Network Working Group A. Bryan, Ed.
Internet-Draft Metalinker Project
Intended status: Standards Track December 2008
Expires: June 02, 2009

The Metalink Download Description Format
draft-bryan-metalink-04

Abstract

This document specifies Metalink Documents, an XML-based download description format.

Status of This Memo

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

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). 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 June 02, 2009.

Copyright Notice

Copyright (c) 2008 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.


Table of Contents

1. Introduction

Metalink is an XML-based document format that describes a file or lists of files to be added to a download queue. Lists are composed of 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.

The primary use case that Metalink addresses is the description of downloadable content in a format so download agents can act intelligently and recover from common errors with little or no user interaction necessary. These errors can include multiple servers going down and data corrupted in transmission.

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">
  <files>
    <file name="example.ext">
      <resources>
        <url>ftp://ftp.example.com/example.ext</url>
        <url>http://example.com/example.ext</url> 
        <metadata type="torrent">
        http://example.com/example.ext.torrent
        </metadata>
      </resources>
    </file>
  </files>
</metalink>

A more extensive, single file Metalink Document:

<?xml version="1.0" encoding="UTF-8"?>
<metalink xmlns="urn:ietf:params:xml:ns:metalink">
  <published>2008-05-15T12:23:23Z</published>
  <files>
    <file name="example.ext">
      <identity>Example</identity>
      <version>1.0</version>
      <description>A description of the example file for 
download.</description>
      <verification>
        <hash type="sha-1">80bc95fd391772fa61c91ed68567f0980bb45fd9
        </hash>
        </verification>
      <resources>
        <url>ftp://ftp.example.com/example.ext</url>
        <url>http://example.com/example.ext</url> 
        <metadata type="torrent">
        http://example.com/example.ext.torrent
        </metadata>
      </resources>
    </file>
  </files>
</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/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]. 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.

Metalink is an extensible format. See Section 7 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

metalinkTextConstruct =
   metalinkCommonAttributes,
   text

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

3.1.1. Text

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

Example 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, Metalink Processors MAY collapse white space (including line breaks) and display the text 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], [W3C.NOTE-datetime-19980827], and [W3C.REC-xmlschema-2-20041028].

<updated>2008-12-13T18:30:02Z</updated>
<updated>2008-12-13T18:30:02.25Z</updated>
<updated>2008-12-13T18:30:02+01:00</updated>
<updated>2008-12-13T18:30:02.25+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 entries 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,
      (metalinkPublished?
       & metalinkOrigin?
       & metalinkGenerator?
       & metalinkUpdated?
       & extensionElement*),
      metalinkFiles
   }

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 "metalink:files" element whose element children consist of metadata elements followed by one or more metalink:file [element.file] child 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 when that element is present, and a non-empty metalink:version [element.version] element, and a non-empty metalink:publisher [element.publisher] element. However, the absence of metalink:description [element.description] is not an error, and Metalink Processors MUST NOT fail to function correctly as a consequence of such an absence.

4.1.2. The "metalink:files" Element

metalinkFiles =
   element metalink:files {
      metalinkCommonAttributes,
      (metalinkIdentity?
       & metalinkVersion?
       & metalinkDescription?
       & metalinkOS?
       & metalinkLogo?
       & metalinkLanguage?
       & metalinkPublisher?
       & metalinkCopyright?
       & metalinkLicense?
       & extensionElement*)
      metalinkFile
   }

The "metalink:files" element acts as a container for metadata and data associated with the listed files. It contains one or more metalink:file [element.file] child elements. Certain elements can be listed either under metalink:files [element.files] or metalink:file [element.file]. If under metalink:files [element.files], they apply to all files listed in each metalink:file [element.file]. If under metalink:file [element.file], then they apply to just that specific file.

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

4.1.3. The "metalink:file" Element

metalinkFile =
   element metalink:file {
      metalinkCommonAttributes,
      attribute name { metalinkTextConstruct },
      (metalinkVerification?
       & metalinkIdentity?
       & metalinkVersion?
       & metalinkDescription?
       & metalinkSize?
       & metalinkOS?
       & metalinkLogo?
       & metalinkLanguage?
       & metalinkPublisher?
       & metalinkCopyright?
       & metalinkLicense?
       & extensionElement*)
      metalinkResources
   }

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

This specification assigns no significance to the order of metalink:file [element.file] elements.

