Internet Engineering Task Force (IETF) E. Hammer-Lahav, Ed.

Internet Engineering Task Force (IETF) E. Hammer-Lahav, Ed.
Request for Comments: 6415 B. Cook
Category: Standards Track October 2011
ISSN: 2070-1721

Web Host Metadata

Abstract

This specification describes a method for locating host metadata as
well as information about individual resources controlled by the
host.

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/rfc6415.

Copyright Notice

Copyright (c) 2011 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.

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Table of Contents

1. Introduction ....................................................2
1.1. Example ....................................................3
1.1.1. Processing Resource-Specific Information ............4
1.2. Notational Conventions .....................................5
2. Obtaining host-meta Documents ...................................6
3. The host-meta Document ..........................................6
3.1. XML Document Format ........................................7
3.1.1. The "Link" Element ..................................7
4. Processing host-meta Documents ..................................8
4.1. Host-Wide Information ......................................9
4.2. Resource-Specific Information ..............................9
5. Security Considerations ........................................10
6. IANA Considerations ............................................11
6.1. The "host-meta" Well-Known URI ............................11
6.2. The "host-meta.json" Well-Known URI .......................11
6.3. The "lrdd" Relation Type ..................................11
Appendix A. JRD Document Format ...................................12
Appendix B. Acknowledgments .......................................15
Normative References ..............................................15

1. Introduction

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 (URIs) with a common URI host [RFC3986], which
can be served by one or more servers.

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.

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.

This document defines a lightweight metadata document format for
describing hosts (thus the name "host-meta"), intended for use by
web-based protocols. This document also registers the well-known URI
suffix "host-meta" in the Well-Known URI Registry established by
[RFC5785].

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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:

o 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.

o 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 "lrdd" relation type.

1.1. Example

The following is a simple host-meta document including both host-wide
and resource-specific information for the 'example.com' host:

<?xml version='1.0' encoding='UTF-8'?>
<XRD xmlns='http://docs.oasis-open.org/ns/xri/xrd-1.0'>



<Property type='http://protocol.example.net/version'>1.0</Property>

<Link rel='copyright'
href='http://example.com/copyright' />



<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>

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The host-wide information that applies to the host in its entirety
provided by the document includes:

o An "http://protocol.example.net/version" host property with a
value of "1.0".

o A link to the host's copyright policy ("copyright").

The resource-specific information provided by the document includes:

o A link template for receiving real-time updates ("hub") about
individual resources. Since the template does not include a
template variable, the target URI is identical for all resources.

o A LRDD document link template ("lrdd") for obtaining additional
resource-specific information contained in a separate document for
each individual resource.

o A link template for finding information about the author of
individual resources ("author").

1.1.1. Processing Resource-Specific Information

When looking for information about 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:

<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' />

The LRDD document for 'http://example.com/xy' (obtained via an HTTP
"GET" request):

<?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>

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<Link rel='hub'
href='http://example.com/another/hub' />

<Link rel='author'
href='http://example.com/john' />
</XRD>

Together, the information available about the individual resource
(presented as an Extensible Resource Descriptor (XRD) document for
illustration purposes) is:

<?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>

Note that the order of links matters and is based on their original
order in the host-meta and LRDD documents. For example, the "hub"
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 "lrdd" link.

On the other hand, the "author" link found in the LRDD document has a
higher priority than the link found in the host-meta document because
it appears after the "lrdd" link.

1.2. Notational Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].

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This document uses the Augmented Backus-Naur Form (ABNF) notation of
[RFC5234]. Additionally, the following rules are included from
[RFC3986]: reserved, unreserved, and pct-encoded.

2. Obtaining host-meta Documents

The client obtains the host-meta document for a given host by sending
an HTTP [RFC2616] or an HTTPS [RFC2818] GET request to the host for
the "/.well-known/host-meta" path, using 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.

The server MUST support at least one protocol but MAY support both.
If both protocols are supported, they MUST produce the same document.

The decision as to which protocol is used to obtain the host-meta
document has significant security ramifications, as described in
Section 5.

For example, the following request is used to obtain the host-meta
document for the 'example.com' host:

GET /.well-known/host-meta HTTP/1.1
Host: example.com

If the server response indicates that the host-meta resource is
located elsewhere (a 301, 302, or 307 response status code), the
client SHOULD 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
code), the client SHOULD infer that metadata is not available via
this mechanism.

