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RFC 5854 - The Metalink Download Description Format


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Internet Engineering Task Force (IETF)                          A. Bryan
Request for Comments: 5854                                  T. Tsujikawa
Category: Standards Track                                       N. McNab
ISSN: 2070-1721
                                                                P. Poeml
                                                             MirrorBrain
                                                               June 2010

                The Metalink Download Description Format

Abstract

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

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5854.

Copyright Notice

   Copyright (c) 2010 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
   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 Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     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
     3.2.  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:file" Element  . . . . . . . . . . . . .  9
       4.1.3.  The "metalink:pieces" Element  . . . . . . . . . . . . 12
     4.2.  Metadata Elements  . . . . . . . . . . . . . . . . . . . . 12
       4.2.1.  The "metalink:copyright" Element . . . . . . . . . . . 12
       4.2.2.  The "metalink:description" Element . . . . . . . . . . 13
       4.2.3.  The "metalink:generator" Element . . . . . . . . . . . 13
       4.2.4.  The "metalink:hash" Element  . . . . . . . . . . . . . 14
       4.2.5.  The "metalink:identity" Element  . . . . . . . . . . . 15
       4.2.6.  The "metalink:language" Element  . . . . . . . . . . . 15
       4.2.7.  The "metalink:logo" Element  . . . . . . . . . . . . . 16
       4.2.8.  The "metalink:metaurl" Element . . . . . . . . . . . . 16
       4.2.9.  The "metalink:origin" Element  . . . . . . . . . . . . 18
       4.2.10. The "metalink:os" Element  . . . . . . . . . . . . . . 18
       4.2.11. The "metalink:published" Element . . . . . . . . . . . 18
       4.2.12. The "metalink:publisher" Element . . . . . . . . . . . 18
       4.2.13. The "metalink:signature" Element . . . . . . . . . . . 19
       4.2.14. The "metalink:size" Element  . . . . . . . . . . . . . 20
       4.2.15. The "metalink:updated" Element . . . . . . . . . . . . 20
       4.2.16. The "metalink:url" Element . . . . . . . . . . . . . . 20
       4.2.17. The "metalink:version" Element . . . . . . . . . . . . 21
   5.  Extending Metalink . . . . . . . . . . . . . . . . . . . . . . 21
     5.1.  Extensions from Non-Metalink Vocabularies  . . . . . . . . 21
     5.2.  Extensions to the Metalink Vocabulary  . . . . . . . . . . 21
     5.3.  Processing Foreign Markup  . . . . . . . . . . . . . . . . 22
     5.4.  Extension Elements . . . . . . . . . . . . . . . . . . . . 22
       5.4.1.  Simple Extension Elements  . . . . . . . . . . . . . . 22
       5.4.2.  Structured Extension Elements  . . . . . . . . . . . . 23
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
     6.1.  XML Namespace Registration . . . . . . . . . . . . . . . . 23
     6.2.  application/metalink4+xml MIME type  . . . . . . . . . . . 23
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 24
     7.1.  Digital Signatures . . . . . . . . . . . . . . . . . . . . 25
     7.2.  URIs and IRIs  . . . . . . . . . . . . . . . . . . . . . . 26
     7.3.  Spoofing . . . . . . . . . . . . . . . . . . . . . . . . . 26
     7.4.  Cryptographic Hashes . . . . . . . . . . . . . . . . . . . 26

   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 27
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 28
   Appendix A.  Acknowledgements and Contributors . . . . . . . . . . 30
   Appendix B.  RELAX NG Compact Schema . . . . . . . . . . . . . . . 31
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

1.  Introduction

   Metalink is a document format based on Extensible Markup Language
   (XML) 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, multiple cryptographic hashes, and a list of Uniform
   Resource Identifiers (URIs) from which it is available.

