The Metalink Download Description Format
draft-bryan-metalink-14

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Abstract

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

Table of Contents

1.  Introduction
    1.1.  Examples
    1.2.  Namespace and Version
    1.3.  Notational Conventions
2.  Metalink Documents
3.  Common Metalink Constructs
    3.1.  Text Constructs
        3.1.1.  Text
    3.2.  Date Constructs
4.  Metalink Element Definitions
    4.1.  Container Elements
        4.1.1.  The "metalink:metalink" Element
        4.1.2.  The "metalink:file" Element
        4.1.3.  The "metalink:pieces" Element
    4.2.  Metadata Elements
        4.2.1.  The "metalink:copyright" Element
        4.2.2.  The "metalink:description" Element
        4.2.3.  The "metalink:dynamic" Element
        4.2.4.  The "metalink:generator" Element
        4.2.5.  The "metalink:hash" Element
        4.2.6.  The "metalink:identity" Element
        4.2.7.  The "metalink:language" Element
        4.2.8.  The "metalink:license" Element
        4.2.9.  The "metalink:logo" Element
        4.2.10.  The "metalink:metaurl" Element
        4.2.11.  The "metalink:origin" Element
        4.2.12.  The "metalink:os" Element
        4.2.13.  The "metalink:published" Element
        4.2.14.  The "metalink:publisher" Element
        4.2.15.  The "metalink:signature" Element
        4.2.16.  The "metalink:size" Element
        4.2.17.  The "metalink:updated" Element
        4.2.18.  The "metalink:url" Element
        4.2.19.  The "metalink:version" Element
5.  Securing Metalink Documents
6.  Extending Metalink
    6.1.  Extensions from Non-Metalink Vocabularies
    6.2.  Extensions to the Metalink Vocabulary
    6.3.  Processing Foreign Markup
    6.4.  Extension Elements
        6.4.1.  Simple Extension Elements
        6.4.2.  Structured Extension Elements
7.  IANA Considerations
    7.1.  XML Namespace Registration
    7.2.  application/metalink+xml MIME type
8.  Security Considerations
    8.1.  URIs and IRIs
    8.2.  Spoofing
    8.3.  Cryptographic Hashes
    8.4.  Signing
9.  References
    9.1.  Normative References
    9.2.  Informative References
Appendix A.  Acknowledgements and Contributors
Appendix B.  RELAX NG Compact Schema
Appendix C.  Document History (to be removed by RFC Editor before publication)
§  Index
§  Authors' Addresses

1.  Introduction

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

Identical copies of a file are frequently accessible in multiple locations on the Internet over a variety of protocols (FTP, HTTP, and Peer-to-Peer). In some cases, Users are shown a list of these multiple download locations (mirrors) and must manually select a single one on the basis of geographical location, priority, or bandwidth. This distributes the load across multiple servers. At times, individual servers can be slow, outdated, or unreachable, but this can not be determined until the download has been initiated. This can lead to the user canceling the download and needing to restart it. During downloads, errors in transmission can corrupt the file. There are no easy ways to repair these files. For large downloads this can be extremely troublesome. Any of the number of problems that can occur during a download lead to frustration on the part of users.

All the information about a download, including mirrors, checksums, digital signatures, and other information can be stored in a machine-readable Metalink file. This Metalink file transfers the knowledge of the download server (and mirror database) to the client. With this knowledge, the client is enabled to work its way to a successful download even under adverse circumstances. All this is done transparently to the user and the download is much more reliable and efficient. In contrast, a traditional HTTP redirect to a mirror conveys only extremely minimal information - one link to one 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.

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

1.1.  Examples

A brief, single file Metalink Document:

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

A more extensive, single file Metalink Document:

<?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">
    <identity>Example</identity>
    <version>1.0</version>
    <language>en</language>
    <description>A description of the example file for
download.</description>
    <hash type="sha-1">
    80bc95fd391772fa61c91ed68567f0980bb45fd9</hash>
    <url location="de" preference="95">
    ftp://ftp.example.com/example.ext</url>
    <url location="fr" preference="100">
    http://example.com/example.ext</url>
    <metaurl type="torrent" preference="90">
    http://example.com/example.ext.torrent
    </metaurl>
  </file>
</metalink>

1.2.  Namespace and Version

The XML Namespaces URI [REC‑xml‑names] (Hollander, D., Bray, T., Tobin, R., and A. Layman, “Namespaces in XML 1.0 (Second Edition),” August 2006.) 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 (Namespace and Version), 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] (Cowan, J. and R. Tobin, “XML Information Set (Second Edition),” February 2004.). 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] (Clark, J., “RELAX NG Compact Syntax,” December 2001.). However, the text of this specification provides the definition of conformance. A complete schema appears in Appendix B (RELAX NG Compact Schema).

