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Network Working Group J. Snell
Internet-Draft January 24, 2006
Expires: July 28, 2006
Atom Syndication Format Link Extensions
draft-snell-atompub-link-extensions-02.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
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 . . . . . . . . . . . . . . . . . . 8
5.2. The 'alternate' Element . . . . . . . . . . . . . . . . . 8
6. Indexing and Automated Downloads . . . . . . . . . . . . . . . 9
6.1. The 'le:follow' attribute . . . . . . . . . . . . . . . . 9
6.2. The 'le:index' attribute . . . . . . . . . . . . . . . . . 10
6.3. The 'le:archive' attribute . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
10. 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
[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;
{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. The attributes are designed to
closely correlate with existing HTTP [RFC2616] mechanisms used to
perform conditional operations, detecting resource modifications and
requesting ranges.
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 may have been
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modified.
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].
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screen
intended for non-paged computer screens
tty
intended for media using a fixed-pitch character
grid, such as teletypes, terminals, or portable
devices with limited display capabilities.
tv
intended for television-type devices (low-resolution,
color, limited scrollability).
projection
intended for projectors
handheld
intended for handheld devices (small screen,
monochrome, bitmapped graphics, limited bandwidth).
print
intended for paged, opaque material and for documents
viewed on screen in print preview mode
braille
intended for braille tactile feedback devices.
aural
intended for speech synthesizers
all
suitable for all devices
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.
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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" />
Comparison operations on the 'group' attribute are performed using a
case-insensitive character-by-character examination. Entity
encodings MUST be interpreted as the character they represent. For
instance, The value "group" is equivalent to
"group", "Group", "GROUP", etc.
5.2. The 'alternate' Element
The 'alternate' Element is used to specify 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, an optional language-sensitive text label, and an optional
'group' attribute used to group associated alternates together into
logical sets. Zero or more 'alternate' elements MAY appear as
children of the atom:link element. The parent link element's 'type',
'length' and 'hreflang' attributes are considered to apply equally to
each of the contained 'alternate' elements.
alternate = element le:alternate {
atomCommonAttributes,
attribute href { atomURI },
attribute title { text }?,
attribute group { text }?
(undefinedContent)
}
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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"
group="Asia"
href="http://jp.example.org/media/myfile.mp3" />
<le:alternate title="Mirror 2"
group="Asia"
href="http://cn.example.org/media/myfile.mp3" />
<le:alternate title="Mirror 3"
group="Europe"
href="http://de.example.org/media/myfile.mp3" />
<le:alternate title="Mirror 4"
group="Europe"
href="http://fr.example.org/media/myfile.mp3" />
</atom:link>
6. Indexing and Automated Downloads
6.1. The 'le:follow' attribute
The 'le:follow" attribute indicates whether applications should
automatically attempt to follow links and referenced content (e.g.,
whether or not enclosure links should be automatically downloaded,
etc). The value of the attribute is either "yes" or "no". A value
of "no" indicates that applications SHOULD NOT attempt to
automatically dereference the linked resource -- rather, the
application should wait until a user explicitly requests the linked
resource to be resolved.
For example,
<entry xmlns="http://www.w3.org/2005/Atom"
xmlns:le="http://purl.org/atompub/link-extensions/1.0">
<id>...</id>
<title>...</title>
<link rel="enclosure"
href="http://example.com/mediafiles/mysong.mp3"
le:follow="no" />
...
</entry>
The 'le:follow' attribute MAY be contained by atom:link elements and
atom:content elements that contain a 'src' attribute.
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6.2. The 'le:index' attribute
The 'le:index' attribute indicates whether applications should index
links and referenced content. The value of the attribute is either
"yes" or "no". A value of "no" indicates that applications SHOULD
NOT index the referenced resource.
For the sake of this specification, 'indexing' refers to the practice
of parsing and processing the content of a resource for the purpose
of populating a database used for searching and other forms of
analysis. The intended purpose of using le:index="no" would be for
the publisher to indicate their preference that the associated
resource not be processed for searching or analysis purposes.
<entry xmlns="http://www.w3.org/2005/Atom"
xmlns:le="http://purl.org/atompub/link-extensions/1.0">
<id>...</id>
<title>...</title>
<link rel="enclosure"
href="http://example.com/mediafiles/mysong.mp3"
le:index="no" />
...
</entry>
The 'le:index' attribute MAY be contained by atom:link elements and
atom:content elements containing a 'src' attribute.
6.3. The 'le:archive' attribute
The 'le:archive' attribute indicates whether applications should
archive the targets of links and content references. The value of
the attribute is either "yes" or "no". A value of "no" indicate that
applications SHOULD NOT archive the referenced resource.
For the sake of this specification, 'archiving' refers to the
practice of maintain a local copy of a resource as part of a
historical record. This is different than the practice of
maintaining locally cached copies of a resource for the sake of
improving transmission performance and reducing network bandwidth.
The intended purpose of using le:archive="no" would be for a
publisher to indicate their preference that local copies of the
asociated resource not be maintained for archival/historical
purposes.
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<entry xmlns="http://www.w3.org/2005/Atom"
xmlns:le="http://purl.org/atompub/link-extensions/1.0">
<id>...</id>
<title>...</title>
<link rel="enclosure"
href="http://example.com/mediafiles/mysong.mp3"
le:archive="no" />
...
</entry>
The 'le:archive' attribute MAY be contained by atom:link elements and
atom:content elements containing a 'src' attribute.
7. Security Considerations
While the mechanisms defined herein may be used to detect
modifications of linked resources, the mechanisms MUST NOT be
considered to provide a guarantee against inappropriate modification.
Other mechanisms, such as XML Digital Signatures and XML Encryption
should be used to properly secure Atom documents.
8. IANA Considerations
This specification does not introduce any IANA considerations.
9. Acknowledgements
The author gratefully acknowledges the feedback from the members of
the Atom Publishing Format and Protocol working group during the
development of this specification.
10. 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.
[RFC4287] Nottingham, M. and R. Sayre, "The Atom Syndication
Format", RFC 4287, December 2005.
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[W3C.REC-html401-19991224]
Hors, A., Jacobs, I., and D. Raggett, "HTML 4.01
Specification", W3C REC REC-html401-19991224,
December 1999.
[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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Acknowledgment
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