GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc5854

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.

Bryan, et al. Standards Track [Page 1] RFC 5854 Metalink Download Description Format June 2010

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

Bryan, et al. Standards Track [Page 2] RFC 5854 Metalink Download Description Format June 2010

 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

Bryan, et al. Standards Track [Page 3] RFC 5854 Metalink Download Description Format June 2010

 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>

Bryan, et al. Standards Track [Page 4] RFC 5854 Metalink Download Description Format June 2010

   <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,

Bryan, et al. Standards Track [Page 5] RFC 5854 Metalink Download Description Format June 2010

 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.

Bryan, et al. Standards Track [Page 6] RFC 5854 Metalink Download Description Format June 2010

 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

Bryan, et al. Standards Track [Page 7] RFC 5854 Metalink Download Description Format June 2010

 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?

Bryan, et al. Standards Track [Page 8] RFC 5854 Metalink Download Description Format June 2010

        & 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

Bryan, et al. Standards Track [Page 9] RFC 5854 Metalink Download Description Format June 2010

 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.

Bryan, et al. Standards Track [Page 10] RFC 5854 Metalink Download Description Format June 2010

 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 "/..".

Bryan, et al. Standards Track [Page 11] RFC 5854 Metalink Download Description Format June 2010

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
    }

Bryan, et al. Standards Track [Page 12] RFC 5854 Metalink Download Description Format June 2010

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).

Bryan, et al. Standards Track [Page 13] RFC 5854 Metalink Download Description Format June 2010

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.

Bryan, et al. Standards Track [Page 14] RFC 5854 Metalink Download Description Format June 2010

 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
    }

Bryan, et al. Standards Track [Page 15] RFC 5854 Metalink Download Description Format June 2010

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.

Bryan, et al. Standards Track [Page 16] RFC 5854 Metalink Download Description Format June 2010

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)
    }

Bryan, et al. Standards Track [Page 17] RFC 5854 Metalink Download Description Format June 2010

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 }?
    }

Bryan, et al. Standards Track [Page 18] RFC 5854 Metalink Download Description Format June 2010

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].

Bryan, et al. Standards Track [Page 19] RFC 5854 Metalink Download Description Format June 2010

 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.

Bryan, et al. Standards Track [Page 20] RFC 5854 Metalink Download Description Format June 2010

 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.

Bryan, et al. Standards Track [Page 21] RFC 5854 Metalink Download Description Format June 2010

 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.

Bryan, et al. Standards Track [Page 22] RFC 5854 Metalink Download Description Format June 2010

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

Bryan, et al. Standards Track [Page 23] RFC 5854 Metalink Download Description Format June 2010

 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.

Bryan, et al. Standards Track [Page 24] RFC 5854 Metalink Download Description Format June 2010

 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.

Bryan, et al. Standards Track [Page 25] RFC 5854 Metalink Download Description Format June 2010

 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.

Bryan, et al. Standards Track [Page 26] RFC 5854 Metalink Download Description Format June 2010

 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/>.

Bryan, et al. Standards Track [Page 27] RFC 5854 Metalink Download Description Format June 2010

 [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.

Bryan, et al. Standards Track [Page 28] RFC 5854 Metalink Download Description Format June 2010

 [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.

Bryan, et al. Standards Track [Page 29] RFC 5854 Metalink Download Description Format June 2010

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.

Bryan, et al. Standards Track [Page 30] RFC 5854 Metalink Download Description Format June 2010

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*)
    }

Bryan, et al. Standards Track [Page 31] RFC 5854 Metalink Download Description Format June 2010

 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
      }

Bryan, et al. Standards Track [Page 32] RFC 5854 Metalink Download Description Format June 2010

 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 }?

Bryan, et al. Standards Track [Page 33] RFC 5854 Metalink Download Description Format June 2010

    }
 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

Bryan, et al. Standards Track [Page 34] RFC 5854 Metalink Download Description Format June 2010

    }
 # 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

Bryan, et al. Standards Track [Page 35] RFC 5854 Metalink Download Description Format June 2010

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

Bryan, et al. Standards Track [Page 36] RFC 5854 Metalink Download Description Format June 2010

 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

Bryan, et al. Standards Track [Page 37] RFC 5854 Metalink Download Description Format June 2010

 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

Bryan, et al. Standards Track [Page 38] RFC 5854 Metalink Download Description Format June 2010

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/

Bryan, et al. Standards Track [Page 39]

/data/webs/external/dokuwiki/data/pages/rfc/rfc5854.txt · Last modified: 2010/06/01 18:30 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki