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Network Working Group                                           A. Bryan
Internet-Draft                                              T. Tsujikawa
Intended status: Standards Track                                N. McNab
Expires: May 30, 2010
                                                                P. Poeml
                                                            Novell, Inc.
                                                       November 26, 2009


                The Metalink Download Description Format
                        draft-bryan-metalink-23

Abstract

   This document specifies Metalink, an XML-based download description
   format.  Metalink describes download locations (mirrors), checksums,
   and other information.  Clients can transparently use this
   information to reliably transfer files.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on May 30, 2010.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal



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   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the BSD License.

   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.































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Namespace and Version  . . . . . . . . . . . . . . . . . .  7
     1.3.  Notational Conventions . . . . . . . . . . . . . . . . . .  7
   2.  Metalink Documents . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Common Metalink Constructs . . . . . . . . . . . . . . . . . .  9
     3.1.  Text Constructs  . . . . . . . . . . . . . . . . . . . . .  9
       3.1.1.  Text . . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.2.  Date Constructs  . . . . . . . . . . . . . . . . . . . . .  9
   4.  Metalink Element Definitions . . . . . . . . . . . . . . . . . 10
     4.1.  Container Elements . . . . . . . . . . . . . . . . . . . . 10
       4.1.1.  The "metalink:metalink" Element  . . . . . . . . . . . 10
       4.1.2.  The "metalink:file" Element  . . . . . . . . . . . . . 11
       4.1.3.  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:dynamic" Element . . . . . . . . . . . . 14
       4.2.4.  The "metalink:generator" Element . . . . . . . . . . . 15
       4.2.5.  The "metalink:hash" Element  . . . . . . . . . . . . . 15
       4.2.6.  The "metalink:identity" Element  . . . . . . . . . . . 16
       4.2.7.  The "metalink:language" Element  . . . . . . . . . . . 17
       4.2.8.  The "metalink:license" Element . . . . . . . . . . . . 17
       4.2.9.  The "metalink:logo" Element  . . . . . . . . . . . . . 17
       4.2.10. The "metalink:metaurl" Element . . . . . . . . . . . . 18
       4.2.11. The "metalink:origin" Element  . . . . . . . . . . . . 19
       4.2.12. The "metalink:os" Element  . . . . . . . . . . . . . . 19
       4.2.13. The "metalink:published" Element . . . . . . . . . . . 19
       4.2.14. The "metalink:publisher" Element . . . . . . . . . . . 19
       4.2.15. The "metalink:signature" Element . . . . . . . . . . . 20
       4.2.16. The "metalink:size" Element  . . . . . . . . . . . . . 20
       4.2.17. The "metalink:updated" Element . . . . . . . . . . . . 21
       4.2.18. The "metalink:url" Element . . . . . . . . . . . . . . 21
       4.2.19. The "metalink:version" Element . . . . . . . . . . . . 21
   5.  Securing Metalink Documents  . . . . . . . . . . . . . . . . . 22
   6.  Extending Metalink . . . . . . . . . . . . . . . . . . . . . . 22
     6.1.  Extensions from Non-Metalink Vocabularies  . . . . . . . . 22
     6.2.  Extensions to the Metalink Vocabulary  . . . . . . . . . . 22
     6.3.  Processing Foreign Markup  . . . . . . . . . . . . . . . . 23
     6.4.  Extension Elements . . . . . . . . . . . . . . . . . . . . 23
       6.4.1.  Simple Extension Elements  . . . . . . . . . . . . . . 23
       6.4.2.  Structured Extension Elements  . . . . . . . . . . . . 23
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
     7.1.  XML Namespace Registration . . . . . . . . . . . . . . . . 24
     7.2.  application/metalink4+xml MIME type  . . . . . . . . . . . 24
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25



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     8.1.  URIs and IRIs  . . . . . . . . . . . . . . . . . . . . . . 25
     8.2.  Spoofing . . . . . . . . . . . . . . . . . . . . . . . . . 25
     8.3.  Cryptographic Hashes . . . . . . . . . . . . . . . . . . . 26
     8.4.  Signing  . . . . . . . . . . . . . . . . . . . . . . . . . 26
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 26
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 28
   Appendix A.  Acknowledgements and Contributors . . . . . . . . . . 28
   Appendix B.  RELAX NG Compact Schema . . . . . . . . . . . . . . . 29
   Appendix C.  Document History (to be removed by RFC Editor
                before publication) . . . . . . . . . . . . . . . . . 34
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 40






































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1.  Introduction

   Metalink is an XML-based document format that describes a file or
   list of files to be downloaded from a server.  Metalinks can list a
   number of files, each with an extensible set of attached metadata.
   Each listed file can have a description, checksum, and a list of URIs
   that it is available from.