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.), 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 (The "metalink:metalink" Element) 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] (Yergeau, F., Paoli, J., Bray, T., Sperberg-McQueen, C., and E. Maler, “Extensible Markup Language (XML) 1.0 (Fourth Edition),” August 2006.) and identified with the "application/metalink+xml" media type.

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

Metalink allows the use of IRIs [RFC3987] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.). Every URI [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) is also an IRI, so a URI may be used wherever below an IRI is named. There is one special consideration: when an IRI that is not also a URI is given for dereferencing, it MUST be mapped to a URI using the steps in Section 3.1 of [RFC3987] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.).

Any element defined by this specification MAY have an xml:base attribute [REC‑xmlbase] (Marsh, J., “XML Base,” June 2001.). When xml:base is used in an Metalink Document, it serves the function described in Section 5.1.1 of [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.), establishing the base URI (or IRI) for resolving any relative references found within the effective scope of the xml:base attribute.

Any element defined by this specification MAY have an xml:lang attribute, whose content indicates the natural language for the element and its descendents. The language context is only significant for elements and attributes declared to be "Language-Sensitive" by this specification. Requirements regarding the content and interpretation of xml:lang are specified in XML 1.0 (Yergeau, F., Paoli, J., Bray, T., Sperberg-McQueen, C., and E. Maler, “Extensible Markup Language (XML) 1.0 (Fourth Edition),” August 2006.) [REC‑xml], Section 2.12.

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

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

3.  Common Metalink Constructs

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

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

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

3.1.  Text Constructs

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

metalinkTextConstruct =
   metalinkCommonAttributes,
   text

3.1.1.  Text

Example metalink:description (The "metalink:description" Element) with text content:

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

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

3.2.  Date Constructs

A Date construct is an element whose content MUST conform to the "date-time" production in [RFC3339] (Klyne, G. and C. Newman, “Date and Time on the Internet: Timestamps,” July 2002.). 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] (International Organization for Standardization, “Data elements and interchange formats - Information interchange - Representation of dates and times,” June 1988.), [W3C.NOTE‑datetime‑19980827] (Wolf, M. and C. Wicksteed, “Date and Time Formats,” August 1998.), and [W3C.REC‑xmlschema‑2‑20041028] (Malhotra, A. and P. Biron, “XML Schema Part 2: Datatypes Second Edition,” October 2004.).

Example Date constructs:

<updated>2009-05-15T18:30:02Z</updated>
<updated>2009-05-15T18:30:02.25Z</updated>
<updated>2009-05-15T18:30:02+01:00</updated>
<updated>2009-05-15T18:30:02.25+01:00</updated>

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

4.  Metalink Element Definitions

4.1.  Container Elements

4.1.1.  The "metalink:metalink" Element

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

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

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

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

4.1.1.1.  Providing Textual Content

Experience teaches that downloads providing textual content are in general more useful than those that do not. Some applications (one example is full-text indexers) require a minimum amount of text to function reliably and predictably. Metalink publishers should be aware of these issues. It is advisable that each metalink:file (The "metalink:file" Element) element contain a non-empty metalink:description (The "metalink:description" Element) element, a non-empty metalink:identity (The "metalink:identity" Element) element when that element is present, and a non-empty metalink:version (The "metalink:version" Element) element, and a non-empty metalink:publisher (The "metalink:publisher" Element) element. However, the absence of metalink:description (The "metalink:description" Element) 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 (The "metalink:file" Element)" element represents an individual file, acting as a container for metadata and data associated with the file.

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

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

This specification assigns no significance to the order of metalink:file (The "metalink:file" Element) elements or to the order of metalink:url (The "metalink:url" Element) or metalink:metaurl (The "metalink:metaurl" Element) elements. Significance is determined by the value of the "preference" attribute of the metalink:url (The "metalink:url" Element) or metalink:metaurl (The "metalink:metaurl" Element) elements.