   Often, identical copies of a file are accessible in multiple
   locations on the Internet over a variety of protocols, such as File
   Transfer Protocol (FTP), Hypertext Transfer Protocol (HTTP), and
   Peer-to-Peer (P2P).  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 cannot 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 bandwidth 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 a
   third party.  This knowledge, together with cryptographic hashes,
   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 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 of
   the protocol over which the Metalink was obtained, it can make a
   resource accessible through other protocols.  If the Metalink was
   obtained 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.

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 mediatype="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>1.0</version>
     <language>en</language>
     <description>
     A description of the example file for download.
     </description>
     <hash type="sha-256">f0ad929cd259957e160ea442eb80986b5f01...</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 mediatype="torrent"
          priority="2">http://example.com/example.ext.torrent</metaurl>
   </file>
   <file name="example2.ext">
     <size>14471447</size>
     <identity>Example2</identity>
     <version>1.0</version>
     <language>en</language>
     <description>
     Another description for a second file.
     </description>
     <hash type="sha-256">2f548ce50c459a0270e85a7d63b2383c5523...</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 mediatype="torrent"
          priority="2">http://example.com/example2.ext.torrent</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
   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 Document Type Definition (DTD) for Metalink Documents,
   and hence it does not require them to be valid (in the sense used by
   XML).

   Metalink allows the use of Internationalized Resource Identifiers
   (IRIs), encoded according to [RFC3987].  Every URI [RFC3986] is also
   an IRI, so a URI may be used wherever an IRI is named below.  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: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:lang { metalinkLanguageTag }?,
      undefinedAttribute*

   All leading and trailing whitespace is part of the element content
   and MUST NOT be ignored.  Consequently, it is disallowed for elements
   where the defined type does not allow whitespace, such as dates,
   integers, or IRIs.  Some XML-generating implementations erroneously
   insert whitespace around values by default, and such implementations
   will generate invalid Metalink Documents.

   Metalink Documents that do not follow this specification are invalid
   and SHOULD NOT be used by Metalink Processors.

   Metalink is an extensible format.  See Section 5 of this document for
   a full description of how Metalink Documents can be extended.

3.  Common Metalink Constructs

   Many Metalink elements share 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.

3.1.  Text Constructs

   A Text construct contains human-readable text, usually short in
   length.

   metalinkTextConstruct =
      metalinkCommonAttributes,
      text

   For example, a 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, whitespace 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.

   metalinkDateConstruct =
      metalinkCommonAttributes,
      xsd:dateTime

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

   Example Date constructs:

   ...
   <updated>2010-05-01T12:15:02Z</updated>
   ...
   <updated>2010-05-01T12:15:02.25Z</updated>
   ...
   <updated>2010-05-01T12:15:02+01:00</updated>
   ...
   <updated>2010-05-01T12:15:02.25+01:00</updated>
   ...

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 that consist of Metadata elements.

   metalinkMetalink =
      element metalink:metalink {
         metalinkCommonAttributes,
         (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):

   o  metalink:metalink elements MUST contain one or more metalink:file
      elements.

   o  metalink:metalink elements MAY contain exactly one metalink:
      generator element and MUST NOT contain more than one such element.

   o  metalink:metalink elements SHOULD contain exactly one metalink:
      origin element and MUST NOT contain more than one such element.

   o  metalink:metalink elements MAY contain exactly one metalink:
      published element and MUST NOT contain more than one such element.

   o  metalink:metalink elements MAY contain exactly one metalink:
      updated element and MUST NOT contain more than one such 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 this.  It is RECOMMENDED that each metalink:file element
   contain a non-empty metalink:description element, a non-empty
   metalink:identity element, a non-empty metalink:version element, and
   a non-empty metalink:publisher element when these elements are
   present.  However, the absence of metalink:description, metalink:
   identity, metalink:version, and metalink:publisher 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.  Each
   unique file described in a Metalink Document MUST have its own
   metalink:file element.