   Often, identical copies of a file are accessible in multiple
   locations on the Internet over a variety of protocols (FTP, HTTP, and
   Peer-to-Peer).  In some cases, users are shown a list of these
   multiple download locations (mirror servers) and must manually select
   one based on geographical location, priority, or bandwidth.  This is
   done to distribute the load across multiple servers, and to give
   human users the opportunity to choose a download location that they
   expect to work best for them.

   At times, individual servers can be slow, outdated, or unreachable,
   but this can not be determined until the download has been initiated.
   This can lead to the user canceling the download and needing to
   restart it.  During downloads, errors in transmission can corrupt the
   file.  There are no easy ways to repair these files.  For large
   downloads this can be especially troublesome.  Any of the number of
   problems that can occur during a download lead to frustration on the
   part of users, and bandwith wasted with retransmission.

   Knowledge about availability of a download on mirror servers can be
   acquired and maintained by the operators of the origin server, or by
   third a party.  This knowledge, together with checksums, digital
   signatures, and more can be stored in a machine-readable Metalink
   file.  The Metalink file can transfer this knowledge to the user
   agent, which can peruse it in automatic ways or present the
   information to a human user.  User agents can fall back to alternate
   mirrors if the current one has an issue.  Thereby, clients are
   enabled to work their way to a successful download even under adverse
   circumstances.  All this can be done transparently to the human user
   and the download is much more reliable and efficient.  In contrast, a
   traditional HTTP redirect to one mirror conveys only comparatively
   minimal information - a referral to a single server, and there is no
   provision in the HTTP protocol to handle failures.

   Other features that some clients provide include multi-source
   downloads, where chunks of a file are downloaded from multiple
   mirrors (and optionally, Peer-to-Peer) simultaneously, which
   frequently results in a faster download.  Metalinks can leverage
   HTTP, FTP and Peer-to-Peer protocols together, because regardless
   over which protocol the Metalink was obtained, it can make a resource
   accessible through other protocols.  If the Metalink was obtained



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   from a trusted source, included verification metadata can solve trust
   issues when downloading files from replica servers operated by third
   parties.  Metalinks also provide structured information about
   downloads that can be indexed by search engines.

   [[ Discussion of this draft should take place on
   discuss@apps.ietf.org.  Past discussion has gone on at the Metalink
   discussion mailing list located at
   metalink-discussion@googlegroups.com /
   http://groups.google.com/group/metalink-discussion . ]]

1.1.  Examples

   A brief, Metalink Document that describes a single file:

   <?xml version="1.0" encoding="UTF-8"?>
   <metalink xmlns="urn:ietf:params:xml:ns:metalink">
     <file name="example.ext">
       <size>
         14471447
       </size>
       <url>
          ftp://ftp.example.com/example.ext
       </url>
       <url>
          http://example.com/example.ext
       </url>
       <metaurl type="torrent">
          http://example.com/example.ext.torrent
       </metaurl>
     </file>
   </metalink>

   A more extensive, Metalink Document that describes two files:

   <?xml version="1.0" encoding="UTF-8"?>
   <metalink xmlns="urn:ietf:params:xml:ns:metalink">
     <published>
        2009-05-15T12:23:23Z
     </published>
     <file name="example.ext">
       <size>
          14471447
       </size>
       <identity>
          Example
       </identity>
       <version>



