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Network Working Group                                      A. Bryan, Ed.
Internet-Draft                                        Metalinker Project
Intended status: Experimental                            August 23, 2008
Expires: February 24, 2009


                The Metalink Download Description Format
                        draft-bryan-metalink-00

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

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

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   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on February 24, 2009.

Abstract

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













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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  . . . . . . . . . . .  8
       4.1.2.  The "metalink:files" Element . . . . . . . . . . . . .  9
       4.1.3.  The "metalink:file" Element  . . . . . . . . . . . . . 10
       4.1.4.  The "metalink:resources" Element . . . . . . . . . . . 12
       4.1.5.  The "metalink:verification" Element  . . . . . . . . . 12
       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  . . . . . . . . . . . 16
       4.2.7.  The "metalink:license" Element . . . . . . . . . . . . 16
       4.2.8.  The "metalink:logo" Element  . . . . . . . . . . . . . 17
       4.2.9.  The "metalink:origin" Element  . . . . . . . . . . . . 17
       4.2.10. The "metalink:os" Element  . . . . . . . . . . . . . . 17
       4.2.11. The "metalink:published" Element . . . . . . . . . . . 17
       4.2.12. The "metalink:publisher" Element . . . . . . . . . . . 18
       4.2.13. The "metalink:signature" Element . . . . . . . . . . . 18
       4.2.14. The "metalink:size" Element  . . . . . . . . . . . . . 18
       4.2.15. The "metalink:type" Element  . . . . . . . . . . . . . 19
       4.2.16. The "metalink:updated" Element . . . . . . . . . . . . 19
       4.2.17. The "metalink:url" Element . . . . . . . . . . . . . . 19
       4.2.18. The "metalink:version" Element . . . . . . . . . . . . 21
   5.  Securing Metalink Documents  . . . . . . . . . . . . . . . . . 21
     5.1.  Digital Signatures . . . . . . . . . . . . . . . . . . . . 21
     5.2.  Encryption . . . . . . . . . . . . . . . . . . . . . . . . 22
     5.3.  Signing and Encrypting . . . . . . . . . . . . . . . . . . 22
   6.  Extending Metalink . . . . . . . . . . . . . . . . . . . . . . 23
     6.1.  Extensions from Non-Metalink Vocabularies  . . . . . . . . 23
     6.2.  Extensions to the Metalink Vocabulary  . . . . . . . . . . 23
     6.3.  Processing Foreign Markup  . . . . . . . . . . . . . . . . 23
     6.4.  Extension Elements . . . . . . . . . . . . . . . . . . . . 23
       6.4.1.  Simple Extension Elements  . . . . . . . . . . . . . . 24



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       6.4.2.  Structured Extension Elements  . . . . . . . . . . . . 24
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25
     8.1.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     8.2.  IRIs . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     8.3.  Spoofing . . . . . . . . . . . . . . . . . . . . . . . . . 26
     8.4.  Encryption and Signing . . . . . . . . . . . . . . . . . . 26
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 26
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 28
   Appendix A.  Contributors  . . . . . . . . . . . . . . . . . . . . 29
   Appendix B.  RELAX NG Compact Schema . . . . . . . . . . . . . . . 30
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 36
   Intellectual Property and Copyright Statements . . . . . . . . . . 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.

1.1.  Examples

   A brief, single file Metalink Document:

   <?xml version="1.0" encoding="UTF-8" ?>
    <metalink version="3.0" xmlns="http://metalinker.org">
      <files>
        <file name="example.ext">
        <resources>
          <url>ftp://ftp.example.com/example.ext</url>
          <url>http://example.com/example.ext</url>
          <url>http://example.com/example.ext.torrent</url>
        </resources>
        </file>
      </files>
    </metalink>





















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   A more extensive, single file Metalink Document:

   <?xml version="1.0" encoding="UTF-8" ?>
    <metalink version="3.0" xmlns="http://metalinker.org">
      <published>2008-05-15T12:23:23Z</published>
      <files>
        <file name="example.ext">
        <description>A description of the example file for download.
        </description>
        <verification>
          <hash type="md5">83b1a04f18d6782cfe0407edadac377f</hash>
          <hash type="sha1">80bc95fd391772fa61c91ed68567f0980bb45fd9
          </hash>
        </verification>
        <resources>
          <url>ftp://ftp.example.com/example.ext</url>
          <url>http://example.com/example.ext</url>
          <url>http://example.com/example.ext.torrent</url>
        </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:

   http://www.metalinker.org/

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



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   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 = "http://www.metalinker.org"
   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.

