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Network Working Group                                      A. Bryan, Ed.
Internet-Draft                                         T. Tsujikawa, Ed.
Intended status: Standards Track                           N. McNab, Ed.
Expires: September 4, 2009                            Metalinker Project
                                                           P. Poeml, Ed.
                                                            Novell, Inc.
                                                           March 3, 2009


                The Metalink Download Description Format
                        draft-bryan-metalink-06

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.  This document may contain material
   from IETF Documents or IETF Contributions published or made publicly
   available before November 10, 2008.  The person(s) controlling the
   copyright in some of this material may not have granted the IETF
   Trust the right to allow modifications of such material outside the
   IETF Standards Process.  Without obtaining an adequate license from
   the person(s) controlling the copyright in such materials, this
   document may not be modified outside the IETF Standards Process, and
   derivative works of it may not be created outside the IETF Standards
   Process, except to format it for publication as an RFC or to
   translate it into languages other than English.

   Internet-Drafts are working documents of the Internet Engineering
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   Drafts.

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   This Internet-Draft will expire on September 4, 2009.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the



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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   This document specifies Metalink Documents, an XML-based download
   description format.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Namespace and Version  . . . . . . . . . . . . . . . . . .  5
     1.3.  Notational Conventions . . . . . . . . . . . . . . . . . .  5
   2.  Metalink Documents . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Common Metalink Constructs . . . . . . . . . . . . . . . . . .  7
     3.1.  Text Constructs  . . . . . . . . . . . . . . . . . . . . .  7
       3.1.1.  Text . . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  Date Constructs  . . . . . . . . . . . . . . . . . . . . .  8
   4.  Metalink Element Definitions . . . . . . . . . . . . . . . . .  8
     4.1.  Container Elements . . . . . . . . . . . . . . . . . . . .  8
       4.1.1.  The "metalink:metalink" Element  . . . . . . . . . . .  9
       4.1.2.  The "metalink:files" Element . . . . . . . . . . . . . 10
       4.1.3.  The "metalink:file" Element  . . . . . . . . . . . . . 11
       4.1.4.  The "metalink:resources" Element . . . . . . . . . . . 12
       4.1.5.  The "metalink:verification" Element  . . . . . . . . . 13
       4.1.6.  The "metalink:pieces" Element  . . . . . . . . . . . . 13
     4.2.  Metadata Elements  . . . . . . . . . . . . . . . . . . . . 14
       4.2.1.  The "metalink:copyright" Element . . . . . . . . . . . 14
       4.2.2.  The "metalink:description" Element . . . . . . . . . . 14
       4.2.3.  The "metalink:generator" Element . . . . . . . . . . . 14
       4.2.4.  The "metalink:hash" Element  . . . . . . . . . . . . . 15
       4.2.5.  The "metalink:identity" Element  . . . . . . . . . . . 16
       4.2.6.  The "metalink:language" Element  . . . . . . . . . . . 17
       4.2.7.  The "metalink:license" Element . . . . . . . . . . . . 17
       4.2.8.  The "metalink:logo" Element  . . . . . . . . . . . . . 17
       4.2.9.  The "metalink:metadata" Element  . . . . . . . . . . . 17
       4.2.10. The "metalink:origin" Element  . . . . . . . . . . . . 18
       4.2.11. The "metalink:os" Element  . . . . . . . . . . . . . . 18
       4.2.12. The "metalink:published" Element . . . . . . . . . . . 19
       4.2.13. The "metalink:publisher" Element . . . . . . . . . . . 19
       4.2.14. The "metalink:signature" Element . . . . . . . . . . . 19



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       4.2.15. The "metalink:size" Element  . . . . . . . . . . . . . 20
       4.2.16. The "metalink:type" Element  . . . . . . . . . . . . . 20
       4.2.17. The "metalink:updated" Element . . . . . . . . . . . . 20
       4.2.18. The "metalink:url" Element . . . . . . . . . . . . . . 21
       4.2.19. The "metalink:version" Element . . . . . . . . . . . . 21
   5.  Client Implementation Considerations . . . . . . . . . . . . . 22
   6.  Securing Metalink Documents  . . . . . . . . . . . . . . . . . 22
   7.  Extending Metalink . . . . . . . . . . . . . . . . . . . . . . 23
     7.1.  Extensions from Non-Metalink Vocabularies  . . . . . . . . 23
     7.2.  Extensions to the Metalink Vocabulary  . . . . . . . . . . 23
     7.3.  Processing Foreign Markup  . . . . . . . . . . . . . . . . 23
     7.4.  Extension Elements . . . . . . . . . . . . . . . . . . . . 23
       7.4.1.  Simple Extension Elements  . . . . . . . . . . . . . . 24
       7.4.2.  Structured Extension Elements  . . . . . . . . . . . . 24
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
     8.1.  XML Namespace Registration . . . . . . . . . . . . . . . . 25
     8.2.  application/metalink+xml MIME type . . . . . . . . . . . . 25
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 26
     9.1.  URIs and IRIs  . . . . . . . . . . . . . . . . . . . . . . 26
     9.2.  Spoofing . . . . . . . . . . . . . . . . . . . . . . . . . 26
     9.3.  Cryptographic Hashes . . . . . . . . . . . . . . . . . . . 26
     9.4.  Signing  . . . . . . . . . . . . . . . . . . . . . . . . . 26
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 27
     10.2. Informative References . . . . . . . . . . . . . . . . . . 28
   Appendix A.  Acknowledgements and Contributors . . . . . . . . . . 29
   Appendix B.  RELAX NG Compact Schema . . . . . . . . . . . . . . . 30
   Appendix C.  Document History  . . . . . . . . . . . . . . . . . . 33
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37





















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1.  Introduction

   Metalink is an XML-based document format that describes a file or
   lists of files to be added to a download queue.  Lists are composed
   of a number of files, each with an extensible set of attached
   metadata.  For example, each file can have a description, checksum,
   and list of URIs that it is available from.

   The primary use case that Metalink addresses is the description of
   downloadable content in a format so download agents can act
   intelligently and recover from common errors with little or no user
   interaction necessary.  These errors can include multiple servers
   going down and data corrupted in transmission.

   Discussion of this draft should take place on discuss@apps.ietf.org
   or the Metalink discussion mailing list located at
   metalink-discussion@googlegroups.com.  To join the list, visit
   http://groups.google.com/group/metalink-discussion .