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

• metalink:file (The "metalink:file" Element) elements MAY contain exactly one metalink:copyright (The "metalink:copyright" Element) element.
• metalink:file (The "metalink:file" Element) elements MAY contain exactly one metalink:description (The "metalink:description" Element) element.
• metalink:file (The "metalink:file" Element) elements MAY contain exactly one metalink:identity (The "metalink:identity" Element) element.
• metalink:file (The "metalink:file" Element) elements MAY contain one or more metalink:hash (The "metalink:hash" Element) elements.
• metalink:file (The "metalink:file" Element) elements MAY contain exactly one metalink:language (The "metalink:language" Element) element.
• metalink:file (The "metalink:file" Element) elements MAY contain exactly one metalink:license (The "metalink:license" Element) element.
• metalink:file (The "metalink:file" Element) elements MAY contain exactly one metalink:logo (The "metalink:logo" Element) element.
• metalink:file (The "metalink:file" Element) elements MAY contain exactly one metalink:os (The "metalink:os" Element) element.
• metalink:file (The "metalink:file" Element) elements MUST contain at least one metalink:url (The "metalink:url" Element) element or at least one metalink:metaurl (The "metalink:metaurl" Element) element. Typically, metalink:file (The "metalink:file" Element) elements contains more than one metalink:url (The "metalink:url" Element) element to provide multiple download sources.
• metalink:file (The "metalink:file" Element) elements MAY contain one or more metalink:pieces (The "metalink:pieces" Element) elements.
• metalink:file (The "metalink:file" Element) elements MAY contain exactly one metalink:publisher (The "metalink:publisher" Element) element.
• metalink:file (The "metalink:file" Element) elements MAY contain one or more metalink:signature (The "metalink:signature" Element) elements.
• metalink:file (The "metalink:file" Element) elements SHOULD contain exactly one metalink:size (The "metalink:size" Element) element.
• metalink:file (The "metalink:file" Element) elements MAY contain exactly one metalink:version (The "metalink:version" Element) element.

4.1.2.1.  The "name" Attribute

metalink:file (The "metalink:file" Element) elements MUST have a "name" attribute, which contains the filename of the file to be downloaded.

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

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

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

4.1.3.  The "metalink:pieces" Element

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

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

4.1.3.1.  The "type" Attribute

metalink:pieces (The "metalink:pieces" Element) elements MUST have a "type" attribute.

The IANA registry named "Hash Function Textual Names" defines values for hash types. If a Metalink Document contains hashes, it SHOULD include "sha-1" which is SHA-1 as specified in [RFC3174] (Eastlake, D. and P. Jones, “US Secure Hash Algorithm 1 (SHA1),” September 2001.), or a stronger hash. It MAY also include other hashes.

4.1.3.2.  The "length" Attribute

metalink:pieces (The "metalink:pieces" Element) elements MUST have a "length" attribute, which is an 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. The last piece extends at most to the end of the file, and can therefore be shorter than the other pieces.

4.2.  Metadata Elements

4.2.1.  The "metalink:copyright" Element

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

metalinkCopyright =
   element metalink:copyright {
     metalinkTextConstruct
   }

4.2.2.  The "metalink:description" Element

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

metalinkDescription =
   element metalink:description {
     metalinkTextConstruct
   }

4.2.3.  The "metalink:dynamic" Element

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

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

4.2.4.  The "metalink:generator" Element

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

metalinkGenerator =
   element metalink:generator {
     metalinkTextConstruct
   }

The content of this element, when present, MUST be a string that is a human-readable name and version, separated by a "/", for the generating agent. For example, "MirrorBrain/2.8.1", where "MirrorBrain" is the name and "2.8.1" is the version. Entities such as "&amp;" and "&lt;" represent their corresponding characters ("&" and "<" respectively), not markup.

4.2.5.  The "metalink:hash" Element

The "metalink:hash (The "metalink:hash" Element)" element is a Text construct that conveys a hash for a file. All hashes are encoded in lowercase hexadecimal format. Hashes are used to verify the integrity of a complete file or portion of a file to determine if the file has been transferred without any errors.

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

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

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

Metalink Documents can also contain hashes for individual pieces of a file. metalink:hash (The "metalink:hash" Element) elements that are inside a metalink:pieces (The "metalink:pieces" Element) 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 (The "metalink:pieces" Element) element they are contained in. metalink:hash (The "metalink:hash" Element) 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.

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

4.2.5.1.  The "type" Attribute

metalink:hash (The "metalink:hash" Element) elements MUST have a "type" attribute, if and only if it contains a hash of the complete file. The IANA registry named "Hash Function Textual Names" defines values for hash types. If a Metalink Document contains hashes, it SHOULD include "sha-1" which is SHA-1 as specified in [RFC3174] (Eastlake, D. and P. Jones, “US Secure Hash Algorithm 1 (SHA1),” September 2001.), or a stronger hash. It MAY also include other hashes.