   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



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   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 6 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:

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




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   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

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.





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

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



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   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.

   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.
   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.



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   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.
   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.




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   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.  Only
   relative paths are allowed.  Metalink Processors MUST NOT allow
   directory traversal.

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.

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

   This specification assigns no significance to the order of 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 one and SHOULD contain
      more than one metalink:url elements.

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 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



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   that has signed it.

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

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

   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 exactly one metalink:
      signature elements.

4.1.6.  The "metalink:pieces" Element

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

   metalinkPieces =
      element metalink:pieces {
         attribute length { metalinkTextConstruct },
         attribute type { "crc32" | "md4" | "md5" | "sha1" |
         "sha256" | "sha384" | "sha512" |  "rmd160" | "tiger" },
         hash+
      }+,

4.1.6.1.  The "type" Attribute

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

   This document defines nine initial values for hash types:
   1.  The value "crc32" signifies that the information is CRC32.
   2.  The value "md4" signifies that the hash information is MD4, as
       specified in [RFC1320].
   3.  The value "md5" signifies that the hash information is MD5, as
       specified in [RFC1321].
   4.  The value "sha1" signifies that the hash information is SHA1, as
       specified in [RFC3174].





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   5.  The value "sha256" signifies that the hash information is SHA-
       256, as specified in [RFC4634].
   6.  The value "sha384" signifies that the hash information is SHA-
       384, as specified in [RFC4634].
   7.  The value "sha512" signifies that the hash information is SHA-
       512, as specified in [RFC4634].
   8.  The value "rmd160" signifies that the hash information is RMD160,
       as specified in [RIPE].
   9.  The value "tiger" signifies that the hash information is TIGER.

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.

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



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   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 human-
   readable hash for a file.

   metalinkHash =
      element metalink:hash {
      attribute piece { xsd:integer }?,
      attribute type { "crc32" | "md4" | "md5" | "sha1" |
          "sha256" | "sha384" | "sha512" |  "rmd160" | "tiger" },
      text
      }

4.2.4.1.  The "type" Attribute

   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.  All hashes are in lowercase hexadecimal format.

   This document defines nine initial values for hash types:
   1.  The value "crc32" signifies that the information is CRC32.
   2.  The value "md4" signifies that the hash information is MD4, as
       specified in [RFC1320].
   3.  The value "md5" signifies that the hash information is MD5, as
       specified in [RFC1321].
   4.  The value "sha1" signifies that the hash information is SHA1, as
       specified in [RFC3174].
   5.  The value "sha256" signifies that the hash information is SHA-
       256, as specified in [RFC4634].
   6.  The value "sha384" signifies that the hash information is SHA-
       384, as specified in [RFC4634].





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   7.  The value "sha512" signifies that the hash information is SHA-
       512, as specified in [RFC4634].
   8.  The value "rmd160" signifies that the hash information is RMD160,
       as specified in [RIPE].
   9.  The value "tiger" signifies that the hash information is TIGER.

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.

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
      }

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



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   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: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)
   }

4.2.10.  The "metalink:os" Element

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

   metalinkOS =
      element metalink:os {
      metalinkTextConstruct
      }

4.2.11.  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.





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   metalinkPublished =
      element metalink:published {
      metalinkDateConstruct
      }

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

4.2.12.  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.

4.2.13.  The "metalink:signature" Element

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

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

4.2.13.1.  The "type" Attribute

   metalink:signature elements MUST have a "type" attribute.  The value
   of "type" MUST be the string that is non-empty and matches "pgp".

4.2.14.  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



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   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
      }

4.2.15.  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"
      }

4.2.16.  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.

4.2.17.  The "metalink:url" Element

   The "metalink:url" element contains the IRI of a file.  All IRIs
   should lead to identical files, except in the case of type
   "bittorrent" where the IRI should lead to a .torrent file.










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   metalinkURL =
      element metalink:url {
         metalinkCommonAttributes,
         attribute location { xsd:string {
           minLength = "2"  maxLength="2"}
           }?,
         attribute preference { xsd:integer }?,
         attribute type { "ftp" | "ftps" | "http" | "https" |
            "rsync" | "bittorrent" |  "magnet" | "ed2k" }?,
         metalinkUri
      }+

4.2.17.1.  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".

4.2.17.2.  The "type" Attribute

   metalink:url elements MAY have a "type" attribute that indicates the
   IRI type.

