One document matched: draft-hammer-hostmeta-14.xml
<?xml version='1.0' encoding='UTF-8' ?>
<!DOCTYPE rfc SYSTEM 'rfc2629.dtd'>
<?xml-stylesheet type='text/xsl' href='rfc2629.xslt' ?>
<rfc category='std' ipr='trust200902' docName='draft-hammer-hostmeta-14'>
<?rfc strict='yes' ?>
<?rfc toc='yes' ?>
<?rfc tocdepth='3' ?>
<?rfc symrefs='yes' ?>
<?rfc sortrefs='yes' ?>
<?rfc compact='yes' ?>
<?rfc subcompact='no' ?>
<front>
<title abbrev='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='2011'/>
<abstract>
<t>
This memo describes a method for locating host metadata as well as information about
individual resources controlled by the host.
</t>
</abstract>
<note title='Editorial Note (to be removed by RFC Editor)'>
<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>
</note>
</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 entity controlling the collection of resources identified by
Uniform Resource Identifiers (URI) with a common URI host <xref target='RFC3986'/>.
</t>
<t>
While web protocols have a wide range of metadata needs, they often use metadata that is
concise, has simple syntax requirements, and can benefit from storing their metadata in a
common location used by other related protocols.
</t>
<t>
Because there is no URI or representation 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 overloading of the root HTTP resource (e.g. 'http://example.com/') with
host metadata that is not specific or relevant to the root resource itself.
</t>
<t>
This memo registers the well-known URI suffix <spanx style='verb'>host-meta</spanx> in
the Well-Known URI Registry established by <xref target='RFC5785' />, and specifies a
simple, general-purpose metadata document format for hosts, to be used by multiple
web-based protocols.
</t>
<t>
In addition, there are times when a host-wide scope for policy or metadata is too
coarse-grained. host-meta provides two mechanisms for providing resource-specific
information:
<list style='symbols'>
<t>
Link Templates - links using a URI template instead of a fixed target URI, providing a
way to define generic rules for generating resource-specific links by applying the
individual resource URI to the template.
</t>
<t>
Link-based Resource Descriptor Documents (LRDD, pronounced 'lard') - descriptor
documents providing resource-specific information, typically information that cannot be
expressed using link templates. LRDD documents are linked to resources or host-meta
documents using link templates with the <spanx style='verb'>lrdd</spanx> relation type.
</t>
</list>
</t>
<section title='Example'>
<t>
The following is a simple host-meta document including both host-wide and
resource-specific information for the 'example.com' host:
</t>
<figure>
<artwork>
<![CDATA[
<?xml version='1.0' encoding='UTF-8'?>
<XRD xmlns='http://docs.oasis-open.org/ns/xri/xrd-1.0'>
<!-- Host-wide Information -->
<Property type='http://protocol.example.net/version'>1.0</Property>
<Link rel='copyright'
href='http://example.com/copyright' />
<!-- Resource-specific Information -->
<Link rel='hub'
template='http://example.com/hub' />
<Link rel='lrdd'
type='application/xrd+xml'
template='http://example.com/lrdd?uri={uri}' />
<Link rel='author'
template='http://example.com/author?q={uri}' />
</XRD>
]]>
</artwork>
</figure>
<t>
The host-wide information which applies to host in its entirety provided by the document
includes:
<list style='symbols'>
<t>
A <spanx style='verb'>http://protocol.example.net/version</spanx> host property with
a value of <spanx style='verb'>1.0</spanx>.
</t>
<t>
A link to the host's copyright policy (<spanx style='verb'>copyright</spanx>).
</t>
</list>
</t>
<t>
The resource-specific information provided by the document includes:
<list style='symbols'>
<t>
A link template for receiving real-time updates (<spanx style='verb'>hub</spanx>)
about individual resources. Since the template does not include a template variable,
the target URI is identical for all resources.
</t>
<t>
A LRDD document link template (<spanx style='verb'>lrdd</spanx>) for obtaining
additional resource-specific information contained in a separate document for each
individual resource.
</t>
<t>
A link template for finding information about the author of individual resources
(<spanx style='verb'>author</spanx>).