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          1.0
       </version>
       <language>
          en
       </language>
       <description>
          A description of the example file for download.
       </description>
       <hash type="sha-1">
          80bc95fd391772fa61c91ed68567f0980bb45fd9
       </hash>
       <url location="de" priority="1">
          ftp://ftp.example.com/example.ext
       </url>
       <url location="fr" priority="1">
          http://example.com/example.ext
       </url>
       <metaurl type="torrent" priority="2">
          http://example.com/example.ext.torrent
       </metaurl>
     </file>
     <file name="example2.ext">
       <size>
          14471447
       </size>
       <identity>
          Example
       </identity>
       <version>
          1.0
       </version>
       <language>
          en
       </language>
       <description>
          Another description for a second file.
       </description>
       <hash type="sha-1">
          7f4afdc84bd1b79238703fb45cd045efd08bee4b
       </hash>
       <url location="de" priority="1">
          ftp://ftp.example.com/example2.ext
       </url>
       <url location="fr" priority="1">
          http://example.com/example2.ext
       </url>
       <metaurl type="torrent" priority="2">
          http://example.com/example2.ext.torrent



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       </metaurl>
     </file>
   </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
   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



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   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/
   metalink4+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, encoded according to [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
   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 Documents that do not follow this specification are invalid,
   and partially or wholly unusable to Metalink Processors.

   Metalink is an extensible format.  See Section 6 of this document for
   a full description of how Metalink Documents can be extended.






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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>
   ...

   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, white space could be collapsed (including line
   breaks) and text could be displayed 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.




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   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>2009-05-15T18:30:02Z</updated>
   <updated>2009-05-15T18:30:02.25Z</updated>
   <updated>2009-05-15T18:30:02+01:00</updated>
   <updated>2009-05-15T18:30:02.25+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 Metalink Documents 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 or more
   metalink:file child elements which consist of metadata elements.

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

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





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   o  metalink:metalink elements MAY contain exactly one metalink:
      dynamic element.
   o  metalink:metalink elements MUST contain one or more metalink:file
      elements.
   o  metalink:metalink elements MAY contain exactly one metalink:
      generator element.
   o  metalink:metalink elements SHOULD contain exactly one metalink:
      origin element.
   o  metalink:metalink elements MAY contain exactly one metalink:
      published 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:file" Element

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

   All metalink:url elements SHOULD lead to identical files.  That is,
   each metalink:url element should be an alternative location for the
   same file and each metalink:metaurl element should provide metadata
   to retrieve the same file in another way, such as a peer to peer
   network.
















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   metalinkFile =
      element metalink:file {
         metalinkCommonAttributes,
         attribute name { text },
         (metalinkCopyright?
          & metalinkDescription?
          & metalinkHash*
          & metalinkIdentity?
          & metalinkLanguage?
          & metalinkLicense?
          & metalinkLogo?
          & metalinkMetaURL*
          & metalinkURL*
          & metalinkOS*
          & metalinkPieces*
          & metalinkPublisher?
          & metalinkSignature?
          & metalinkSize?
          & metalinkVersion?
          & extensionElement*)
      }

   This specification assigns no significance to the order of metalink:
   file elements or to the order of metalink:url or metalink:metaurl
   elements.  Significance is determined by the value of the "priority"
   attribute of the metalink:url or metalink:metaurl 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 MAY contain exactly one metalink:copyright
      element.
   o  metalink:file elements MAY contain exactly one metalink:
      description element.
   o  metalink:file elements MAY contain exactly one metalink:identity
      element.
   o  metalink:file elements MAY contain one or more metalink:hash
      elements.
   o  metalink:file elements MAY contain exactly one metalink:language
      element.
   o  metalink:file elements MAY contain exactly one metalink:license
      element.
   o  metalink:file elements MAY contain exactly one metalink:logo
      element.
   o  metalink:file elements MAY contain one or more metalink:os
      element.





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   o  metalink:file elements MUST contain at least one metalink:url
      element or at least one metalink:metaurl element.  Typically,
      metalink:file elements contain more than one metalink:url element
      to provide multiple download sources.
   o  metalink:file elements MAY contain one or more metalink:pieces
      elements.
   o  metalink:file elements MAY contain exactly one metalink:publisher
      element.
   o  metalink:file elements MAY contain one or more metalink:signature
      elements.
   o  metalink:file elements SHOULD contain exactly one metalink:size
      element.
   o  metalink:file elements MAY contain exactly one metalink:version
      element.

