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Internet Engineering Task Force (IETF) A. Bryan
Request for Comments: 5854 T. Tsujikawa
Category: Standards Track N. McNab
ISSN: 2070-1721
P. Poeml
MirrorBrain
June 2010
The Metalink Download Description Format
Abstract
This document specifies Metalink, an XML-based download description
format. Metalink describes download locations (mirrors),
cryptographic hashes, and other information. Clients can
transparently use this information to reliably transfer files.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5854.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Bryan, et al. Standards Track [Page 1]
RFC 5854 Metalink Download Description Format June 2010
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Namespace and Version . . . . . . . . . . . . . . . . . . 5
1.3. Notational Conventions . . . . . . . . . . . . . . . . . . 5
2. Metalink Documents . . . . . . . . . . . . . . . . . . . . . . 6
3. Common Metalink Constructs . . . . . . . . . . . . . . . . . . 7
3.1. Text Constructs . . . . . . . . . . . . . . . . . . . . . 7
3.2. Date Constructs . . . . . . . . . . . . . . . . . . . . . 8
4. Metalink Element Definitions . . . . . . . . . . . . . . . . . 8
4.1. Container Elements . . . . . . . . . . . . . . . . . . . . 8
4.1.1. The "metalink:metalink" Element . . . . . . . . . . . 8
4.1.2. The "metalink:file" Element . . . . . . . . . . . . . 9
4.1.3. The "metalink:pieces" Element . . . . . . . . . . . . 12
4.2. Metadata Elements . . . . . . . . . . . . . . . . . . . . 12
4.2.1. The "metalink:copyright" Element . . . . . . . . . . . 12
4.2.2. The "metalink:description" Element . . . . . . . . . . 13
4.2.3. The "metalink:generator" Element . . . . . . . . . . . 13
4.2.4. The "metalink:hash" Element . . . . . . . . . . . . . 14
4.2.5. The "metalink:identity" Element . . . . . . . . . . . 15
4.2.6. The "metalink:language" Element . . . . . . . . . . . 15
4.2.7. The "metalink:logo" Element . . . . . . . . . . . . . 16
4.2.8. The "metalink:metaurl" Element . . . . . . . . . . . . 16
4.2.9. The "metalink:origin" Element . . . . . . . . . . . . 18
4.2.10. The "metalink:os" Element . . . . . . . . . . . . . . 18
4.2.11. The "metalink:published" Element . . . . . . . . . . . 18
4.2.12. The "metalink:publisher" Element . . . . . . . . . . . 18
4.2.13. The "metalink:signature" Element . . . . . . . . . . . 19
4.2.14. The "metalink:size" Element . . . . . . . . . . . . . 20
4.2.15. The "metalink:updated" Element . . . . . . . . . . . . 20
4.2.16. The "metalink:url" Element . . . . . . . . . . . . . . 20
4.2.17. The "metalink:version" Element . . . . . . . . . . . . 21
5. Extending Metalink . . . . . . . . . . . . . . . . . . . . . . 21
5.1. Extensions from Non-Metalink Vocabularies . . . . . . . . 21
5.2. Extensions to the Metalink Vocabulary . . . . . . . . . . 21
5.3. Processing Foreign Markup . . . . . . . . . . . . . . . . 22
5.4. Extension Elements . . . . . . . . . . . . . . . . . . . . 22
5.4.1. Simple Extension Elements . . . . . . . . . . . . . . 22
5.4.2. Structured Extension Elements . . . . . . . . . . . . 23
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
6.1. XML Namespace Registration . . . . . . . . . . . . . . . . 23
6.2. application/metalink4+xml MIME type . . . . . . . . . . . 23
7. Security Considerations . . . . . . . . . . . . . . . . . . . 24
7.1. Digital Signatures . . . . . . . . . . . . . . . . . . . . 25
7.2. URIs and IRIs . . . . . . . . . . . . . . . . . . . . . . 26
7.3. Spoofing . . . . . . . . . . . . . . . . . . . . . . . . . 26
7.4. Cryptographic Hashes . . . . . . . . . . . . . . . . . . . 26
Bryan, et al. Standards Track [Page 2]
RFC 5854 Metalink Download Description Format June 2010
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
8.1. Normative References . . . . . . . . . . . . . . . . . . . 27
8.2. Informative References . . . . . . . . . . . . . . . . . . 28
Appendix A. Acknowledgements and Contributors . . . . . . . . . . 30
Appendix B. RELAX NG Compact Schema . . . . . . . . . . . . . . . 31
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
1. Introduction
Metalink is a document format based on Extensible Markup Language
(XML) that describes a file or list of files to be downloaded from a
server. Metalinks can list a number of files, each with an
extensible set of attached metadata. Each listed file can have a
description, multiple cryptographic hashes, and a list of Uniform
Resource Identifiers (URIs) from which it is available.
Often, identical copies of a file are accessible in multiple
locations on the Internet over a variety of protocols, such as File
Transfer Protocol (FTP), Hypertext Transfer Protocol (HTTP), and
Peer-to-Peer (P2P). In some cases, users are shown a list of these
multiple download locations (mirror servers) and must manually select
one based on geographical location, priority, or bandwidth. This is
done to distribute the load across multiple servers, and to give
human users the opportunity to choose a download location that they
expect to work best for them.
At times, individual servers can be slow, outdated, or unreachable,
but this cannot be determined until the download has been initiated.
This can lead to the user canceling the download and needing to
restart it. During downloads, errors in transmission can corrupt the
file. There are no easy ways to repair these files. For large
downloads, this can be especially troublesome. Any of the number of
problems that can occur during a download lead to frustration on the
part of users, and bandwidth wasted with retransmission.
Knowledge about availability of a download on mirror servers can be
acquired and maintained by the operators of the origin server or by a
third party. This knowledge, together with cryptographic hashes,
digital signatures, and more, can be stored in a machine-readable
Metalink file. The Metalink file can transfer this knowledge to the
user agent, which can peruse it in automatic ways or present the
information to a human user. User agents can fall back to alternate
mirrors if the current one has an issue. Thereby, clients are
enabled to work their way to a successful download under adverse
circumstances. All this can be done transparently to the human user
and the download is much more reliable and efficient. In contrast, a
Bryan, et al. Standards Track [Page 3]
RFC 5854 Metalink Download Description Format June 2010
traditional HTTP redirect to one mirror conveys only comparatively
minimal information -- a referral to a single server, and there is no
provision in the HTTP protocol to handle failures.
Other features that some clients provide include multi-source
downloads, where chunks of a file are downloaded from multiple
mirrors (and optionally, Peer-to-Peer) simultaneously, which
frequently results in a faster download. Metalinks can leverage
HTTP, FTP, and Peer-to-Peer protocols together, because regardless of
the protocol over which the Metalink was obtained, it can make a
resource accessible through other protocols. If the Metalink was
obtained from a trusted source, included verification metadata can
solve trust issues when downloading files from replica servers
operated by third parties. Metalinks also provide structured
information about downloads that can be indexed by search engines.