</t>
</list>
</t>
<section title='Processing Resource-Specific Information' anchor='resource_example'>
<t>
When looking for information about the an individual resource, for example, the resource
identified by 'http://example.com/xy', the resource URI is applied to the
templates found, producing the following links:
</t>
<figure>
<artwork>
<![CDATA[
<Link rel='hub'
href='http://example.com/hub' />
<Link rel='lrdd'
type='application/xrd+xml'
href='http://example.com/lrdd?uri=http%3A%2F%2Fexample.com%2Fxy' />
<Link rel='author'
href='http://example.com/author?q=http%3A%2F%2Fexample.com%2Fxy' />
]]>
</artwork>
</figure>
<t>
The LRDD document for 'http://example.com/xy' is obtained using an HTTP
<spanx style='verb'>GET</spanx> request:
</t>
<figure>
<artwork>
<![CDATA[
<?xml version='1.0' encoding='UTF-8'?>
<XRD xmlns='http://docs.oasis-open.org/ns/xri/xrd-1.0'>
<Subject>http://example.com/xy</Subject>
<Property type='http://spec.example.net/color'>red</Property>
<Link rel='hub'
href='http://example.com/another/hub' />
<Link rel='author'
href='http://example.com/john' />
</XRD>
]]>
</artwork>
</figure>
<t>
Together, the information available about the individual resource (presented as an XRD
document for illustration purposes) is:
</t>
<figure>
<artwork>
<![CDATA[
<?xml version='1.0' encoding='UTF-8'?>
<XRD xmlns='http://docs.oasis-open.org/ns/xri/xrd-1.0'>
<Subject>http://example.com/xy</Subject>
<Property type='http://spec.example.net/color'>red</Property>
<Link rel='hub'
href='http://example.com/hub' />
<Link rel='hub'
href='http://example.com/another/hub' />
<Link rel='author'
href='http://example.com/john' />
<Link rel='author'
href='http://example.com/author?q=http%3A%2F%2Fexample.com%2Fxy' />
</XRD>
]]>
</artwork>
</figure>
<t>
Note that the order of links matters and is based on their original order in the
host-meta and LRDD documents. For example, the <spanx style='verb'>hub</spanx> link
obtained from the host-meta link template has a higher priority than the link found in
the LRDD document because the host-meta link appears before the
<spanx style='verb'>lrdd</spanx> link.
</t>
<t>
On the other hand, the <spanx style='verb'>author</spanx> link found in the LRDD document
has a higher priority than the link found in the host-meta document because it appears
after the <spanx style='verb'>lrdd</spanx> link.
</t>
</section>
</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 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, and pct-encoded.
</t>
</section>
</section>
<section title='Obtaining host-meta Documents'>
<t>
The client obtains the host-meta document for a given host by sending an HTTP
<xref target='RFC2616' /> or an HTTPS <xref target='RFC2818' /> GET request to the host for
the <spanx style='verb'>/.well-known/host-meta</spanx> path. The client MUST only use the
default ports defined for each protocol (e.g. port 80 for HTTP and port 443 for HTTPS). The
scope and meaning of host-meta documents obtained via other protocols or ports is
undefined.
</t>
<t>
The server MUST support at least one protocol but MAY support both. If both protocols are
supported, they MUST produce the same document.
</t>
<t>
The decision which protocol is used to obtain the host-meta document have significant
security ramifications as described in <xref target='security' />.
</t>
<figure>
<preamble>
For example, the following request is used to obtain the host-meta document for the
'example.com' host:
</preamble>
<artwork>
<![CDATA[
GET /.well-known/host-meta HTTP/1.1
Host: example.com
]]>
</artwork>
</figure>
<t>
If the server response indicates that the host-meta resource is located elsewhere (a 301,
302, or 307 response status code), the client MUST try to obtain the resource from the
location provided in the response. This means that the host-meta document for one host
MAY be retrieved from another host. Likewise, if the resource is not available or does
not exist (e.g. a 404 or 410 response status codes) using both the HTTP and HTTPS
protocols, the client should infer that metadata is not available via this mechanism.
</t>
<t>
The host-meta document SHOULD be served with the <spanx style='verb'>application/xrd+xml</spanx>
media type. [[ media type registration pending ]]
</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 schema
for describing resources. This memo defines additional processing rules needed to describe
hosts. Documents MAY include any XRD element not explicitly excluded.
</t>
<t>
The host-meta document root MUST be an <spanx style='verb'>XRD</spanx> element. The
document SHOULD NOT include a <spanx style='verb'>Subject</spanx> element, as at this time
no URI is available to identify hosts. The use of the <spanx style='verb'>Alias</spanx>
element in host-meta is undefined and NOT RECOMMENDED.
</t>
<t>
The subject (or "context resource" as defined by <xref target='RFC5988' />)
of the XRD <spanx style='verb'>Property</spanx> and <spanx style='verb'>Link</spanx>
elements is the host described by the host-meta document. However, the subject of
<spanx style='verb'>Link</spanx> elements with a <spanx style='verb'>template</spanx>
attribute is the individual resource whose URI is applied to the link template as described
in <xref target='Link element' />.
</t>
<section title="The 'Link' Element" anchor='Link element'>
<t>
The XRD <spanx style='verb'>Link</spanx> element, when used with the
<spanx style='verb'>href</spanx> attribute, conveys a link relation between the host
described by the document and a common target URI.