   All metalink:url elements contained in each metalink:file element
   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.  Refer to
   Sections 4.2.8 and 4.2.16 for more information.

   metalinkFile =
      element metalink:file {
         metalinkCommonAttributes,
         attribute name { text },
         (metalinkCopyright?
          & metalinkDescription?
          & metalinkHash*
          & metalinkIdentity?
          & metalinkLanguage*
          & metalinkLogo?
          & metalinkMetaURL*
          & metalinkOS*
          & metalinkPieces*
          & metalinkPublisher?
          & metalinkSignature?
          & metalinkSize?
          & metalinkURL*
          & 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 (the
   presence of some of them is required):

   o  metalink:file elements MAY contain exactly one metalink:copyright
      element and MUST NOT contain more than one such element.

   o  metalink:file elements MAY contain exactly one metalink:
      description element and MUST NOT contain more than one such
      element.

   o  metalink:file elements MAY contain exactly one metalink:identity
      element and MUST NOT contain more than one such element.

   o  metalink:file elements MAY contain one or more metalink:hash
      elements.

   o  metalink:file elements MAY contain one or more metalink:language
      elements.

   o  metalink:file elements MAY contain exactly one metalink:logo
      element and MUST NOT contain more than one such element.

   o  metalink:file elements MAY contain one or more metalink:os
      element.

   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 and MUST NOT contain more than one such element.

   o  metalink:file elements MAY contain one or more metalink:signature
      elements.

   o  metalink:file elements SHOULD contain exactly one metalink:size
      element and MUST NOT contain more than one such element.

   o  metalink:file elements MAY contain exactly one metalink:version
      element and MUST NOT contain more than one such element.

4.1.2.1.  The "name" Attribute

   metalink:file elements MUST have a "name" attribute, which contains
   the local file name to which the downloaded file will be written.
   Hence, if a Metalink Document contains multiple metalink:file
   elements, the value of the "name" attribute MUST be unique for each.

   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.

      Security Note: 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
   cryptographic hashes of contiguous, non-overlapping pieces of the
   file.  The cryptographic hashes MUST be listed in the same order as
   the corresponding pieces appear in the file, starting at the
   beginning of the file.  Metalink Documents MAY contain one or
   multiple metalink:pieces container elements, if each "type" attribute
   of metalink:pieces has a unique value.

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

4.1.3.1.  The "type" Attribute

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

   The Internet Assigned Numbers Authority (IANA) registry named "Hash
   Function Textual Names" defines values for hash types.  See
   Section 7.4 for security implications.

4.1.3.2.  The "length" Attribute

   metalink:pieces elements MUST have a "length" attribute, which is a
   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 MUST be the same size, apart from the last piece, which is the
   remainder.  The last piece extends to the end of the file, and it
   therefore MAY 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.  It is Language-Sensitive.

   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 file description.  It is Language-Sensitive.

   metalinkDescription =
      element metalink:description {
        metalinkTextConstruct
      }

4.2.3.  The "metalink:generator" Element

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

   metalinkGenerator =
      element metalink:generator {
        metalinkTextConstruct
      }

   The metalink:generator element's content is defined below in ABNF
   notation [RFC5234].

   token          = 1*<any CHAR except CTLs or separators>
   separators     = "(" / ")" / "<" / ">" / "@"
                  / "," / ";" / ":" / "\" / DQUOTE
                  / "/" / "[" / "]" / "?" / "="
                  / "{" / "}" / SP / HTAB
   agent         = token ["/" agent-version]
   agent-version = token

   Examples:

   ...
   <generator>MirrorBrain/2.11</generator>
   ...
   <generator>MirrorManager/1.2.11</generator>
   ...
   <generator>metalinktools/0.3.6</generator>
   ...
   <generator>MetalinkEditor/1.2.0</generator>
   ...

   Although any token character MAY appear in an agent-version, this
   token SHOULD only be used for a version identifier (i.e., successive
   versions of the same agent SHOULD only differ in the agent-version
   portion of the agent value).