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

4.1.3.1. The "name" Attribute

metalink:file [element.file] elements MUST have a "name" attribute, which contains the filename of the file 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 be relative. The path MUST NOT begin with a "/", "./" or "../", contain "/../", or end with "/..". Metalink Processors MUST NOT allow directory traversal.

A Metalink Processor MAY alter the name of the subdirectory or file if they contain characters which are invalid in the destination filesystem.

4.1.4. The "metalink:resources" Element

metalinkResources =
   element metalink:resources {
      metalinkCommonAttributes,
      extensionElement*
      metalinkURL*
   }

The "metalink:resources" element acts as a container for metadata and data associated with the listed files. It contains one or more metalink:url [element.url] child elements.

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

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

4.1.5. The "metalink:verification" Element

metalinkVerification =
   element metalink:verification {
      metalinkCommonAttributes,
     (metalinkHash*
      & metalinkPieces*
      & metalinkSignature
      & extensionElement*)
   }

The "metalink:verification" element acts as a container for metadata and data associated with verifying the listed files. This information is in the form of checksums and digital signatures. Checksums are used to verify the integrity of a complete file or portion of a file to determine if the files have been transferred without any errors. Digital signatures verify that a file is from the entity that has signed it.

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

4.1.6. The "metalink:pieces" Element

metalinkPieces =
   element metalink:pieces {
      attribute length { metalinkTextConstruct },
      attribute type { metalinkTextConstruct },
      hash+
   }+,

The "metalink:pieces [element.pieces]" element acts as a container for metadata and data associated with verifying the listed files. This information is in the form of checksums for a portion of a file.

4.1.6.1. 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. Metalink Generators and Processors supporting verification SHOULD at least implement "sha-1" which is SHA1, as specified in [RFC3174].

4.1.6.2. The "length" Attribute

metalink:pieces [element.pieces] elements MUST have a "length" attribute, which is an integer that describes the length of the piece of the file in octets.

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 description for a file.

4.2.3. The "metalink:generator" Element

metalinkGenerator = element metalink:generator {
   metalinkCommonAttributes,
   attribute uri { metalinkUri }?,
   attribute version { text }?,
   text
}

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

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

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

The metalink:generator [element.generator] element MAY have a "version" attribute that indicates the version of the generating agent.

4.2.4. The "metalink:hash" Element

metalinkHash =
   element metalink:hash {
   attribute piece { xsd:integer }?,
   attribute type { metalinkTextConstruct },
   text
   }

The "metalink:hash [element.hash]" element is a Text construct that conveys a hash for a file. All hashes are encoded in lowercase hexadecimal format.

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

Metalinks can contain multiples hashes for a complete file, for example both SHA-1 and SHA-256.

...
<verification>
  <hash type="sha-1">a97fcf6ba9358f8a6f62beee4421863d3e52b080</hash>
  <hash type="sha-256">fc87941af7fd7f03e53b34af393f4c14923d74
  825f51116ff591336af4880227</hash>
  <pieces length="1048576" type="sha-1">
    <hash piece="0">d96b9a4b92a899c2099b7b31bddb5ca423bb9b30</hash>
    <hash piece="1">10d68f4b1119014c123da2a0a6baf5c8a6d5ba1e</hash>
    <hash piece="2">3e84219096435c34e092b17b70a011771c52d87a</hash>
    <hash piece="3">67183e4c3ab892d3ebe8326b7d79eb62d077f487</hash>
  </pieces>
</verification>
...

Metalinks can also contain hashes for individual pieces of a file.

metalink:hash [element.hash] elements MUST have a "type" attribute or a "piece" attribute. metalink:hash [element.hash] elements with a "type" attribute contain a hash of the whole file. metalink:hash [element.hash] elements with a "piece" attribute contain a hash for that specific piece or chunk of the file.

4.2.4.1. The "type" Attribute

The IANA registry named "Hash Function Textual Names" defines values for hash types. Metalink Generators and Processors supporting verification SHOULD at least implement "sha-1" which is SHA1, as specified in [RFC3174].

4.2.4.2. The "piece" Attribute

metalink:hash [element.hash] elements MAY have a "piece" attribute, only when they are a sub element of metalink:pieces [element.pieces]. The value of "piece" starts at "0" and increases, depending on the "length" attribute of metalink:pieces [element.pieces] and the size of the file. Depending on the size of a file, the last piece may not be the same size as the others.

