Network Working Group J. Snell Internet-Draft June 8, 2010 Updates: 4287 (if approved) Intended status: Informational Expires: December 10, 2010 Atom Link Extensions draft-snell-atompub-link-extensions-06.txt Abstract This specification adds additional attributes to the Atom Syndication Format link and content elements that may be used to express additional metadata about linked resources. Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on December 10, 2010. Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Snell Expires December 10, 2010 [Page 1]
Internet-Draft Atom Link Extensions June 2010 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3 3. Hash Attributes . . . . . . . . . . . . . . . . . . . . . . . . 3 3.1. Computing Hash Digests . . . . . . . . . . . . . . . . . . 3 3.2. The 'hash' attributes . . . . . . . . . . . . . . . . . . . 3 4. The 'etag' attribute . . . . . . . . . . . . . . . . . . . . . 4 5. The 'modified' attribute . . . . . . . . . . . . . . . . . . . 5 6. The 'accessed' attribute . . . . . . . . . . . . . . . . . . . 5 7. The 'media' attribute . . . . . . . . . . . . . . . . . . . . . 5 8. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7 10. Normative References . . . . . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7 Snell Expires December 10, 2010 [Page 2]
Internet-Draft Atom Link Extensions June 2010 1. Introduction This specification adds additional attribute to the Atom Syndication Format [RFC4287] link and content elements that may be used to express additional metadata about linked resources. 2. Notational Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14, [RFC2119] This specification uses XML Namespaces [W3C.REC-xml-names-19990114] to uniquely identify XML element names. It uses the following namespace prefix for the indicated namespace URI; "atom": "http://www.w3.org/2005/Atom" 3. Hash Attributes 3.1. Computing Hash Digests When the resource referenced by atom:link or atom:content elements is retrievable using HTTP, hash digest values are computed by first performing an HTTP GET request on the URL specified by the @href or @src attributes, extracting the returned entity-body, then following the steps specified in Section 14.15 of [RFC2616]. It should be noted, however, that there are a variety of factors that influence whether the entity-body returned by the HTTP GET will yield a hash digest value matching that specified by a hash attribute contained by the atom:link or atom:content elements. Accordingly, hash attribute values MUST be considered to be strictly advisory and cannot be used reliably as an end-to-end integrity check. 3.2. The 'hash' attributes The 'hash' Attribute specifies a whitespace-delimited list of hash digest values calculated over the resource identified by the atom: link/@href or atom:content/@src attributes. Each digest value is represented as a token identifying the hash algorithm and the hex- encoded digest separated by an ASCII colon (":"). The 'hash' attribute MAY appear as a child of the atom:link and atom:content elements. Snell Expires December 10, 2010 [Page 3]
Internet-Draft Atom Link Extensions June 2010 hash = attribute hash { digest-list } digest-value = token ":" 1*HEXDIG digest-list = digest-value *( LWSP digest-value ) An example MD5 and SHA-256 digests of an enclosed MP3 file: <atom:link rel="enclosure" href="http://example.org/media/myfile.mp3" hash="md5:9e107d9d372bb6826bd81d3542a419d6 sha-256:6bf05cbbde96d6...d5ffe91e29272d805c98b988dc" /> This specification defines the following hash algorithm tokens: "md2", "md5", "sha-1", "sha-224", "sha-256", "sha-384", and "sha- 512". Additional hash algorithms MAY be used. 4. The 'etag' attribute The 'etag' Attribute specifies an Entity Tag [RFC2616] for the resource identified by the atom:link or atom:content element as provided by the server hosting the resource. The 'etag' attribute MAY appear as a child of the atom:link and atom:content elements. etag = attribute le:etag { entity-tag } entity-tag = [ weak ] opaque-tag weak = "W/" opaque-tag = quoted-string An example weak entity tag for an enclosed MP3 file: <atom:link rel="enclosure" href="http://example.org/media/myfile.mp3" etag='W/"xyzzy"' /> Note that HTTP defines the Entity Tag production such that quotes are significant. For example, the values "W/xyzzy" and W/"xyzzy" represent two distinctly different Entity Tags, the former being considered a "strong" entity tag, the latter a "weak" entity tag. The etag attribute value MUST include the appropriate double quotation marks. The presence and placement of the quotes in the entity tag value can introduce some difficulty when inserting the value into the etag attribute. Producers of Atom documents must either use single quotes when specifying the value of the etag attribute, e.g. etag='W/"xyzzy"' or use the " entity reference to escape the double quotes within the etag value, e.g. etag="W/"xyzzy"". Snell Expires December 10, 2010 [Page 4]