4.2.6.  The "metalink:identity" Element

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

metalinkIdentity =
   element metalink:identity {
     metalinkTextConstruct
   }

4.2.7.  The "metalink:language" Element

The "metalink:language (The "metalink:language" Element)" element is a Text construct that conveys a code for the language of a file, per [RFC4646] (Phillips, A. and M. Davis, “Tags for Identifying Languages,” September 2006.).

metalinkLanguage =
   element metalink:language {
     metalinkTextConstruct
   }

4.2.8.  The "metalink:license" Element

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

metalinkLicense =
   element metalink:license {
     metalinkCommonAttributes,
     attribute url { metalinkUri }?,
     attribute name { text }?
   }

The metalink:license (The "metalink:license" Element) element MAY have a "url" attribute whose value MUST be an IRI reference [RFC3987] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.). When dereferenced, the resulting URI (mapped from an IRI, if necessary) SHOULD produce a representation that is relevant to that agent.

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

4.2.9.  The "metalink:logo" Element

The "metalink:logo (The "metalink:logo" Element)" element's content is an IRI reference [RFC3987] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.) 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.

4.2.10.  The "metalink:metaurl" Element

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

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

4.2.10.1.  The "preference" Attribute

metalink:metaurl (The "metalink:metaurl" Element) elements MAY have a preference attribute, whose value MUST be a number from 1 to 100 for priority, with 100 used first and 1 used last. Multiple metalink:metaurl (The "metalink:metaurl" Element) elements can have the same preference, i.e. three BitTorrent .torrent files could have preference="100". See also the "preference" attribute of the metalink:url (The "metalink:url" Element) element.

4.2.10.2.  The "type" Attribute

metalink:metaurl (The "metalink:metaurl" Element) elements MUST have a "type" attribute that indicates the MIME type of the metadata available at the IRI. In the case of BitTorrent as specified in [BITTORRENT] (Cohen, B., “The BitTorrent Protocol Specification,” February 2008.), the value "torrent" is required. Types without "/" are reserved. Currently, "torrent" is the only reserved value.

4.2.10.3.  The "name" Attribute

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

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

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

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

4.2.11.  The "metalink:origin" Element

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

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

4.2.12.  The "metalink:os" Element

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

metalinkOS =
   element metalink:os {
     metalinkTextConstruct
   }

4.2.13.  The "metalink:published" Element

The "metalink:published (The "metalink:published" Element)" 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 (The "metalink:published" Element) will be associated with the initial creation or first availability of the resource.

4.2.14.  The "metalink:publisher" Element

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

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

The metalink:publisher (The "metalink:publisher" Element) element MAY have a "url" attribute whose value MUST be an IRI reference [RFC3987] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.). When dereferenced, the resulting URI (mapped from an IRI, if necessary) SHOULD produce a representation that is relevant to that agent.

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

4.2.15.  The "metalink:signature" Element

The "metalink:signature (The "metalink:signature" Element)" element is a Text construct that conveys a digital signature for a file described in a Metalink Document. Digital signatures verify that a file is from the entity that has signed it.

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

4.2.15.1.  The "type" Attribute

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

4.2.16.  The "metalink:size" Element

The "metalink:size (The "metalink:size" Element)" element indicates the length of the linked content in octets; it is a hint about the content length of the representation returned when the IRI is mapped to a URI and dereferenced.

metalinkSize =
   element metalink:size {
     metalinkTextConstruct
   }

4.2.17.  The "metalink:updated" Element

The "metalink:updated (The "metalink:updated" Element)" 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 (The "metalink:updated" Element) value.

metalinkUpdated =
   element metalink:updated {
     metalinkDateConstruct
   }

Publishers MAY change the value of this element over time.

4.2.18.  The "metalink:url" Element

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

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

4.2.18.1.  The "preference" Attribute

metalink:url (The "metalink:url" Element) elements MAY have a preference attribute, whose value MUST be a number from 1 to 100 for priority, with 100 used first and 1 used last. Multiple metalink:url (The "metalink:url" Element) elements can have the same preference, i.e. ten mirrors could have preference="100".

