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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Unused Reference: 'MULREL' is defined on line 220, but no explicit reference was found in the text ** Obsolete normative reference: RFC 822 (Obsoleted by RFC 2822) ** Obsolete normative reference: RFC 1521 (ref. 'MIME') (Obsoleted by RFC 2045, RFC 2046, RFC 2047, RFC 2048, RFC 2049) -- Possible downref: Non-RFC (?) normative reference: ref. 'URL' ** Downref: Normative reference to an Experimental RFC: RFC 1874 (ref. 'MULREL') Summary: 12 errors (**), 0 flaws (~~), 2 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 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 IETF logo 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