Network Working Group A. Bryan
Internet-Draft T. Tsujikawa
Intended status: Standards Track N. McNab
Expires: July 03, 2010 P. Poeml
Novell, Inc.
January 2010

The Metalink Download Description Format
draft-bryan-metalink-25

Abstract

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

Status of This Memo

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

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http:/⁠/⁠datatracker.ietf.org/⁠drafts/⁠current/⁠.

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

This Internet-Draft will expire on July 03, 2010.

Copyright Notice

Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http:/⁠/⁠trustee.ietf.org/⁠license-⁠info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.


Table of Contents

1. Introduction

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

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

At times, individual servers can be slow, outdated, or unreachable, but this can not be determined until the download has been initiated. This can lead to the user canceling the download and needing to restart it. During downloads, errors in transmission can corrupt the file. There are no easy ways to repair these files. For large downloads this can be especially troublesome. Any of the number of problems that can occur during a download lead to frustration on the part of users, and 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 checksums, digital signatures, and more can be stored in a machine-readable Metalink file. The Metalink file can transfer this knowledge to the user agent, which can peruse it in automatic ways or present the information to a human user. User agents can fall back to alternate mirrors if the current one has an issue. Thereby, clients are enabled to work their way to a successful download even under adverse circumstances. All this can be done transparently to the human user and the download is much more reliable and efficient. In contrast, a traditional HTTP redirect to one mirror conveys only comparatively minimal information - a referral to a single server, and there is no provision in the HTTP protocol to handle failures.

Other features that some clients provide include multi-source downloads, where chunks of a file are downloaded from multiple mirrors (and optionally, Peer-to-Peer) simultaneously, which frequently results in a faster download. Metalinks can leverage HTTP, FTP and Peer-to-Peer protocols together, because regardless over which protocol the Metalink was obtained, it can make a resource accessible through other protocols. If the Metalink was obtained from a trusted source, included verification metadata can solve trust issues when downloading files from replica servers operated by third parties. Metalinks also provide structured information about downloads that can be indexed by search engines.

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

1.1. Examples

A brief, Metalink Document that describes a single file:

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

A more extensive, Metalink Document that describes two files:

<?xml version="1.0" encoding="UTF-8"?>
<metalink xmlns="urn:ietf:params:xml:ns:metalink">
  <published>2009-05-15T12:23:23Z</published>
  <file name="example.ext">
    <size>14471447</size>
    <identity>
    Example
    </identity>
    <version>1.0</version>
    <language>en</language>	  
    <description>
    A description of the example file for download.
    </description>
    <hash type="sha-256">f0ad929cd259957e160ea442eb80986b5f01...</hash>
    <url location="de" 
         priority="1">ftp://ftp.example.com/example.ext</url>
    <url location="fr" 
         priority="1">http://example.com/example.ext</url> 
    <metaurl type="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 type="torrent" 
         priority="2">http://example.com/example2.ext.torrent</metaurl>
  </file>
</metalink>

1.2. Namespace and Version

The XML Namespaces URI [REC-xml-names] for the XML data format described in this specification is:

urn:ietf:params:xml:ns:metalink

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

1.3. Notational Conventions

This specification describes conformance of Metalink Documents. Additionally, it places some requirements on Metalink Processors.

This specification uses the namespace prefix "metalink:" for the Namespace URI identified in Section 1.2, above. Note that the choice of namespace prefix is arbitrary and not semantically significant.

Metalink is specified using terms from the XML Infoset [REC-xml-infoset]. However, this specification uses a shorthand for two common terms: the phrase "Information Item" is omitted when naming Element Information Items and Attribute Information Items. Therefore, when this specification uses the term "element," it is referring to an Element Information Item in Infoset terms. Likewise, when it uses the term "attribute," it is referring to an Attribute Information Item.

Some sections of this specification are illustrated with fragments of a non-normative RELAX NG Compact schema [RELAX-NG]. However, the text of this specification provides the definition of conformance. A complete schema appears in Appendix 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.metalink] element.

namespace metalink = "urn:ietf:params:xml:ns:metalink"	 		
start = metalinkMetalink

Metalink Documents are specified in terms of the XML Information Set, serialized as XML 1.0 [REC-xml] and identified with the "application/metalink4+xml" media type.

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

Metalink allows the use of IRIs, encoded according to [RFC3987]. Every URI [RFC3986] is also an IRI, so a URI may be used wherever below an IRI is named. There is one special consideration: when an IRI that is not also a URI is given for dereferencing, it MUST be mapped to a URI using the steps in Section 3.1 of [RFC3987].