   This document defines eight initial values for IRI types:
   1.  The value "ftp" signifies that the IRI is accessed via FTP as
       specified in [RFC959].
   2.  The value "ftps" signifies that the IRI is accessed via FTPS as
       specified in [RFC4217].
   3.  The value "http" signifies that the IRI is accessed via HTTP as
       specified in [RFC2616].
   4.  The value "https" signifies that the IRI is accessed via HTTPS as
       specified in [RFC2818].
   5.  The value "rsync" signifies that the IRI is accessed via rsync.
   6.  The value "bittorrent" signifies that the IRI is a BitTorrent
       .torrent file as specified in [BITTORRENT].  Metalink Processors
       that do not support BitTorrent should ignore this type and also
       ignore metalink:url elements that end in ".torrent".
   7.  The value "magnet" signifies that the IRI is accessed via Magnet
       Link.
   8.  The value "ed2k" signifies that the IRI is accessed via ED2K
       network.

4.2.17.3.  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.



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4.2.18.  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
   would be "3.0".

   metalinkVersion =
      element metalink:version {
      metalinkTextConstruct
      }


5.  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 and/or
   encrypting 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.

5.1.  Digital Signatures

   The root of a Metalink Document (i.e., metalink:metalink MAY have an
   Enveloped Signature, as described by XML-Signature and Syntax
   Processing [REC-xmldsig-core].

   Metalink Processors MUST NOT reject an Metalink Document containing
   such a signature because they are not capable of verifying it; they
   MUST continue processing and MAY inform the user of their failure to
   validate the signature.

   In other words, the presence of an element with the namespace URI
   "http://www.w3.org/2000/09/xmldsig#" and a local name of "Signature"
   as a child of the document element MUST NOT cause an Metalink
   Processor to fail merely because of its presence.

   Other elements in an Metalink Document MUST NOT be signed unless
   their definitions explicitly specify such a capability.



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   Section 6.5.1 of [REC-xmldsig-core] requires support for Canonical
   XML [REC-xml-c14n].  However, many implementers do not use it because
   signed XML documents enclosed in other XML documents have their
   signatures broken.  Thus, Metalink Processors that verify signed
   Metalink Documents MUST be able to canonicalize with the exclusive
   XML canonicalization method identified by the URI
   "http://www.w3.org/2001/10/xml-exc-c14n#", as specified in Exclusive
   XML Canonicalization [REC-xml-exc-c14n].

   Section 4.4.2 of [REC-xmldsig-core] requires support for DSA
   signatures and recommends support for RSA signatures.  However,
   because of the much greater popularity in the market of RSA versus
   DSA, Metalink Processors that verify signed Metalink Documents MUST
   be able to verify RSA signatures, but do not need be able to verify
   DSA signatures.  Due to security issues that can arise if the keying
   material for message authentication code (MAC) authentication is not
   handled properly, Metalink Documents SHOULD NOT use MACs for
   signatures.

5.2.  Encryption

   The root of a Metalink Document (i.e., metalink:metalink in a
   Metalink Document MAY be encrypted, using the mechanisms described by
   XML Encryption Syntax and Processing [REC-xmlenc-core].

   Section 5.1 of [REC-xmlenc-core] requires support of TripleDES, AES-
   128, and AES-256.  Metalink Processors that decrypt Metalink
   Documents MUST be able to decrypt with AES-128 in Cipher Block
   Chaining (CBC) mode.

   Encryption based on [REC-xmlenc-core] does not ensure integrity of
   the original document.  There are known cryptographic attacks where
   someone who cannot decrypt a message can still change bits in a way
   where part or all the decrypted message makes sense but has a
   different meaning.  Thus, Metalink Processors that decrypt Metalink
   Documents SHOULD check the integrity of the decrypted document by
   verifying the hash in the signature (if any) in the document, or by
   verifying a hash of the document within the document (if any).

5.3.  Signing and Encrypting

   When an Metalink Document is to be both signed and encrypted, it is
   generally a good idea to first sign the document, then encrypt the
   signed document.  This provides integrity to the base document while
   encrypting all the information, including the identity of the entity
   that signed the document.  Note that, if MACs are used for
   authentication, the order MUST be that the document is signed and
   then encrypted, and not the other way around.



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6.  Extending Metalink

6.1.  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.

6.2.  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".

6.3.  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.

6.4.  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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6.4.1.  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.

6.4.2.  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.


