One document matched: draft-hammer-hostmeta-02.xml
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<rfc category="info" ipr="trust200902" docName="draft-hammer-hostmeta-02">
<?rfc strict="yes" ?>
<?rfc toc="yes" ?>
<?rfc tocdepth="2" ?>
<?rfc symrefs="yes" ?>
<?rfc sortrefs="yes" ?>
<?rfc compact="yes" ?>
<?rfc subcompact="no" ?>
<front>
<title>Host-Meta: Web Host Metadata</title>
<author initials="E" surname="Hammer-Lahav" fullname="Eran Hammer-Lahav">
<organization>
Yahoo!
</organization>
<address>
<email>eran@hueniverse.com</email>
<uri>http://hueniverse.com</uri>
</address>
</author>
<date year="2009"/>
<abstract>
<t>
This memo describes a method for locating host metadata for Web-based protocols.
</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>
Web-based protocols often require the discovery of host policy or metadata, where host is
not a single resource but the collection of resources identified by URIs with a common
scheme and authority as defined by <xref target="RFC3986"/>. While these protocols have a
wide range of metadata needs, they often define metadata that is concise, has simple syntax
requirements, and can benefit from sharing its metadata store with other related protocols.
</t>
<t>
Because there is no URI or a resource available to describe a host, many of the methods
used for associating per-resource metadata (such as HTTP headers) are not available. This
often leads to the usage of the root HTTP resource as a placeholder for host metadata that
is not specific to the root resource, and often has nothing to do it.
</t>
<t>
This memo registers the "well-known" URI suffix 'host-meta' in the Well-Known URI Registry
established by <xref target="I-D.nottingham-site-meta" />, and specifies a simple,
general-purpose metadata document for hosts, to be used by multiple Web-based protocols.
The name 'host-meta' was chosen based on the HTTP 'Host' header defined by <xref target="RFC2616"/>.
</t>
<t>
Please discuss this draft on the
<eref target="https://www.ietf.org/mailman/listinfo/apps-discuss">apps-discuss@ietf.org</eref>
mailing list.
</t>
<section title="Example">
<t>
A simple host-meta document for the 'example.com' and 'www.example.com' authorities and
'http' scheme with a link providing scope-wide copyright information and a link template
providing a URI for obtaining resource-specific metadata for each resource within the
host-meta document scope:
</t>
<figure>
<artwork xml:space="preserve"><![CDATA[
<?xml version='1.0' encoding='UTF-8'?>
<XRD xmlns='http://docs.oasis-open.org/ns/xri/xrd-1.0'
xmlns:host-meta='http://host-meta.net/ns/1.0'>
<host-meta:Scope>http://example.com</host-meta:Scope>
<host-meta:Scope>http://www.example.com</host-meta:Scope>
<Link>
<Title xml:lang='en-us'>Site License Policy</Title>
<Rel>license</Rel>
<URI>http://example.com/license</URI>
</Link>
<Link>
<Title xml:lang='en-us'>Resource Descriptor</Title>
<Rel>describedby</Rel>
<URITemplate>http://meta.example.com?uri={uri}</URITemplate>
</Link>
</XRD>
]]></artwork>
</figure>
</section>
<section title="Namespace and Version" anchor="ns">
<t>
The host-meta document uses the XRD 1.0 XML namespace URI <xref target="W3C.REC-xml-names-19990114" />:
</t>
<figure>
<artwork xml:space="preserve"><![CDATA[
http://docs.oasis-open.org/ns/xri/xrd-1.0
]]></artwork>
</figure>
<t>
The XML namespace URI for the host-meta specific extension elements defined in this
specification is:
</t>
<figure>
<artwork xml:space="preserve"><![CDATA[
http://host-meta.net/ns/1.0
]]></artwork>
</figure>
</section>
<section title="Notational Conventions">
<t>
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
<xref target="RFC2119" />.
