
Describing the Format of Internet Message Bodies,"
September 1993, RFC 1521.
[URL] Berners-Lee, T., Masinter, L., and McCahill, M., "Uniform
Resource Locators (URL)," December 1994.
[MULREL] E. Levinson, "The MIME Multipart/Related Content-type,"
December 1995, RFC 1874.
5. Acknowledgments
The original concept of "mid" and "cid" URLs were part of the Tim
Berners-Lee's original vision of the World Wide Web. The ideas and
design have benefited greatly by discussions with Harald Alvestrand,
Dan Connolly, Roy Fielding, Larry Masinter, Jacob Palme, and others
in the MHTML working group.
6. Author's Address
Edward Levinson
47 Clive Street
Metuchen, NJ 08840-1060
USA
+1 908 549 3716
<XIson@cnj.digex.net>
AI ELI5
On this page
§1Introduction
The cid: (Content-ID) and mid: (Message-ID) URL schemes allow references to messages and body parts of messages. The primary use is within a single multipart message, where one HTML body part might include embedded references to other parts of the same message.
For example, an HTML part can refer to an image contained elsewhere in the same message using a cid: URL, avoiding the need to attach the image separately or reference it by an external URL.
Email messages can have several parts, like a letter with pictures. These URL types let one part point to another part in the same message.
§2The cid: and mid: URL Schemes
The cid: URL scheme takes the form cid:<content-id>, where the content-id is URL-encoded. It refers to a specific body part (MIME entity) by its Content-ID header value. The use of cid: URLs is generally limited to references to other body parts in the same message as the referring body part.
The mid: URL scheme takes the form mid:<message-id> or mid:<message-id>/<content-id>. With only a message-id, it refers to an entire message. With the appended content-id, it refers to a specific body part within that message. This long form was designed to support interoperability with message storage systems that do not index body parts independently of their context.
Both message-id and content-id must be globally unique. Characters not allowed in URLs (such as "/" in a message-id) must be hex-encoded using the %hh escape mechanism.
A cid: address says "the picture inside this message with this label." A mid: address says "the message with this ID" or "the part with this label inside that message."
§3Practical Examples
A common example is an HTML email that includes images. The HTML might contain <IMG SRC="cid:image-part@example.com"> to reference an image body part with Content-ID image-part@example.com. The email client retrieves the image from the message itself rather than from the network.
Another example is a message that points to a previous message using a mid: URL that includes both the message-id and a content-id, allowing a thread to build references across multiple emails. This is useful when a reply needs to refer to a specific part within an earlier message.
An email with a picture can say "show the picture labeled image-part@example.com from inside this message." A reply can point to a specific part from an earlier message.