7.  IANA Considerations

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




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   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.
   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


8.  Security Considerations

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

8.1.  URIs

   Metalink Processors handle URIs.  See Section 7 of [RFC3986].







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8.2.  IRIs

   Metalink Processors handle IRIs.  See Section 8 of [RFC3987].

8.3.  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.

8.4.  Encryption and Signing

   Metalink Documents can be encrypted and signed using
   [REC-xmlenc-core] and [REC-xmldsig-core], respectively, and are
   subject to the security considerations implied by their use.

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


9.  References

9.1.  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.




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   [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-c14n]
              Boyer, J., "Canonical XML Version 1.0", W3C REC REC-xml-
              c14n-20010315, March 2001,
              <http://www.w3.org/TR/2001/REC-xml-c14n-20010315>.

   [REC-xml-exc-c14n]
              Eastlake, D., Boyer, J., and J. Reagle, "Exclusive XML
              Canonicalization Version 1.0", W3C REC REC-xml-exc-c14n-
              20020718, July 2002,
              <http://www.w3.org/TR/2002/REC-xml-exc-c14n-20020718>.

   [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]
              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>.

   [REC-xmlenc-core]
              Eastlake, D. and J. Reagle, "XML Encryption Syntax and
              Processing", World Wide Web Consortium Recommendation REC-
              xmlenc-core-20021210, December 2002,
              <http://www.w3.org/TR/2002/REC-xmlenc-core-20021210>.

   [RFC1320]  Rivest, R., "The MD4 Message-Digest Algorithm", RFC 1320,



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              April 1992.

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              April 1992.

   [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.

   [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.

   [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.

   [RFC4217]  Ford-Hutchinson, P., "Securing FTP with TLS", RFC 4217,
              October 2005.

   [RFC4634]  Eastlake, D. and P. Jones, "US Secure Hash Algorithms (SHA
              and HMAC-SHA)", RFC 4634, July 2006.

   [RFC959]   Postel, J. and J. Reynolds, "FILE TRANSFER PROTOCOL
              (FTP)", RFC 959, October 1985.

   [RIPE]     Dobbertin, H., Bosselaers, A., and B. Preneel, "RIPEMD-
              160: A Strengthened Version of RIPEMD", RIPE RIPE,
              April 1996.

9.2.  Informative References

   [ISO.8601.1988]
              International Organization for Standardization, "Data
              elements and interchange formats - Information interchange



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              - 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/
              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.  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, Tatsuhiro Tsujikawa, A. Bram
   Neijt, Max Velasques, Manolo Valdes, Urs Wolfer, Frederick Cheung,
   Nils Maier, Hampus Wessman, Neil McNab, Hayden Legendre, Danny Ayers,
   Nick Dominguez, Rene Leonhardt, Per Oyvind Karlsen, Gary Zellerbach,
   James Clark, Tim Bray, Dan Brickley, Daniel Stenberg, Peter Poeml,
   and Matt Domsch.  The content and concepts within are a product of
   the Metalink community.

   The Metalink community has dozens of very active contributors who
   proposed ideas and wording for this document, 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 and Rachel Hewett, Ryan and Darren Cronin,



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   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, Markus 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, 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 6.2 and 6.3.

   # -*- rnc -*-
   # RELAX NG Compact Syntax Grammar for the
   # Metalink Format Specification Version 1

      namespace metalink = "http://www.metalinker.org"
      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




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      metalinkDateConstruct =
         metalinkCommonAttributes,
         xsd:dateTime


   start =
     element metalink {
       attribute version { "3.0" },
       element generator {
         attribute uri { metalinkUri }?,
         attribute version { text }?,
         metalinkTextConstruct
       }
       element origin { metalinkUri }?,
       element type { "static" | "dynamic"  }?,
       element published { metalinkDateConstruct }?,
       element updated { metalinkDateConstruct }?,
       element files {
         element file {
           attribute name { metalinkTextConstruct },
           element identity { metalinkTextConstruct }?,
           element version { metalinkTextConstruct }?,
           element size { xsd:integer }?,
           element description { metalinkTextConstruct }?,
           element license {
             attribute uri { metalinkUri }?,
             attribute name { metalinkTextConstruct }?,
           }?,
           element logo { metalinkUri }?,
           element publisher {
             attribute uri { metalinkUri }?,
             attribute name { metalinkTextConstruct }?,
           }?,
           element language { metalinkTextConstruct }?,
           element copyright { metalinkTextConstruct }?,
           element license { metalinkTextConstruct }?,
           element os { metalinkTextConstruct }?,
           element verification {
             hash+,
             element pieces {
               attribute length { metalinkTextConstruct },
               attribute type { "crc32" | "md4" | "md5" | "sha1" |
               "sha256" | "sha384" | "sha512" |  "rmd160" | "tiger" },
               hash+
             }+,
             element signature {
              attribute type { "pgp" },
              text