1.1.  Examples

   A brief, single file Metalink Document:

   <?xml version="1.0" encoding="UTF-8"?>
   <metalink xmlns="urn:ietf:params:xml:ns:metalink">
     <files>
       <file name="example.ext">
         <resources>
           <url>ftp://ftp.example.com/example.ext</url>
           <url>http://example.com/example.ext</url>
           <metadata type="torrent">
           http://example.com/example.ext.torrent
           </metadata>
         </resources>
       </file>
     </files>
   </metalink>














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   A more extensive, single file Metalink Document:

   <?xml version="1.0" encoding="UTF-8"?>
   <metalink xmlns="urn:ietf:params:xml:ns:metalink">
     <published>2008-05-15T12:23:23Z</published>
     <files>
       <file name="example.ext">
         <identity>Example</identity>
         <version>1.0</version>
         <description>A description of the example file for
   download.</description>
         <verification>
           <hash type="sha-1">80bc95fd391772fa61c91ed68567f0980bb45fd9
           </hash>
           </verification>
         <resources>
           <url>ftp://ftp.example.com/example.ext</url>
           <url>http://example.com/example.ext</url>
           <metadata type="torrent">
           http://example.com/example.ext.torrent
           </metadata>
         </resources>
       </file>
     </files>
   </metalink>

1.2.  Namespace and Version

   The XML Namespaces URI [REC-xml-names] for the XML data format
   described in this specification is:

   urn:ietf:params:xml:ns:metalink

   For convenience, this data format may be referred to as "Metalink",
   which this specification uses internally.

1.3.  Notational Conventions

   This specification describes conformance of Metalink Documents.
   Additionally, it places some requirements on Metalink Processors.

   This specification uses the namespace prefix "metalink:" for the
   Namespace URI identified in Section 1.2, above.  Note that the choice
   of namespace prefix is arbitrary and not semantically significant.

   Metalink is specified using terms from the XML Infoset
   [REC-xml-infoset].  However, this specification uses a shorthand for
   two common terms: the phrase "Information Item" is omitted when



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   naming Element Information Items and Attribute Information Items.
   Therefore, when this specification uses the term "element," it is
   referring to an Element Information Item in Infoset terms.  Likewise,
   when it uses the term "attribute," it is referring to an Attribute
   Information Item.

   Some sections of this specification are illustrated with fragments of
   a non-normative RELAX NG Compact schema [RELAX-NG].  However, the
   text of this specification provides the definition of conformance.  A
   complete schema appears in Appendix B.

   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], as
   scoped to those conformance targets.


2.  Metalink Documents

   This specification describes Metalink Documents.

   A Metalink Document describes a file or group of files, how to access
   them, and metadata that identifies them.  Its root is the metalink:
   metalink element.

   namespace metalink = "urn:ietf:params:xml:ns:metalink"
   start = metalinkMetalink

   Metalink Documents are specified in terms of the XML Information Set,
   serialized as XML 1.0 [REC-xml] and identified with the "application/
   metalink+xml" media type.

   Metalink Documents MUST be well-formed XML.  This specification does
   not define a DTD for Metalink Documents, and hence does not require
   them to be valid (in the sense used by XML).

   Metalink allows the use of IRIs [RFC3987].  Every URI [RFC3986] is
   also an IRI, so a URI may be used wherever below an IRI is named.
   There is one special consideration: when an IRI that is not also a
   URI is given for dereferencing, it MUST be mapped to a URI using the
   steps in Section 3.1 of [RFC3987].

   Any element defined by this specification MAY have an xml:base
   attribute [REC-xmlbase].  When xml:base is used in an Metalink
   Document, it serves the function described in Section 5.1.1 of
   [RFC3986], establishing the base URI (or IRI) for resolving any
   relative references found within the effective scope of the xml:base
   attribute.



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   Any element defined by this specification MAY have an xml:lang
   attribute, whose content indicates the natural language for the
   element and its descendents.  The language context is only
   significant for elements and attributes declared to be "Language-
   Sensitive" by this specification.  Requirements regarding the content
   and interpretation of xml:lang are specified in XML 1.0 [REC-xml],
   Section 2.12.

   metalinkCommonAttributes =
      attribute xml:base { metalinkUri }?,
      attribute xml:lang { metalinkLanguageTag }?,
      undefinedAttribute*

   Metalink is an extensible format.  See Section 7 of this document for
   a full description of how Metalink Documents can be extended.


3.  Common Metalink Constructs

   Many of Metalink's elements share a few common structures.  This
   section defines those structures and their requirements for
   convenient reference by the appropriate element definitions.

   When an element is identified as being a particular kind of
   construct, it inherits the corresponding requirements from that
   construct's definition in this section.

   Note that there MUST NOT be any white space in a Date construct or in
   any IRI.  Some XML-emitting implementations erroneously insert white
   space around values by default, and such implementations will emit
   invalid Metalink Documents.

3.1.  Text Constructs

   A Text construct contains human-readable text, usually in small
   quantities.  The content of Text constructs is Language-Sensitive.

   metalinkTextConstruct =
      metalinkCommonAttributes,
      text

3.1.1.  Text

   Example metalink:description with text content:







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   ...
   <description>
   A description of the example file for download.
   </description>
   ...

   The content of the Text construct MUST NOT contain child elements.
   Such text is intended to be presented to humans in a readable
   fashion.  Thus, Metalink Processors MAY collapse white space
   (including line breaks) and display the text using typographic
   techniques such as justification and proportional fonts.

3.2.  Date Constructs

   A Date construct is an element whose content MUST conform to the
   "date-time" production in [RFC3339].  In addition, 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.

   metalinkDateConstruct =
      metalinkCommonAttributes,
      xsd:dateTime

   Such date values happen to be compatible with the following
   specifications: [ISO.8601.1988], [W3C.NOTE-datetime-19980827], and
   [W3C.REC-xmlschema-2-20041028].

   Example Date constructs:

   <updated>2008-12-13T18:30:02Z</updated>
   <updated>2008-12-13T18:30:02.25Z</updated>
   <updated>2008-12-13T18:30:02+01:00</updated>
   <updated>2008-12-13T18:30:02.25+01:00</updated>

   Date values SHOULD be as accurate as possible.  For example, it would
   be generally inappropriate for a publishing system to apply the same
   timestamp to several entries that were published during the course of
   a single day.


4.  Metalink Element Definitions

4.1.  Container Elements







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4.1.1.  The "metalink:metalink" Element

   The "metalink:metalink" element is the document (i.e., top-level)
   element of a Metalink Document, acting as a container for metadata
   and data associated with the listed files.  It contains one
   "metalink:files" element whose element children consist of metadata
   elements followed by one or more metalink:file child elements.

   metalinkMetalink =
      element metalink:metalink {
         metalinkCommonAttributes,
         (metalinkPublished?
          & metalinkOrigin?
          & metalinkGenerator?
          & metalinkUpdated?
          & extensionElement*),
         metalinkFiles
      }

   The following child elements are defined by this specification (note
   that the presence of some of these elements is required):

   o  metalink:metalink elements MUST contain exactly one metalink:files
      element.
   o  If metalink:type is "dynamic", metalink:metalink elements MAY
      contain exactly one metalink:origin element.
   o  metalink:metalink elements MAY contain exactly one metalink:type
      element.
   o  metalink:metalink elements MAY contain exactly one metalink:
      generator element.
   o  metalink:metalink elements MAY contain exactly one metalink:
      published element.
   o  If metalink:type is "dynamic", metalink:metalink elements MAY
      contain exactly one metalink:updated element.

4.1.1.1.  Providing Textual Content

   Experience teaches that downloads providing textual content are in
   general more useful than those that do not.  Some applications (one
   example is full-text indexers) require a minimum amount of text to
   function reliably and predictably.  Metalink publishers should be
   aware of these issues.  It is advisable that each metalink:file
   element contain a non-empty metalink:description element, a non-empty
   metalink:identity element when that element is present, and a non-
   empty metalink:version element, and a non-empty metalink:publisher
   element.  However, the absence of metalink:description is not an
   error, and Metalink Processors MUST NOT fail to function correctly as
   a consequence of such an absence.