4.2.4.  The "metalink:hash" Element

   The "metalink:hash" element is a Text construct that conveys a
   cryptographic 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 Documents MAY contain one or multiples hashes of a complete
   file. metalink:hash elements with a "type" attribute MUST contain a
   hash of the complete file.  In this example, both SHA-1 and SHA-256
   hashes of the complete file are included.

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

   Metalink Documents MAY 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 in
   which they are contained.  Metalink Documents MAY contain one or
   multiple metalink:pieces container elements, if each "type" attribute
   of metalink:pieces has a unique value.

   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, apart from the last piece, which is the remainder.  See
   Section 4.1.3.2 for more information on the size of pieces.

   In this example, SHA-1 and SHA-256 hashes of the complete file are
   included, along with four SHA-1 piece hashes.

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

4.2.4.1.  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.
   metalink:hash elements MUST NOT have a "type" attribute, if they are
   inside a metalink:pieces container element.  See Section 7.4 for
   security implications.

4.2.5.  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
      }

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 [RFC5646].

   Multiple metalink:language elements are allowed, for instance, to
   describe a file such as an binary installation program that provides
   multiple language options, a movie with multiple language tracks, or
   a document in multiple languages.

   metalinkLanguage =
      element metalink:language {
        metalinkTextConstruct
      }

4.2.7.  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.

4.2.8.  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 {
            maxInclusive = "999999"}}?,
         attribute mediatype { text },
         attribute name { text }?,
         (metalinkUri)
      }

4.2.8.1.  The "priority" Attribute

   metalink:metaurl elements MAY have a priority attribute.  Values MUST
   be positive integers between 1 and 999999.  Lower values indicate a
   higher priority. metalink:metaurl elements without a priority
   attribute are considered to have the lowest priority, i.e., 999999.
   The priority values of metalink:metaurl and metalink:url elements are
   compared and those with the lowest values, starting with 1, are used
   first.  Multiple metalink:metaurl and metalink:url elements MAY have
   the same priority, i.e., one BitTorrent .torrent file and three FTP
   URIs could have priority="1".  See also the "priority" attribute of
   the metalink:url element.

4.2.8.2.  The "mediatype" Attribute

   metalink:metaurl elements MUST have a "mediatype" attribute that
   indicates the Multipurpose Internet Mail Extensions (MIME) media type
   [RFC4288] of the metadata file 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.

   Values for this attribute are defined below in ABNF notation
   [RFC5234].

   media-type = (type-name "/" subtype-name) / media-reserved
   media-reserved = "torrent"
   type-name = <Defined in Section 4.2 of RFC 4288>
   subtype-name = <Defined in Section 4.2 of RFC 4288>

4.2.8.3.  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
        mediatype="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
   relative.  The path MUST NOT begin with a "/", "./", or "../";
   contain "/../"; or end with "/..".

4.2.9.  The "metalink:origin" Element

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

   metalinkOrigin =
      element metalink:origin {
        metalinkCommonAttributes,
        attribute dynamic { xsd:boolean }?,
        (metalinkUri)
      }

4.2.9.1.  The "dynamic" Attribute

   The metalink:origin element MAY have a "dynamic" attribute, set to
   "true" or "false", which tells if a Metalink at the origin IRI will
   contain dynamic updated information or if it is static and not likely
   to be updated.

4.2.10.  The "metalink:os" Element

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

   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.

   metalinkPublished =
      element metalink:published {
        metalinkDateConstruct
      }

   Typically, metalink:published will be associated with the initial
   creation or first availability of the resource.  The metalink:updated
   element is used when a Metalink Document has been updated after
   initial publication.

4.2.12.  The "metalink:publisher" Element

   The "metalink:publisher" element contains a human-readable group or
   other entity that has published the file described in the Metalink
   Document and an IRI for more information.

