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1 MIMESGML Working Group E. Levinson
2 Internet Draft: CID and MID URLs 5 January 1997
3
5 Content-ID and Message-ID Uniform Resource Locators
7 This draft document is being circulated for comment. Please send your
8 comments to the authors or to the mhtml mail list
9 . If consensus is reached, this Access Type may
10 be registered with IANA and this document may be submitted to the RFC
11 editor as an Informational protocol specification.
13 Status of this Memo
15 This document is an Internet Draft; Internet Drafts are working
16 documents of the Internet Engineering Task Force (IETF) its Areas, and
17 Working Groups. Note that other groups may also distribute working
18 documents as Internet Drafts.
20 Internet Drafts are draft documents valid for a maximum of six months.
21 They may be updated, replaced, or obsoleted by other documents at any
22 time. It is not appropriate to use Internet Drafts as reference
23 material or to cite them other than as a "working draft" or "work in
24 progress".
26 Please check the abstract listing in each Internet Draft directory for
27 the current status of this or any other Internet Draft.
29 Abstract
31 The Uniform Resource Locator (URL) schemes, "cid:" and "mid:" allow
32 references to messages and the body parts of messages. For example,
33 within a single multipart message, one HTML body part might include
34 embedded references to other parts of the same message.
36 Changes from previous draft (02)
38 Completed the paragraph "In limited circumstances ... body parts that
39 have the same Content-ID. ... will". This paragraph ended with an
40 incomplete sentence. The missing phrase was a **requirement** for the
41 method of choosing one of the body parts from a multipart/alternative
42 MIME entity - use the rules for multipart/alternative. Similar language
43 was in draft 01.
45 MUST and MAY replaced by "is required to" and "can choose to, but are
46 not required to," respectively.
48 Deleted the paragraph "A msgmid (cidurl) ... can be converted to a
49 Internet Draft Message- & Content-ID URLs
51 message-id, content-id pair." The deleted paragraph duplicated the
52 preceding one, "A 'cid' URL is converted ... in a similar fashion." and
53 used terminology that had been superseded.
55 1. Introduction
57 The use of [MIME] within email to convey Web pages and their associated
58 images requires a URL scheme to permit the HTML to refer to the images
59 or other data included in the message. The Content-ID Uniform Resource
60 Locator, "cid:", serves that purpose.
62 Similarly Net News readers use Message-IDs to link related messages
63 together. The Message-ID URL provides a scheme, "mid:", to refer to
64 such messages as a "resource".
66 The "mid" (Message-ID) and "cid" (Content-ID) URL schemes provide
67 identifiers for messages and their body parts. The "mid" scheme uses (a
68 part of) the message-id of an email message to refer to a specific
69 message. The "cid" scheme refers to a specific body part of a message;
70 its use is generally limited to references to other body parts in the
71 same message as the referring body part. The "mid" scheme may also
72 refer to a specific body part within a designated message, by including
73 the content-ID's address.
75 A note on terminology. The terms "body part" and "MIME entity" are used
76 interchangeably. They refer to the headers and body of a MIME message,
77 either the message itself or one of the body parts contained in a
78 Multipart message.
80 2. The MID and CID URL Schemes
82 RFC1738 [URL] reserves the "mid" and "cid" schemes for Message-ID and
83 Content-ID respectively. This memorandum defines the syntax for those
84 URLs. Because they use the same syntactic elements they are presented
85 together.
87 The URLs take the form
89 content-id = url-addr-spec
91 message-id = url-addr-spec
93 url-addr-spec = addr-spec ; URL encoding of RFC 822 addr-spec
95 cid-url = "cid" ":" content-id
97 mid-url = "mid" ":" message-id [ "/" content-id ]
99 Internet Draft Message- & Content-ID URLs
101 Note: in Internet mail messages, the addr-spec in a Content-ID
102 [MIME] or Message-ID [822] header are enclosed in angle brackets
103 (<>). Since addr-spec in a Message-ID or Content-ID might contain
104 characters not allowed within a URL; any such character (including
105 "/", which is reserved within the "mid" scheme) must be hex-encoded
106 using the %hh escape mechanism in [URL].
108 A "mid" URL with only a "message-id" refers to an entire message. With
109 the appended "content-id", it refers to a body part within a message, as
110 does a "cid" URL. The Content-ID of a MIME body part is required to be
111 globally unique. However, in many systems that store messages, body
112 parts are not indexed independently their context (message). The "mid"
113 URL long form was designed to supply the context needed to support
114 interoperability with such systems.