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4.1.2.  The "metalink:files" Element

   The "metalink:files" element acts as a container for metadata and
   data associated with the listed files.  It contains one or more
   metalink:file child elements.  Certain elements can be listed either
   under metalink:files or metalink:file.  If under metalink:files, they
   apply to all files listed in each metalink:file.  If under metalink:
   file, then they apply to just that specific file.  If an element is
   listed both under metalink:files and metalink:file, then the element
   under metalink:file has precedence and the metalink:files element
   does not apply to that particular file.

   metalinkFiles =
      element metalink:files {
         metalinkCommonAttributes,
         (metalinkIdentity?
          & metalinkVersion?
          & metalinkDescription?
          & metalinkOS?
          & metalinkLogo?
          & metalinkLanguage?
          & metalinkPublisher?
          & metalinkCopyright?
          & metalinkLicense?
          & extensionElement*)
         metalinkFile
      }

   The following child elements are defined by this specification (note
   that the presence of some of these elements is required):

   o  metalink:files element MUST contain one or more metalink:file
      elements.
   o  metalink:files elements SHOULD contain exactly one metalink:
      identity element.
   o  metalink:files elements SHOULD contain exactly one metalink:
      version element.
   o  metalink:files elements MAY contain exactly one metalink:
      description element.
   o  metalink:files elements MAY contain exactly one metalink:os
      element.
   o  metalink:files elements MAY contain exactly one metalink:logo
      element.
   o  metalink:files elements MAY contain exactly one metalink:language
      element.
   o  metalink:files elements MAY contain exactly one metalink:publisher
      element.




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   o  metalink:files elements MAY contain exactly one metalink:copyright
      element.
   o  metalink:files elements MAY contain exactly one metalink:license
      element.

4.1.3.  The "metalink:file" Element

   The "metalink:file" element represents an individual file, acting as
   a container for metadata and data associated with the file.

   metalinkFile =
      element metalink:file {
         metalinkCommonAttributes,
         attribute name { metalinkTextConstruct },
         (metalinkVerification?
          & metalinkIdentity?
          & metalinkVersion?
          & metalinkDescription?
          & metalinkSize?
          & metalinkOS?
          & metalinkLogo?
          & metalinkLanguage?
          & metalinkPublisher?
          & metalinkCopyright?
          & metalinkLicense?
          & extensionElement*)
         metalinkResources
      }

   This specification assigns no significance to the order of metalink:
   file elements.

   The following child elements are defined by this specification (note
   that it requires the presence of some of these elements):

   o  metalink:file elements MUST contain exactly one metalink:resources
      element.
   o  metalink:file elements SHOULD contain exactly one metalink:
      verification element.
   o  metalink:file elements SHOULD contain exactly one metalink:
      identity element.
   o  metalink:file elements SHOULD contain exactly one metalink:version
      element.
   o  metalink:file elements MAY contain exactly one metalink:
      description element.
   o  metalink:file elements SHOULD contain exactly one metalink:size
      element.




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   o  metalink:file elements MAY contain exactly one metalink:os
      element.
   o  metalink:file elements MAY contain exactly one metalink:logo
      element.
   o  metalink:file elements MAY contain exactly one metalink:language
      element.
   o  metalink:file elements MAY contain exactly one metalink:publisher
      element.
   o  metalink:file elements MAY contain exactly one metalink:copyright
      element.
   o  metalink:file elements MAY contain exactly one metalink:license
      element.

4.1.3.1.  The "name" Attribute

   metalink:file elements MUST have a "name" attribute, which contains
   the filename of the file downloaded.

   Directory information can also be contained in a "path/file" format
   only, as in:

   <file name="debian-amd64/sarge/Contents-amd64.gz">

   In this example, a subdirectory debian-amd64/sarge/ will be created
   and a file named Contents-amd64.gz will be created inside it.  The
   path MUST be relative.  The path MUST NOT begin with a "/", "./" or
   "../", contain "/../", or end with "/..".  Metalink Processors MUST
   NOT allow directory traversal.

   A Metalink Processor MAY alter the name of the subdirectory or file
   if they contain characters which are invalid in the destination
   filesystem.

4.1.4.  The "metalink:resources" Element

   The "metalink:resources" element acts as a container for metadata and
   data associated with the listed files.  It contains one or more
   metalink:url child elements.  It can also contain one or more
   metalink:metadata child elements.

   metalinkResources =
      element metalink:resources {
         metalinkCommonAttributes,
         extensionElement*
         metalinkURL*
         metalinkMetadata*
      }




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   This specification assigns no significance to the order of metalink:
   url elements.  Significance is determined by the value of the
   "preference" attribute of the metalink:url elements.

   The following child elements are defined by this specification (note
   that the presence of some of these elements is required):

   o  metalink:resources element MUST contain at least one metalink:
      metadata element or at least one metalink:url element.  Typically,
      metalink:resources element contains more than one metalink:url
      element to provide multiple download sources.

4.1.5.  The "metalink:verification" Element

   The "metalink:verification" element acts as a container for metadata
   and data associated with verifying the listed files.  This
   information is in the form of checksums and digital signatures.
   Checksums are used to verify the integrity of a complete file or
   portion of a file to determine if the files have been transferred
   without any errors.  Digital signatures verify that a file is from
   the entity that has signed it.

   metalinkVerification =
      element metalink:verification {
         metalinkCommonAttributes,
        (metalinkHash*
         & metalinkPieces*
         & metalinkSignature?
         & extensionElement*)
      }

   The following child elements are defined by this specification:

   o  metalink:verification element MAY contain one or more metalink:
      hash elements.
   o  metalink:verification element MAY contain one or more metalink:
      pieces elements.
   o  metalink:verification element MAY contain one or more metalink:
      signature elements.

4.1.6.  The "metalink:pieces" Element

   The "metalink:pieces" element acts as a container for metadata and
   data associated with verifying the listed files.  This information is
   in the form of checksums for a portion of a file.






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   metalinkPieces =
      element metalink:pieces {
         attribute length { metalinkTextConstruct },
         attribute type { metalinkTextConstruct },
         hash+
      }+,

4.1.6.1.  The "type" Attribute

   metalink:pieces elements MUST have a "type" attribute.

   The IANA registry named "Hash Function Textual Names" defines values
   for hash types.  Metalink Generators and Processors supporting
   verification SHOULD at least implement "sha-1" which is SHA1, as
   specified in [RFC3174].

4.1.6.2.  The "length" Attribute

   metalink:pieces elements MUST have a "length" attribute, which is an
   integer that describes the length of the piece of the file in octets.