4.2.18.2.  The "location" Attribute

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

4.2.19.  The "metalink:version" Element

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

metalinkVersion =
   element metalink:version {
     metalinkTextConstruct
   }

5.  Securing Metalink Documents

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

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

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

Metalink Processors that verify signed Metalink Documents MUST at least support XML-Signature and Syntax Processing (Solo, D., Reagle, J., and D. Eastlake, “XML-Signature Syntax and Processing (Second Edition),” June 2008.) [REC‑xmldsig‑core].

6.  Extending Metalink

6.1.  Extensions from Non-Metalink Vocabularies

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

6.2.  Extensions to the Metalink Vocabulary

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

6.3.  Processing Foreign Markup

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

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

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

6.4.  Extension Elements

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

6.4.1.  Simple Extension Elements

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

simpleExtensionElement =
   element * - metalink:* {
      text
   }

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

6.4.2.  Structured Extension Elements

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

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

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

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

7.  IANA Considerations

7.1.  XML Namespace Registration

This document makes use of the XML registry specified in [RFC3688] (Mealling, M., “The IETF XML Registry,” January 2004.). Accordingly, IANA has made the following registration:

Registration request for the Metalink namespace:

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

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

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

7.2.  application/metalink+xml MIME type

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

MIME media type name:

application

MIME subtype name:

metalink4+xml

Mandatory parameters:

None.

Optional parameters:

"charset":

This parameter has semantics identical to the charset parameter of the "application/xml" media type as specified in [RFC3023] (Murata, M., St. Laurent, S., and D. Kohn, “XML Media Types,” January 2001.).

Encoding considerations:

Identical to those of "application/xml" as described in [RFC3023] (Murata, M., St. Laurent, S., and D. Kohn, “XML Media Types,” January 2001.), 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] (Murata, M., St. Laurent, S., and D. Kohn, “XML Media Types,” January 2001.), Section 10.

Interoperability considerations:

There are no known interoperability issues.

Published specification:

This specification.

Applications that use this media type:

No known applications currently use this media type.

Additional information:

Magic number(s):

As specified for "application/xml" in [RFC3023] (Murata, M., St. Laurent, S., and D. Kohn, “XML Media Types,” January 2001.), Section 3.2.

File extension:

.metalink

Fragment identifiers:

As specified for "application/xml" in [RFC3023] (Murata, M., St. Laurent, S., and D. Kohn, “XML Media Types,” January 2001.), Section 5.

Base URI:

As specified in [RFC3023] (Murata, M., St. Laurent, S., and D. Kohn, “XML Media Types,” January 2001.), Section 6.

Macintosh File Type code:

TEXT

Person and email address to contact for further information:

Anthony Bryan <anthonybryan@gmail.com>

Intended usage:

COMMON

Author/Change controller:

IESG

8.  Security Considerations

Publishers are encouraged to offer Metalink documents via authenticated HTTP under TLS as specified in [RFC2818] (Rescorla, E., “HTTP Over TLS,” May 2000.). Publishers are also encouraged to include digital signatures of the files within the Metalink Documents if they are available.

8.1.  URIs and IRIs

Metalink Processors handle URIs and IRIs. See Section 7 of [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) and Section 8 of [RFC3987] (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.) for security considerations related to their handling and use.

8.2.  Spoofing

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

8.3.  Cryptographic Hashes

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

If a Metalink Document contains hashes, it SHOULD include "sha-1" which is SHA-1, as specified in [RFC3174] (Eastlake, D. and P. Jones, “US Secure Hash Algorithm 1 (SHA1),” September 2001.). It MAY also include other hashes.

8.4.  Signing

Metalink Documents SHOULD be signed using [REC‑xmldsig‑core] (Solo, D., Reagle, J., and D. Eastlake, “XML-Signature Syntax and Processing (Second Edition),” June 2008.) and are subject to the security considerations implied by its use. This addresses the issue of spoofing.

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

9.  References

9.1. Normative References

9.2. Informative References

Appendix A.  Acknowledgements and Contributors

The layout and content of this document relies heavily on work pioneered in the Atom Syndication Format as specified in [RFC4287] (Nottingham, M. and R. Sayre, “The Atom Syndication Format,” December 2005.).