4.1.2.1.  The "name" Attribute

   metalink:file elements MUST have a "name" attribute, which contains
   the filename of the file to be 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 NOT contain any directory traversal directives or
   information.  The path MUST be relative.  The path MUST NOT begin
   with a "/", "./" or "../", contain "/../", or end with "/..".

4.1.3.  The "metalink:pieces" Element

   The "metalink:pieces" element acts as a container for a list of
   checksums of non-overlapping pieces of the file.  The checksums MUST
   be listed in the same order as the corresponding pieces appear in the
   file, starting at the beginning of the file.

   metalinkPieces =
      element metalink:pieces {
         attribute length { xsd:positiveInteger },
         attribute type { text },
         metalinkHash+
      }







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4.1.3.1.  The "type" Attribute

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

   The IANA registry named "Hash Function Textual Names" defines values
   for hash types.  If a Metalink Document contains hashes, it SHOULD
   include "sha-1" which is SHA-1 as specified in [RFC3174], or a
   stronger hash.  It MAY also include other hashes.

4.1.3.2.  The "length" Attribute

   metalink:pieces elements MUST have a "length" attribute, which is an
   positive integer that describes the length of the pieces of the file
   in octets.  The whole file is divided into non-overlapping pieces of
   this length, starting from the beginning of the file.  That is, every
   piece should be the same size, apart from the last piece which is the
   remainder.  The last piece extends to the end of the file, and can
   therefore be shorter than the other pieces.

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:dynamic" Element

   The "metalink:dynamic" element is a Text construct that describes
   whether the IRI from "metalink:origin" in a Metalink will contain
   dynamic updated information or if it is static and not likely to be
   updated.




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   metalinkDynamic =
      element metalink:dynamic {
         "true" | "false"
      }

4.2.4.  The "metalink:generator" Element

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

   metalinkGenerator =
      element metalink:generator {
        metalinkTextConstruct
      }

   The content of this element, when present, MUST be a string that is a
   human-readable name and version, separated by a "/", for the
   generating agent.  For example, "MirrorBrain/2.9.2", where
   "MirrorBrain" is the name and "2.9.2" is the version.  Entities such
   as "&amp;" and "&lt;" represent their corresponding characters ("&"
   and "<" respectively), not markup.

4.2.5.  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.
   Hashes are used to verify the integrity of a complete file or portion
   of a file to determine if the file has been transferred without any
   errors.

   metalinkHash =
      element metalink:hash {
        attribute type { text }?,
        text
      }

   metalink:hash elements with a "type" attribute MUST contain a hash of
   the complete file.  Metalink Documents can contain one or multiples
   hashes of a complete file.  In this example, both SHA-1 and SHA-256
   hashes are included.

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




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   Metalink Documents can also contain hashes for individual pieces of a
   file. metalink:hash elements that are inside a metalink:pieces
   container element have a hash for that specific piece or chunk of the
   file, and are of the same hash type as the metalink:pieces element
   they are contained in. metalink:hash elements without a "type"
   attribute MUST contain a hash for that specific piece or chunk of the
   file and MUST be listed in the same order as the corresponding pieces
   appear in the file, starting at the beginning of the file.  The size
   of the piece is equal to the value of the "length" attribute of the
   metalink:pieces element.  The whole file is divided into non-
   overlapping pieces of this length, starting from the beginning of the
   file.  That is, every piece should be the same size, apart from the
   last piece which is the remainder.  The last piece extends to the end
   of the file, and can therefore be shorter than the other pieces.

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

4.2.5.1.  The "type" Attribute

   metalink:hash elements MUST have a "type" attribute, if and only if
   it contains a hash of the complete file.  The IANA registry named
   "Hash Function Textual Names" defines values for hash types.  If a
   Metalink Document contains hashes, it SHOULD include "sha-1" which is
   SHA-1 as specified in [RFC3174], or a stronger hash.  It MAY also
   include additional hash types.