4.2.  Metadata Elements

4.2.1.  The "metalink:copyright" Element

   The "metalink:copyright" element is a Text construct that conveys a
   human-readable copyright for a file.

   metalinkCopyright =
      element metalink:copyright {
      metalinkTextConstruct
      }

4.2.2.  The "metalink:description" Element

   The "metalink:description" element is a Text construct that conveys a
   human-readable description for a file.

   metalinkDescription =
      element metalink:description {
      metalinkTextConstruct
      }

4.2.3.  The "metalink:generator" Element

   The "metalink:generator" element's content identifies the agent used
   to generate a Metalink Document, for debugging and other purposes.




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   metalinkGenerator = element metalink:generator {
      metalinkCommonAttributes,
      attribute uri { metalinkUri }?,
      attribute version { text }?,
      text
   }

   The content of this element, when present, MUST be a string that is a
   human-readable name for the generating agent.  Entities such as
   "&amp;" and "&lt;" represent their corresponding characters ("&" and
   "<" respectively), not markup.

   The metalink:generator element MAY have a "uri" attribute whose value
   MUST be an IRI reference [RFC3987].  When dereferenced, the resulting
   URI (mapped from an IRI, if necessary) SHOULD produce a
   representation that is relevant to that agent.

   The metalink:generator element MAY have a "version" attribute that
   indicates the version of the generating agent.

4.2.4.  The "metalink:hash" Element

   The "metalink:hash" element is a Text construct that conveys a hash
   for a file.  All hashes are encoded in lowercase hexadecimal format.

   metalinkHash =
      element metalink:hash {
      attribute piece { xsd:integer }?,
      attribute type { metalinkTextConstruct },
      text
      }

   Metalinks can contain multiples hashes for a complete file, for
   example both SHA-1 and SHA-256.

   ...
   <verification>
     <hash type="sha-1">a97fcf6ba9358f8a6f62beee4421863d3e52b080</hash>
     <hash type="sha-256">fc87941af7fd7f03e53b34af393f4c14923d74
     825f51116ff591336af4880227</hash>
   </verification>
   ...

   Metalinks can also contain hashes for individual pieces of a file.







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   ...
   <verification>
     <hash type="sha-1">a97fcf6ba9358f8a6f62beee4421863d3e52b080</hash>
     <hash type="sha-256">fc87941af7fd7f03e53b34af393f4c14923d74
     825f51116ff591336af4880227</hash>
     <pieces length="1048576" type="sha-1">
       <hash piece="0">d96b9a4b92a899c2099b7b31bddb5ca423bb9b30</hash>
       <hash piece="1">10d68f4b1119014c123da2a0a6baf5c8a6d5ba1e</hash>
       <hash piece="2">3e84219096435c34e092b17b70a011771c52d87a</hash>
       <hash piece="3">67183e4c3ab892d3ebe8326b7d79eb62d077f487</hash>
     </pieces>
   </verification>
   ...

   metalink:hash elements MUST have a "type" attribute or a "piece"
   attribute. metalink:hash elements with a "type" attribute contain a
   hash of the whole file. metalink:hash elements with a "piece"
   attribute contain a hash for that specific piece or chunk of the
   file.

4.2.4.1.  The "type" Attribute

   The IANA registry named "Hash Function Textual Names" defines values
   for hash types.  Metalink Generators and Processors supporting
   verification SHOULD at least implement "sha-1" which is SHA1, as
   specified in [RFC3174].

4.2.4.2.  The "piece" Attribute

   metalink:hash elements MAY have a "piece" attribute, only when they
   are a sub element of metalink:pieces.  The value of "piece" starts at
   "0" and increases, depending on the "length" attribute of metalink:
   pieces and the size of the file.  Depending on the size of a file,
   the last piece may not be the same size as the others.

4.2.5.  The "metalink:identity" Element

   The "metalink:identity" element is a Text construct that conveys a
   human-readable identity for a file.  The identity of OpenOffice.org
   3.0 would be "OpenOffice.org".

   metalinkIdentity =
      element metalink:identity {
      metalinkTextConstruct
      }






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4.2.6.  The "metalink:language" Element

   The "metalink:language" element is a Text construct that conveys a
   code for the language of a file, per [ISO639-2].

   metalinkLanguage =
      element metalink:language {
      metalinkTextConstruct
      }

4.2.7.  The "metalink:license" Element

   The "metalink:license" element is a Text construct that conveys a
   human-readable license name for a file.

   metalinkLicense =
      element metalink:license {
      metalinkCommonAttributes,
      attribute uri { metalinkUri }?,
      attribute name { metalinkTextConstruct }?,
      }

   The metalink:license element MAY have a "uri" attribute whose value
   MUST be an IRI reference [RFC3987].  When dereferenced, the resulting
   URI (mapped from an IRI, if necessary) SHOULD produce a
   representation that is relevant to that agent.

   The metalink:license element MAY have a "name" attribute that
   indicates the name of the license.

4.2.8.  The "metalink:logo" Element

   The "metalink:logo" element's content is an IRI reference [RFC3987]
   that identifies an image that provides visual identification for a
   file.

   metalinkLogo = element metalink:logo {
      metalinkCommonAttributes,
      (metalinkUri)
   }

   The image SHOULD have an aspect ratio of one (horizontal) to one
   (vertical) and SHOULD be suitable for presentation at a small size.

4.2.9.  The "metalink:metadata" Element

   The "metalink:metadata" element contains the IRI of metadata about a
   resource to download.  For example, this could be the IRI of a



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   BitTorrent .torrent file or a Metalink Document.

   metalinkMetadata =
      element metalink:metadata {
         metalinkCommonAttributes,
         attribute preference { xsd:integer }?,
         attribute type { metalinkTextConstruct },
         metalinkUri
      }+

4.2.9.1.  The "preference" Attribute

   metalink:metadata elements MAY have a preference attribute, whose
   value MUST be a number from 1 to 100 for priority, with 100 used
   first and 1 used last.  See the "preference" attribute of the
   metalink:url element for more information.

4.2.9.2.  The "type" Attribute

   metalink:metadata elements MUST have a "type" attribute that
   indicates the MIME type of the metadata available at the IRI.  In the
   case of BitTorrent as specified in [BITTORRENT], the value "torrent"
   is required.  Types without "/" are reserved.  Currently, "torrent"
   is the only reserved value.

   Metalink Processors that do not support a specified type of metadata
   about a resource to download MUST ignore that metadata.

4.2.10.  The "metalink:origin" Element

   The "metalink:origin" element is an IRI where the Metalink Document
   was originally published.  If metalink:type is "dynamic", then
   updated versions of the Metalink can be found at this IRI.

   metalinkOrigin = element metalink:origin {
      metalinkCommonAttributes,
      (metalinkUri)
   }

4.2.11.  The "metalink:os" Element

   The "metalink:os" element is a Text construct that conveys a human-
   readable Operating System for a file.  The IANA registry named
   "Operating System Names" defines values for OS types.

   metalinkOS =
      element metalink:os {
      metalinkTextConstruct



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      }

4.2.12.  The "metalink:published" Element

   The "metalink:published" element is a Date construct indicating an
   instant in time associated with an event early in the life cycle of
   the entry.

   metalinkPublished =
      element metalink:published {
      metalinkDateConstruct
      }

   Typically, metalink:published will be associated with the initial
   creation or first availability of the resource.