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              }
           }?,
           element resources {
             element url {
               attribute location { xsd:string {
               minLength = "2"  maxLength="2"}
               }?,
               attribute preference { xsd:integer }?,
               attribute type { "ftp" | "ftps" | "http" | "https" |
               "rsync" | "bittorrent" |  "magnet" | "ed2k" }?,
               metalinkUri
             }+
           }
         }+
       }
     }
   hash =
     element hash {
       attribute piece { metalinkTextConstruct }?,
       attribute type { "crc32" | "md4" | "md5" | "sha1" |
       "sha256" | "sha384" | "sha512" |  "rmd160" | "tiger" },
       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)*))



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      }

   # Other Extensibility

   extensionElement =
      simpleExtensionElement | structuredExtensionElement

   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


Index

   A
      application/metalink+xml Media Type  24

   C
      copyright XML element  14

   D
      description XML element  14

   F
      file XML element  10
      files XML element  9

   G
      generator XML element  14
      Grammar
         metalinkCommonAttributes  7



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         metalinkCopyright  14
         metalinkDateConstruct  8
         metalinkDescription  14
         metalinkFile  11
         metalinkFiles  10
         metalinkGenerator  14
         metalinkHash  15
         metalinkIdentity  16
         metalinkLanguage  16
         metalinkLicense  16
         metalinkLogo  17
         metalinkMetalink  9
         metalinkOrigin  17
         metalinkOS  17
         metalinkPieces  13
         metalinkPublished  18
         metalinkPublisher  18
         metalinkResources  12
         metalinkSignature  18
         metalinkSize  19
         metalinkTextConstruct  7
         metalinkType  19
         metalinkUpdated  19
         metalinkURL  20
         metalinkVerification  13
         metalinkVersion  21
         simpleExtensionElement  24
         structuredExtensionElement  24

   H
      hash XML element  15

   I
      identity XML element  16

   L
      language XML element  16
      license XML element  16
      logo XML element  17

   M
      Media Type
         application/metalink+xml  24
      metalink XML element  8
      metalinkCommonAttributes grammar production  7
      metalinkCopyright grammar production  14
      metalinkDateConstruct grammar production  8
      metalinkDescription grammar production  14



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      metalinkFile grammar production  11
      metalinkFiles grammar production  10
      metalinkGenerator grammar production  14
      metalinkHash grammar production  15
      metalinkIdentity grammar production  16
      metalinkLanguage grammar production  16
      metalinkLicense grammar production  16
      metalinkLogo grammar production  17
      metalinkMetalink grammar production  9
      metalinkOrigin grammar production  17
      metalinkOS grammar production  17
      metalinkPieces grammar production  13
      metalinkPublished grammar production  18
      metalinkPublisher grammar production  18
      metalinkResources grammar production  12
      metalinkSignature grammar production  18
      metalinkSize grammar production  19
      metalinkTextConstruct grammar production  7
      metalinkType grammar production  19
      metalinkUpdated grammar production  19
      metalinkURL grammar production  20
      metalinkVerification grammar production  13
      metalinkVersion grammar production  21

   O
      origin XML element  17
      os XML element  17

   P
      pieces XML element  13
      published XML element  17
      publisher XML element  18

   R
      resources XML element  12

   S
      signature XML element  18
      simpleExtensionElement grammar production  24
      size XML element  18
      structuredExtensionElement grammar production  24

   T
      type XML element  19

   U
      updated XML element  19
      url XML element  19



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   V
      verification XML element  12
      version XML element  21

   X
      XML Elements
         copyright  14
         description  14
         entry  10
         files  9
         generator  14
         hash  15
         identity  16
         language  16
         license  16
         logo  17
         metalink  8
         origin  17
         os  17
         pieces  13
         published  17
         publisher  18
         resources  12
         signature  18
         size  18
         type  19
         updated  19
         url  19
         verification  12
         version  21


Author's Address

   Anthony Bryan (editor)
   Metalinker Project

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












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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

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


Intellectual Property

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

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

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











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