Any element defined by this specification MAY have an xml:base attribute [REC-xmlbase]. When xml:base is used in an Metalink Document, it serves the function described in Section 5.1.1 of [RFC3986], establishing the base URI (or IRI) for resolving any relative references found within the effective scope of the xml:base attribute.

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

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.

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 white space around values by default, and such implementations will generate invalid Metalink Documents.

Metalink Documents that do not follow this specification are invalid, and partially or wholly unusable to Metalink Processors.

Metalink is an extensible format. See Section 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

metalinkTextConstruct =
   metalinkCommonAttributes,
   text

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

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

For example, a metalink:description [element.description] with text content:

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

metalinkDateConstruct =
   metalinkCommonAttributes,
   xsd:dateTime

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.

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

...
<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>
...

Example Date constructs:

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

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

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 [element.file] child elements which consist of metadata elements.

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

4.1.1.1. Providing Textual Content

Experience teaches that downloads providing textual content are in general more useful than those that do not. Some applications (one example is full-text indexers) require a minimum amount of text to function reliably and predictably. Metalink publishers should be aware of these issues. It is advisable that each metalink:file [element.file] element contain a non-empty metalink:description [element.description] element, a non-empty metalink:identity [element.identity] element, a non-empty metalink:version [element.version] element, and a non-empty metalink:publisher [element.publisher] element when these elements are present. However, the absence of metalink:description [element.description], metalink:identity [element.identity], metalink:version [element.version], and metalink:publisher [element.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.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.file] element.

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

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

This specification assigns no significance to the order of metalink:file [element.file] elements or to the order of metalink:url [element.url] or metalink:metaurl [element.metaurl] elements. Significance is determined by the value of the "priority" attribute of the metalink:url [element.url] or metalink:metaurl [element.metaurl] elements.

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

4.1.2.1. The "name" Attribute

metalink:file [element.file] elements MUST have a "name" attribute, which contains the local filename that the downloaded file will be written to. Hence, if a Metalink Document contains multiple metalink:file [element.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. 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

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

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

4.1.3.1. The "type" Attribute

metalink:pieces [element.pieces] elements MUST have a "type" attribute.

The 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 [element.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 should be the same size, apart from the last piece which is the remainder. The last piece extends to the end of the file, and can therefore be shorter than the other pieces.

4.2. Metadata Elements

4.2.1. The "metalink:copyright" Element

metalinkCopyright =
   element metalink:copyright {
     metalinkTextConstruct
   }

The "metalink:copyright [element.copyright]" element is a Text construct that conveys a human-readable copyright for a file.

4.2.2. The "metalink:description" Element

metalinkDescription =
   element metalink:description {
     metalinkTextConstruct
   }

The "metalink:description [element.description]" element is a Text construct that conveys a human-readable file description.

4.2.3. The "metalink:generator" Element

metalinkGenerator = 
   element metalink:generator {
     metalinkTextConstruct
   }

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

agent         = token ["/" agent-version]
agent-version = token

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

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

Examples:

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

4.2.4. The "metalink:hash" Element

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

The "metalink:hash [element.hash]" element is a Text construct that conveys a hash, also known as a checksum, 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.

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

Metalink Documents MAY contain one or multiples hashes of a complete file. metalink:hash [element.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.

Metalink Documents MAY also contain hashes for individual pieces of a file. metalink:hash [element.hash] elements that are inside a metalink:pieces [element.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.pieces] element they are contained in.

metalink:hash [element.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.pieces] element. See Section 4.1.3.2 for more information on the size of pieces.

...
  <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>
...

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

4.2.4.1. The "type" Attribute

metalink:hash [element.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. See Section 7.4 for security implications.

4.2.5. The "metalink:identity" Element

metalinkIdentity =
   element metalink:identity {
     metalinkTextConstruct
   }

The "metalink:identity [element.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".

4.2.6. The "metalink:language" Element

metalinkLanguage =
   element metalink:language {
     metalinkTextConstruct
   }

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

4.2.7.

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

The "metalink:logo [element.logo]" element's content is an IRI reference [RFC3987] that identifies an image that provides visual identification for a file.

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

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

The "metalink:metaurl [element.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.hash] element does not apply to these metadata files, but to the files that are described by them.

4.2.8.1. The "priority" Attribute

metalink:metaurl [element.metaurl] elements MAY have a priority attribute. Values MUST be positive integers between 1 and 999999. Lower values indicate a higher priority. metalink:metaurl [element.metaurl] elements without a priority attribute are considered to have the lowest priority, i.e. 999999. The priority values of metalink:metaurl [element.metaurl] and metalink:url [element.url] elements are compared and those with the lowest values, starting with 1, are used first. Multiple metalink:metaurl [element.metaurl] and metalink:url [element.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.url] element.