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. The identity of OpenOffice.org 3.0 would be "OpenOffice.org".

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 [ISO639-2].

4.2.7. The "metalink:license" Element

metalinkLicense =
   element metalink:license {
   metalinkCommonAttributes,
   attribute uri { metalinkUri }?,
   attribute name { metalinkTextConstruct }?,
   }

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

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

The metalink:license [element.license] element MAY have a "name" attribute that indicates the name of the license.

4.2.8.

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.

4.2.9. The "metalink:metadata" Element

metalinkMetadata =
   element metalink:metadata {
      metalinkCommonAttributes,
      attribute preference { xsd:integer }?,
      attribute type { metalinkTextConstruct },
      metalinkUri
   }+

The "metalink:metadata [element.metadata]" 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.

4.2.9.1. The "preference" Attribute

metalink:metadata [element.metadata] 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. See the "preference" attribute of the metalink:url [element.url] element for more information.

4.2.9.2. The "type" Attribute

metalink:metadata [element.metadata] 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], the value "torrent" is required. Types without "/" are reserved. Currently, "torrent" is the only reserved value.

Metalink Processors that do not support a specified type of metadata about a resource to download MUST ignore that metadata.

4.2.10. The "metalink:origin" Element

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

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

4.2.11. The "metalink:os" Element

metalinkOS =
   element metalink:os {
   metalinkTextConstruct
   }

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

4.2.12. 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.

4.2.13. The "metalink:publisher" Element

metalinkPublisher =
   element metalink:publisher {
   metalinkCommonAttributes,
   attribute uri { metalinkUri }?,
   attribute name { metalinkTextConstruct }?,
}

The "metalink:publisher [element.publisher]" element indicates a group or other entity which has published the file.

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

The metalink:publisher [element.publisher] element MAY have a "name" attribute that indicates the name of the publisher.

4.2.14. The "metalink:signature" Element

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

The "metalink:signature [element.signature]" element is a Text construct that conveys a digital signature for a file described in a Metalink Document.

4.2.14.1. The "type" Attribute

metalink:signature [element.signature] elements MUST have a "type" attribute. The inital 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.

4.2.15. The "metalink:size" Element

metalinkSize =
   element metalink:size {
   metalinkTextConstruct
   }

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. Note that the "metalink:size [element.size]" element MUST override the actual content length of the representation as reported by the underlying protocol, i.e. files with different sizes should be discarded.

4.2.16. The "metalink:type" Element

metalinkType =
   element metalink:type {
   "static" | "dynamic"
   }

The "metalink:type [element.type]" element is a Text construct that describes whether the IRI from "metalink:origin [element.origin]" a Metalink will contain dynamic updated Metalinks or static content that is not updated.

4.2.17. 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.

4.2.18. The "metalink:url" Element

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

The "metalink:url [element.url]" element contains the IRI of a file. All IRIs MUST lead to identical files.

4.2.18.1. The "preference" Attribute

metalink:url [element.url] 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 [element.url] elements can have the same preference, i.e. ten mirrors could have preference="100". A Metalink Processor MAY download different segments of a file from more than one IRI simultaneously, and when doing so SHOULD first use the highest priority IRIs and then use lower ones.

When one or more metalink:url [element.url] elements have a preference attribute value of "100", other metalink:url [element.url] elements SHOULD NOT be used, unless the elements with a preference of 100 cannot be processed (e.g. if they are of a metalink:metadata [element.metadata] element type which is not supported by the Metalink Processor, such as BitTorrent, or if the servers are unavailable).

Any metalink:url [element.url] elements with a preference attribute value of "1" SHOULD NOT be used unless all other metalink:url [element.url] elements cannot be processed (e.g. if they are of a metalink:metadata [element.metadata] element type which is not supported by the Metalink Processor, such as BitTorrent, or if the servers are unavailable).

4.2.18.2. The "location" Attribute

metalink:url [element.url] elements MAY have a "location" attribute, which is a [ISO3166] 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

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 OpenOffice.org 3.0 would be "3.0".