Internet-Draft Atom Link Extensions June 2010 A strong entity tag would be encoded as either etag='"xyzzy"' or etag=""xyzzy"". 5. The 'modified' attribute The 'modified' Attribute specifies the date and time when the resource identified by the atom:link or atom:content element was last modified. The value MUST conform to the "date-time" production defined by [RFC3339]. An uppercase "T" character MUST be used to separate date and time, and an uppercase "Z" character MUST be present in the absence of a numeric time zone offset. The 'modified' attribute MAY appear as a child of the atom:link and atom:content elements. modified = attribute modified { xsd:dateTime } An example last-modified attribute for an enclosed MP3 file: <atom:link rel="enclosure" href="http://example.org/media/myfile.mp3" modified="2010-12-12T12:12:12Z" /> 6. The 'accessed' attribute The 'accessed' Attribute specifies the most recent date and time when the resource identified by the atom:link or atom:content element was accessed. The value MUST conform to the "date-time" production defined by [RFC3339]. An uppercase "T" character MUST be used to separate date and time, and an uppercase "Z" character MUST be present in the absence of a numeric time zone offset. The 'accessed' attribute MAY appear as a child of the atom:link and atom:content elements. accessed = attribute accessed { xsd:dateTime } An example accessed attribute for an enclosed MP3 file: <atom:link rel="enclosure" href="http://example.org/media/myfile.mp3" accessed="2010-12-12T12:12:12Z" /> 7. The 'media' attribute The 'media' attribute identifies the optimum media for the resource identified by the atom:link/@href attribute. The value MUST be a Snell Expires December 10, 2010 [Page 5]
Internet-Draft Atom Link Extensions June 2010 valid media query as specified by the Media Queries Specification [W3C.CR-css3-mediaqueries-20090915]. The media attribute MUST be considered to be purely advisory and identifies for which type of media the resource in question was designed. The 'media' attribute MAY appear as a child of the atom:link element. media = attribute media { media-query } An example media attribute on an atom:link element: <atom:link rel="alternate" href="http://example.org/media/index.html" media="handheld and (min-width: 20em)" /> Section 4.1.2 of the Atom specification [RFC4287] limits the ability for an atom:entry to contain multiple atom:link elements with a rel attribute value of "alternate". Specifically, the specification states that an atom:entry "MUST NOT contain more than one atom:link element with a rel attribute value of "alternate" that has the same combination of type and hreflang attribute values." This restriction limits the ability of an Atom document publisher to provide multiple media-targeted alternate links of the same content type. Each alternate link MUST specify a different media type or language. Other types of links do not share the same limitation. 8. Security Considerations The 'hash', 'etag' and 'modified' attributes are intended to allow an Atom publisher the means of describing the state of a linked resource at a point-in-time -- usually at the moment specified by the 'accessed' attribute or at the moment the atom:link or atom:content element was created. An Atom consumer that is aware of these attributes can use their values as an integrity check to determine if the linked resource has been modified since the attribute values were established by the publisher. The 'hash' attribute is intended for use when the publisher of an Atom document requires the ability to link to a specific version of a resource that is expected to remain stable and unchanged for a useful period of time. If publishers fall into the habit of regularly including hash digests for resources whose states change frequently, there is a danger that consumers of feeds containing large numbers of invalid digests will simply begin to ignore them and completely undermine the utility of the attribute. Snell Expires December 10, 2010 [Page 6]
Internet-Draft Atom Link Extensions June 2010 9. IANA Considerations No IANA actions are required by this document. 10. Normative References [RFC1864] Myers, J. and M. Rose, "The Content-MD5 Header Field", RFC 1864, October 1995. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. [RFC3339] Klyne, G., Ed. and C. Newman, "Date and Time on the Internet: Timestamps", RFC 3339, July 2002. [RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom Syndication Format", RFC 4287, December 2005. [W3C.CR-css3-mediaqueries-20090915] Celik, T., Glazman, D., Lie, H., and A. Kesteren, "Media Queries", World Wide Web Consortium CR CR-css3- mediaqueries-20090915, September 2009, <http://www.w3.org/TR/2009/CR-css3-mediaqueries-20090915>. [W3C.REC-xml-names-19990114] Hollander, D., Layman, A., and T. Bray, "Namespaces in XML", World Wide Web Consortium FirstEdition REC-xml- names-19990114, January 1999, <http://www.w3.org/TR/1999/REC-xml-names-19990114>. Author's Address James M Snell Phone: Email: jasnell@us.ibm.com URI: http://ibm.com Snell Expires December 10, 2010 [Page 7]