</t>
<t>
This specification uses the namespace prefix "host-meta:" for the extension Namespace URI
identified in <xref target="ns" />. Note that the choice of namespace prefix is arbitrary
and not semantically significant. Element names without a namespace prefix belong to the
XRD 1.0 XML namespace identified in <xref target="ns" />.
</t>
<t>
This document uses the Augmented Backus-Naur Form (ABNF) notation of <xref target="RFC5234" />.
Additionally, the following rules are included from <xref target="RFC3986" />: reserved,
unreserved, authority, and scheme.
</t>
</section>
</section>
<section title="Metadata Scope">
<t>
The scope of each host-meta document is defined by one or more scheme-authority pairs, each
applies to the collection of resources identified by URIs with the same scheme and authority
as defined by <xref target="RFC3986"/>. The scope MUST be expressed explicitly within the
document by using the one or more <xref target="Scope element">'host-meta:Scope' elements</xref>.
</t>
<t>
The host-meta scope does not apply to a subset, nor does it apply to other combinations of
scheme and authority (e.g., using another scheme, port, or a different hostname in the same
domain) not explicitly declared. For example, resources with URIs beginning with
'http://example.com/', 'http://example.com:8080/', 'https://example.com/', and
'http://www.example.com/' all have different and non-overlapping scopes.
</t>
<t>
The scope declared within the host-meta document MUST match the scheme and authority used
to <xref target="obtain">obtain</xref> the document. However, protocols MAY use a different
scheme to obtain the document than the one they are interested in, as long as both scheme
and authority combinations are declared. For example, a protocol can use HTTP to obtain
metadata about a scheme other than 'http' for which a host-meta document cannot be obtained
using the method described in <xref target="obtain" />.
</t>
<t>
Protocols MAY place additional requirements on protocol-specific metadata retrieved from
multiple host-meta documents. For example, protocols can require that protocol-specific
metadata obtained for the 'example.com:80' authority to be the same in both host-meta
documents for the 'http' and 'https' schemes.
</t>
<t>
Any changes in scope, the retrieval of metadata for one scheme using another, and other
requirements should only occur after a careful consideration of their security implications
and authoritativeness.
</t>
</section>
<section title="The host-meta Document Format">
<t>
The host-meta document uses the XRD 1.0 document format as defined by
<xref target="OASIS.XRD-1.0" />, which provides a simple and extensible XML-based format
for describing resources. Since the host-meta XRD document does not describe a single
resource but a collection of resources, this memo defines additional XRD elements and
processing rules. The XRD elements not mentioned in this memo are allowed and used as
defined in <xref target="OASIS.XRD-1.0" />.
</t>
<t>
host-meta document SHOULD NOT include the 'Subject' or 'Alias' XRD elements since these
elements require a valid URI to identify the resource being described, which is not
available for the host-meta scope. The use of these elements in host-meta is undefined.
Instead, host-meta defines the <xref target="Scope element">'host-meta:Scope' element</xref> for
declaring document scope.
</t>
<t>
The subject (or "context resource" as defined by <xref target="I-D.nottingham-http-link-header" />)
of the XRD 'Type' and 'Link' elements is the entire collection of resources included in the
document scope, with the exception of 'Link' elements with a 'URITemplate' child element as
defined in <xref target="Link element" />.
</t>
<section title="The 'host-meta:Scope' Element" anchor="Scope element">
<t>
The 'host-meta:Scope" element is used to declare the scope of the host-meta document and
is defined as a child element of the root 'XRD' element. The parent 'XRD' element MUST
include one but MAY include more 'host-meta:Scope' elements (order does not matter).
</t>
<figure>
<preamble>
The element value syntax ABNF:
</preamble>
<artwork xml:space="preserve"><![CDATA[
Scope = scheme "://" authority
]]></artwork>
</figure>
<t>
If the authority component does not include a port number, the port is implicitly set to
the default port number for the given scheme, and does not include any other port number.