5. Client Implementation Considerations

Metalink Processors that support HTTP MUST support transparent content negotiation with HTTP [RFC2295]. Transparent content negotiation is accomplished by adding the Metalink media type to the Accept request header. Metalink Processors MUST check the returned content type, and if the Metalink media type is used, it MUST process the Metalink. If the content type does not match the Metalink media type, then Metalink Processors SHOULD handle the response as a normal response. Metalink Processors MUST NOT add the Metalink media type to Accept when requesting a URI from a metalink:url [element.url] element, thus avoiding loops. Metalink Processors SHOULD handle external redirects that might lead to a Metalink.

When multiple hash types methods are provided, a Metalink Processor MAY verify using more than one of these hash types. Metalink Processors are encouraged to check all hash types given which they are able to verify.

6. Securing Metalink Documents

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

Producers of Metalinks 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 encypt Metalinks that list digital songs that have been purchased. Of course, many other examples exist as well.

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

Metalink Processors that verify signed Metalink Documents MUST at least support XML-Signature and Syntax Processing [REC-xmldsig-core].

7. Extending Metalink

7.1. 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.

7.2. 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".

7.3. 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.

7.4. 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. Child elements of Person constructs are considered to apply to the construct. The role of other foreign markup is undefined by this specification.

7.4.1. 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.

7.4.2. 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.

8. IANA Considerations

8.1. 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.

8.2. 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:
metalink+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:
.metalink
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

9. Security Considerations

Publishers are encouraged to offer Metalink documents via authenticated HTTP under TLS as specified in [RFC2818]. Publishers are also encouraged to include digital signatures of the files within the Metalink Documents if they are available.

9.1. 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.

9.2. Spoofing

Metalink Processors should be aware of the potential for spoofing attacks where the attacker publishes Metalinks 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.

9.3. 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 match.

Metalink Generators and Processors supporting verification SHOULD at least implement "sha-1" which is SHA1, as specified in [RFC3174].

9.4. Signing

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

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

10. References

10.1. 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.
[RFC2295] Holtman, K. and A. Mutz, "Transparent Content Negotiation in HTTP", RFC 2295, March 1998.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3023] Murata, M., St. Laurent, S. and D. Kohn, "XML Media Types", RFC 3023, January 2001.
[RFC3174] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)", RFC 3174, September 2001.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet: Timestamps", RFC 3339, July 2002.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, January 2004.
[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.
[REC-xml-infoset] Cowan, J. and R. Tobin, "XML Information Set (Second Edition)", World Wide Web Consortium Recommendation REC-xml-infoset-20040204, February 2004.
[REC-xmlbase] Marsh, J, "XML Base", W3C REC W3C.REC-xmlbase-20010627, June 2001.
[REC-xml-names] Hollander, D., Bray, T., Tobin, R. and A. Layman, "Namespaces in XML 1.0 (Second Edition)", World Wide Web Consortium Recommendation REC-xml-names-20060816, August 2006.
[REC-xml] Yergeau, F., Paoli, J., Bray, T., Sperberg-McQueen, C. and E. Maler, "Extensible Markup Language (XML) 1.0 (Fourth Edition)", World Wide Web Consortium Recommendation REC-xml-20060816, August 2006.
[REC-xmldsig-core] Solo, D., Reagle, J. and D. Eastlake, "XML-Signature Syntax and Processing", World Wide Web Consortium Recommendation REC-xmldsig-core-20020212, February 2002.
[ISO639-2] International Organization for Standardization, "ISO 639-2:1998 - Codes for the representation of names of languages -- Part 2: Alpha-3 code - edition 1, 1998-11-01, 66 pages, prepared by a Joint Working Group of ISO TC46/SC4 and ISO TC37/SC2.", ISO Standard 639-2, 1998.
[ISO3166] International Organization for Standardization, "ISO 3166:1988 (E/F) - Codes for the representation of names of countries - The International Organization for Standardization, 3rd edition, 1988-08-15.", ISO Standard 3166, 1988.

10.2. 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.
[W3C.REC-xmlschema-2-20041028] Malhotra, A and P Biron, "XML Schema Part 2: Datatypes Second Edition", W3C REC REC-xmlschema-2-20041028, October 2004.
[W3C.NOTE-datetime-19980827] Wolf, M and C Wicksteed, "Date and Time Formats", W3C NOTE NOTE-datetime-19980827, August 1998.
[RELAX-NG] Clark, J, "RELAX NG Compact Syntax", December 2001.
[RFC4287] Nottingham, M. and R. Sayre, "The Atom Syndication Format", RFC 4287, December 2005.