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

4.2.12.1.  The "name" Attribute

   The metalink:publisher element MUST have a "name" attribute that
   indicates the human-readable name of the publisher.

4.2.12.2.  The "url" Attribute

   The metalink:publisher element MAY have a "url" attribute whose value
   MUST be an IRI reference [RFC3987].

4.2.13.  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.

   Support in Metalink Processors for digital signatures included in
   this element is OPTIONAL.  Note that the signing of Metalink
   Documents, as opposed to a digital signature of a file described in a
   Metalink Document, is covered in Section 7.1.

   metalinkSignature =
      element metalink:signature {
        attribute mediatype { text },
        metalinkTextConstruct
      }

   Example with an OpenPGP signature [RFC4880]:

   <signature mediatype="application/pgp-signature">
   -----BEGIN PGP SIGNATURE-----
   Version: GnuPG v1.4.10 (GNU/Linux)

   iEYEABECAAYFAkrxdXQACgkQeOEcayedXJHqFwCfd1p/HhRf/iDvYhvFbTrQPz+p
   p3oAoO9lKHoOqOE0EMB3zmMcLoYUrNkg
   =ggAf
   -----END PGP SIGNATURE-----
   </signature>

4.2.13.1.  The "mediatype" Attribute

   metalink:signature elements MUST have a "mediatype" attribute that
   indicates the MIME media type [RFC4288] of the included digital
   signature.

   Values for this attribute are defined below in ABNF notation
   [RFC5234].

   media-type = type-name "/" subtype-name
   type-name = <Defined in Section 4.2 of RFC 4288>
   subtype-name = <Defined in Section 4.2 of RFC 4288>

4.2.14.  The "metalink:size" Element

   The "metalink:size" element indicates the length of the linked
   content in octets.  This is the content length of the representation
   returned when the IRI is mapped to a URI and dereferenced.  Note that
   the "metalink:size" element MUST override the actual content length
   of the representation as reported by the underlying protocol, and
   those that do not match MUST be discarded by Metalink Processors.
   This value MUST be a non-negative integer.

   metalinkSize =
      element metalink:size {
        xsd:nonNegativeInteger
      }

4.2.15.  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.

4.2.16.  The "metalink:url" Element

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

   The metalink:url elements SHOULD be resolvable and, if resolvable,
   SHOULD lead to identical files.

   Metalink Processors MUST filter out invalid files obtained from
   "metalink:url" elements by using information in the metalink:size
   element and metalink:hash elements.

   metalinkURL =
      element metalink:url {
         metalinkCommonAttributes,
         attribute location { xsd:string {
            minLength = "2"  maxLength="2"}
         }?,
         attribute priority { xsd:positiveInteger {
            maxInclusive = "999999"}}?,
         (metalinkUri)
      }

4.2.16.1.  The "priority" Attribute

   metalink:url elements MAY have a priority attribute.  Values MUST be
   positive integers between 1 and 999999.  Lower values indicate a
   higher priority. metalink:url elements without a priority attribute
   are considered to have the lowest priority, i.e., 999999.  Multiple
   metalink:url elements can have the same priority, i.e., ten different
   mirrors could have priority="1".

4.2.16.2.  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.

4.2.17.  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".

   metalinkVersion =
      element metalink:version {
        metalinkTextConstruct
      }

5.  Extending Metalink

5.1.  Extensions from Non-Metalink Vocabularies

   This specification describes Metalink's XML vocabulary.

5.2.  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".

5.3.  Processing Foreign Markup

   Metalink Processors that encounter foreign markup in a location that
   is legal according to this specification MUST ignore such foreign
   markup, in particular they 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.

5.4.  Extension Elements

   Metalink allows foreign markup anywhere in a 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.

5.4.1.  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.

5.4.2.  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.

6.  IANA Considerations

6.1.  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 "Authors' Addresses" section of this
   document.