</t>
<figure>
<preamble>
For example, these two scope declarations are equivalent:
</preamble>
<artwork>
<![CDATA[
<host-meta:Scope>http://example.com</host-meta:Scope>
<host-meta:Scope>http://example.com:80</host-meta:Scope>
]]></artwork>
</figure>
</section>
<section title="The 'Link' Element" anchor="Link element">
<t>
The XRD 'Link' element, when used with the 'URI' child element, conveys a link relation
between the collection of resources included in the host-meta document scope and a common
target URI.
</t>
<figure>
<preamble>
For example, the following link declares a common author for the entire scope:
</preamble>
<artwork xml:space="preserve"><![CDATA[
<Link>
<Rel>author</Rel>
<URI>http://example.com/author</URI>
</Link>
]]></artwork>
</figure>
<t>
However, there are cases in which individual resources within the same scope do not share
the same target URI, but follow a common pattern in how the target URI is constructed. A
'Link' element with a 'URITemplate' child element conveys relations whose context are
individual resources within the host-meta document scope, and whose target is constructed
by applying the context URI to a template.
</t>
<figure>
<preamble>
For example, a blog with multiple authors can provide information about each article's
author by appending a suffix (such as ';by') to the URI of each article. Each article
has a unique author, but all share the same pattern of where that information is located:
</preamble>
<artwork xml:space="preserve"><![CDATA[
<Link>
<Rel>author</Rel>
<URITemplate>{+uri};by</URITemplate>
</Link>
]]></artwork>
</figure>
<section title="Template Syntax" anchor="template_syntax">
<t>
host-meta defines a simple template syntax and a vocabulary for URI transformation. A
template is a string containing brace-enclosed ("{}") variable names marking the parts
of the string that are to be substituted by the corresponding variable values.
</t>
<t>
Before substituting template variables, any value character other than reserved and
unreserved (as defined by <xref target="RFC3986" />) MUST be percent-encoded per
<xref target="RFC3986" />. If a variable name is not prefixed by a '+' character, any
reserved character SHALL also be percent-encoded.
</t>
<t>
To construct a URI using a template, the context URI is parsed into its URI components
and each component value assigned to a variable name. The variable set is based on the
URI vocabulary defined by <xref target="RFC3986" /> section 3 and includes: 'scheme',
'authority', 'path', 'query', 'fragment', 'userinfo', 'host', and 'port'. In addition,
this memo defines the 'uri' variable as the entire context URI excluding the fragment
component and the '#' fragment separator.
</t>
<figure>
<artwork xml:space="preserve"><![CDATA[
foo://william@example.com:8080/over/there?name=ferret#nose
\_/ \______________________/\_________/ \_________/ \__/
| | | | |
scheme authority path query fragment
foo://william@example.com:8080/over/there?name=ferret#nose
\_____/ \_________/ \__/
| | |
userinfo host port
foo://william@example.com:8080/over/there?name=ferret#nose
\___________________________________________________/
|
uri
]]></artwork>
</figure>
<t>
Protocols MAY define additional variables and syntax rules, but SHOULD only do so for
protocol-specific relation types, and MUST NOT change the meaning of the variables
defined above. If a template uses an unknown syntax or contains unknown variable names,
the parent 'Link' element SHOULD be ignored.
