cid: and mid: URLs: Content-ID and Message-ID Uniform Resource Locators
Two web address schemes for pointing at an email message or one of its parts.
Levinson, E., "Content-ID and Message-ID Uniform Resource Locators", RFC 2392, DOI 10.17487/RFC2392, August 1998, <https://www.rfc-editor.org/info/rfc2392>.
@misc{rfc2392,
series = {Request for Comments},
number = 2392,
howpublished = {RFC 2392},
publisher = {RFC Editor},
doi = {10.17487/RFC2392},
url = {https://www.rfc-editor.org/info/rfc2392},
author = {E. Levinson},
title = {{Content-ID and Message-ID Uniform Resource Locators}},
pagetotal = 6,
year = 1998,
month = aug,
} RFC text
Contents
Network Working Group E. Levinson
Request for Comments: 2392 August 1998
Obsoletes: 2111
Category: Standards Track
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.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
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.
Changes from (RFC 2111)
Clarified the example on page 3 on of converting cid URLs to
Content-IDs. The example now uses a cid URL instead of an mid.
Corrected the example messages to have the correct Content-ID form;
they now use the angle brackets. Added a Message-ID header to the
second example.
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
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.
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. In those cases, conforming implementations are
required to use the rules of the containing MIME entity (e.g.,
multipart/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 the % encoded
character to their equivalent US-ASCII characters, and enclosing the
remaining parts with an angle bracket pair, "<" and ">". For
example, "cid:foo4%25foo1@bar.net" corresponds to
Content-ID: <foo4%25foo1@bar.net>
Reversing the process and converting URL special characters to their
% encodings produces the original cid.
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
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
Message-ID: <970701.32784@VIers.none.com>
Content-type: text/html; charset=usascii
<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, STD 11, RFC 822, August 1982.
[MIME] Borenstein, N., and N. Freed, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[URL] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
Resource Locators (URL)", RFC 1738, December 1994.
[MULREL] Levinson, E., "The MIME Multipart/Related Content-type",
RFC 2387, August 1998.
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
Phone: +1 908 549 3716
EMail: XIson@cnj.digex.net
7. Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
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ISO Plain Language
On this page
§1Why These Two Schemes Exist
Using MIME (Multipurpose Internet Mail Extensions) inside email to carry a web page and the images that go with it needs a way for the HTML to refer to those images. That is what the Content-ID URL, the cid: scheme, is for.
News readers have the same kind of problem in reverse: they use Message-IDs to link related messages together. The Message-ID URL, the mid: scheme, gives a way to refer to such a message as a resource.
Both schemes provide identifiers. The mid scheme uses part or all of the message-id of an email message to name one message. The cid scheme names one body part of a message, and its use is generally limited to references to other body parts in the same message. The mid scheme can also name a specific body part within a named message, by including the content-ID's address.
Email can carry a web page and the images it shows. The cid: scheme lets the page's HTML point at those images. The mid: scheme lets a news reader point at a message by its identifier.
§1Body Part and MIME Entity
The document uses two terms interchangeably. "Body part" and "MIME entity" both mean the headers and body of a MIME message, whether that message is the whole message or one of the body parts contained in a Multipart message. The note is there so that a reader does not read "body part" as the content alone and leave the headers out.
Two words mean the same thing here. "Body part" and "MIME entity" both mean the headers and content of a message, or of one piece of a message that holds several pieces.
§2The Syntax of cid and mid URLs
RFC 1738 reserves the "mid" and "cid" schemes for Message-ID and Content-ID, and this document defines the syntax of both. Because they use the same syntactic elements, they are presented together. Each carries an addr-spec, the address form of RFC 822, with URL encoding applied to it. A cid URL is the scheme name, a colon and a Content-ID. A mid URL is the scheme name, a colon, a Message-ID and, optionally, a slash and a Content-ID.
In Internet mail messages, the address in a Content-ID or Message-ID header is enclosed in angle brackets. Because that address may contain characters that are not allowed within a URL, any such character, including the "/" that is reserved within the "mid" scheme, must be hex-encoded with the %hh escape mechanism described in RFC 1738.
Both schemes wrap an address in the same way. A cid: address holds a Content-ID, and a mid: address holds a Message-ID and, after a slash, an optional Content-ID. Characters a web address cannot carry are written as %hh codes.
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 ]| Scheme | Carries | Names |
|---|---|---|
| cid | A Content-ID address | One body part, usually in the same message |
| mid | A Message-ID address, optionally followed by / and a Content-ID | A whole message, or one part of it |
§2Turning a cid URL into a Content-ID Header
A mid URL that carries only a Message-ID names an entire message. With the Content-ID appended, it names 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 of their context, the message they belong to. The long form of the mid URL was designed to supply that context, so that such systems can interoperate.
To turn a cid URL into the matching Content-ID header, remove the cid: prefix, convert each %-encoded character to the equivalent US-ASCII character, and enclose what remains in a pair of angle brackets. For example, cid:foo4%25foo1@bar.net corresponds to Content-ID: <foo4%25foo1@bar.net>. Reversing the process, converting URL special characters to their % encodings, produces the original cid URL.
An implementation that conforms to this specification is required to support the long form of the mid URL, the message-id and content-id pair. It may choose to, but need not, use the uniqueness of a Content-ID to interpret a cid URL as naming any body part within the message store. In limited circumstances, for example within a multipart/alternate entity, a single message can contain several body parts that have the same Content-ID. That happens when identical data can be reached by different methods, and a conforming implementation must then use the rules of the containing MIME entity to select the body part meant.
To find the part a cid: address names, drop the prefix, turn each %hh code back into its character, and put the rest in angle brackets. That gives the exact Content-ID header to look for.
cid:foo4%25foo1@bar.net
Content-ID: <foo4%25foo1@bar.net>§2Two Worked Examples
The first example is a message whose body parts sit inside a Multipart/Related entity. The first part is HTML, and its IMG tag points at the image in the second part with a cid URL, cid:foo4*foo1@bar.net. The second part carries the matching Content-ID header, <foo4*foo1@bar.net>, and holds an image in base64 encoding.
The second example is a plain HTML message that points at another message with a mid URL in the long form, mid:960830.1639@XIson.com/partA.960830.1639@XIson.com. The document notes that it points to another message, hopefully still in the recipient's message store.
The first example is a message that carries an HTML page and an image. The page points at the image with a cid: address. The second example points from one message to another message in the reader's store.
§3Security Considerations
The URLs defined here provide an addressing or referencing mechanism. Their values disclose no more about the originator's environment than the corresponding Message-ID and Content-ID values do. 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.
These schemes only point at things. They reveal no more about the sender than the Message-ID and Content-ID headers already do, so the same care about those headers is enough.
§5Where the Idea Came From
The original concept of "mid" and "cid" URLs was part of Tim Berners-Lee's original vision of the World Wide Web. The ideas and the design benefited greatly from discussions with Harald Alvestrand, Dan Connolly, Roy Fielding, Larry Masinter, Jacob Palme and others in the MHTML working group.
This document obsoletes RFC 2111. It clarifies the example on page 3 that converts a cid URL to a Content-ID, so that the example now uses a cid URL instead of a mid URL. It also corrects the example messages so that their Content-IDs use angle brackets, and adds a Message-ID header to the second example.
The idea came from Tim Berners-Lee's original plan for the web. People in the MHTML working group, including Harald Alvestrand, Dan Connolly, Roy Fielding, Larry Masinter and Jacob Palme, worked out the details.