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Network Working Group                                           J. Snell
Internet-Draft                                             November 2005
Expires: May 5, 2006


                Atom Syndication Format Link Extensions
               draft-snell-atompub-link-extensions-00.txt

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   This Internet-Draft will expire on May 5, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This specification defines extensions to the Atom Syndication Format
   link and content elements that may be used to express additional
   metadata about linked resources.









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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  3
   3.  Entity Attributes  . . . . . . . . . . . . . . . . . . . . . .  3
     3.1.  The 'md5' Attribute  . . . . . . . . . . . . . . . . . . .  3
     3.2.  The 'etag' Attribute . . . . . . . . . . . . . . . . . . .  4
     3.3.  The 'last-modified' Attribute  . . . . . . . . . . . . . .  4
     3.4.  The 'range' Attribute  . . . . . . . . . . . . . . . . . .  5
   4.  Media Descriptors  . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  The 'media' Attribute  . . . . . . . . . . . . . . . . . .  6
   5.  Link Grouping and Alternates . . . . . . . . . . . . . . . . .  7
     5.1.  The 'group' Attribute  . . . . . . . . . . . . . . . . . .  7
     5.2.  The 'alternate' Element  . . . . . . . . . . . . . . . . .  7
   6.  Link Decoration  . . . . . . . . . . . . . . . . . . . . . . .  8
     6.1.  The 'description' Element  . . . . . . . . . . . . . . . .  8
     6.2.  The 'icon' Element . . . . . . . . . . . . . . . . . . . .  8
   7.  Link Relations . . . . . . . . . . . . . . . . . . . . . . . .  9
     7.1.  Hierarchy Links  . . . . . . . . . . . . . . . . . . . . .  9
     7.2.  Informational Links  . . . . . . . . . . . . . . . . . . . 10
     7.3.  Processing Links . . . . . . . . . . . . . . . . . . . . . 11
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 14
























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1.  Introduction

   This specification defines extensions to the Atom Syndication Format
   [I-D.ietf-atompub-format] 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;

   {Ed.  Note: this namespace MUST be changed to a proper IETF namespace
   scheme prior to publication}

    "le": "http://purl.org/atompub/link-extensions/1.0"


3.  Entity Attributes

   The following link Entity Attributes may be used to specify metadata
   about the resource referenced by an atom:link element or atom:content
   element using the 'src' attribute.

3.1.  The 'md5' Attribute

   The 'md5' Attribute specifies a Base64-encoded MD5 digest [RFC1864]
   of the resource identified by the atom:link/@href or atom:content/@
   src attributes.  The value of the digest is calculated as specified
   in [RFC2616] and [RFC1864].  The 'md5' attribute MAY appear as a
   child of the atom:link and atom:content (using the 'src' attribute)
   elements.

     md5 = attribute le:md5 { md5-digest }

     md5-digest   = <base64 of 128 bit MD5 digest as per RFC 1864>

   While MD5 digests are not a guarantee against malicious modifications
   of a resource, the digest specified may be used as a simple integrity
   check to verify whether or not the referenced resource has been
   modified.





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   An example MD5 digest of an enclosed MP3 file

     <atom:link rel="enclosure"
       href="http://example.org/media/myfile.mp3"
       le:md5="Q2hlY2sgSW50ZWdyaXR5IQ==" />

3.2.  The 'etag' Attribute

   The 'etag' Attribute specifies an Entity Tag [RFC2616] for the
   resource identified by the atom:link or atom:content element.  The
   'etag' attribute MAY appear as a child of the atom:link and atom:
   content (using the 'src' attribute) elements.

     etag = attribute le:etag { entity-tag }

     entity-tag = [ weak ] opaque-tag
     weak       = "W/"
     opaque-tag = quoted-string

   As specified by [RFC2616], Entity Tags are usable for the purpose of
   comparing multiple entities served from the same resource URI and are
   typically used in the context of conditional-retrievals of resource
   representations.