4.2.13.  The "metalink:publisher" Element

   The "metalink:publisher" element indicates a group or other entity
   which has published the file.

   metalinkPublisher =
      element metalink:publisher {
      metalinkCommonAttributes,
      attribute uri { metalinkUri }?,
      attribute name { metalinkTextConstruct }?,
   }

   The metalink:publisher element MAY have a "uri" attribute whose value
   MUST be an IRI reference [RFC3987].  When dereferenced, the resulting
   URI (mapped from an IRI, if necessary) SHOULD produce a
   representation that is relevant to that agent.

   The metalink:publisher element MAY have a "name" attribute that
   indicates the name of the publisher.

4.2.14.  The "metalink:signature" Element

   The "metalink:signature" element is a Text construct that conveys a
   digital signature for a file described in a Metalink Document.

   metalinkSignature =
      element metalink:signature {
      attribute type { "pgp" },
      metalinkTextConstruct
      }





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4.2.14.1.  The "type" Attribute

   metalink:signature elements MUST have a "type" attribute.  The inital
   value of "type" is the string that is non-empty and matches "pgp".
   It may be useful to extend Metalink documents with new types of
   digital signatures, so unknown types are allowed.

4.2.15.  The "metalink:size" Element

   The "metalink:size" element indicates the length of the linked
   content in octets; it is a hint about the content length of the
   representation returned when the IRI is mapped to a URI and
   dereferenced.  Note that the "metalink:size" element MUST override
   the actual content length of the representation as reported by the
   underlying protocol, i.e. files with different sizes should be
   discarded.

   metalinkSize =
      element metalink:size {
      metalinkTextConstruct
      }

4.2.16.  The "metalink:type" Element

   The "metalink:type" element is a Text construct that describes
   whether the IRI from "metalink:origin" a Metalink will contain
   dynamic updated Metalinks or static content that is not updated.

   metalinkType =
      element metalink:type {
      "static" | "dynamic"
      }

4.2.17.  The "metalink:updated" Element

   The "metalink:updated" element is a Date construct indicating the
   most recent instant in time when a Metalink was modified in a way the
   publisher considers significant.  Therefore, not all modifications
   necessarily result in a changed metalink:updated value.

   metalinkUpdated =
      element metalink:updated {
      metalinkDateConstruct
      }

   Publishers MAY change the value of this element over time.





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4.2.18.  The "metalink:url" Element

   The "metalink:url" element contains the IRI of a file.  All IRIs MUST
   lead to identical files.

   metalinkURL =
      element metalink:url {
         metalinkCommonAttributes,
         attribute location { xsd:string {
           minLength = "2"  maxLength="2"}
           }?,
         attribute preference { xsd:integer }?,
         metalinkUri
      }+

4.2.18.1.  The "preference" Attribute

   metalink:url elements MAY have a preference attribute, whose value
   MUST be a number from 1 to 100 for priority, with 100 used first and
   1 used last.  Multiple metalink:url elements can have the same
   preference, i.e. ten mirrors could have preference="100".  A Metalink
   Processor MAY download different segments of a file from more than
   one IRI simultaneously, and when doing so SHOULD first use the
   highest priority IRIs and then use lower ones.

   When one or more metalink:url elements have a preference attribute
   value of "100", other metalink:url elements SHOULD NOT be used,
   unless the elements with a preference of 100 cannot be processed
   (e.g. if they are of a metalink:metadata element type which is not
   supported by the Metalink Processor, such as BitTorrent, or if the
   servers are unavailable).

   Any metalink:url elements with a preference attribute value of "1"
   SHOULD NOT be used unless all other metalink:url elements cannot be
   processed (e.g. if they are of a metalink:metadata element type which
   is not supported by the Metalink Processor, such as BitTorrent, or if
   the servers are unavailable).

4.2.18.2.  The "location" Attribute

   metalink:url elements MAY have a "location" attribute, which is a
   [ISO3166] alpha-2 two letter country code for the geographical
   location of the physical server an IRI is used to access.

4.2.19.  The "metalink:version" Element

   The "metalink:version" element is a Text construct that conveys a
   human-readable version for a file.  The version of OpenOffice.org 3.0



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   would be "3.0".

   metalinkVersion =
      element metalink:version {
      metalinkTextConstruct
      }


5.  Client Implementation Considerations

   Metalink Processors that support HTTP MUST support transparent
   content negotiation with HTTP [RFC2295].  Transparent content
   negotiation is accomplished by adding the Metalink media type to the
   Accept request header.  Metalink Processors MUST check the returned
   content type, and if the Metalink media type is used, it MUST process
   the Metalink.  If the content type does not match the Metalink media
   type, then Metalink Processors SHOULD handle the response as a normal
   response.  Metalink Processors MUST NOT add the Metalink media type
   to Accept when requesting a URI from a metalink:url element, thus
   avoiding loops.  Metalink Processors SHOULD handle external redirects
   that might lead to a Metalink.

   When multiple hash types methods are provided, a Metalink Processor
   MAY verify using more than one of these hash types.  Metalink
   Processors are encouraged to check all hash types given which they
   are able to verify.


6.  Securing Metalink Documents

   Because Metalink is an XML-based format, existing XML security
   mechanisms can be used to secure its content.

   Producers of Metalinks may have sound reasons for signing otherwise-
   unprotected content.  For example, a merchant might digitally sign a
   Metalink that lists a file download to verify its origin.  Other
   merchants may wish to sign and encypt Metalinks that list digital
   songs that have been purchased.  Of course, many other examples exist
   as well.

   The algorithm requirements in this section pertain to the Metalink
   Processor.  They require that a recipient, at a minimum, be able to
   handle messages that use the specified cryptographic algorithms.
   These requirements do not limit the algorithms that the sender can
   choose.

   Metalink Processors that verify signed Metalink Documents MUST at
   least support XML-Signature and Syntax Processing [REC-xmldsig-core].



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7.  Extending Metalink

7.1.  Extensions from Non-Metalink Vocabularies

   This specification describes Metalink's XML markup vocabulary.
   Markup from other vocabularies ("foreign markup") can be used in an
   Metalink Document.

7.2.  Extensions to the Metalink Vocabulary

   The Metalink namespace is reserved for future forward-compatible
   revisions of Metalink.  Future versions of this specification could
   add new elements and attributes to the Metalink markup vocabulary.
   Software written to conform to this version of the specification will
   not be able to process such markup correctly and, in fact, will not
   be able to distinguish it from markup error.  For the purposes of
   this discussion, unrecognized markup from the Metalink vocabulary
   will be considered "foreign markup".

7.3.  Processing Foreign Markup

   Metalink Processors that encounter foreign markup in a location that
   is legal according to this specification MUST NOT stop processing or
   signal an error.  It might be the case that the Metalink Processor is
   able to process the foreign markup correctly and does so.  Otherwise,
   such markup is termed "unknown foreign markup".

   When unknown foreign markup is encountered as a child of metalink:
   file, metalink:metalink, Metalink Processors MAY bypass the markup
   and any textual content and MUST NOT change their behavior as a
   result of the markup's presence.

   When unknown foreign markup is encountered in a Text Construct,
   software SHOULD ignore the markup and process any text content of
   foreign elements as though the surrounding markup were not present.