</t>
<figure>
<preamble>
For example, the following link declares a common copyright license for the entire
scope:
</preamble>
<artwork>
<![CDATA[
<Link rel='copyright' href='http://example.com/copyright' />
]]>
</artwork>
</figure>
<t>
However, a <spanx style='verb'>Link</spanx> element with a
<spanx style='verb'>template</spanx> attribute conveys a relation whose context is an
individual resource within the host-meta document scope, and whose target is constructed
by applying the context resource URI to the template. The template string MAY contain a URI
string without any variables to represent a resource-level relation that is identical for
every individual resource.
</t>
<figure>
<preamble>
For example, a blog with multiple authors can provide information about each article's
author by providing an endpoint with a parameter set 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>
<![CDATA[
<Link rel='author'
template='http://example.com/author?article={uri}' />
]]>
</artwork>
</figure>
<section title='Template Syntax' anchor='template_syntax'>
<t>
This memo defines a simple template syntax 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, values MUST be encoded using UTF-8 and any
character other than unreserved (as defined by <xref target='RFC3986' />) MUST be
percent-encoded per <xref target='RFC3986' />.
</t>
<t>
This memo defines a single variable - <spanx style='verb'>uri</spanx> - as the entire
context resource URI. Protocols MAY define additional relation-specific variables and
syntax rules, but SHOULD only do so for protocol-specific relation types, and MUST NOT
change the meaning of the <spanx style='verb'>uri</spanx> variable. If a client is
unable to successfully process a template (e.g. unknown variable names, unknown or
incompatible syntax) the parent <spanx style='verb'>Link</spanx> element SHOULD be
ignored.
</t>
<figure>
<preamble>
The template syntax ABNF:
</preamble>
<artwork>
<![CDATA[
URI-Template = *( uri-char / variable )
variable = "{" var-name "}"
uri-char = ( reserved / unreserved / pct-encoded )
var-name = %x75.72.69 / ( 1*var-char ) ; "uri" or other names
var-char = ALPHA / DIGIT / "." / "_"
]]>
</artwork>
</figure>
<figure>
<preamble>
For example:
</preamble>
<artwork>
<![CDATA[
Input: http://example.com/r?f=1
Template: http://example.org/?q={uri}
Output: http://example.org/?q=http%3A%2F%2Fexample.com%2Fr%3Ff%3D1
]]>
</artwork>
</figure>
</section>
</section>
</section>
<section title='Processing host-meta Documents'>
<t>
Once the host-meta document has been obtained, the client processes its content based on
the type of information desired: host-wide or resource-specific.
</t>
<t>
Clients usually look for a link with a specific relation type or other attributes. In
such cases, the client does not need to process the entire host-meta document and all
linked LRDD documents, but instead, process the various documents in their prescribed order
until the desired information is found.
</t>
<t>
Protocols using host-meta must indicate whether the information they seek is host-wide or
resource-specific. For example, "obtain the first host-meta resource-specific link using
the 'author' relation type". If both types are used for the same purpose (e.g. first look
for resource-specific, then look for host-wide), the protocol must specify the processing
order.
</t>
<section title='Host-Wide Information'>
<t>
When looking for host-wide information, the client MUST ignore any
<spanx style='verb'>Link</spanx> elements with a <spanx style='verb'>template</spanx>
attribute, as well as any link using the <spanx style='verb'>lrdd</spanx> relation type.
All other elements are scoped as host-wide.
</t>
</section>
<section title='Resource-Specific Information'>
<t>
Unlike host-wide information which is contained solely within the host-meta document,
resource-specific information is obtained from host-meta link templates, as well as from
linked LRDD documents.
</t>
<t>
When looking for resource-specific information, the client constructs a resource
descriptor by collecting and processing all the host-meta link templates. For each link
template:
<list style='numbers'>
<t>
The client applies the URI of the desired resource to the template, producing a
resource-specific link.
</t>
<t>
If the link's relation type is other than <spanx style='verb'>lrdd</spanx>, the
client adds the link to the resource descriptor in order.
</t>
<t>
If the link's relation type is <spanx style='verb'>lrdd</spanx>:
<list style='format 3.%d'>
<t>
The client obtains the LRDD document by following the scheme-specific rules
for the LRDD document URI. If the document URI scheme is
<spanx style='verb'>http</spanx> or <spanx style='verb'>https</spanx>, the
document is obtained via an HTTP <spanx style='verb'>GET</spanx> request to
the identified URI. If the HTTP response status code is 301, 302, or 307, the
client MUST follow the redirection response and repeat the request with the
provided location.
</t>
<t>
The client adds any links found in the LRDD document to the resource
descriptor in order, except for any link using the
<spanx style='verb'>lrdd</spanx> relation type (processing is limited to a single
level of inclusion). When adding links, the client
SHOULD retain any extension attributes and child elements if present (e.g.