4.2.6.  The "metalink:identity" Element

   The "metalink:identity" element is a Text construct that conveys a
   human-readable identity for a file.  For example, the identity of
   Firefox 3.5 would be "Firefox".

   metalinkIdentity =
      element metalink:identity {
        metalinkTextConstruct
      }





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4.2.7.  The "metalink:language" Element

   The "metalink:language" element is a Text construct that conveys a
   code for the language of a file, per [RFC5646].

   metalinkLanguage =
      element metalink:language {
        metalinkTextConstruct
      }

4.2.8.  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 url { metalinkUri }?,
        attribute name { text }?
      }

   The metalink:license element MAY have a "url" 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.

4.2.9.  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.







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4.2.10.  The "metalink:metaurl" Element

   The "metalink:metaurl" element contains the IRI of a metadata file,
   also known as a metainfo file, about a resource to download.  For
   example, this could be the IRI of a BitTorrent .torrent file, a
   Metalink Document, or other type of metadata file.  Note that the
   information in the metalink:hash element does not apply to these
   metadata files, but to the files that are described by them.

   metalinkMetaURL =
      element metalink:metaurl {
         metalinkCommonAttributes,
         attribute priority { xsd:positiveInteger }?,
         attribute type { text },
         attribute name { text }?,
         metalinkUri
      }

4.2.10.1.  The "priority" Attribute

   metalink:metaurl elements MAY have a priority attribute.  Values MUST
   be positive integers.  Lower values indicate a higher priority.
   Multiple metalink:metaurl elements can have the same priority, i.e.
   three BitTorrent .torrent files could have priority="1".  See also
   the "priority" attribute of the metalink:url element.

4.2.10.2.  The "type" Attribute

   metalink:metaurl elements MUST have a "type" attribute that indicates
   the MIME media type [RFC4288] 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.

4.2.10.3.  The "name" Attribute

   metalink:metaurl elements MAY have a "name" attribute that indicates
   a specific file in a BitTorrent .torrent file or a Metalink Document
   that describes multiple files.

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

   <metaurl type="torrent" name="debian-amd64/sarge/Contents-amd64.gz">

   In this example, a file named "Contents-amd64.gz" is indicated, in a
   "debian-amd64/sarge/" subdirectory.  The path MUST NOT contain any
   directory traversal directives or information.  The path MUST be



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   relative.  The path MUST NOT begin with a "/", "./" or "../", contain
   "/../", or end with "/..".

4.2.11.  The "metalink:origin" Element

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

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

4.2.12.  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
      }

4.2.13.  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.

4.2.14.  The "metalink:publisher" Element

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







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   metalinkPublisher =
      element metalink:publisher {
        metalinkCommonAttributes,
        attribute url { metalinkUri }?,
        attribute name { text }?
      }

   The metalink:publisher element MAY have a "url" 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.

4.2.15.  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.
   Digital signatures verify that a file is from the entity that has
   signed it.

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

4.2.15.1.  The "type" Attribute

   metalink:signature elements MUST have a "type" attribute.  The
   initial 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.

4.2.16.  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.  This value MUST be a non-negative integer.

   metalinkSize =
      element metalink:size {
        xsd:nonNegativeInteger
      }





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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.

4.2.18.  The "metalink:url" Element

   The "metalink:url" element contains the IRI of a file.  Most Metalink
   Documents will contain multiple metalink:url elements, and each one
   SHOULD be a valid alternative to download the same file.

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

4.2.18.1.  The "priority" Attribute

   metalink:url elements MAY have a priority attribute.  Values MUST be
   positive integers.  Lower values indicate a higher priority.
   Multiple metalink:url elements can have the same priority, i.e. ten
   mirrors could have priority="1".

4.2.18.2.  The "location" Attribute

   metalink:url elements MAY have a "location" attribute, which is a
   [ISO3166-1] 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 Firefox 3.5 would
   be "3.5".



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   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 Metalink Documents 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 encrypt Metalink
   Documents that list digital songs that have been purchased.  Of
   course, many other examples are conceivable 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].


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".





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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.  The role of other foreign markup is undefined by
   this specification.

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



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   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

7.1.  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.

7.2.  application/metalink4+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:  metalink4+xml
   Mandatory parameters:  None.