1.1. Examples
A brief, Metalink Document that describes a single file:
<?xml version="1.0" encoding="UTF-8"?>
<metalink xmlns="urn:ietf:params:xml:ns:metalink">
<file name="example.ext">
<size>14471447</size>
<url>ftp://ftp.example.com/example.ext</url>
<url>http://example.com/example.ext</url>
<metaurl mediatype="torrent">
http://example.com/example.ext.torrent</metaurl>
</file>
</metalink>
A more extensive Metalink Document that describes two files:
<?xml version="1.0" encoding="UTF-8"?>
<metalink xmlns="urn:ietf:params:xml:ns:metalink">
<published>2009-05-15T12:23:23Z</published>
<file name="example.ext">
<size>14471447</size>
<identity>Example</identity>
<version>1.0</version>
<language>en</language>
<description>
A description of the example file for download.
</description>
<hash type="sha-256">f0ad929cd259957e160ea442eb80986b5f01...</hash>
<url location="de"
priority="1">ftp://ftp.example.com/example.ext</url>
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<url location="fr"
priority="1">http://example.com/example.ext</url>
<metaurl mediatype="torrent"
priority="2">http://example.com/example.ext.torrent</metaurl>
</file>
<file name="example2.ext">
<size>14471447</size>
<identity>Example2</identity>
<version>1.0</version>
<language>en</language>
<description>
Another description for a second file.
</description>
<hash type="sha-256">2f548ce50c459a0270e85a7d63b2383c5523...</hash>
<url location="de"
priority="1">ftp://ftp.example.com/example2.ext</url>
<url location="fr"
priority="1">http://example.com/example2.ext</url>
<metaurl mediatype="torrent"
priority="2">http://example.com/example2.ext.torrent</metaurl>
</file>
</metalink>
1.2. Namespace and Version
The XML Namespaces URI [REC-xml-names] for the XML data format
described in this specification is:
urn:ietf:params:xml:ns:metalink
For convenience, this data format may be referred to as "Metalink",
which this specification uses internally.
1.3. Notational Conventions
This specification describes conformance of Metalink Documents.
Additionally, it places some requirements on Metalink Processors.
This specification uses the namespace prefix "metalink:" for the
Namespace URI identified in Section 1.2, above. Note that the choice
of namespace prefix is arbitrary and not semantically significant.
Metalink is specified using terms from the XML Infoset
[REC-xml-infoset]. However, this specification uses a shorthand for
two common terms: the phrase "Information Item" is omitted when
naming Element Information Items and Attribute Information Items.
Therefore, when this specification uses the term "element," it is
referring to an Element Information Item in Infoset terms. Likewise,
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RFC 5854 Metalink Download Description Format June 2010
when it uses the term "attribute," it is referring to an Attribute
Information Item.
Some sections of this specification are illustrated with fragments of
a non-normative RELAX NG Compact schema [RELAX-NG]. However, the
text of this specification provides the definition of conformance. A
complete schema appears in Appendix B.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, [RFC2119], as
scoped to those conformance targets.
2. Metalink Documents
This specification describes Metalink Documents.
A Metalink Document describes a file or group of files, how to access
them, and metadata that identifies them. Its root is the metalink:
metalink element.
namespace metalink = "urn:ietf:params:xml:ns:metalink"
start = metalinkMetalink
Metalink Documents are specified in terms of the XML Information Set,
serialized as XML 1.0 [REC-xml] and identified with the "application/
metalink4+xml" media type.
Metalink Documents MUST be well-formed XML. This specification does
not define a Document Type Definition (DTD) for Metalink Documents,
and hence it does not require them to be valid (in the sense used by
XML).
Metalink allows the use of Internationalized Resource Identifiers
(IRIs), encoded according to [RFC3987]. Every URI [RFC3986] is also
an IRI, so a URI may be used wherever an IRI is named below. There
is one special consideration: when an IRI that is not also a URI is
given for dereferencing, it MUST be mapped to a URI using the steps
in Section 3.1 of [RFC3987].
Any element defined by this specification MAY have an xml:lang
attribute, whose content indicates the natural language for the
element and its descendents. The language context is only
significant for elements and attributes declared to be "Language-
Sensitive" by this specification. Requirements regarding the content
and interpretation of xml:lang are specified in XML 1.0 [REC-xml],
Section 2.12.
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RFC 5854 Metalink Download Description Format June 2010
metalinkCommonAttributes =
attribute xml:lang { metalinkLanguageTag }?,
undefinedAttribute*
All leading and trailing whitespace is part of the element content
and MUST NOT be ignored. Consequently, it is disallowed for elements
where the defined type does not allow whitespace, such as dates,
integers, or IRIs. Some XML-generating implementations erroneously
insert whitespace around values by default, and such implementations
will generate invalid Metalink Documents.
Metalink Documents that do not follow this specification are invalid
and SHOULD NOT be used by Metalink Processors.
Metalink is an extensible format. See Section 5 of this document for
a full description of how Metalink Documents can be extended.
3. Common Metalink Constructs
Many Metalink elements share common structures. This section defines
those structures and their requirements for convenient reference by
the appropriate element definitions.
When an element is identified as being a particular kind of
construct, it inherits the corresponding requirements from that
construct's definition in this section.
3.1. Text Constructs
A Text construct contains human-readable text, usually short in
length.
metalinkTextConstruct =
metalinkCommonAttributes,
text
For example, a metalink:description with text content:
...
<description>
A description of the example file for download.
</description>
...
The content of the Text construct MUST NOT contain child elements.
Such text is intended to be presented to humans in a readable
fashion. Thus, whitespace could be collapsed (including line
Bryan, et al. Standards Track [Page 7]
RFC 5854 Metalink Download Description Format June 2010
breaks), and text could be displayed using typographic techniques
such as justification and proportional fonts.
3.2. Date Constructs
A Date construct is an element whose content MUST conform to the
"date-time" production in [RFC3339]. In addition, an uppercase "T"
character MUST be used to separate date and time, and an uppercase
"Z" character MUST be present in the absence of a numeric time zone
offset.
metalinkDateConstruct =
metalinkCommonAttributes,
xsd:dateTime
Such date values happen to be compatible with the following
specifications: [ISO.8601.1988], [NOTE-datetime-19980827], and
[REC-xmlschema-2-20041028].
Example Date constructs:
...
<updated>2010-05-01T12:15:02Z</updated>
...
<updated>2010-05-01T12:15:02.25Z</updated>
...
<updated>2010-05-01T12:15:02+01:00</updated>
...
<updated>2010-05-01T12:15:02.25+01:00</updated>
...
4. Metalink Element Definitions
4.1. Container Elements
4.1.1. The "metalink:metalink" Element
The "metalink:metalink" element is the document (i.e., top-level)
element of a Metalink Document, acting as a container for metadata
and data associated with the listed files. It contains one or more
metalink:file child elements that consist of Metadata elements.
metalinkMetalink =
element metalink:metalink {
metalinkCommonAttributes,
(metalinkFile+
& metalinkGenerator?
& metalinkOrigin?
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RFC 5854 Metalink Download Description Format June 2010
& metalinkPublished?
& metalinkUpdated?
& extensionElement*)
}
The following child elements are defined by this specification (note
that the presence of some of these elements is required):
o metalink:metalink elements MUST contain one or more metalink:file
elements.
o metalink:metalink elements MAY contain exactly one metalink:
generator element and MUST NOT contain more than one such element.
o metalink:metalink elements SHOULD contain exactly one metalink:
origin element and MUST NOT contain more than one such element.
o metalink:metalink elements MAY contain exactly one metalink:
published element and MUST NOT contain more than one such element.
o metalink:metalink elements MAY contain exactly one metalink:
updated element and MUST NOT contain more than one such element.
