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Snell 3 Internet-Draft May 17, 2010 4 Updates: 4287 (if approved) 5 Intended status: Informational 6 Expires: November 18, 2010 8 Atom Link Extensions 9 draft-snell-atompub-link-extensions-05.txt 11 Abstract 13 This specification adds additional attributes to the Atom Syndication 14 Format link and content elements that may be used to express 15 additional metadata about linked resources. 17 Status of this Memo 19 This Internet-Draft is submitted to IETF in full conformance with the 20 provisions of BCP 78 and BCP 79. 22 Internet-Drafts are working documents of the Internet Engineering 23 Task Force (IETF). Note that other groups may also distribute 24 working documents as Internet-Drafts. The list of current Internet- 25 Drafts is at http://datatracker.ietf.org/drafts/current/. 27 Internet-Drafts are draft documents valid for a maximum of six months 28 and may be updated, replaced, or obsoleted by other documents at any 29 time. It is inappropriate to use Internet-Drafts as reference 30 material or to cite them other than as "work in progress." 32 This Internet-Draft will expire on November 18, 2010. 34 Copyright Notice 36 Copyright (c) 2010 IETF Trust and the persons identified as the 37 document authors. All rights reserved. 39 This document is subject to BCP 78 and the IETF Trust's Legal 40 Provisions Relating to IETF Documents 41 (http://trustee.ietf.org/license-info) in effect on the date of 42 publication of this document. Please review these documents 43 carefully, as they describe your rights and restrictions with respect 44 to this document. Code Components extracted from this document must 45 include Simplified BSD License text as described in Section 4.e of 46 the Trust Legal Provisions and are provided without warranty as 47 described in the Simplified BSD License. 49 Table of Contents 51 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 52 2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3 53 3. Hash Attributes . . . . . . . . . . . . . . . . . . . . . . . . 3 54 3.1. Computing Hash Digests . . . . . . . . . . . . . . . . . . 3 55 3.2. The 'hash' attributes . . . . . . . . . . . . . . . . . . . 3 56 4. The 'etag' attribute . . . . . . . . . . . . . . . . . . . . . 4 57 5. The 'modified' attribute . . . . . . . . . . . . . . . . . . . 5 58 6. The 'media' attribute . . . . . . . . . . . . . . . . . . . . . 5 59 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 60 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 61 9. Normative References . . . . . . . . . . . . . . . . . . . . . 6 62 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6 64 1. Introduction 66 This specification adds additional attribute to the Atom Syndication 67 Format [RFC4287] link and content elements that may be used to 68 express additional metadata about linked resources. 70 2. Notational Conventions 72 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 73 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 74 document are to be interpreted as described in BCP 14, [RFC2119] 76 This specification uses XML Namespaces [W3C.REC-xml-names-19990114] 77 to uniquely identify XML element names. It uses the following 78 namespace prefix for the indicated namespace URI; 80 "atom": "http://www.w3.org/2005/Atom" 82 3. Hash Attributes 84 3.1. Computing Hash Digests 86 When the resource referenced by atom:link or atom:content elements is 87 retrievable using HTTP, hash digest values are computed by first 88 performing an HTTP GET request on the URL specified by the @href or 89 @src attributes, extracting the returned entity-body, then following 90 the steps specified in Section 14.15 of [RFC2616]. 92 It should be noted, however, that there are a variety of factors that 93 influence whether the entity-body returned by the HTTP GET will yield 94 a hash digest value matching that specified by a hash attribute 95 contained by the atom:link or atom:content elements. Accordingly, 96 hash attribute values MUST be considered to be strictly advisory and 97 cannot be used reliably as an end-to-end integrity check. 