<Property> or <Title> elements).
</t>
<t>
The client adds any resource properties found in the LRDD document to the
resource descriptor in order (e.g. <Alias> or <Property> child
elements of the LRDD document <XRD> root element).
</t>
</list>
</t>
</list>
</t>
</section>
</section>
<section title='Security Considerations' anchor='security'>
<t>
The host-meta document is designed to be used by other applications explicitly "opting-in"
to use the facility. Therefore, any such application MUST review the specific security
implications of using host-meta documents. By itself, this specification does not provide
any protections or guarantees that any given host-meta document is under the control of the
appropriate entity as required by each application.
</t>
<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 the applications
using it.
</t>
<t>
Applications utilizing the host-meta document for sensitive or security related information
MUST require the use of the HTTPS protocol and MUST NOT produce a host-meta document using
other means. In addition, such applications MUST require that any redirection leading to
the retrieval of a host-meta document also utilize the HTTPS protocol.
</t>
<t>
Since the host-meta document is authoritative for the entire host, not just the authority
(combination of scheme, host, and port) of the host-meta document server, applications MUST
ensure that using a host-meta document for another URI authority does not represent a
potential security exploit.
</t>
<t>
Protocols using host-meta templates must evaluate the construction of their templates as
well as any protocol-specific variables or syntax to ensure that the templates cannot be
abused by an attacker. For example, a client can be tricked into following a malicious link
due to a poorly constructed template which produces unexpected results when its variable
values contain unexpected characters.
</t>
</section>
<section title='IANA Considerations'>
<section title="The 'host-meta' Well-Known URI">
<t>
This memo registers the <spanx style='verb'>host-meta</spanx> well-known URI in the
Well-Known URI Registry as defined by <xref target='RFC5785' />.
<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:'>
The <spanx style='verb'>host-meta</spanx> documents obtained from the same host using
the HTTP and HTTPS protocols (using default ports) MUST be identical.
</t>
</list>
</t>
</section>
<section title="The 'lrdd' Relation Type">
<t>
This specification registers the <spanx style='verb'>lrdd</spanx> relation type in the
Link Relation Type Registry defined by <xref target='RFC5988' />:
<list style='hanging'>
<t hangText='Relation Name:'>
lrdd
</t>
<t hangText='Description:'>
<spanx style='verb'>lrdd</spanx> (pronounced 'lard') is an acronym for Link-based
Resource Descriptor Document. It is used by the host-meta document processor to
locate resource-specific information about individual resources. When used elsewhere
(e.g. in HTTP <spanx style='verb'>Link</spanx> header fields or in HTML <LINK>
elements), it operates as an include directive, identifying the location of
additional links and other metadata. Multiple links with the 'lrdd' relation indicate
multiple sources to include, not alternative sources of the same information. An
<spanx style='verb'>application/xrd+xml</spanx> representation MUST be available, and
this media type MAY appear in a link's <spanx style='verb'>type</spanx> attribute.
Additional representations MAY be available (using the HTTP
<spanx style='verb'>Accept</spanx> request header field), in which case the link's
<spanx style='verb'>type</spanx> attribute SHOULD be omitted.
</t>
<t hangText='Reference:'>
[[ This specification ]]
</t>
</list>
</t>
</section>
</section>
<appendix title='Acknowledgments'>
<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,
Eve Maler, Breno de Medeiros, Brad Fitzpatrick, James Manger, Will Norris, Mark Nottingham,
John Panzer, Drummond Reed, and Peter Saint-Andre.
</t>
</appendix>
</middle>
<back>
<references title='Normative References'>
<?rfc include='http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml'?>
<?rfc include='http://xml.resource.org/public/rfc/bibxml/reference.RFC.2616.xml'?>
<?rfc include='http://xml.resource.org/public/rfc/bibxml/reference.RFC.2818.xml'?>
<?rfc include='http://xml.resource.org/public/rfc/bibxml/reference.RFC.3986.xml'?>
<?rfc include='http://xml.resource.org/public/rfc/bibxml/reference.RFC.5234.xml'?>
<?rfc include='http://xml.resource.org/public/rfc/bibxml/reference.RFC.5785.xml'?>
<?rfc include='http://xml.resource.org/public/rfc/bibxml/reference.RFC.5988.xml'?>
<reference anchor='OASIS.XRD-1.0' target='http://docs.oasis-open.org/xri/xrd/v1.0/xrd-1.0.html'>
<front>
<title>Extensible Resource Descriptor (XRD) Version 1.0</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>
<format type='HTML' target='http://docs.oasis-open.org/xri/xrd/v1.0/xrd-1.0.html' />
</reference>
</references>
</back>
</rfc>| PAFTECH AB 2003-2026 | 2026-04-24 13:38:19 |