4.1.1.1. Providing Textual Content
Experience teaches that downloads providing textual content are, in
general, more useful than those that do not. Some applications (one
example is full-text indexers) require a minimum amount of text to
function reliably and predictably. Metalink publishers should be
aware of this. It is RECOMMENDED that each metalink:file element
contain a non-empty metalink:description element, a non-empty
metalink:identity element, a non-empty metalink:version element, and
a non-empty metalink:publisher element when these elements are
present. However, the absence of metalink:description, metalink:
identity, metalink:version, and metalink:publisher is not an error,
and Metalink Processors MUST NOT fail to function correctly as a
consequence of such an absence.
4.1.2. The "metalink:file" Element
The "metalink:file" element represents an individual file, acting as
a container for metadata and data associated with the file. Each
unique file described in a Metalink Document MUST have its own
metalink:file element.
All metalink:url elements contained in each metalink:file element
SHOULD lead to identical files. That is, each metalink:url element
should be an alternative location for the same file and each
Bryan, et al. Standards Track [Page 9]
RFC 5854 Metalink Download Description Format June 2010
metalink:metaurl element should provide metadata to retrieve the same
file in another way, such as a Peer-to-Peer network. Refer to
Sections 4.2.8 and 4.2.16 for more information.
metalinkFile =
element metalink:file {
metalinkCommonAttributes,
attribute name { text },
(metalinkCopyright?
& metalinkDescription?
& metalinkHash*
& metalinkIdentity?
& metalinkLanguage*
& metalinkLogo?
& metalinkMetaURL*
& metalinkOS*
& metalinkPieces*
& metalinkPublisher?
& metalinkSignature?
& metalinkSize?
& metalinkURL*
& metalinkVersion?
& extensionElement*)
}
This specification assigns no significance to the order of metalink:
file elements or to the order of metalink:url or metalink:metaurl
elements. Significance is determined by the value of the "priority"
attribute of the metalink:url or metalink:metaurl elements.
The following child elements are defined by this specification (the
presence of some of them is required):
o metalink:file elements MAY contain exactly one metalink:copyright
element and MUST NOT contain more than one such element.
o metalink:file elements MAY contain exactly one metalink:
description element and MUST NOT contain more than one such
element.
o metalink:file elements MAY contain exactly one metalink:identity
element and MUST NOT contain more than one such element.
o metalink:file elements MAY contain one or more metalink:hash
elements.
o metalink:file elements MAY contain one or more metalink:language
elements.
Bryan, et al. Standards Track [Page 10]
RFC 5854 Metalink Download Description Format June 2010
o metalink:file elements MAY contain exactly one metalink:logo
element and MUST NOT contain more than one such element.
o metalink:file elements MAY contain one or more metalink:os
element.
o metalink:file elements MUST contain at least one metalink:url
element or at least one metalink:metaurl element. Typically,
metalink:file elements contain more than one metalink:url element
to provide multiple download sources.
o metalink:file elements MAY contain one or more metalink:pieces
elements.
o metalink:file elements MAY contain exactly one metalink:publisher
element and MUST NOT contain more than one such element.
o metalink:file elements MAY contain one or more metalink:signature
elements.
o metalink:file elements SHOULD contain exactly one metalink:size
element and MUST NOT contain more than one such element.
o metalink:file elements MAY contain exactly one metalink:version
element and MUST NOT contain more than one such element.
4.1.2.1. The "name" Attribute
metalink:file elements MUST have a "name" attribute, which contains
the local file name to which the downloaded file will be written.
Hence, if a Metalink Document contains multiple metalink:file
elements, the value of the "name" attribute MUST be unique for each.
Directory information can also be contained in a "path/file" format
only, as in:
<file name="debian-amd64/sarge/Contents-amd64.gz">
In this example, a subdirectory "debian-amd64/sarge/" will be created
and a file named "Contents-amd64.gz" will be created inside it.
Security Note: The path MUST NOT contain any directory traversal
directives or information. The path MUST be relative. The path
MUST NOT begin with a "/", "./", or "../"; contain "/../"; or end
with "/..".
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4.1.3. The "metalink:pieces" Element
The "metalink:pieces" element acts as a container for a list of
cryptographic hashes of contiguous, non-overlapping pieces of the
file. The cryptographic hashes MUST be listed in the same order as
the corresponding pieces appear in the file, starting at the
beginning of the file. Metalink Documents MAY contain one or
multiple metalink:pieces container elements, if each "type" attribute
of metalink:pieces has a unique value.
metalinkPieces =
element metalink:pieces {
attribute length { xsd:positiveInteger },
attribute type { text },
metalinkHash+
}
4.1.3.1. The "type" Attribute
metalink:pieces elements MUST have a "type" attribute.
The Internet Assigned Numbers Authority (IANA) registry named "Hash
Function Textual Names" defines values for hash types. See
Section 7.4 for security implications.
4.1.3.2. The "length" Attribute
metalink:pieces elements MUST have a "length" attribute, which is a
positive integer that describes the length of the pieces of the file
in octets. The whole file is divided into non-overlapping pieces of
this length, starting from the beginning of the file. That is, every
piece MUST be the same size, apart from the last piece, which is the
remainder. The last piece extends to the end of the file, and it
therefore MAY be shorter than the other pieces.
4.2. Metadata Elements
4.2.1. The "metalink:copyright" Element
The "metalink:copyright" element is a Text construct that conveys a
human-readable copyright for a file. It is Language-Sensitive.
metalinkCopyright =
element metalink:copyright {
metalinkTextConstruct
}
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4.2.2. The "metalink:description" Element
The "metalink:description" element is a Text construct that conveys a
human-readable file description. It is Language-Sensitive.
metalinkDescription =
element metalink:description {
metalinkTextConstruct
}
4.2.3. The "metalink:generator" Element
The "metalink:generator" element's content identifies the generating
agent name and version used to generate a Metalink Document, for
debugging and other purposes.
metalinkGenerator =
element metalink:generator {
metalinkTextConstruct
}
The metalink:generator element's content is defined below in ABNF
notation [RFC5234].
token = 1*<any CHAR except CTLs or separators>
separators = "(" / ")" / "<" / ">" / "@"
/ "," / ";" / ":" / "\" / DQUOTE
/ "/" / "[" / "]" / "?" / "="
/ "{" / "}" / SP / HTAB
agent = token ["/" agent-version]
agent-version = token
Examples:
...
<generator>MirrorBrain/2.11</generator>
...
<generator>MirrorManager/1.2.11</generator>
...
<generator>metalinktools/0.3.6</generator>
...
<generator>MetalinkEditor/1.2.0</generator>
...
Although any token character MAY appear in an agent-version, this
token SHOULD only be used for a version identifier (i.e., successive
versions of the same agent SHOULD only differ in the agent-version
portion of the agent value).
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4.2.4. The "metalink:hash" Element
The "metalink:hash" element is a Text construct that conveys a
cryptographic hash for a file. All hashes are encoded in lowercase
hexadecimal format. Hashes are used to verify the integrity of a
complete file or portion of a file to determine if the file has been
transferred without any errors.
metalinkHash =
element metalink:hash {
attribute type { text }?,
text
}
Metalink Documents MAY contain one or multiples hashes of a complete
file. metalink:hash elements with a "type" attribute MUST contain a
hash of the complete file. In this example, both SHA-1 and SHA-256
hashes of the complete file are included.