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

6.2.  application/metalink4+xml MIME type

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

   Type name:  application

   Subtype name:  metalink4+xml

   Required 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:  File transfer applications.

   Additional information:

   Magic number(s):  None.

   File extension:  .meta4

   Macintosh File Type code:  TEXT

   Person and email address to contact for further information:
      Anthony Bryan <anthonybryan@gmail.com>

   Intended usage:  COMMON

   Restrictions on usage:  None.

   Author:  Anthony Bryan <anthonybryan@gmail.com>

   Change controller:  IESG

7.  Security Considerations

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

   Publishers 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.  Many
   other examples are conceivable.

   Publishers are encouraged to offer Metalink documents via
   authenticated HTTP under Transport Layer Security (TLS) as specified
   in [RFC2818].  The choice of a secure content layer rests entirely
   with the content providers.

   Publishers are also encouraged to include digital signatures of the
   files within the Metalink Documents, if they are available, as
   described in Section 4.2.13.

   Normally, a publisher is in the best position to know how strong the
   protective signing ought to be on their content.  Thus, a publisher
   can choose weak or strong cryptography, and a Metalink Processor
   SHOULD normally accept that.  There are potential applications where
   the Metalink Processor chooses to reject weak cryptography, but that
   is not envisioned as the common use case.

7.1.  Digital Signatures

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

   Although signing and verifying signatures are both OPTIONAL, an
   implementation that supports either feature SHOULD implement RSA with
   a minimum key size of 2048 with SHA-256.

   Metalink Processors that support verifying signatures MUST reject
   Metalink Documents with invalid signatures.

   Metalink Processors MUST NOT reject a 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 a Metalink
   Processor to fail merely because of its presence.

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

   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 Digital
   Signature Algorithm (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.

7.2.  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.

7.3.  Spoofing

   There is potential for spoofing attacks where the attacker publishes
   Metalink Documents with false information.  Malicious publishers
   might create Metalink Documents containing inaccurate information
   anywhere in the document.  Unaware downloaders could be deceived into
   downloading malicious or worthless content.  Malicious publishers
   could attempt a distributed denial-of-service attack by inserting
   unrelated IRIs into Metalink Documents.

   Digital signatures address the issue of spoofing.

7.4.  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 that are not suitable
   for cryptographically strong verification.  Malicious parties 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.

   Metalink Generators and Processors MUST support "sha-256", which is
   SHA-256, as specified in [FIPS-180-3], and MAY support stronger
   hashes.

   If a Metalink Document contains hashes, it SHOULD include "sha-256",
   which is SHA-256, or stronger.  It MAY also include other hashes from
   the IANA registry named "Hash Function Textual Names".

8.  References

8.1.  Normative References

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

   [FIPS-180-3] National Institute of Standards and Technology (NIST),
                "Secure Hash Standard (SHS)", FIPS PUB 180-3,
                October 2008.

   [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
                (Fifth Edition)", W3C REC-xml-20081126, November 2008,
                <http://www.w3.org/TR/2008/REC-xml-20081126/>.

   [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)", W3C 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 (Third Edition)", W3C REC-xml-
                names-20091208, December 2009,
                <http://www.w3.org/TR/2009/REC-xml-names-20091208/>.

   [REC-xmldsig-core]
                Solo, D., Reagle, J., and D. Eastlake, "XML-Signature
                Syntax and Processing (Second Edition)",
                W3C 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.

   [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.

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

   [RFC5234]    Crocker, D., Ed. and P. Overell, "Augmented BNF for
                Syntax Specifications: ABNF", STD 68, January 2008.

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

8.2.  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.

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

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

   [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.

   [RFC4880]    Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and
                R. Thayer, "OpenPGP Message Format", RFC 4880,
                November 2007.