   An example Entity Tag for an enclosed MP3 file

     <atom:link rel="enclosure"
       href="http://example.org/media/myfile.mp3"
       le:etag="W/xyzzy" />

3.3.  The 'last-modified' Attribute

   The 'last-modified' Attribute specifies the date and time when the
   resource identified by the atom:link or atom:content element was last
   modified as specified by [RFC2616].  The value of the attribute must
   conform to the HTTP-date construct from [RFC2616].  The 'last-
   modified' attribute MAY appear as a child of the atom:link and atom:
   content (using the 'src' attribute) elements.













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     last-modified = attribute le:last-modified { HTTP-date }

     HTTP-date    = rfc1123-date | rfc850-date | asctime-date
     rfc1123-date = wkday "," SP date1 SP time SP "GMT"
     rfc850-date  = weekday "," SP date2 SP time SP "GMT"
     asctime-date = wkday SP date3 SP time SP 4DIGIT
     date1        = 2DIGIT SP month SP 4DIGIT
                    ; day month year (e.g., 02 Jun 1982)
     date2        = 2DIGIT "-" month "-" 2DIGIT
                    ; day-month-year (e.g., 02-Jun-82)
     date3        = month SP ( 2DIGIT | ( SP 1DIGIT ))
                    ; month day (e.g., Jun  2)
     time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT
                    ; 00:00:00 - 23:59:59
     wkday        = "Mon" | "Tue" | "Wed"
                  | "Thu" | "Fri" | "Sat" | "Sun"
     weekday      = "Monday" | "Tuesday" | "Wednesday"
                  | "Thursday" | "Friday" | "Saturday" | "Sunday"
     month        = "Jan" | "Feb" | "Mar" | "Apr"
                  | "May" | "Jun" | "Jul" | "Aug"
                  | "Sep" | "Oct" | "Nov" | "Dec"

   An example last-modified attribute for an enclosed MP3 file

     <atom:link rel="enclosure"
       href="http://example.org/media/myfile.mp3"
       le:last-modified="Tue, 29 Nov 2005 20:37:00 PST" />

3.4.  The 'range' Attribute

   The 'range' Attribute specifies a subset of the resource identified
   by the atom:link or atom:content element as specified by the standard
   HTTP Range header defined by [RFC2616].  The 'range' attribute MAY
   appear as a child of the atom:link and atom:content (using the 'src'
   attribute) elements.

     range = attribute le:range { ranges-specifier }

     ranges-specifier = byte-ranges-specifier
     byte-ranges-specifier = bytes-unit "=" byte-range-set
     byte-range-set  = 1#( byte-range-spec | suffix-byte-range-spec )
     byte-range-spec = first-byte-pos "-" [last-byte-pos]
     first-byte-pos  = 1*DIGIT
     last-byte-pos   = 1*DIGIT

   The only Range Specifier defined by [RFC2616] is the "bytes" range
   specifier used to request a specific sequence of bytes from a
   resource.  Other range specifiers MAY be used.  Unknown range



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   specifiers SHOULD be ignored.

   An example range attribute specifying the first 500 bytes of an
   enclosed MP3 file

     <atom:link rel="enclosure"
       href="http://example.org/media/myfile.mp3"
       le:range="bytes 0-499" />


4.  Media Descriptors

   Resources referenced by atom:link and atom:content elements may be
   optimized for rendering on specific types of devices.

4.1.  The 'media' Attribute

   The 'media' attribute MAY be used to identify the types of devices
   for which a resource referenced by an atom:link or atom:content
   (using the src attribute) has been targeted.  The definition of the
   media attribute is the same as the 'media' attribute on HTML
   [W3C.REC-html401-19991224] and XHTML [W3C.REC-xhtml1-20020801] link
   elements.  The 'media' attribute MAY appear as a child of the atom:
   link and atom:content elements.

     media = attribute le:media { media-descriptors }

   The value of the 'media' attribute is a comma separated list of
   media-descriptor tokens.  If not specified, the value of attribute is
   assumed to be "all".  The following lists the standard media-
   descriptor definitions as specified by [W3C.REC-html401-19991224].

   o  screen: intended for non-paged computer screens
   o  tty: Intended for media using a fixed-pitch character grid, such
      as teletypes, terminals, or portable devices with limited display
      capabilities.
   o  tv: Intended for television-type devices (low resolution, color,
      limited scrollability).
   o  projection: Intended for projectors.
   o  handheld: Intended for handheld devices (small screen, monochrome,
      bitmapped graphics, limited bandwidth).
   o  print: Intended for paged, opaque material and for documents
      viewed on screen in print preview mode.
   o  braille: Intended for braille tactile feedback devices.
   o  aural: Intended for speech synthesizers.
   o  all: Suitable for all devices.