...
<hash type="sha-1">a97fcf6ba9358f8a6f62beee4421863d3e52b080</hash>
<hash type="sha-256">fc87941af7fd7f03e53b34af393f4c14923d74...</hash>
...
Metalink Documents MAY also contain hashes for individual pieces of a
file. metalink:hash elements that are inside a metalink:pieces
container element have a hash for that specific piece or chunk of the
file, and are of the same hash type as the metalink:pieces element in
which they are contained. Metalink Documents MAY contain one or
multiple metalink:pieces container elements, if each "type" attribute
of metalink:pieces has a unique value.
metalink:hash elements without a "type" attribute MUST contain a hash
for that specific piece or chunk of the file and MUST be listed in
the same order as the corresponding pieces appear in the file,
starting at the beginning of the file. The size of the piece is
equal to the value of the "length" attribute of the metalink:pieces
element, apart from the last piece, which is the remainder. See
Section 4.1.3.2 for more information on the size of pieces.
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In this example, SHA-1 and SHA-256 hashes of the complete file are
included, along with four SHA-1 piece hashes.
...
<hash type="sha-1">a97fcf6ba9358f8a6f62beee4421863d3e52b080</hash>
<hash type="sha-256">fc87941af7fd7f03e53b34af393f4c14923d74...</hash>
<pieces length="1048576" type="sha-1">
<hash>d96b9a4b92a899c2099b7b31bddb5ca423bb9b30</hash>
<hash>10d68f4b1119014c123da2a0a6baf5c8a6d5ba1e</hash>
<hash>3e84219096435c34e092b17b70a011771c52d87a</hash>
<hash>67183e4c3ab892d3ebe8326b7d79eb62d077f487</hash>
</pieces>
...
4.2.4.1. The "type" Attribute
metalink:hash elements MUST have a "type" attribute, if and only if
it contains a hash of the complete file. The IANA registry named
"Hash Function Textual Names" defines values for hash types.
metalink:hash elements MUST NOT have a "type" attribute, if they are
inside a metalink:pieces container element. See Section 7.4 for
security implications.
4.2.5. The "metalink:identity" Element
The "metalink:identity" element is a Text construct that conveys a
human-readable identity for a file. For example, the identity of
Firefox 3.5 would be "Firefox".
metalinkIdentity =
element metalink:identity {
metalinkTextConstruct
}
4.2.6. The "metalink:language" Element
The "metalink:language" element is a Text construct that conveys a
code for the language of a file, per [RFC5646].
Multiple metalink:language elements are allowed, for instance, to
describe a file such as an binary installation program that provides
multiple language options, a movie with multiple language tracks, or
a document in multiple languages.
metalinkLanguage =
element metalink:language {
metalinkTextConstruct
}
Bryan, et al. Standards Track [Page 15]
RFC 5854 Metalink Download Description Format June 2010
4.2.7. The "metalink:logo" Element
The "metalink:logo" element's content is an IRI reference [RFC3987]
that identifies an image that provides visual identification for a
file.
metalinkLogo =
element metalink:logo {
metalinkCommonAttributes,
(metalinkUri)
}
The image SHOULD have an aspect ratio of one (horizontal) to one
(vertical) and SHOULD be suitable for presentation at a small size.
4.2.8. The "metalink:metaurl" Element
The "metalink:metaurl" element contains the IRI of a metadata file,
also known as a metainfo file, about a resource to download. For
example, this could be the IRI of a BitTorrent .torrent file, a
Metalink Document, or other type of metadata file. Note that the
information in the metalink:hash element does not apply to these
metadata files but to the files that are described by them.
metalinkMetaURL =
element metalink:metaurl {
metalinkCommonAttributes,
attribute priority { xsd:positiveInteger {
maxInclusive = "999999"}}?,
attribute mediatype { text },
attribute name { text }?,
(metalinkUri)
}
4.2.8.1. The "priority" Attribute
metalink:metaurl elements MAY have a priority attribute. Values MUST
be positive integers between 1 and 999999. Lower values indicate a
higher priority. metalink:metaurl elements without a priority
attribute are considered to have the lowest priority, i.e., 999999.
The priority values of metalink:metaurl and metalink:url elements are
compared and those with the lowest values, starting with 1, are used
first. Multiple metalink:metaurl and metalink:url elements MAY have
the same priority, i.e., one BitTorrent .torrent file and three FTP
URIs could have priority="1". See also the "priority" attribute of
the metalink:url element.
Bryan, et al. Standards Track [Page 16]
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4.2.8.2. The "mediatype" Attribute
metalink:metaurl elements MUST have a "mediatype" attribute that
indicates the Multipurpose Internet Mail Extensions (MIME) media type
[RFC4288] of the metadata file available at the IRI. In the case of
BitTorrent as specified in [BITTORRENT], the value "torrent" is
REQUIRED. Types without "/" are reserved. Currently, "torrent" is
the only reserved value.
Values for this attribute are defined below in ABNF notation
[RFC5234].
media-type = (type-name "/" subtype-name) / media-reserved
media-reserved = "torrent"
type-name = <Defined in Section 4.2 of RFC 4288>
subtype-name = <Defined in Section 4.2 of RFC 4288>
4.2.8.3. The "name" Attribute
metalink:metaurl elements MAY have a "name" attribute that indicates
a specific file in a BitTorrent .torrent file or a Metalink Document
that describes multiple files.
Directory information can also be contained in a "path/file" format
only, as in:
<metaurl
mediatype="torrent" name="debian-amd64/sarge/Contents-amd64.gz">
In this example, a file named "Contents-amd64.gz" is indicated, in a
"debian-amd64/sarge/" subdirectory. The path MUST NOT contain any
directory traversal directives or information. The path MUST be
relative. The path MUST NOT begin with a "/", "./", or "../";
contain "/../"; or end with "/..".
4.2.9. The "metalink:origin" Element
The "metalink:origin" element is an IRI where the Metalink Document
was originally published. If the dynamic attribute of metalink:
origin is "true", then updated versions of the Metalink can be found
at this IRI.
metalinkOrigin =
element metalink:origin {
metalinkCommonAttributes,
attribute dynamic { xsd:boolean }?,
(metalinkUri)
}
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4.2.9.1. The "dynamic" Attribute
The metalink:origin element MAY have a "dynamic" attribute, set to
"true" or "false", which tells if a Metalink at the origin IRI will
contain dynamic updated information or if it is static and not likely
to be updated.
4.2.10. The "metalink:os" Element
The "metalink:os" element is a Text construct that conveys an
Operating System that a file is suitable for. The IANA registry
named "Operating System Names" defines values for OS types.
metalinkOS =
element metalink:os {
metalinkTextConstruct
}
4.2.11. The "metalink:published" Element
The "metalink:published" element is a Date construct indicating an
instant in time associated with an event early in the life cycle of
the entry.
metalinkPublished =
element metalink:published {
metalinkDateConstruct
}
Typically, metalink:published will be associated with the initial
creation or first availability of the resource. The metalink:updated
element is used when a Metalink Document has been updated after
initial publication.