Appendix A.  Acknowledgements and Contributors

   The layout and shape 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.  Key contributors provided early implementations: A. Bram
   Neijt, Hampus Wessman, Darius Liktorius, Manuel Subredu, Michael
   Burford, Giorgio Maone, Nils Maier, Max Velasques, Manolo Valdes,
   Hayden Legendre, Frederick Cheung, Rene Leonhardt, Per Oyvind
   Karlsen, Matt Domsch, Yazsoft, KGet developers, Free Download Manager
   developers, Orbit developers, Arne Babenhauserheide, Mathias
   Berchtold, Xienzhenyu and TheWorld Browser developers, Xi Software,
   Agostino Russo, and James Antill.

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

   Paul Burkhead, Kristian Weston, Nicolas Alvarez, Urs Wolfer, Bridget
   and Ethan Fletcher, Patrick Ruckstuhl, Sebastien Willemijns, Micah
   Cowan, Ruben Kerkhof, Danny Ayers, Nick Dominguez, Gary Zellerbach,
   James Clark, Daniel Stenberg, John and Sandra Sowder, Salvatore
   Musumeci, Steve Eshelman, Lucas Hewett, Ryan Cronin, Dave Winquist,
   Bob Denison, Wes Shelton, Josh Colbert, Steve Kleisath, Chad Neptune,
   Derick Cordoba, Nick Carrabba, Chris Carrabba, Erin Solari, Ryan
   Alexander, 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, Kees Cook, Dedric Carter, and Debi Goulding.
   We also thank the Anthony Family, the Bryan Family, Juanita Anthony,
   and Zimmy Bryan.

   Special thanks to Eran Hammer-Lahav, document shepherd, and Lisa
   Dusseault, Area Director.  We also thank the following contributors
   for assistance and review: Mark Nottingham, Peter Saint-Andre, Julian
   Reschke, Chris Newman, Ian Macfarlane, Dave Cridland, Barry Leiba,
   Uri Blumenthal, Paul Hoffman, Felix Sasaki, Matthias Fuchs, Mark
   Baker, Scott Cantor, Brian Carpenter, Alexey Melnikov, Lars Eggert,
   Pasi Eronen, Tim Polk, Dan Romascanu, and Bjoern Hoehrmann.

   Peter Poeml wishes to acknowledge the support of SUSE Linux Products
   GmbH / Novell Inc., where he was employed during much of the work on
   this document.

   This document is dedicated to Sonora 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 5.2 and 5.3.

   # -*- rnc -*-
   # RELAX NG Compact Syntax Grammar for the
   # Metalink Format Specification Version 4
   # Based on RFC 4287 schema

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

   # Common attributes

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

   # Text Constructs

   metalinkTextConstruct =
      metalinkCommonAttributes,
      text

   # Date Construct

   metalinkDateConstruct =
      metalinkCommonAttributes,
      xsd:dateTime

   start = metalinkMetalink

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

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

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

   metalinkCopyright =
      element metalink:copyright {
         metalinkTextConstruct
      }

   metalinkDescription =
      element metalink:description {
         metalinkTextConstruct
      }

   metalinkGenerator =
      element metalink:generator {
         metalinkTextConstruct
      }

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

   metalinkIdentity =
      element metalink:identity {
         metalinkTextConstruct
      }

   metalinkLanguage =
      element metalink:language {
         metalinkTextConstruct
      }

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

   metalinkMetaURL =
      element metalink:metaurl {
         metalinkCommonAttributes,
         attribute priority { xsd:positiveInteger {
            maxInclusive = "999999"}}?,
         attribute mediatype { text },
         attribute name { text }?,
         (metalinkUri)
      }

   metalinkOrigin =
      element metalink:origin {
         metalinkCommonAttributes,
         attribute dynamic { xsd:boolean }?,
         (metalinkUri)
      }

   metalinkOS =
      element metalink:os {
         metalinkTextConstruct
      }

   metalinkPublished =
      element metalink:published {
         metalinkDateConstruct
      }