</t>
<figure>
<preamble>
The template syntax ABNF:
</preamble>
<artwork xml:space="preserve"><![CDATA[
URI-Template = *( uri-char | variable )
variable = "{" [ "+" ] var-name "}"
uri-char = ( reserved | unreserved )
var-name = uri-var | ( 1*var-char )
uri-var = "scheme" | "authority" | "path" |
"query" | "fragment" | "userinfo" |
"host" | "port" | "uri"
var-char = ALPHA / DIGIT / "." / "_"
]]></artwork>
</figure>
<figure>
<preamble>
For example, given the input URI 'http://example.com/r/1?f=xml#top', each of the
following templates will produce the associated output URI:
</preamble>
<artwork xml:space="preserve"><![CDATA[
{+uri}&test -->
http://example.com/r/1?f=xml&test
http://example.org?q={uri} -->
http://example.org?q=http%3A%2F%2Fexample.com%2Fr%2F1%3Ff%3Dxml
http://meta.{host}:8080{+path}?{+query} -->
http://meta.example.com:8080/r/1?f=xml
]]></artwork>
</figure>
</section>
</section>
</section>
<section title="Obtaining host-meta Documents" anchor="obtain">
<t>
The host-meta document for a given scheme-authority pair is identified by a URI constructed
using the desired scheme, authority, and '/.well-known/host-meta' as the URI path. For
example, given the 'http' scheme and 'www.example.com:80' authority, the host-meta URI is:
</t>
<figure>
<artwork xml:space="preserve"><![CDATA[
http://www.example.com/.well-known/host-meta
]]></artwork>
</figure>
<t>
The host-meta document is obtained by dereferencing the request host-meta URI. The
semantics of the protocol used to obtain the host-meta document apply. Therefore, if the
server indicates that the host-meta resource is located elsewhere (in HTTP, 3xx response
status codes), the client MUST try to obtain the resource from the location provided. This
means that the host-meta document for one authority MAY be retrieved from a different
authority. Likewise, if the resource is not available or exists (in HTTP, the 404 or 410
response status codes), the client SHOULD infer that metadata is not available via this
mechanism.
</t>
<t>
If a representation is successfully obtained, but is not in the format described above,
clients SHOULD infer that the authority is using this URI for other purposes, and not
process it as a host-meta document. To aid in this process, authorities using this
mechanism SHOULD correctly label host-meta responses with the "application/xrd+xml" internet
media type.
</t>
</section>
<section title="Security Considerations" anchor="Security">
<t>
The metadata returned by the host-meta resource is presumed to be under the control of the
appropriate authority and representative of all the resources described by it. If this
resource is compromised or otherwise under the control of another party, it may represent a
risk to the security of the server and data served by it, depending on what protocols use it.
</t>
<t>
Scoping metadata to a single scheme-authority pair is the default host-meta scope. Protocols
that change the scope without careful consideration can incur security risks.
</t>
</section>
<section title="IANA Considerations">
<section title="The host-meta Well-Known URI">
<t>
This memo registers the 'host-meta' well-known URI in the Well-Known URI Registry as defined by
<xref target="I-D.nottingham-site-meta" />.
<list style="hanging">
<t hangText="URI suffix:">
host-meta
</t>
<t hangText="Change controller:">
IETF
</t>
<t hangText="Specification document(s):">
[[ this document ]]
</t>
<t hangText="Related information:">
None
</t>
</list>
</t>
</section>
</section>
<appendix title="Acknowledgments">
<t>
This memo was initially based on <xref target="I-D.nottingham-site-meta" />.
</t>
<t>
The author would like to acknowledge the contributions of everyone who
provided feedback and use cases for this memo; in particular, Dirk Balfanz, DeWitt Clinton,
Blaine Cook, Breno de Medeiros, Brad Fitzpatrick, Will Norris, Mark Nottingham, John Panzer,
and Drummond Reed.
</t>
</appendix>
<appendix title="Document History">
<t>
[[ to be removed by the RFC editor before publication as an RFC ]]
</t>
<t>
-02
<list style="symbols">
<t>
Changed Scope element syntax from attributes to URI-like string value.
</t>
</list>
</t>
<t>
-01
<list style="symbols">
<t>
Editorial rewrite.
</t>
<t>
Redefined scope as a scheme-authority pair.
</t>
<t>
Added document structure section.
</t>
</list>
</t>
<t>
-00
<list style="symbols">
<t>
Initial draft.
</t>
</list>
</t>
</appendix>
</middle>
<back>
<references title="Normative References">
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<front>
<title>Extensible Resource Descriptor (XRD) Version 1.0 (work in progress)</title>
<author initials="E.H" surname="Hammer-Lahav" fullname="Eran">
<organization>Yahoo!</organization>
</author>
<author initials="W.N" surname="Norris" fullname="Will Norris">
<organization>Internet2</organization>
</author>
</front>
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</rfc>| PAFTECH AB 2003-2026 | 2026-04-24 13:38:17 |