116 A implementation conforming to this specification is required to support
117 the "mid" URL long form (message-id/content-id). Conforming implementa-
118 tions can choose to, but are not required to, take advantage of the con-
119 tent-id's uniqueness and interpret a "cid" URL to refer to any body part
120 within the message store.
122 In limited circumstances (e.g., within multipart/alternate), a single
123 message may contain several body parts that have the same Content-ID.
124 For example when identical data can be accessed through different meth-
125 ods [MIME, sect. 7.2.3]. In those cases, conforming implementations are
126 required to use the rules of the containing MIME entity (e.g., multi-
127 part/alternate) to select the body part to which the Content-ID refers.
129 A "cid" URL is converted to the corresponding Content-ID message header
130 [MIME] by removing the "cid:" prefix, converting %hh hex-escaped charac-
131 ters to their ASCII equivalents and enclosing the remaining parts with
132 an angle bracket pair, "<" and ">". For example,
133 "mid:foo4%25foo1@bar.net" corresponds to
135 Message-ID:
137 A "mid" URL is converted to a Message-ID or Message-ID/Content-ID pair
138 in a similar fashion.
140 Both message-id and content-id are required to be globally unique. That
141 is, no two different messages will ever have the same Message-ID addr-
142 spec; no different body parts will ever have the same Content-ID addr-
143 spec. A common technique used by many message systems is to use a time
144 and date stamp along with the local host's domain name, e.g.,
145 950124.162336@XIson.com.
147 Internet Draft Message- & Content-ID URLs
149 Some Examples
151 The following message contains an HTML body part that refers to an image
152 contained in another body part. Both body parts are contained in a Mul-
153 tipart/Related MIME entity. The HTML IMG tag contains a cidurl which
154 points to the image.
156 From: foo1@bar.net
157 To: foo2@bar.net
158 Subject: A simple example
159 Mime-Version: 1.0
160 Content-Type: multipart/related; boundary="boundary-example-1";
161 type=Text/HTML
163 --boundary-example 1
164 Content-Type: Text/HTML; charset=US-ASCII
166 ... text of the HTML document, which might contain a hyperlink
167 to the other body part, for example through a statement such as:
168
170 --boundary-example-1
171 Content-ID: foo4*foo1@bar.net
172 Content-Type: IMAGE/GIF
173 Content-Transfer-Encoding: BASE64
175 R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk5
176 NSBJRVRGLiBVbmF1dGhvcml6ZWQgZHVwbGljYXRpb24gcHJvaGliaXRlZC4A
177 etc...
179 --boundary-example-1--
181 The following message points to another message (hopefully still in the
182 recipient's message store).
184 From: bar@none.com
185 To: phooey@all.com
186 Subject: Here's how to do it
187 Content-type: text/html; charset=usascii
189 ... The items in my
190
191 previous message, shows how the approach you propose can be
192 used to accomplish ...
194 Internet Draft Message- & Content-ID URLs
196 3. Security Considerations
198 The URLs defined here provide an addressing or referencing mechanism.
199 The values of these URLs disclose no more about the originators environ-
200 ment than the corresponding Message-ID and Content-ID values. Where
201 concern exists about such disclosures the originator of a message using
202 mid and cid URLs must take precautions to insure that confidential
203 information is not disclosed. Those precautions should already be in
204 place to handle existing mail use of the Message-ID and Content-ID.
206 4. References
208 [822] Crocker, D., "Standard for the Format of ARPA Internet Text
209 Messages," August 1982, University of Delaware, STD 11, RFC
210 822.
212 [MIME] N. Borenstein, N. Freed, "MIME (Multipurpose Internet Mail
213 Extensions) Part One: Mechanisms for Specifying and Describ-
214 ing the Format of Internet Message Bodies," September 1993,
215 RFC 1521.
217 [URL] Berners-Lee, T., Masinter, L., and McCahill, M., "Uniform
218 Resource Locators (URL)," December 1994.
220 [MULREL] E. Levinson, "The MIME Multipart/Related Content-type," Decem-
221 ber 1995, RFC 1874.
223 5. Acknowledgments
225 The original concept of "mid" and "cid" URLs were part of the Tim Bern-
226 ers-Lee's original vision of the World Wide Web. The ideas and design
227 have benefited greatly by discussions with Harald Alvestrand, Dan Con-
228 nolly, Roy Fielding, Larry Masinter, Jacob Palme, and others in the
229 MHTML working group.
231 6. Author's Address
233 Edward Levinson
234 47 Clive Street
235 Metuchen, NJ 08840-1060
236 USA
237 +1 908 549 3716
238