4.2.12. The "metalink:publisher" Element
The "metalink:publisher" element contains a human-readable group or
other entity that has published the file described in the Metalink
Document and an IRI for more information.
metalinkPublisher =
element metalink:publisher {
metalinkCommonAttributes,
attribute name { text },
attribute url { metalinkUri }?
}
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RFC 5854 Metalink Download Description Format June 2010
4.2.12.1. The "name" Attribute
The metalink:publisher element MUST have a "name" attribute that
indicates the human-readable name of the publisher.
4.2.12.2. The "url" Attribute
The metalink:publisher element MAY have a "url" attribute whose value
MUST be an IRI reference [RFC3987].
4.2.13. The "metalink:signature" Element
The "metalink:signature" element is a Text construct that conveys a
digital signature for a file described in a Metalink Document.
Digital signatures verify that a file is from the entity that has
signed it.
Support in Metalink Processors for digital signatures included in
this element is OPTIONAL. Note that the signing of Metalink
Documents, as opposed to a digital signature of a file described in a
Metalink Document, is covered in Section 7.1.
metalinkSignature =
element metalink:signature {
attribute mediatype { text },
metalinkTextConstruct
}
Example with an OpenPGP signature [RFC4880]:
<signature mediatype="application/pgp-signature">
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.10 (GNU/Linux)
iEYEABECAAYFAkrxdXQACgkQeOEcayedXJHqFwCfd1p/HhRf/iDvYhvFbTrQPz+p
p3oAoO9lKHoOqOE0EMB3zmMcLoYUrNkg
=ggAf
-----END PGP SIGNATURE-----
</signature>
4.2.13.1. The "mediatype" Attribute
metalink:signature elements MUST have a "mediatype" attribute that
indicates the MIME media type [RFC4288] of the included digital
signature.
Values for this attribute are defined below in ABNF notation
[RFC5234].
Bryan, et al. Standards Track [Page 19]
RFC 5854 Metalink Download Description Format June 2010
media-type = type-name "/" subtype-name
type-name = <Defined in Section 4.2 of RFC 4288>
subtype-name = <Defined in Section 4.2 of RFC 4288>
4.2.14. The "metalink:size" Element
The "metalink:size" element indicates the length of the linked
content in octets. This is the content length of the representation
returned when the IRI is mapped to a URI and dereferenced. Note that
the "metalink:size" element MUST override the actual content length
of the representation as reported by the underlying protocol, and
those that do not match MUST be discarded by Metalink Processors.
This value MUST be a non-negative integer.
metalinkSize =
element metalink:size {
xsd:nonNegativeInteger
}
4.2.15. The "metalink:updated" Element
The "metalink:updated" element is a Date construct indicating the
most recent instant in time when a Metalink was modified in a way the
publisher considers significant. Therefore, not all modifications
necessarily result in a changed metalink:updated value.
metalinkUpdated =
element metalink:updated {
metalinkDateConstruct
}
Publishers MAY change the value of this element over time.
4.2.16. The "metalink:url" Element
The "metalink:url" element contains a file IRI. Most metalink:file
container elements will contain multiple metalink:url elements, and
each one SHOULD be a valid alternative to download the same file.
The metalink:url elements SHOULD be resolvable and, if resolvable,
SHOULD lead to identical files.
Metalink Processors MUST filter out invalid files obtained from
"metalink:url" elements by using information in the metalink:size
element and metalink:hash elements.
Bryan, et al. Standards Track [Page 20]
RFC 5854 Metalink Download Description Format June 2010
metalinkURL =
element metalink:url {
metalinkCommonAttributes,
attribute location { xsd:string {
minLength = "2" maxLength="2"}
}?,
attribute priority { xsd:positiveInteger {
maxInclusive = "999999"}}?,
(metalinkUri)
}
4.2.16.1. The "priority" Attribute
metalink:url elements MAY have a priority attribute. Values MUST be
positive integers between 1 and 999999. Lower values indicate a
higher priority. metalink:url elements without a priority attribute
are considered to have the lowest priority, i.e., 999999. Multiple
metalink:url elements can have the same priority, i.e., ten different
mirrors could have priority="1".
4.2.16.2. The "location" Attribute
metalink:url elements MAY have a "location" attribute, which is a
[ISO3166-1] alpha-2 two letter country code for the geographical
location of the physical server an IRI is used to access.
4.2.17. The "metalink:version" Element
The "metalink:version" element is a Text construct that conveys a
human-readable version for a file. The version of Firefox 3.5 would
be "3.5".
metalinkVersion =
element metalink:version {
metalinkTextConstruct
}
5. Extending Metalink
5.1. Extensions from Non-Metalink Vocabularies
This specification describes Metalink's XML vocabulary.
5.2. Extensions to the Metalink Vocabulary
The Metalink namespace is reserved for future forward-compatible
revisions of Metalink. Future versions of this specification could
add new elements and attributes to the Metalink markup vocabulary.
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RFC 5854 Metalink Download Description Format June 2010
Software written to conform to this version of the specification will
not be able to process such markup correctly and, in fact, will not
be able to distinguish it from markup error. For the purposes of
this discussion, unrecognized markup from the Metalink vocabulary
will be considered "foreign markup".
5.3. Processing Foreign Markup
Metalink Processors that encounter foreign markup in a location that
is legal according to this specification MUST ignore such foreign
markup, in particular they MUST NOT stop processing or signal an
error. It might be the case that the Metalink Processor is able to
process the foreign markup correctly and does so. Otherwise, such
markup is termed "unknown foreign markup".
When unknown foreign markup is encountered as a child of metalink:
file, metalink:metalink, Metalink Processors MAY bypass the markup
and any textual content and MUST NOT change their behavior as a
result of the markup's presence.
5.4. Extension Elements
Metalink allows foreign markup anywhere in a Metalink document,
except where it is explicitly forbidden. Child elements of metalink:
file and metalink:metalink are considered Metadata elements and are
described below. The role of other foreign markup is undefined by
this specification.
5.4.1. Simple Extension Elements
A Simple Extension element MUST NOT have any attributes or child
elements. The element MAY contain character data or be empty.
Simple Extension elements are not Language-Sensitive.
simpleExtensionElement =
element * - metalink:* {
text
}
The element can be interpreted as a simple property (or name/value
pair) of the parent element that encloses it. The pair consisting of
the namespace URI of the element and the local name of the element
can be interpreted as the name of the property. The character data
content of the element can be interpreted as the value of the
property. If the element is empty, then the property value can be
interpreted as an empty string.
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RFC 5854 Metalink Download Description Format June 2010
5.4.2. Structured Extension Elements
The root element of a Structured Extension element MUST have at least
one attribute or child element. It MAY have attributes, it MAY
contain well-formed XML content (including character data), or it MAY
be empty. Structured Extension elements are Language-Sensitive.
structuredExtensionElement =
element * - metalink:* {
(attribute * { text }+,
(text|anyElement)*)
| (attribute * { text }*,
(text?, anyElement+, (text|anyElement)*))
}
The structure of a Structured Extension element, including the order
of its child elements, could be significant.
This specification does not provide an interpretation of a Structured
Extension element. The syntax of the XML contained in the element
(and an interpretation of how the element relates to its containing
element) is defined by the specification of the Metalink extension.