4.2.8.2. The "type" Attribute

metalink:metaurl [element.metaurl] elements MUST have a "type" attribute that indicates the MIME media type [RFC4288] of the metadata available at the IRI. In the case of BitTorrent as specified in [BITTORRENT], the value "torrent" is required. Types without "/" are reserved. Currently, "torrent" is the only reserved value.

4.2.8.3. The "name" Attribute

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

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

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

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

4.2.9. The "metalink:origin" Element

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

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

4.2.9.1. The "dynamic" Attribute

The metalink:origin [element.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

metalinkOS =
   element metalink:os {
     metalinkTextConstruct
   }

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

4.2.11. The "metalink:published" Element

metalinkPublished =
   element metalink:published { 
     metalinkDateConstruct
   }

The "metalink:published [element.published]" element is a Date construct indicating an instant in time associated with an event early in the life cycle of the entry.

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

4.2.12. The "metalink:publisher" Element

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

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

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

The metalink:publisher [element.publisher] element MAY have a "url" attribute whose value MUST be an IRI reference [RFC3987]. When dereferenced, the resulting URI (mapped from an IRI, if necessary) SHOULD produce a representation that is relevant to that agent.

4.2.13. The "metalink:signature" Element

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

The "metalink:signature [element.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.

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

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

Example:

4.2.13.1. The "type" Attribute

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

4.2.14. The "metalink:size" Element

metalinkSize =
   element metalink:size {
     xsd:nonNegativeInteger
   }

The "metalink:size [element.size]" element indicates the length of the linked content in octets; it is a hint about the content length of the representation returned when the IRI is mapped to a URI and dereferenced. This value MUST be a non-negative integer.

4.2.15. The "metalink:updated" Element

metalinkUpdated =
   element metalink:updated { 
     metalinkDateConstruct
   }

The "metalink:updated [element.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 [element.updated] value.

Publishers MAY change the value of this element over time.

4.2.16. The "metalink:url" Element

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

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

4.2.16.1. The "priority" Attribute

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

4.2.16.2. The "location" Attribute

metalink:url [element.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

metalinkVersion =
   element metalink:version {
     metalinkTextConstruct
   }

The "metalink:version [element.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".

5. Extending Metalink

5.1. 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.

5.2. Extensions to the Metalink Vocabulary

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

5.3. Processing Foreign Markup

Metalink Processors that encounter foreign markup in a location that is legal according to this specification MUST 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 [element.file], metalink:metalink [element.metalink], Metalink Processors MAY bypass the markup and any textual content and MUST NOT change their behavior as a result of the markup's presence.

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

5.4. Extension Elements

Metalink allows foreign markup anywhere in an Metalink document, except where it is explicitly forbidden. Child elements of metalink:file [element.file] and metalink:metalink [element.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

simpleExtensionElement =
   element * - metalink:* {
      text
   }

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.

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

5.4.2. Structured Extension Elements

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

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.

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 "Author's Address" 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:

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].

Encoding considerations:
Identical to those of "application/xml" as described in [RFC3023], Section 3.2.
Security considerations:
As defined in this specification.
In addition, as this media type uses the "+xml" convention, it shares the same security considerations as described in [RFC3023], Section 10.
Interoperability considerations:
There are no known interoperability issues.
Published specification:
This specification.
Applications that use this media type:
No known applications currently use this media type.

Additional information:

Magic number(s):
As specified for "application/xml" in [RFC3023], Section 3.2.
File extension:
.meta4
Fragment identifiers:
As specified for "application/xml" in [RFC3023], Section 5.
Base URI:
As specified in [RFC3023], Section 6.
Macintosh File Type code:
TEXT
Person and email address to contact for further information:
Anthony Bryan <anthonybryan@gmail.com>
Intended usage:
COMMON
Author/Change controller:
IESG

7. Security Considerations

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.

Publishers are encouraged to offer Metalink documents via authenticated HTTP under TLS as specified in [RFC2818]. The choice of a secure content layer is entirely possible for 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.

7.1. URIs and IRIs

Metalink Processors handle URIs and IRIs. See Section 7 of [RFC3986] and Section 8 of [RFC3987] for security considerations related to their handling and use.

7.2. 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. Unaware downloaders could be deceived into downloading a malicious or worthless file. Malicious publishers could attempt a distributed denial of service attack by inserting unrelated IRIs into Metalink Documents.