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

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

Nicolas Alvarez, Patrick Ruckstuhl, Sebastien Willemijns, Micah Cowan, Yazsoft, KGet developers, FDM Team, Orbit Team, Arne Babenhauserheide, Mathias Berchtold, Xienzhenyu and TheWorld Browser Team, Xi Software, Bridget and Ethan Fletcher, Ruben Kerkhof, Agostino Russo, Salvatore and Robin Musumeci, Steve and Rachel Eshelman, Lucas Hewett, Ryan and Darren Cronin, Dave Winquist, Bob Denison, Wes Shelton, Kees Cook, Josh Colbert, Steve Kleisath, Chad Neptune, Nick Carrabba, Chris Carrabba, Erin Solari, Derick Cordoba, Ryan Alexander, John Sowder, Sandra Amisano, Tom Mainville, Janie Wargo, Jason Hansen, Tim Bray, Dan Brickley, Markus Hofmann, Dan Connolly, Tim Berners-Lee, Louis Suarez-Potts, Ross Smith, Jeff Covey, Ed Lee, Shawn Wilsher, Mike Connor, Johan Svedberg, Dedric Carter, James Antill, Debi Goulding, the Anthony Family, the Bryan Family, Juanita Anthony and Zimmy Bryan.

Appendix B.  RELAX NG Compact Schema

This appendix is informative.

The Relax NG schema explicitly excludes elements in the Metalink namespace that are not defined in this revision of the specification. Requirements for Metalink Processors encountering such markup are given in Sections 6.2 (Extensions to the Metalink Vocabulary) and 6.3 (Processing Foreign Markup).

# -*- rnc -*-
# RELAX NG Compact Syntax Grammar for the
# Metalink Format Specification Version 4

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

# Common attributes

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

# Text Constructs

metalinkTextConstruct =
   metalinkCommonAttributes,
   text

# Date Construct

metalinkDateConstruct =
   metalinkCommonAttributes,
   xsd:dateTime

start = metalinkMetalink

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

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

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

metalinkCopyright =
   element metalink:copyright {
      metalinkTextConstruct
   }

metalinkDescription =
   element metalink:description {
      metalinkTextConstruct
   }

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

metalinkGenerator =
   element metalink:generator {
      metalinkTextConstruct
   }

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

metalinkIdentity =
   element metalink:identity {
      metalinkTextConstruct
   }

metalinkLanguage =
   element metalink:language {
      metalinkTextConstruct
   }

metalinkLicense =
   element metalink:license {
      metalinkCommonAttributes,
      attribute url { metalinkUri }?,
      attribute name { text }?
   }

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

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

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

metalinkOS =
   element metalink:os {
      metalinkTextConstruct
   }

metalinkPublished =
   element metalink:published {
      metalinkDateConstruct
   }

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

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

metalinkSize =
   element metalink:size {
      metalinkTextConstruct
   }

metalinkUpdated =
   element metalink:updated {
      metalinkDateConstruct
   }

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

metalinkVersion =
   element metalink:version {
      metalinkTextConstruct
   }

# As defined in RFC 3066 and compatible with RFC 4646
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:base | xml:lang | local:*) { text }

undefinedContent = (text|anyForeignElement)*

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

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

# EOF

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

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

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

Known issues concerning this draft:

• None.

-14 : August , 2009.

• Update abstract and introduction.

-13 : August 21, 2009.

• Remove files, resources, verification container elements.
• MIME type: application/metalink4+xml

-12 : August 18, 2009.

• Remove "piece" attribute from hash elements in pieces container elements.
• Rename "uri" attribute of license and publisher elements to "url".

-11 : August 08, 2009.

• Renamed type element (static or dynamic values) to dynamic element (true or false values).
• Removed metadata inheritance and most other elements from files element.

-10 : July 28, 2009.

• Schema fixes.
• Rename metadata element to metaurl, add name attribute to it similar to file element's name attribute.
• Update REC-xmldsig-core reference to second edition.

-09 : July 11, 2009.

• Replace ISO639-2 references with RFC 4646.
• Add ISO3166-1.

-08 : July 04, 2009.

• Clarifications.
• Remove "uri" and "version" attributes from generator element.

-07 : June 18, 2009.

• This ID describes the Metalink document format/schema.
• Remove "Client Implementation Considerations" section.
• Expand "Known issues" section of Document History.

-06 : March 3, 2009.

• Add authors and this Document History section.

-05 : January 13, 2009.

• Clarifications.

-04 : December 31, 2008.

• New IPR notice as required by IETF.
• Correct "metalink:pieces" Element text.
• Add hash examples.
• Slim down "Securing Metalink Documents" section.
• Recommend at least SHA-1.

-03 : September 19, 2008.

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

-02 : September 4, 2008.

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

-01 : August 28, 2008.

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

-00 : August 23, 2008.

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

Index

Authors' Addresses