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   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:  .meta4
   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 as specified in [RFC2818].  Publishers
   are also encouraged to include digital signatures of the files within
   the Metalink Documents if they are available.

8.1.  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.

8.2.  Spoofing

   There is potential for spoofing attacks where the attacker publishes
   Metalink Documents with false information.  Malicious publishers



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   might create Metalink Documents containing inaccurate information
   anywhere in the document.  Unaware downloaders could be deceived into
   downloading a malicious or worthless file.  Malicious publishers
   could attempt a distributed denial of service attack by inserting
   unrelated IRIs into Metalink Documents.

8.3.  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 of the intended file
   and a substituted malicious file could match.

   If a Metalink Document contains hashes, it SHOULD include "sha-1"
   which is SHA-1, as specified in [RFC3174], or stronger.  It MAY also
   include other hashes from the IANA registry named "Hash Function
   Textual Names".

8.4.  Signing

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

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


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>.

   [ISO3166-1]
              International Organization for Standardization, "ISO 3166-
              1:2006.  Codes for the representation of names of
              countries and their subdivisions -- Part 1: Country
              codes", November 2006.

   [REC-xml]  Yergeau, F., Paoli, J., Bray, T., Sperberg-McQueen, C.,
              and E. Maler, "Extensible Markup Language (XML) 1.0



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              (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]
              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 (Second Edition)", World Wide Web
              Consortium Recommendation REC-xmldsig-core-20080610,
              June 2008,
              <http://www.w3.org/TR/2008/REC-xmldsig-core-20080610>.

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

   [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



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              Identifiers (IRIs)", RFC 3987, January 2005.

   [RFC4288]  Freed, N. and J. Klensin, "Media Type Specifications and
              Registration Procedures", BCP 13, RFC 4288, December 2005.

   [RFC5646]  Phillips, A. and M. Davis, "Tags for Identifying
              Languages", BCP 47, RFC 5646, September 2009.

9.2.  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/
              compact-20021121.html>.

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

   [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 content and concepts within are a product of the Metalink
   community.

   The Metalink community has dozens of contributors who proposed ideas



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   and wording for this document, or contributed to the evolution of
   Metalink, including:

   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, Yazsoft, KGet developers, FDM Team, Orbit Team, Arne
   Babenhauserheide, Mathias Berchtold, Xienzhenyu and TheWorld Browser
   Team, Xi Software, Nicolas Alvarez, Patrick Ruckstuhl, Sebastien
   Willemijns, Micah Cowan, Bridget and Ethan Fletcher, Ruben Kerkhof,
   Agostino Russo, Salvatore Musumeci, Steve Eshelman, Lucas Hewett,
   Ryan 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 & Sandra Sowder,
   Tom Mainville, Janie Wargo, Jason Hansen, Tim Bray, Dan Brickley,
   Markus Hofmann, Dan Connolly, Tim Berners-Lee, Louis Suarez-Potts,
   Ross Smith, Jeff Covey, Ed Lee, Shawn Wilsher, Mike Connor, Johan
   Svedberg, Dedric Carter, James Antill, Debi Goulding, Chris Newman,
   Lisa Dusseault, Ian Macfarlane, Dave Cridland, Julian Reschke, Barry
   Leiba, Uri Blumenthal, Paul Hoffman, Felix Sasaki, Matthias Fuchs,
   Mark Baker, Eran Hammer-Lahav, Mark Nottingham, and Peter Saint-
   Andre.  We also thank 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 4

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

   # Common attributes

   metalinkCommonAttributes =
      attribute xml:base { metalinkUri }?,



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      attribute xml:lang { metalinkLanguageTag }?,
      undefinedAttribute*

   # Text Constructs

   metalinkTextConstruct =
      metalinkCommonAttributes,
      text

   # Date Construct

   metalinkDateConstruct =
      metalinkCommonAttributes,
      xsd:dateTime

   start = metalinkMetalink

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

   metalinkFile =
      element metalink:file {
         metalinkCommonAttributes,
         attribute name { text },
         (metalinkCopyright?
          & metalinkDescription?
          & metalinkIdentity?
          & metalinkHash*
          & metalinkLanguage?
          & metalinkLicense?
          & metalinkLogo?
          & metalinkMetaURL*
          & metalinkURL*
          & metalinkOS*
          & metalinkPieces*
          & metalinkPublisher?
          & metalinkSignature?
          & metalinkSize?
          & metalinkVersion?