7.3. Signing

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

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

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 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-256" which is SHA-256, as specified in [FIPS-180-3], 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.
[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. 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.
[REC-xml-infoset] Cowan, J. and R. Tobin, "XML Information Set (Second Edition)", W3C REC-xml-infoset-20040204, February 2004.
[REC-xmlbase] Marsh, J and R Tobin, "XML Base (Second Edition)", W3C REC-xmlbase-20090128, January 2009.
[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.
[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.
[REC-xmldsig-core] Solo, D., Reagle, J. and D. Eastlake, "XML-Signature Syntax and Processing (Second Edition)", W3C REC-xmldsig-core-20080610, June 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.
[FIPS-180-3] National Institute of Standards and Technology (NIST), "Secure Hash Standard (SHS)", FIPS PUB 180-3, October 2008.

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.
[REC-xmlschema-2-20041028] Malhotra, A and P Biron, "XML Schema Part 2: Datatypes Second Edition", W3C REC-xmlschema-2-20041028, October 2004.
[NOTE-datetime-19980827] Wolf, M and C Wicksteed, "Date and Time Formats", W3C NOTE-datetime-19980827, August 1998.
[RELAX-NG] Clark, J, "RELAX NG Compact Syntax", December 2001.
[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.

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 proposed ideas and wording for this document, or contributed to the evolution of Metalink, 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, Kees Cook, Josh Colbert, Steve Kleisath, Chad Neptune, Nick Carrabba, Chris Carrabba, Erin Solari, Derick Cordoba, 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, Dedric Carter, and Debi Goulding. We also thank the Anthony Family, the Bryan Family, Juanita Anthony and Zimmy Bryan.

We also thank the following contributors for assistance and review: Eran Hammer-Lahav, Lisa Dusseault, 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, and Brian Carpenter.

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: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,
      (metalinkFile+
       & metalinkGenerator?
       & metalinkOrigin?
       & metalinkPublished?
       & metalinkUpdated?
       & extensionElement*)
   }

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

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

metalinkCopyright =
   element metalink:copyright {
      metalinkTextConstruct
   }

metalinkDescription =
   element metalink:description {
      metalinkTextConstruct
   }

metalinkGenerator =
   element metalink:generator {
      metalinkTextConstruct
   }

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

metalinkIdentity =
   element metalink:identity {
      metalinkTextConstruct
   }

metalinkLanguage =
   element metalink:language {
      metalinkTextConstruct
   }

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

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

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

metalinkOS =
   element metalink:os {
      metalinkTextConstruct
   }

metalinkPublished =
   element metalink:published {
      metalinkDateConstruct
   }

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

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

metalinkSize =
   element metalink:size {
      xsd:nonNegativeInteger
   }

metalinkUpdated =
   element metalink:updated {
      metalinkDateConstruct
   }

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

metalinkVersion =
   element metalink:version {
      metalinkTextConstruct
   }

# As defined in RFC 3066 and compatible with RFC 5646
metalinkLanguageTag = xsd:string {
   pattern = "[A-Za-z]{1,8}(-[A-Za-z0-9]{1,8})*"
}

# Unconstrained; it's not entirely clear how IRI fit into
# xsd:anyURI so let's not try to constrain it here
metalinkUri = text

# Simple Extension

simpleExtensionElement =
   element * - metalink:* {
      text
   }

# Structured Extension

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

# Other Extensibility

extensionElement =
   simpleExtensionElement | structuredExtensionElement

undefinedAttribute =
  attribute * - (xml: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:

-25 : December xx, 2009.

-24 : December 08, 2009.

-23 : November 26, 2009.

-22 : November 09, 2009.

-21 : October 13, 2009.

-20 : October 12, 2009.

-19 : October 5, 2009.

-18 : October 4, 2009.

-17 : September 28, 2009.

-16 : August 31, 2009.

-15 : August 26, 2009.

-14 : August 24, 2009.

-13 : August 21, 2009.

-12 : August 18, 2009.

-11 : August 08, 2009.

-10 : July 28, 2009.

-09 : July 11, 2009.

-08 : July 04, 2009.

-07 : June 18, 2009.

-06 : March 3, 2009.

-05 : January 13, 2009.

-04 : December 31, 2008.

-03 : September 19, 2008.

-02 : September 4, 2008.

-01 : August 28, 2008.

-00 : August 23, 2008.

Authors' Addresses

Anthony Bryan Pompano Beach, FL USA EMail: anthonybryan@gmail.com URI: http://www.metalinker.org
Tatsuhiro Tsujikawa EMail: tatsuhiro.t@gmail.com URI: http://aria2.sourceforge.net
Neil McNab EMail: neil@nabber.org URI: http://www.nabber.org
Peter Poeml Novell, Inc. EMail: poeml@mirrorbrain.org URI: http://www.mirrorbrain.org/