99 3.2. The 'hash' attributes 101 The 'hash' Attribute specifies a whitespace-delimited list of hash 102 digest values calculated over the resource identified by the atom: 103 link/@href or atom:content/@src attributes. Each digest value is 104 represented as a token identifying the hash algorithm and the hex- 105 encoded digest separated by an ASCII colon (":"). The 'hash' 106 attribute MAY appear as a child of the atom:link and atom:content 107 elements. 109 hash = attribute hash { digest-list } 110 digest-value = token ":" 1*HEXDIG 111 digest-list = digest-value *( LWSP digest-value ) 113 An example MD5 and SHA-256 digests of an enclosed MP3 file: 115 120 This specification defines the following hash algorithm tokens: 121 "md2", "md5", "sha-1", "sha-224", "sha-256", "sha-384", and "sha- 122 512". Additional hash algorithms MAY be used. 124 4. The 'etag' attribute 126 The 'etag' Attribute specifies an Entity Tag [RFC2616] for the 127 resource identified by the atom:link or atom:content element. The 128 'etag' attribute MAY appear as a child of the atom:link and atom: 129 content elements. 131 etag = attribute le:etag { entity-tag } 133 entity-tag = [ weak ] opaque-tag 134 weak = "W/" 135 opaque-tag = quoted-string 137 An example weak entity tag for an enclosed MP3 file: 139 143 Note that HTTP defines the Entity Tag production such that quotes are 144 significant. For example, the values "W/xyzzy" and W/"xyzzy" 145 represent two distinctly different Entity Tags, the former being 146 considered a "strong" entity tag, the latter a "weak" entity tag. 147 The etag attribute value MUST include the appropriate double 148 quotation marks. 150 The presence and placement of the quotes in the entity tag value can 151 introduce some difficulty when inserting the value into the etag 152 attribute. Producers of Atom documents must either use single quotes 153 when specifying the value of the etag attribute, e.g. 154 etag='W/"xyzzy"' or use the " entity reference to escape the 155 double quotes within the etag value, e.g. etag="W/"xyzzy"". 157 A strong entity tag would be encoded as either etag='"xyzzy"' or 158 etag=""xyzzy"". 160 5. The 'modified' attribute 162 The 'modified' Attribute specifies the date and time when the 163 resource identified by the atom:link or atom:content element was last 164 modified. The value MUST conform to the "date-time" production 165 defined by [RFC3339]. An uppercase "T" character MUST be used to 166 separate date and time, and an uppercase "Z" character MUST be 167 present in the absence of a numeric time zone offset. The 'modified' 168 attribute MAY appear as a child of the atom:link and atom:content 169 elements. 171 modified = attribute modified { xsd:dateTime } 173 An example last-modified attribute for an enclosed MP3 file: 175 179 6. The 'media' attribute 181 The 'media' attribute identifies the optimum media for the resource 182 identified by the atom:link/@href attribute. The value MUST be a 183 valid media query as specified by the Media Queries Specification 184 [TODO: Link]. The media attribute MUST be considered to be purely 185 advisory and identifies for which type of media the resource in 186 question was designed. The 'media' attribute MAY appear as a child 187 of the atom:link element. 189 media = attribute media { media-query } 191 An example media attribute on an atom:link element: 193 197 7. Security Considerations 199 TBD 201 8. IANA Considerations 203 No IANA actions are required by this document. 205 9. Normative References 207 [RFC1864] Myers, J. and M. Rose, "The Content-MD5 Header Field", 208 RFC 1864, October 1995. 210 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 211 Requirement Levels", BCP 14, RFC 2119, March 1997. 213 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., 214 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext 215 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 217 [RFC3339] Klyne, G., Ed. and C. Newman, "Date and Time on the 218 Internet: Timestamps", RFC 3339, July 2002. 220 [RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom 221 Syndication Format", RFC 4287, December 2005. 223 [W3C.REC-xml-names-19990114] 224 Hollander, D., Layman, A., and T. Bray, "Namespaces in 225 XML", World Wide Web Consortium FirstEdition REC-xml- 226 names-19990114, January 1999, 227 . 229 Author's Address 231 James M Snell 233 Phone: 234 Email: jasnell@us.ibm.com 235 URI: http://ibm.com