7.4.  Extension Elements

   Metalink allows foreign markup anywhere in an Metalink document,
   except where it is explicitly forbidden.  Child elements of metalink:
   file and metalink:metalink are considered Metadata elements and are
   described below.  Child elements of Person constructs are considered
   to apply to the construct.  The role of other foreign markup is
   undefined by this specification.







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7.4.1.  Simple Extension Elements

   A Simple Extension element MUST NOT have any attributes or child
   elements.  The element MAY contain character data or be empty.
   Simple Extension elements are not Language-Sensitive.

   simpleExtensionElement =
      element * - metalink:* {
         text
      }

   The element can be interpreted as a simple property (or name/value
   pair) of the parent element that encloses it.  The pair consisting of
   the namespace-URI of the element and the local name of the element
   can be interpreted as the name of the property.  The character data
   content of the element can be interpreted as the value of the
   property.  If the element is empty, then the property value can be
   interpreted as an empty string.

7.4.2.  Structured Extension Elements

   The root element of a Structured Extension element MUST have at least
   one attribute or child element.  It MAY have attributes, it MAY
   contain well-formed XML content (including character data), or it MAY
   be empty.  Structured Extension elements are Language-Sensitive.

   structuredExtensionElement =
      element * - metalink:* {
         (attribute * { text }+,
            (text|anyElement)*)
       | (attribute * { text }*,
          (text?, anyElement+, (text|anyElement)*))
      }

   The structure of a Structured Extension element, including the order
   of its child elements, could be significant.

   This specification does not provide an interpretation of a Structured
   Extension element.  The syntax of the XML contained in the element
   (and an interpretation of how the element relates to its containing
   element) is defined by the specification of the Metalink extension.


8.  IANA Considerations







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8.1.  XML Namespace Registration

   This document makes use of the XML registry specified in [RFC3688].
   Accordingly, IANA has made the following registration:

   Registration request for the Metalink namespace:

   URI: urn:ietf:params:xml:ns:metalink

   Registrant Contact: See the "Author's Address" section of this
   document.

   XML: None.  Namespace URIs do not represent an XML specification.

8.2.  application/metalink+xml MIME type

   A Metalink Document, when serialized as XML 1.0, can be identified
   with the following media type:

   MIME media type name:  application
   MIME subtype name:  metalink+xml
   Mandatory parameters:  None.
   Optional parameters:
      "charset":  This parameter has semantics identical to the charset
         parameter of the "application/xml" media type as specified in
         [RFC3023].
   Encoding considerations:  Identical to those of "application/xml" as
      described in [RFC3023], Section 3.2.
   Security considerations:  As defined in this specification.
      In addition, as this media type uses the "+xml" convention, it
      shares the same security considerations as described in [RFC3023],
      Section 10.
   Interoperability considerations:  There are no known interoperability
      issues.
   Published specification:  This specification.
   Applications that use this media type:  No known applications
      currently use this media type.

   Additional information:

   Magic number(s):  As specified for "application/xml" in [RFC3023],
      Section 3.2.
   File extension:  .metalink
   Fragment identifiers:  As specified for "application/xml" in
      [RFC3023], Section 5.






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   Base URI:  As specified in [RFC3023], Section 6.
   Macintosh File Type code:  TEXT
   Person and email address to contact for further information:  Anthony
      Bryan <anthonybryan@gmail.com>
   Intended usage:  COMMON
   Author/Change controller:  IESG


9.  Security Considerations

   Publishers are encouraged to offer Metalink documents via
   authenticated HTTP under TLS as specified in [RFC2818].  Publishers
   are also encouraged to include digital signatures of the files within
   the Metalink Documents if they are available.

9.1.  URIs and IRIs

   Metalink Processors handle URIs and IRIs.  See Section 7 of [RFC3986]
   and Section 8 of [RFC3987] for security considerations related to
   their handling and use.

9.2.  Spoofing

   Metalink Processors should be aware of the potential for spoofing
   attacks where the attacker publishes Metalinks with false
   information.  Malicious publishers might create Metalink Documents
   containing inaccurate information anywhere in the document.  At best,
   this could deceive unaware downloaders that they are downloading a
   malicious or worthless file.  At worst, malicious publishers could
   attempt a distributed denial of service attack by inserting unrelated
   IRIs into Metalink Documents.

9.3.  Cryptographic Hashes

   Currently, some of the hash types defined in the IANA registry named
   "Hash Function Textual Names" are considered insecure.  These include
   the whole Message Digest family of algorithms which are not suitable
   for cryptographically strong verification.  Malicious people could
   provide files that appear to be identical to another file because of
   a collision, i.e. the weak cryptographic hashes match.

   Metalink Generators and Processors supporting verification SHOULD at
   least implement "sha-1" which is SHA1, as specified in [RFC3174].

9.4.  Signing

   Metalink Documents SHOULD be signed using [REC-xmldsig-core] and are
   subject to the security considerations implied by its use.  This



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   addresses the issue of spoofing.

   Digital signatures provide authentication, message integrity, and
   non-repudiation with proof of origin.


10.  References

10.1.  Normative References

   [BITTORRENT]
              Cohen, B., "The BitTorrent Protocol Specification",
              BITTORRENT 11031, February 2008,
              <http://www.bittorrent.org/beps/bep_0003.html>.

   [ISO3166]  International Organization for Standardization, "ISO 3166:
              1988 (E/F) - Codes for the representation of names of
              countries - The International Organization for
              Standardization, 3rd edition, 1988-08-15.", ISO Standard
              3166, 1988.

   [ISO639-2]
              International Organization for Standardization, "ISO 639-
              2:1998 - Codes for the representation of names of
              languages -- Part 2: Alpha-3 code  - edition 1,
              1998-11-01, 66 pages, prepared by a Joint Working Group of
              ISO TC46/SC4 and ISO TC37/SC2.", ISO Standard 639-2, 1998.

   [REC-xml]  Yergeau, F., Paoli, J., Bray, T., Sperberg-McQueen, C.,
              and E. Maler, "Extensible Markup Language (XML) 1.0
              (Fourth Edition)", World Wide Web Consortium
              Recommendation REC-xml-20060816, August 2006,
              <http://www.w3.org/TR/2006/REC-xml-20060816>.

   [REC-xml-infoset]
              Cowan, J. and R. Tobin, "XML Information Set (Second
              Edition)", World Wide Web Consortium Recommendation REC-
              xml-infoset-20040204, February 2004,
              <http://www.w3.org/TR/2004/REC-xml-infoset-20040204>.

   [REC-xml-names]
              Hollander, D., Bray, T., Tobin, R., and A. Layman,
              "Namespaces in XML 1.0 (Second Edition)", World Wide Web
              Consortium Recommendation REC-xml-names-20060816,
              August 2006,
              <http://www.w3.org/TR/2006/REC-xml-names-20060816>.

   [REC-xmlbase]



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              Marsh, J., "XML Base", W3C REC W3C.REC-xmlbase-20010627,
              June 2001,
              <http://www.w3.org/TR/2001/REC-xmlbase-20010627>.

