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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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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Copyright Statement
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except as set forth therein, the authors retain all their rights.
Acknowledgment
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Internet Society.
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