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   Two alternate atom:link elements respectively targeted at screen and
   handheld devices

     <atom:link rel="alternate"
       href="http://example.org/a.html"
       le:media="screen" />
     <atom:link rel="alternate"
       href="http://example.org/b.html"
       le:media="handheld" />


5.  Link Grouping and Alternates

   The Link Grouping and Alternate mechanisms defined below make it
   possible to group related atom:link elements into logical sets or to
   identify alternate IRI's (e.g. mirrors) for linked resources.

5.1.  The 'group' Attribute

   The 'group' Attribute MAY appear as a child of atom:link elements and
   MAY be used to group associated links together into a logical set of
   mutually-exclusive options.  The value of the attribute is a case-
   insensitive opaque text string.  Link elements with 'group'
   attributes specifying the same value are considered to belong to the
   same logical set.

     group = attribute le:group { text }

   The following illustrates an example of how the 'group' attribute may
   be used to identify alternate encodings of the same enclosed audio
   content.

     <atom:link rel="enclosure"
       href="http://example.org/media/myfile.mp3"
       le:group="mypodcast" />
     <atom:link rel="enclosure"
       href="http://example.org/media/myfile.wma"
       le:group="mypodcast" />

5.2.  The 'alternate' Element

   The 'alternate' Element is used to specifying one or more alternative
   IRI's for the resource identified by an atom:link element.  For
   instance, an enclosure referenced by a link may be downloadable from
   multiple mirror locations.  The 'alternate' Element consists of an
   IRI and optional language-sensitive text label.  Zero or more
   'alternate' elements MAY appear as children of the atom:link element.
   The parent link element's 'type', 'length' and 'hreflang' attributes



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   are considered to apply equally to each of the contained 'alternate'
   elements.

     alternate = element le:alternate {
      atomCommonAttributes,
      attribute href,
      attribute title?,
      (undefinedContent)
     }

   An example atom:link that specifies two alternate download locations
   for the enclosed MP3 file

     <atom:link rel="enclosure"
       type="audio/mpeg"
       href="http://example.org/media/myfile.mp3">
       <le:alternate title="Mirror 1"
         href="http://west.example.org/media/myfile.mp3" />
       <le:alternate title="Mirror 2"
         href="http://east.example.org/media/myfile.mp3" />
     </atom:link>


6.  Link Decoration

6.1.  The 'description' Element

   The 'description' element is an Atom Text Construct that is used to
   associate a short, human-readable textual summary with an atom:link
   element.  The element MAY appear as a child of atom:link.

     description = element le:description { atomTextConstruct }

   An example atom:link element with an associated description.

     <atom:link rel="enclosure"
       href="http://example.org/media/myfile.mp3">
       <le:description type="text">
         My first podcast.  Isn't it great!
       </le:description>
     </atom:link>

6.2.  The 'icon' Element

   The 'icon' element identifies an image that provides an iconic visual
   identification for an atom:link.  List the standard atom:icon
   element, which is used to associate an icon image with Atom Feed
   Documents, images referenced by the 'icon' element SHOULD have a



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   1-to-1 aspect ratio and be suitable for presentation at a small size.
   The element MAY contain an optional 'type' attribute whose value
   specifies the media-type of the referenced icon.

     icon = element le:icon {
       atomCommonAttributes,
       attribute type { media-type }?,
       (atomURI)
     }

   An example atom:link element with an associated PNG icon

     <atom:link rel="enclosure"
       href="http://example.org/media/myfile.mp3">
       <le:icon>http://example.org/icons/podcast.png</le:icon>
     </atom:link>


7.  Link Relations

   This specification adds the following values to the IANA Registry of
   Link Relations as specified by [I-D.ietf-atompub-format].