   [REC-xmldsig-core]
              Solo, D., Reagle, J., and D. Eastlake, "XML-Signature
              Syntax and Processing", World Wide Web Consortium
              Recommendation REC-xmldsig-core-20020212, February 2002,
              <http://www.w3.org/TR/2002/REC-xmldsig-core-20020212>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2295]  Holtman, K. and A. Mutz, "Transparent Content Negotiation
              in HTTP", RFC 2295, March 1998.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC3023]  Murata, M., St. Laurent, S., and D. Kohn, "XML Media
              Types", RFC 3023, January 2001.

   [RFC3174]  Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1
              (SHA1)", RFC 3174, September 2001.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, July 2002.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              January 2004.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, January 2005.

10.2.  Informative References

   [ISO.8601.1988]
              International Organization for Standardization, "Data
              elements and interchange formats - Information interchange
              - Representation of dates and times", ISO Standard 8601,
              June 1988.

   [RELAX-NG]
              Clark, J., "RELAX NG Compact Syntax", December 2001, <http
              ://www.oasis-open.org/committees/relax-ng/



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              compact-20021121.html>.

   [RFC4287]  Nottingham, M. and R. Sayre, "The Atom Syndication
              Format", RFC 4287, December 2005.

   [W3C.NOTE-datetime-19980827]
              Wolf, M. and C. Wicksteed, "Date and Time Formats", W3C
              NOTE NOTE-datetime-19980827, August 1998,
              <http://www.w3.org/TR/1998/NOTE-datetime-19980827>.

   [W3C.REC-xmlschema-2-20041028]
              Malhotra, A. and P. Biron, "XML Schema Part 2: Datatypes
              Second Edition", W3C REC REC-xmlschema-2-20041028,
              October 2004,
              <http://www.w3.org/TR/2004/REC-xmlschema-2-20041028>.


Appendix A.  Acknowledgements and Contributors

   The layout and content of this document relies heavily on work
   pioneered in the Atom Syndication Format as specified in [RFC4287].

   The following people contributed to preliminary versions of this
   document: Paul Burkhead, Kristian Weston, Darius Liktorius, Michael
   Burford, Giorgio Maone, Manuel Subredu, A. Bram Neijt, Max Velasques,
   Manolo Valdes, Urs Wolfer, Frederick Cheung, Nils Maier, Hampus
   Wessman, Hayden Legendre, Danny Ayers, Nick Dominguez, Rene
   Leonhardt, Per Oyvind Karlsen, Gary Zellerbach, James Clark, Daniel
   Stenberg, Matt Domsch, Chris Newman, Lisa Dusseault, Ian Macfarlane,
   Dave Cridland, Julian Reschke, Barry Leiba, Uri Blumenthal, Paul
   Hoffman, Eran Hammer-Lahav, and Mark Nottingham.  The content and
   concepts within are a product of the Metalink community.

   The Metalink community has dozens of contributors who proposed ideas
   and wording for this document, or contributed to the evolution of
   Metalink, including:

   Nicolas Alvarez, Patrick Ruckstuhl, Mike Wells, Sebastien Willemijns,
   Micah Cowan, Dan Fandrich, Francis Giannaros, Yazsoft, Lukas
   Appelhans, KGet developers, FDM Team, Orbit Team, Arne
   Babenhauserheide, Mathias Berchtold, Xienzhenyu and TheWorld Browser
   Team, Xi Software, Bridget and Ethan Fletcher, Ruben Kerkhof,
   Agostino Russo, Gervase Markham, Salvatore and Robin Musumeci, Steve
   and Rachel Eshelman, Lucas Hewett, Ryan and Darren Cronin, Dave
   Winquist, Bob Denison, Wes Shelton, Kees Cook, Josh Colbert, Steve
   Kleisath, Chad Neptune, Nick Carrabba, Chris Carrabba, Erin Solari,
   Derick Cordoba, Ryan Alexander, John Sowder, Sandra Amisano, Tom
   Mainville, Janie Wargo, Jason Hansen, Tim Bray, Dan Brickley, Markus



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   Hofmann, Dan Connolly, Tim Berners-Lee, Harry Chen, Adrien Macneil,
   Louis Suarez-Potts, Ross Smith, Rahul Sundaram, Jesse Keating, Michal
   Bentkowski, Andrew Pantyukhin, Judd Vinet, Charles Landemaine, Pascal
   Bleser, Jeff@BLAG, Yuichiro Nakada, Jereme Hancock, Marcel Hauser,
   Jeff Covey, Doug Lang, Seth Brown, Alexander Lazic, Mayank Sharma,
   Robin Heggelund Hansen, Steve Langasek, Federico Parodi, Stefano
   Verna, Jason Green, James Linden, Matt Nederlanden, Aren Olsen, Dag
   Odenhall, Troy Sobotka, Corey Farwell, Ed Lee, Shawn Wilsher, Mike
   Connor, Anand Muttagi, Johan Svedberg, Dedric Carter, James Antill,
   Debi Goulding, the Anthony Family, the Bryan Family, Juanita Anthony
   and Zimmy Bryan.


Appendix B.  RELAX NG Compact Schema

   This appendix is informative.

   The Relax NG schema explicitly excludes elements in the Metalink
   namespace that are not defined in this revision of the specification.
   Requirements for Metalink Processors encountering such markup are
   given in Sections 7.2 and 7.3.

   # -*- rnc -*-
   # RELAX NG Compact Syntax Grammar for the
   # Metalink Format Specification Version 2

      namespace metalink = "urn:ietf:params:xml:ns:metalink"
      namespace xsd = "http://www.w3.org/2001/XMLSchema"


      # Common attributes

      metalinkCommonAttributes =
         attribute xml:base { metalinkUri }?,
         attribute xml:lang { metalinkLanguageTag }?,
         undefinedAttribute*

      # Text Constructs

      metalinkTextConstruct =
         metalinkCommonAttributes,
         text

      # Date Construct

      metalinkDateConstruct =
         metalinkCommonAttributes,
         xsd:dateTime



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   start =
     element metalink:metalink {
       element metalink:generator {
         attribute uri { metalinkUri }?,
         attribute version { text }?,
         metalinkTextConstruct
       }
       element metalink:origin { metalinkUri }?,
       element metalink:type { "static" | "dynamic"  }?,
       element metalink:published { metalinkDateConstruct }?,
       element metalink:updated { metalinkDateConstruct }?,
       element metalink:files {
         element metalink:file {
           attribute name { metalinkTextConstruct },
           element metalink:identity { metalinkTextConstruct }?,
           element metalink:version { metalinkTextConstruct }?,
           element metalink:size { xsd:integer }?,
           element metalink:description { metalinkTextConstruct }?,
           element metalink:license {
             attribute uri { metalinkUri }?,
             attribute name { metalinkTextConstruct }?,
           }?,
           element metalink:logo { metalinkUri }?,
           element metalink:publisher {
             attribute uri { metalinkUri }?,
             attribute name { metalinkTextConstruct }?,
           }?,
           element metalink:language { metalinkTextConstruct }?,
           element metalink:copyright { metalinkTextConstruct }?,
           element metalink:license { metalinkTextConstruct }?,
           element metalink:os { metalinkTextConstruct }?,
           element metalink:verification {
             hash+,
             element metalink:pieces {
               attribute length { metalinkTextConstruct },
               attribute type { metalinkTextConstruct },
               hash+
             }+,
             element metalink:signature {
              attribute type { "pgp" },
              text
              }+
           }?,
           element metalink:resources {
             element metalink:metadata {
               attribute preference { xsd:integer }?,
               attribute type { metalinkTextConstruct },
               metalinkUri