6. IANA Considerations
6.1. XML Namespace Registration
This document makes use of the XML registry specified in [RFC3688].
Accordingly, IANA has made the following registration:
Registration request for the Metalink namespace:
URI: urn:ietf:params:xml:ns:metalink
Registrant Contact: See the "Authors' Addresses" section of this
document.
XML: None. Namespace URIs do not represent an XML specification.
6.2. application/metalink4+xml MIME type
A Metalink Document, when serialized as XML 1.0, can be identified
with the following media type:
Type name: application
Subtype name: metalink4+xml
Bryan, et al. Standards Track [Page 23]
RFC 5854 Metalink Download Description Format June 2010
Required parameters: None.
Optional parameters:
"charset": This parameter has semantics identical to the charset
parameter of the "application/xml" media type as specified in
[RFC3023].
Encoding considerations: Identical to those of "application/xml" as
described in [RFC3023], Section 3.2.
Security considerations: As defined in this specification.
In addition, as this media type uses the "+xml" convention, it
shares the same security considerations as described in [RFC3023],
Section 10.
Interoperability considerations: There are no known interoperability
issues.
Published specification: This specification.
Applications that use this media type: File transfer applications.
Additional information:
Magic number(s): None.
File extension: .meta4
Macintosh File Type code: TEXT
Person and email address to contact for further information:
Anthony Bryan <anthonybryan@gmail.com>
Intended usage: COMMON
Restrictions on usage: None.
Author: Anthony Bryan <anthonybryan@gmail.com>
Change controller: IESG
7. Security Considerations
Because Metalink is an XML-based format, existing XML security
mechanisms can be used to secure its content.
Bryan, et al. Standards Track [Page 24]
RFC 5854 Metalink Download Description Format June 2010
Publishers of Metalink Documents may have sound reasons for signing
otherwise-unprotected content. For example, a merchant might
digitally sign a Metalink that lists a file download to verify its
origin. Other merchants may wish to sign and encrypt Metalink
Documents that list digital songs that have been purchased. Many
other examples are conceivable.
Publishers are encouraged to offer Metalink documents via
authenticated HTTP under Transport Layer Security (TLS) as specified
in [RFC2818]. The choice of a secure content layer rests entirely
with the content providers.
Publishers are also encouraged to include digital signatures of the
files within the Metalink Documents, if they are available, as
described in Section 4.2.13.
Normally, a publisher is in the best position to know how strong the
protective signing ought to be on their content. Thus, a publisher
can choose weak or strong cryptography, and a Metalink Processor
SHOULD normally accept that. There are potential applications where
the Metalink Processor chooses to reject weak cryptography, but that
is not envisioned as the common use case.
7.1. Digital Signatures
The root of a Metalink Document (i.e., metalink:metalink) or any
metalink:file element MAY have an Enveloped Signature, as described
by XML-Signature and Syntax Processing [REC-xmldsig-core].
Although signing and verifying signatures are both OPTIONAL, an
implementation that supports either feature SHOULD implement RSA with
a minimum key size of 2048 with SHA-256.
Metalink Processors that support verifying signatures MUST reject
Metalink Documents with invalid signatures.
Metalink Processors MUST NOT reject a Metalink Document containing
such a signature because they are not capable of verifying it; they
MUST continue processing and MAY inform the user of their failure to
validate the signature.
In other words, the presence of an element with the namespace URI
"http://www.w3.org/2000/09/xmldsig#" and a local name of "Signature"
as a child of the document element MUST NOT cause a Metalink
Processor to fail merely because of its presence.
Other elements in a Metalink Document MUST NOT be signed unless their
definitions explicitly specify such a capability.
Bryan, et al. Standards Track [Page 25]
RFC 5854 Metalink Download Description Format June 2010
Section 6.5.1 of [REC-xmldsig-core] requires support for Canonical
XML [REC-xml-c14n]. However, many - implementers do not use it
because signed XML documents - enclosed in other XML documents have
their signatures - broken. Thus, Metalink Processors that verify
signed Metalink Documents MUST be able to canonicalize with the
exclusive XML canonicalization method identified by the URI
"http://www.w3.org/2001/10/xml-exc-c14n#", as specified in Exclusive
XML Canonicalization [REC-xml-exc-c14n].
Section 4.4.2 of [REC-xmldsig-core] requires support for Digital
Signature Algorithm (DSA) signatures and recommends support for RSA
signatures. However, because of the much greater popularity in the
market of RSA versus DSA, Metalink Processors that verify signed
Metalink Documents MUST be able to verify RSA signatures, but do not
need be able to verify DSA signatures. Due to security issues that
can arise if the keying material for message authentication code
(MAC) authentication is not handled properly, Metalink Documents
SHOULD NOT use MACs for signatures.
7.2. URIs and IRIs
Metalink Processors handle URIs and IRIs. See Section 7 of [RFC3986]
and Section 8 of [RFC3987] for security considerations related to
their handling and use.
7.3. Spoofing
There is potential for spoofing attacks where the attacker publishes
Metalink Documents with false information. Malicious publishers
might create Metalink Documents containing inaccurate information
anywhere in the document. Unaware downloaders could be deceived into
downloading malicious or worthless content. Malicious publishers
could attempt a distributed denial-of-service attack by inserting
unrelated IRIs into Metalink Documents.
Digital signatures address the issue of spoofing.
7.4. Cryptographic Hashes
Currently, some of the hash types defined in the IANA registry named
"Hash Function Textual Names" are considered insecure. These include
the whole Message Digest family of algorithms that are not suitable
for cryptographically strong verification. Malicious parties could
provide files that appear to be identical to another file because of
a collision, i.e., the weak cryptographic hashes of the intended file
and a substituted malicious file could match.
Bryan, et al. Standards Track [Page 26]
RFC 5854 Metalink Download Description Format June 2010
Metalink Generators and Processors MUST support "sha-256", which is
SHA-256, as specified in [FIPS-180-3], and MAY support stronger
hashes.
If a Metalink Document contains hashes, it SHOULD include "sha-256",
which is SHA-256, or stronger. It MAY also include other hashes from
the IANA registry named "Hash Function Textual Names".
8. References
8.1. Normative References
[BITTORRENT] Cohen, B., "The BitTorrent Protocol Specification",
BITTORRENT 11031, February 2008,
<http://www.bittorrent.org/beps/bep_0003.html>.
[FIPS-180-3] National Institute of Standards and Technology (NIST),
"Secure Hash Standard (SHS)", FIPS PUB 180-3,
October 2008.
[ISO3166-1] International Organization for Standardization, "ISO
3166- 1:2006. Codes for the representation of names of
countries and their subdivisions -- Part 1: Country
codes", November 2006.
[REC-xml] Yergeau, F., Paoli, J., Bray, T., Sperberg-McQueen, C.,
and E. Maler, "Extensible Markup Language (XML) 1.0
(Fifth Edition)", W3C REC-xml-20081126, November 2008,
<http://www.w3.org/TR/2008/REC-xml-20081126/>.
[REC-xml-c14n]
Boyer, J., "Canonical XML Version 1.0", W3C REC REC-xml-
c14n-20010315, March 2001,
<http://www.w3.org/TR/2001/REC-xml-c14n-20010315>.