7.1.  Hierarchy Links

   The Hierarchy Link Relations may be used to organize Atom Feed and
   Entry documents into a hierarchical tree.

   o  link[@rel='origin'] : The 'origin' link refers to the logical root
      of a hierarchical tree of entities.
   o  link[@rel='parent'] : The 'parent' link refers to the logical
      parent of the current entity in a hierarchical tree of entities.
   o  link[@rel='child'] : The 'child' link refers to a logical
      subordinate of the current document in a hierarchical tree of
      entities.
   o  link[@rel='sibling'] : The 'sibling' link refers to a logical
      sibling of the current document in a hierarchical tree of
      entities.













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   An example of an Atom document hierarchy

     <!-- Origin -->
     <?xml version="1.0"?>
     <feed xmlns="http://www.w3.org/2005/Atom">
       ...
       <link rel="child"
             href="child_feed_A.xml" />
       <link rel="child"
             href="child_feed_B.xml" />
     </feed>

     <!-- Child Feed A -->
     <?xml version="1.0"?>
     <feed xmlns="http://www.w3.org/2005/Atom">
       <link rel="origin"
             href="origin.xml" />
       <link rel="parent"
             href="origin.xml" />
       <link rel="sibling"
             href="child_feed_B.xml" />
       <link rel="child"
             href="child_feed_C.xml" />
     </feed>

     <!-- Child Feed C -->
     <?xml version="1.0"?>
     <feed xmlns="http://www.w3.org/2005/Atom">
       <link rel="origin"
             href="origin.xml" />
       <link rel="parent"
             href="child_feed_A.xml" />
     </feed>

7.2.  Informational Links

   {To Be Completed}

   The following link relations may be used to reference informational
   resources about the containing Atom Feed or Entry.

   o  link[@rel='help'] : The 'help' link refers to a resource offering
      help information.
   o  link[@rel='about'] : The 'about' link refers to a resource
      offering information about the containing Feed or Entry.






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7.3.  Processing Links

   {To Be Completed}

   The following link relations may be used to reference resources that
   clients may use to process the content of an Atom entry.

   o  link[@rel='stylesheet'] : The 'stylesheet' link refers to a
      stylesheet that may be applied to the content of Atom entries.
   o  link[@rel='transformation'] : The 'transformation' link refers to
      a resource that may be applied to the content of Atom entries.


8.  Security Considerations

   TBD


9.  IANA Considerations

   There are no IANA considerations introduced by this specification.


10.  Acknowledgements

   TBD

11.  References

   [I-D.ietf-atompub-format]
              Sayre, R. and M. Nottingham, "The Atom Syndication
              Format", draft-ietf-atompub-format-11 (work in progress),
              August 2005.

   [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.

   [W3C.REC-html401-19991224]
              Hors, A., Jacobs, I., and D. Raggett, "HTML 4.01
              Specification", W3C REC REC-html401-19991224,
              December 1999.



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   [W3C.REC-xhtml1-20020801]
              Pemberton, S., "XHTML[TM] 1.0 The Extensible HyperText
              Markup Language (Second Edition)", W3C REC REC-xhtml1-
              20020801, August 2002.

   [W3C.REC-xml-infoset-20040204]
              Tobin, R. and J. Cowan, "XML Information Set (Second
              Edition)", W3C REC REC-xml-infoset-20040204,
              February 2004.

   [W3C.REC-xml-names-19990114]
              Hollander, D., Bray, T., and A. Layman, "Namespaces in
              XML", W3C REC REC-xml-names-19990114, January 1999.






































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Author's Address

   James M Snell


   Phone:
   Email: jasnell@gmail.com
   URI:   http://snellspace.com











































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Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.


Disclaimer of Validity

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Copyright Statement

   Copyright (C) The Internet Society (2005).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.


Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.




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