Appendix A. Contributors

The layout and content of this document relies heavily on work pioneered in the Atom Syndication Format as specified in [RFC4287].

The following people contributed to preliminary versions of this document: Paul Burkhead, Kristian Weston, Darius Liktorius, Michael Burford, Giorgio Maone, Manuel Subredu, Tatsuhiro Tsujikawa, A. Bram Neijt, Max Velasques, Manolo Valdes, Urs Wolfer, Frederick Cheung, Nils Maier, Hampus Wessman, Neil McNab, Hayden Legendre, Danny Ayers, Nick Dominguez, Rene Leonhardt, Per Oyvind Karlsen, Gary Zellerbach, James Clark, Daniel Stenberg, Peter Poeml, Matt Domsch, Chris Newman, Lisa Dusseault, Ian Macfarlane, Dave Cridland, Julian Reschke, Barry Leiba, Uri Blumenthal, and Paul Hoffman. 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, Mike Wells, Sebastien Willemijns, Micah Cowan, Dan Fandrich, Francis Giannaros, Yazsoft, Lukas Appelhans, 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, Gervase Markham, 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, Harry Chen, Adrien Macneil, Louis Suarez-Potts, Ross Smith, Rahul Sundaram, Jesse Keating, Michal Bentkowski, Andrew Pantyukhin, Judd Vinet, Charles Landemaine, Pascal Bleser, Jeff@BLAG, Yuichiro Nakada, Jereme Hancock, Marcel Hauser, Jeff Covey, Doug Lang, Seth Brown, Alexander Lazic, Mayank Sharma, Robin Heggelund Hansen, Steve Langasek, Federico Parodi, Stefano Verna, Jason Green, James Linden, Matt Nederlanden, Aren Olsen, Dag Odenhall, Troy Sobotka, Corey Farwell, Ed Lee, Shawn Wilsher, Mike Connor, Anand Muttagi, Dedric Carter, 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 7.2 and 7.3.

# -*- rnc -*-
# RELAX NG Compact Syntax Grammar for the
# Metalink Format Specification Version 1

   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 =
  element metalink:metalink {
    element metalink:generator {
      attribute uri { metalinkUri }?,
      attribute version { text }?,
      metalinkTextConstruct
    }
    element metalink:origin { metalinkUri }?,
    element metalink:type { "static" | "dynamic"  }?,
    element metalink:published { metalinkDateConstruct }?,
    element metalink:updated { metalinkDateConstruct }?,
    element metalink:files {
      element metalink:file {
        attribute name { metalinkTextConstruct },
        element metalink:identity { metalinkTextConstruct }?,
        element metalink:version { metalinkTextConstruct }?,
        element metalink:size { xsd:integer }?,
        element metalink:description { metalinkTextConstruct }?,
        element metalink:license {
          attribute uri { metalinkUri }?,
          attribute name { metalinkTextConstruct }?,
        }?,
        element metalink:logo { metalinkUri }?,
        element metalink:publisher {
          attribute uri { metalinkUri }?,
          attribute name { metalinkTextConstruct }?,
        }?,
        element metalink:language { metalinkTextConstruct }?,
        element metalink:copyright { metalinkTextConstruct }?,
        element metalink:license { metalinkTextConstruct }?,
        element metalink:os { metalinkTextConstruct }?,
        element metalink:verification {
          hash+,
          element metalink:pieces {
            attribute length { metalinkTextConstruct },
            attribute type { metalinkTextConstruct },
            hash+
          }+,
          element metalink:signature {
           attribute type { "pgp" },
           text
           }
        }?,
        element metalink:resources {
          element metalink:metadata {
            attribute preference { xsd:integer }?,
            attribute type { metalinkTextConstruct },
            metalinkUri
          element metalink:url {
            attribute location { xsd:string { 
            minLength = "2"  maxLength="2"} 
            }?,
            attribute preference { xsd:integer }?,
            metalinkUri
          }+
        }
      }+
    }
  }
hash =
  element metalink:hash {
    attribute piece { metalinkTextConstruct }?,
    attribute type { metalinkTextConstruct },
    text
  }

# As defined in RFC 3066
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

Author's Address

Anthony Bryan (editor) Metalinker Project EMail: anthonybryan@gmail.com URI: http://www.metalinker.org