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          & extensionElement*)
      }

   metalinkPieces =
      element metalink:pieces {
         attribute length { xsd:positiveInteger },
         attribute type { text },
         metalinkHash+
      }

   metalinkCopyright =
      element metalink:copyright {
         metalinkTextConstruct
      }

   metalinkDescription =
      element metalink:description {
         metalinkTextConstruct
      }

   metalinkDynamic =
      element metalink:dynamic {
         "true" | "false"
      }

   metalinkGenerator =
      element metalink:generator {
         metalinkTextConstruct
      }

   metalinkHash =
        element metalink:hash {
          attribute type { text }?,
          text
        }

   metalinkIdentity =
      element metalink:identity {
         metalinkTextConstruct
      }

   metalinkLanguage =
      element metalink:language {
         metalinkTextConstruct
      }

   metalinkLicense =
      element metalink:license {



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         metalinkCommonAttributes,
         attribute url { metalinkUri }?,
         attribute name { text }?
      }

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

   metalinkMetaURL =
      element metalink:metaurl {
         metalinkCommonAttributes,
         attribute priority { xsd:positiveInteger }?,
         attribute type { text },
         attribute name { text }?,
         metalinkUri
      }

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

   metalinkOS =
      element metalink:os {
         metalinkTextConstruct
      }

   metalinkPublished =
      element metalink:published {
         metalinkDateConstruct
      }

   metalinkPublisher =
      element metalink:publisher {
         metalinkCommonAttributes,
         attribute url { metalinkUri }?,
         attribute name { text }?
      }

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



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   metalinkSize =
      element metalink:size {
         xsd:nonNegativeInteger
      }

   metalinkUpdated =
      element metalink:updated {
         metalinkDateConstruct
      }

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

   metalinkVersion =
      element metalink:version {
         metalinkTextConstruct
      }

   # As defined in RFC 3066 and compatible with RFC 5646
   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 }*,



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          (text?, anyElement+, (text|anyElement)*))
      }

   # 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


Appendix C.  Document History (to be removed by RFC Editor before
             publication)

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

   Updated versions can be found at
   http://tools.ietf.org/html/draft-bryan-metalink with frequent updates
   in Subversion at
   http://metalinks.svn.sourceforge.net/viewvc/metalinks/internetdraft/

   Known issues concerning this draft:
   o  None.

   -23 : November 26, 2009.
   o  Apps Area AD review changes, Change RFC3688 from Normative to
      Informative Reference.





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   o  Schema: integer changed to positiveInteger or nonNegativeInteger
      where fitting.

   -22 : November 09, 2009.
   o  Clarifications.

   -21 : October 13, 2009.
   o  Update author details.

   -20 : October 12, 2009.
   o  RFC 5646 updates RFC 4646.

   -19 : October 5, 2009.
   o  Remove organization for independent authors.

   -18 : October 4, 2009.
   o  File extension: .meta4
   o  Hashes clarification, modified to allow multiple metalink:os
      elements, add size element to example.

   -17 : September 28, 2009.
   o  Typo correction.

   -16 : August 31, 2009.
   o  Clarifications.

   -15 : August 26, 2009.
   o  Rename "preference" attribute of metaurl and url elements to
      "priority", where lower values indicate higher priority.

   -14 : August 24, 2009.
   o  Update abstract and introduction.

   -13 : August 21, 2009.
   o  Remove files, resources, verification container elements.
   o  MIME type: application/metalink4+xml

   -12 : August 18, 2009.
   o  Remove "piece" attribute from hash elements in pieces container
      elements.
   o  Rename "uri" attribute of license and publisher elements to "url".

   -11 : August 08, 2009.
   o  Renamed type element (static or dynamic values) to dynamic element
      (true or false values).
   o  Removed metadata inheritance and most other elements from files
      element.