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             element metalink:url {
               attribute location { xsd:string {
               minLength = "2"  maxLength="2"}
               }?,
               attribute preference { xsd:integer }?,
               metalinkUri
             }+
           }
         }+
       }
     }
   hash =
     element metalink:hash {
       attribute piece { metalinkTextConstruct }?,
       attribute type { metalinkTextConstruct },
       text
     }

   # As defined in RFC 3066
   metalinkLanguageTag = xsd:string {
      pattern = "[A-Za-z]{1,8}(-[A-Za-z0-9]{1,8})*"
   }

   # Unconstrained; it's not entirely clear how IRI fit into
   # xsd:anyURI so let's not try to constrain it here
   metalinkUri = text

   # Simple Extension

   simpleExtensionElement =
      element * - metalink:* {
         text
      }

   # Structured Extension

   structuredExtensionElement =
      element * - metalink:* {
         (attribute * { text }+,
            (text|anyElement)*)
       | (attribute * { text }*,
          (text?, anyElement+, (text|anyElement)*))
      }

   # Other Extensibility

   extensionElement =
      simpleExtensionElement | structuredExtensionElement



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   undefinedAttribute =
     attribute * - (xml:base | xml:lang | local:*) { text }

   undefinedContent = (text|anyForeignElement)*

   anyElement =
      element * {
         (attribute * { text }
          | text
          | anyElement)*
      }

   anyForeignElement =
      element * - metalink:* {
         (attribute * { text }
          | text
          | anyElement)*
   }

   # EOF


Appendix C.  Document History

   [[ to be removed by the RFC editor before publication as an RFC. ]]

   Updated versions are kept in Subversion at
   http://metalinks.svn.sourceforge.net/viewvc/metalinks/internetdraft/
   and http://tools.ietf.org/html/draft-bryan-metalink

   Remaining issues: Section 5 - Content negotiation vs HTTP Link
   header.  Section 4.2.14 - Need to allow signatures other than PGP.

   -06
   o  Add authors and this Document History section.

   -05
   o  Clarifications.

   -04
   o  New IPR notice as required by IETF.
   o  Correct "metalink:pieces" Element text.
   o  Add hash examples.
   o  Slim down "Securing Metalink Documents" section.
   o  Recommend at least SHA1.

   -03




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   o  New namespace - urn:ietf:params:xml:ns:metalink
   o  Use the IANA registry named "Operating System Names" to define
      values for OS types.
   o  Add "Client Implementation Considerations" section, which includes
      Content Negotiation.

   -02
   o  Use the IANA registry named "Hash Function Textual Names" for hash
      types.
   o  metadata Element for listing .torrent, .metalink, etc.
   o  Remove type attribute for url Element.

   -01
   o  Clarify directory info in name attribute, hash types, add text for
      preference attribute.

   -00
   o  Initial draft; Text largely based on RFC 4287, ideas from Metalink
      3.0 specification.


Index

   A
      application/metalink+xml Media Type  25

   C
      copyright XML element  14

   D
      description XML element  14

   F
      file XML element  11
      files XML element  10

   G
      generator XML element  14
      Grammar
         metalinkCommonAttributes  7
         metalinkCopyright  14
         metalinkDateConstruct  8
         metalinkDescription  14
         metalinkFile  11
         metalinkFiles  10
         metalinkGenerator  15
         metalinkHash  15
         metalinkIdentity  16



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         metalinkLanguage  17
         metalinkLicense  17
         metalinkLogo  17
         metalinkMetalink  9
         metalinkOrigin  18
         metalinkOS  18
         metalinkPieces  14
         metalinkPublished  19
         metalinkPublisher  19
         metalinkResources  12
         metalinkSignature  19
         metalinkSize  20
         metalinkTextConstruct  7
         metalinkType  20
         metalinkUpdated  20
         metalinkURL  18, 21
         metalinkVerification  13
         metalinkVersion  22
         simpleExtensionElement  24
         structuredExtensionElement  24

   H
      hash XML element  15

   I
      identity XML element  16

   L
      language XML element  17
      license XML element  17
      logo XML element  17

   M
      Media Type
         application/metalink+xml  25
      metadata XML element  17
      metalink XML element  9
      metalinkCommonAttributes grammar production  7
      metalinkCopyright grammar production  14
      metalinkDateConstruct grammar production  8
      metalinkDescription grammar production  14
      metalinkFile grammar production  11
      metalinkFiles grammar production  10
      metalinkGenerator grammar production  15
      metalinkHash grammar production  15
      metalinkIdentity grammar production  16
      metalinkLanguage grammar production  17
      metalinkLicense grammar production  17



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      metalinkLogo grammar production  17
      metalinkMetalink grammar production  9
      metalinkOrigin grammar production  18
      metalinkOS grammar production  18
      metalinkPieces grammar production  14
      metalinkPublished grammar production  19
      metalinkPublisher grammar production  19
      metalinkResources grammar production  12
      metalinkSignature grammar production  19
      metalinkSize grammar production  20
      metalinkTextConstruct grammar production  7
      metalinkType grammar production  20
      metalinkUpdated grammar production  20
      metalinkURL grammar production  18, 21
      metalinkVerification grammar production  13
      metalinkVersion grammar production  22

   O
      origin XML element  18
      os XML element  18

   P
      pieces XML element  13
      published XML element  19
      publisher XML element  19

   R
      resources XML element  12

   S
      signature XML element  19
      simpleExtensionElement grammar production  24
      size XML element  20
      structuredExtensionElement grammar production  24

   T
      type XML element  20

   U
      updated XML element  20
      url XML element  21

   V
      verification XML element  13
      version XML element  21

   X
      XML Elements



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         copyright  14
         description  14
         entry  11
         files  10
         generator  14
         hash  15
         identity  16
         language  17
         license  17
         logo  17
         metadata  17
         metalink  9
         origin  18
         os  18
         pieces  13
         published  19
         publisher  19
         resources  12
         signature  19
         size  20
         type  20
         updated  20
         url  21
         verification  13
         version  21


Authors' Addresses

   Anthony Bryan (editor)
   Metalinker Project

   Email: anthonybryan@gmail.com
   URI:   http://www.metalinker.org


   Tatsuhiro Tsujikawa (editor)
   Metalinker Project

   Email: tatsuhiro.t@gmail.com
   URI:   http://aria2.sourceforge.net










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   Neil McNab (editor)
   Metalinker Project

   Email: nabber00@gmail.com
   URI:   http://www.nabber.org


   Peter Poeml (editor)
   Novell, Inc.

   Email: info@mirrorbrain.org
   URI:   http://www.mirrorbrain.org/







































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