[REC-xml-exc-c14n]
Eastlake, D., Boyer, J., and J. Reagle, "Exclusive XML
Canonicalization Version 1.0", W3C REC REC-xml-exc-c14n-
20020718, July 2002,
<http://www.w3.org/TR/2002/REC-xml-exc-c14n-20020718/>.
[REC-xml-infoset]
Cowan, J. and R. Tobin, "XML Information Set (Second
Edition)", W3C REC-xml-infoset-20040204, February 2004,
<http://www.w3.org/TR/2004/REC-xml-infoset-20040204/>.
Bryan, et al. Standards Track [Page 27]
RFC 5854 Metalink Download Description Format June 2010
[REC-xml-names]
Hollander, D., Bray, T., Tobin, R., and A. Layman,
"Namespaces in XML 1.0 (Third Edition)", W3C REC-xml-
names-20091208, December 2009,
<http://www.w3.org/TR/2009/REC-xml-names-20091208/>.
[REC-xmldsig-core]
Solo, D., Reagle, J., and D. Eastlake, "XML-Signature
Syntax and Processing (Second Edition)",
W3C REC-xmldsig- core-20080610, June 2008,
<http://www.w3.org/TR/2008/REC-xmldsig-core-20080610/>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media
Types", RFC 3023, January 2001.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, July 2002.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288,
December 2005.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, January 2008.
[RFC5646] Phillips, A. and M. Davis, "Tags for Identifying
Languages", BCP 47, RFC 5646, September 2009.
8.2. Informative References
[ISO.8601.1988]
International Organization for Standardization, "Data
elements and interchange formats - Information
interchange - Representation of dates and times",
ISO Standard 8601, June 1988.
Bryan, et al. Standards Track [Page 28]
RFC 5854 Metalink Download Description Format June 2010
[NOTE-datetime-19980827]
Wolf, M. and C. Wicksteed, "Date and Time Formats",
W3C NOTE-datetime-19980827, August 1998,
<http://www.w3.org/TR/1998/NOTE-datetime-19980827>.
[REC-xmlschema-2-20041028]
Malhotra, A. and P. Biron, "XML Schema Part 2: Datatypes
Second Edition", W3C REC-xmlschema-2-20041028,
October 2004,
<http://www.w3.org/TR/2004/REC-xmlschema-2-20041028/>.
[RELAX-NG] Clark, J., "RELAX NG Compact Syntax", December 2001,
<http ://www.oasis-open.org/committees/relax-ng/
compact-20021121.html>.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
January 2004.
[RFC4287] Nottingham, M. and R. Sayre, "The Atom Syndication
Format", RFC 4287, December 2005.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and
R. Thayer, "OpenPGP Message Format", RFC 4880,
November 2007.
Bryan, et al. Standards Track [Page 29]
RFC 5854 Metalink Download Description Format June 2010
Appendix A. Acknowledgements and Contributors
The layout and shape of this document relies heavily on work
pioneered in the Atom Syndication Format as specified in [RFC4287].
The content and concepts within are a product of the Metalink
community. Key contributors provided early implementations: A. Bram
Neijt, Hampus Wessman, Darius Liktorius, Manuel Subredu, Michael
Burford, Giorgio Maone, Nils Maier, Max Velasques, Manolo Valdes,
Hayden Legendre, Frederick Cheung, Rene Leonhardt, Per Oyvind
Karlsen, Matt Domsch, Yazsoft, KGet developers, Free Download Manager
developers, Orbit developers, Arne Babenhauserheide, Mathias
Berchtold, Xienzhenyu and TheWorld Browser developers, Xi Software,
Agostino Russo, and James Antill.
The Metalink community has dozens of contributors who contributed to
the evolution of Metalink or proposed ideas and wording for this
document, including:
Paul Burkhead, Kristian Weston, Nicolas Alvarez, Urs Wolfer, Bridget
and Ethan Fletcher, Patrick Ruckstuhl, Sebastien Willemijns, Micah
Cowan, Ruben Kerkhof, Danny Ayers, Nick Dominguez, Gary Zellerbach,
James Clark, Daniel Stenberg, John and Sandra Sowder, Salvatore
Musumeci, Steve Eshelman, Lucas Hewett, Ryan Cronin, Dave Winquist,
Bob Denison, Wes Shelton, Josh Colbert, Steve Kleisath, Chad Neptune,
Derick Cordoba, Nick Carrabba, Chris Carrabba, Erin Solari, Ryan
Alexander, Tom Mainville, Janie Wargo, Jason Hansen, Tim Bray, Dan
Brickley, Markus Hofmann, Dan Connolly, Tim Berners-Lee, Louis
Suarez-Potts, Ross Smith, Jeff Covey, Ed Lee, Shawn Wilsher, Mike
Connor, Johan Svedberg, Kees Cook, Dedric Carter, and Debi Goulding.
We also thank the Anthony Family, the Bryan Family, Juanita Anthony,
and Zimmy Bryan.
Special thanks to Eran Hammer-Lahav, document shepherd, and Lisa
Dusseault, Area Director. We also thank the following contributors
for assistance and review: Mark Nottingham, Peter Saint-Andre, Julian
Reschke, Chris Newman, Ian Macfarlane, Dave Cridland, Barry Leiba,
Uri Blumenthal, Paul Hoffman, Felix Sasaki, Matthias Fuchs, Mark
Baker, Scott Cantor, Brian Carpenter, Alexey Melnikov, Lars Eggert,
Pasi Eronen, Tim Polk, Dan Romascanu, and Bjoern Hoehrmann.
Peter Poeml wishes to acknowledge the support of SUSE Linux Products
GmbH / Novell Inc., where he was employed during much of the work on
this document.
This document is dedicated to Sonora Bryan.
Bryan, et al. Standards Track [Page 30]
RFC 5854 Metalink Download Description Format June 2010
Appendix B. RELAX NG Compact Schema
This appendix is informative.
The Relax NG schema explicitly excludes elements in the Metalink
namespace that are not defined in this revision of the specification.
Requirements for Metalink Processors encountering such markup are
given in Sections 5.2 and 5.3.
# -*- rnc -*-
# RELAX NG Compact Syntax Grammar for the
# Metalink Format Specification Version 4
# Based on RFC 4287 schema
namespace local = ""
namespace metalink = "urn:ietf:params:xml:ns:metalink"
namespace xsd = "http://www.w3.org/2001/XMLSchema"
# Common attributes
metalinkCommonAttributes =
attribute xml:lang { metalinkLanguageTag }?,
undefinedAttribute*
# Text Constructs
metalinkTextConstruct =
metalinkCommonAttributes,
text
# Date Construct
metalinkDateConstruct =
metalinkCommonAttributes,
xsd:dateTime
start = metalinkMetalink
metalinkMetalink =
element metalink:metalink {
metalinkCommonAttributes,
(metalinkFile+
& metalinkGenerator?
& metalinkOrigin?
& metalinkPublished?
& metalinkUpdated?
& extensionElement*)
}
Bryan, et al. Standards Track [Page 31]
RFC 5854 Metalink Download Description Format June 2010
metalinkFile =
element metalink:file {
metalinkCommonAttributes,
attribute name { text },
(metalinkCopyright?
& metalinkDescription?
& metalinkHash*
& metalinkIdentity?
& metalinkLanguage*
& metalinkLogo?