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   -10 : July 28, 2009.
   o  Schema fixes.
   o  Rename metadata element to metaurl, add name attribute to it
      similar to file element's name attribute.
   o  Update REC-xmldsig-core reference to second edition.

   -09 : July 11, 2009.
   o  Replace ISO639-2 references with RFC 4646.
   o  Add ISO3166-1.

   -08 : July 04, 2009.
   o  Clarifications.
   o  Remove "uri" and "version" attributes from generator element.

   -07 : June 18, 2009.
   o  This ID describes the Metalink document format/schema.
   o  Remove "Client Implementation Considerations" section.
   o  Expand "Known issues" section of Document History.

   -06 : March 3, 2009.
   o  Add authors and this Document History section.

   -05 : January 13, 2009.
   o  Clarifications.

   -04 : December 31, 2008.
   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 SHA-1.

   -03 : September 19, 2008.
   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 : September 4, 2008.
   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 : August 28, 2008.





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

   -00 : August 23, 2008.
   o  Initial draft; Text largely based on RFC 4287, ideas from Metalink
      3.0 specification.


Index

   A
      application/metalink4+xml Media Type  25

   C
      copyright XML element  15

   D
      description XML element  15

   F
      file XML element  12

   G
      generator XML element  16
      Grammar
         metalinkCommonAttributes  9
         metalinkCopyright  15
         metalinkDateConstruct  11
         metalinkDescription  15
         metalinkFile  13
         metalinkGenerator  16
         metalinkHash  16
         metalinkIdentity  17
         metalinkLanguage  18
         metalinkLicense  18
         metalinkLogo  18
         metalinkMetalink  11
         metalinkOrigin  20
         metalinkOS  20
         metalinkPieces  14
         metalinkPublished  20
         metalinkPublisher  21
         metalinkSignature  21
         metalinkSize  21
         metalinkTextConstruct  10
         metalinkType  16
         metalinkUpdated  22
         metalinkURL  19, 22



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         metalinkVersion  23
         simpleExtensionElement  24
         structuredExtensionElement  25

   H
      hash XML element  16

   I
      identity XML element  17

   L
      language XML element  18
      license XML element  18
      logo XML element  18

   M
      Media Type
         application/metalink4+xml  25
      metadata XML element  19
      metalink XML element  11
      metalinkCommonAttributes grammar production  9
      metalinkCopyright grammar production  15
      metalinkDateConstruct grammar production  11
      metalinkDescription grammar production  15
      metalinkFile grammar production  13
      metalinkGenerator grammar production  16
      metalinkHash grammar production  16
      metalinkIdentity grammar production  17
      metalinkLanguage grammar production  18
      metalinkLicense grammar production  18
      metalinkLogo grammar production  18
      metalinkMetalink grammar production  11
      metalinkOrigin grammar production  20
      metalinkOS grammar production  20
      metalinkPieces grammar production  14
      metalinkPublished grammar production  20
      metalinkPublisher grammar production  21
      metalinkSignature grammar production  21
      metalinkSize grammar production  21
      metalinkTextConstruct grammar production  10
      metalinkType grammar production  16
      metalinkUpdated grammar production  22
      metalinkURL grammar production  19, 22
      metalinkVersion grammar production  23

   O
      origin XML element  20
      os XML element  20



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   P
      pieces XML element  14
      published XML element  20
      publisher XML element  20

   S
      signature XML element  21
      simpleExtensionElement grammar production  24
      size XML element  21
      structuredExtensionElement grammar production  25

   T
      type XML element  15

   U
      updated XML element  22
      url XML element  22

   V
      version XML element  22

   X
      XML Elements
         copyright  15
         description  15
         entry  12
         generator  16
         hash  16
         identity  17
         language  18
         license  18
         logo  18
         metadata  19
         metalink  11
         origin  20
         os  20
         pieces  14
         published  20
         publisher  20
         signature  21
         size  21
         type  15
         updated  22
         url  22
         version  22






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Authors' Addresses

   Anthony Bryan
   Pompano Beach, FL
   USA

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


   Tatsuhiro Tsujikawa

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


   Neil McNab

   Email: neil@nabber.org
   URI:   http://www.nabber.org


   Peter Poeml
   Novell, Inc.

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
























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