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

      }

   metalinkSignature =
      element metalink:signature {
         attribute mediatype { text },
         metalinkTextConstruct
      }

   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 {
            maxInclusive = "999999"}}?,
         (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 }*,
          (text?, anyElement+, (text|anyElement)*))
      }

   # Other Extensibility

   extensionElement =
      simpleExtensionElement | structuredExtensionElement

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

   undefinedContent = (text|anyForeignElement)*

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

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

   # EOF

Index

   A
      ABNF
         metalinkGenerator  13
         metaurl mediatype  17
         signature mediatype  19
      application/metalink4+xml Media Type  23

   C
      copyright XML element  12

   D
      description XML element  13

   F
      file XML element  10

   G
      generator XML element  13
      Grammar
         metalinkCommonAttributes  7
         metalinkCopyright  13
         metalinkDateConstruct  8
         metalinkDescription  13
         metalinkFile  10
         metalinkGenerator  13
         metalinkHash  14
         metalinkIdentity  15
         metalinkLanguage  15
         metalinkLogo  16
         metalinkMetalink  8
         metalinkMetaURL  16
         metalinkOrigin  17
         metalinkOS  18
         metalinkPieces  12
         metalinkPublished  18
         metalinkPublisher  18
         metalinkSignature  19
         metalinkSize  20
         metalinkTextConstruct  7
         metalinkUpdated  20
         metalinkURL  21
         metalinkVersion  21
         simpleExtensionElement  22
         structuredExtensionElement  23

   H
      hash XML element  14

   I
      identity XML element  15

   L
      language XML element  15
      logo XML element  16

   M
      Media Type
         application/metalink4+xml  23
      metalink XML element  8
      metalinkCommonAttributes grammar production  7
      metalinkCopyright grammar production  12
      metalinkDateConstruct grammar production  8
      metalinkDescription grammar production  13
      metalinkFile grammar production  10
      metalinkGenerator ABNF  13
      metalinkGenerator grammar production  13
      metalinkHash grammar production  14
      metalinkIdentity grammar production  15
      metalinkLanguage grammar production  15
      metalinkLogo grammar production  16
      metalinkMetalink grammar production  8
      metalinkMetaURL grammar production  16
      metalinkOrigin grammar production  17
      metalinkOS grammar production  18
      metalinkPieces grammar production  12
      metalinkPublished grammar production  18
      metalinkPublisher grammar production  18
      metalinkSignature grammar production  19
      metalinkSize grammar production  20
      metalinkTextConstruct grammar production  7
      metalinkUpdated grammar production  20
      metalinkURL grammar production  21
      metalinkVersion grammar production  21
      metaurl mediatype ABNF  16
      metaurl XML element  16

   O
      origin XML element  17
      os XML element  18

   P
      pieces XML element  12
      published XML element  18
      publisher XML element  18

   S
      signature mediatype ABNF  19
      signature XML element  19
      simpleExtensionElement grammar production  22
      size XML element  20
      structuredExtensionElement grammar production  23

   U
      updated XML element  20
      url XML element  20

   V
      version XML element  21

   X
      XML Elements
         copyright  12
         description  13
         file  9
         generator  13
         hash  14
         identity  15
         language  15
         logo  16
         metalink  8
         metaurl  16
         origin  17
         os  18
         pieces  12
         published  18
         publisher  18
         signature  19
         size  20
         updated  20
         url  20
         version  21

Authors' Addresses

   Anthony Bryan
   Pompano Beach, FL
   USA

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

   Tatsuhiro Tsujikawa
   Shiga
   Japan

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

   Neil McNab
   San Diego, CA
   USA

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

   Dr. med. Peter Poeml
   MirrorBrain
   Venloer Str. 317
   Koeln  50823
   DE

   Phone: +49 221 6778 333 8
   EMail: peter@poeml.de
   URI:   http://mirrorbrain.org/~poeml/

 

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