& metalinkMetaURL*
& metalinkOS*
& metalinkPieces*
& metalinkPublisher?
& metalinkSignature?
& metalinkSize?
& metalinkURL*
& metalinkVersion?
& extensionElement*)
}
metalinkPieces =
element metalink:pieces {
attribute length { xsd:positiveInteger },
attribute type { text },
metalinkHash+
}
metalinkCopyright =
element metalink:copyright {
metalinkTextConstruct
}
metalinkDescription =
element metalink:description {
metalinkTextConstruct
}
metalinkGenerator =
element metalink:generator {
metalinkTextConstruct
}
metalinkHash =
element metalink:hash {
attribute type { text }?,
text
}
Bryan, et al. Standards Track [Page 32]
RFC 5854 Metalink Download Description Format June 2010
metalinkIdentity =
element metalink:identity {
metalinkTextConstruct
}
metalinkLanguage =
element metalink:language {
metalinkTextConstruct
}
metalinkLogo =
element metalink:logo {
metalinkCommonAttributes,
(metalinkUri)
}
metalinkMetaURL =
element metalink:metaurl {
metalinkCommonAttributes,
attribute priority { xsd:positiveInteger {
maxInclusive = "999999"}}?,
attribute mediatype { text },
attribute name { text }?,
(metalinkUri)
}
metalinkOrigin =
element metalink:origin {
metalinkCommonAttributes,
attribute dynamic { xsd:boolean }?,
(metalinkUri)
}
metalinkOS =
element metalink:os {
metalinkTextConstruct
}
metalinkPublished =
element metalink:published {
metalinkDateConstruct
}
metalinkPublisher =
element metalink:publisher {
metalinkCommonAttributes,
attribute name { text },
attribute url { metalinkUri }?
Bryan, et al. Standards Track [Page 33]
RFC 5854 Metalink Download Description Format June 2010
}
metalinkSignature =
element metalink:signature {
attribute mediatype { text },
metalinkTextConstruct
}
metalinkSize =
element metalink:size {
xsd:nonNegativeInteger
}
metalinkUpdated =
element metalink:updated {
metalinkDateConstruct
}
metalinkURL =
element metalink:url {
metalinkCommonAttributes,
attribute location { xsd:string {
minLength = "2" maxLength="2"}
}?,
attribute priority { xsd:positiveInteger {
maxInclusive = "999999"}}?,
(metalinkUri)
}
metalinkVersion =
element metalink:version {
metalinkTextConstruct
}
# As defined in RFC 3066 and compatible with RFC 5646
metalinkLanguageTag = xsd:string {
pattern = "[A-Za-z]{1,8}(-[A-Za-z0-9]{1,8})*"
}
# Unconstrained; it's not entirely clear how IRI fit into
# xsd:anyURI so let's not try to constrain it here
metalinkUri = text
# Simple Extension
simpleExtensionElement =
element * - metalink:* {
text
Bryan, et al. Standards Track [Page 34]
RFC 5854 Metalink Download Description Format June 2010
}
# Structured Extension
structuredExtensionElement =
element * - metalink:* {
(attribute * { text }+,
(text|anyElement)*)
| (attribute * { text }*,
(text?, anyElement+, (text|anyElement)*))
}
# Other Extensibility
extensionElement =
simpleExtensionElement | structuredExtensionElement
undefinedAttribute =
attribute * - (xml:lang | local:*) { text }
undefinedContent = (text|anyForeignElement)*
anyElement =
element * {
(attribute * { text }
| text
| anyElement)*
}
anyForeignElement =
element * - metalink:* {
(attribute * { text }
| text
| anyElement)*
}
# EOF
Bryan, et al. Standards Track [Page 35]
RFC 5854 Metalink Download Description Format June 2010
Index
A
ABNF
metalinkGenerator 13
metaurl mediatype 17
signature mediatype 19
application/metalink4+xml Media Type 23
C
copyright XML element 12
D
description XML element 13
F
file XML element 10
G
generator XML element 13
Grammar
metalinkCommonAttributes 7
metalinkCopyright 13
metalinkDateConstruct 8
metalinkDescription 13
metalinkFile 10
metalinkGenerator 13
metalinkHash 14
metalinkIdentity 15
metalinkLanguage 15
metalinkLogo 16
metalinkMetalink 8
metalinkMetaURL 16
metalinkOrigin 17
metalinkOS 18
metalinkPieces 12
metalinkPublished 18
metalinkPublisher 18
metalinkSignature 19
metalinkSize 20
metalinkTextConstruct 7
metalinkUpdated 20
metalinkURL 21
metalinkVersion 21
simpleExtensionElement 22
structuredExtensionElement 23
Bryan, et al. Standards Track [Page 36]
RFC 5854 Metalink Download Description Format June 2010
H
hash XML element 14
I
identity XML element 15
L
language XML element 15
logo XML element 16
M
Media Type
application/metalink4+xml 23
metalink XML element 8
metalinkCommonAttributes grammar production 7
metalinkCopyright grammar production 12
metalinkDateConstruct grammar production 8
metalinkDescription grammar production 13
metalinkFile grammar production 10
metalinkGenerator ABNF 13
metalinkGenerator grammar production 13
metalinkHash grammar production 14
metalinkIdentity grammar production 15
metalinkLanguage grammar production 15
metalinkLogo grammar production 16
metalinkMetalink grammar production 8
metalinkMetaURL grammar production 16
metalinkOrigin grammar production 17
metalinkOS grammar production 18
metalinkPieces grammar production 12
metalinkPublished grammar production 18
metalinkPublisher grammar production 18
metalinkSignature grammar production 19
metalinkSize grammar production 20
metalinkTextConstruct grammar production 7
metalinkUpdated grammar production 20
metalinkURL grammar production 21
metalinkVersion grammar production 21
metaurl mediatype ABNF 16
metaurl XML element 16
O
origin XML element 17
os XML element 18
Bryan, et al. Standards Track [Page 37]
RFC 5854 Metalink Download Description Format June 2010
P
pieces XML element 12
published XML element 18
publisher XML element 18
S
signature mediatype ABNF 19
signature XML element 19
simpleExtensionElement grammar production 22
size XML element 20
structuredExtensionElement grammar production 23
U
updated XML element 20
url XML element 20
V
version XML element 21
X
XML Elements
copyright 12
description 13
file 9
generator 13
hash 14
identity 15
language 15
logo 16
metalink 8
metaurl 16
origin 17
os 18
pieces 12
published 18
publisher 18
signature 19
size 20
updated 20
url 20
version 21
Bryan, et al. Standards Track [Page 38]
RFC 5854 Metalink Download Description Format June 2010
Authors' Addresses
Anthony Bryan
Pompano Beach, FL
USA
EMail: anthonybryan@gmail.com
URI: http://www.metalinker.org
Tatsuhiro Tsujikawa
Shiga
Japan
EMail: tatsuhiro.t@gmail.com
URI: http://aria2.sourceforge.net
Neil McNab
San Diego, CA
USA
EMail: neil@nabber.org
URI: http://www.nabber.org
Dr. med. Peter Poeml
MirrorBrain
Venloer Str. 317
Koeln 50823
DE
Phone: +49 221 6778 333 8
EMail: peter@poeml.de
URI: http://mirrorbrain.org/~poeml/
Bryan, et al. Standards Track [Page 39]