The host-meta document SHOULD be served with the
"application/xrd+xml" media type.

3. The host-meta Document

The host-meta document uses the XRD 1.0 document format as defined by
[OASIS.XRD-1.0], which provides a simple and extensible XML-based
schema for describing resources. This specification defines
additional processing rules needed to describe hosts. Documents MAY
include any elements included in the XRD 1.0 schema that are not
explicitly excluded by this specification.

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The server MAY offer alternative representations of any XRD document
it serves (host-meta, LRDD, or other XRD-based documents). The
client MAY request a particular representation using the HTTP
"Accept" request header field. If no "Accept" request header field
is included with the request, or if the client requests an
"application/xrd+xml" representation, the server MUST respond using
the REQUIRED XRD 1.0 XML representation described in Section 3.1.

Applications using the host-meta document MAY require the server to
provide a specific alternative representation in addition to the
XRD 1.0 XML representation when explicitly requested by the client.

A JavaScript Object Notation (JSON) Resource Descriptor, known as
JRD, is described in Appendix A. It is RECOMMENDED that servers
offer the JRD representation in addition to the XRD representation.

3.1. XML Document Format

The host-meta document root MUST be an "XRD" element. The document
SHOULD NOT include a "Subject" element, as at this time no URI is
available to identify hosts. The use of the "Alias" element in
host-meta is undefined and NOT RECOMMENDED.

The subject (or "context IRI", as defined by [RFC5988]) of the XRD
"Property" and "Link" elements is the host described by the host-meta
document. However, the subject of "Link" elements with a "template"
attribute is the individual resource whose URI is applied to the link
template, as described in Section 3.1.1.

3.1.1. The "Link" Element

The XRD "Link" element, when used with the "href" attribute, conveys
a link relation between the host described by the document and a
common target URI.

For example, the following link declares a common copyright license
for the entire scope:

<Link rel='copyright' href='http://example.com/copyright' />

However, a "Link" element with a "template" 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.

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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:

<Link rel='author'
template='http://example.com/author?article={uri}' />

3.1.1.1. Template Syntax

This specification 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.

Before substituting template variables, values MUST be encoded using
UTF-8, and any character other than unreserved (as defined by
[RFC3986]) MUST be percent-encoded per [RFC3986].

This specification defines a single variable -- "uri" -- 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 "uri" variable. If a client is unable to successfully process
a template (e.g., unknown variable names, unknown or incompatible
syntax), the parent "Link" element SHOULD be ignored.

The template syntax ABNF follows:

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 / "." / "_"

For example:

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

4. Processing host-meta Documents

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.

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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.

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.

4.1. Host-Wide Information

When looking for host-wide information, the client MUST ignore any
"Link" elements with a "template" attribute, as well as any link
using the "lrdd" relation type. All other elements are scoped as
host-wide.

4.2. Resource-Specific Information

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.

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:

1. The client applies the URI of the desired resource to the
template, producing a resource-specific link.

2. If the link's relation type is other than "lrdd", the client adds
the link to the resource descriptor in order.

3. If the link's relation type is "lrdd":

3.1. The client obtains the LRDD document by following the
scheme-specific rules for the LRDD document URI. If the
document URI scheme is "http" or "https", the document is
obtained via an HTTP "GET" 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.

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3.2. The client adds any links found in the LRDD document to the
resource descriptor in order, except for any link using the
"lrdd" 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).

3.3. 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).

5. Security Considerations

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.

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.

Applications utilizing the host-meta document where the authenticity
of the information is necessary 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.

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.

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 that produces unexpected results
when its variable values contain unexpected characters.

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6. IANA Considerations

6.1. The "host-meta" Well-Known URI

This specification registers the "host-meta" well-known URI in the
Well-Known URI Registry as defined by [RFC5785].

URI suffix: host-meta

Change controller: IETF

Specification document(s): RFC 6415

Related information: The "host-meta" documents obtained from the
same host using the HTTP and HTTPS protocols (using default ports)
MUST be identical.

6.2. The "host-meta.json" Well-Known URI

This specification registers the "host-meta.json" well-known URI in
the Well-Known URI Registry as defined by [RFC5785].

URI suffix: host-meta.json

Change controller: IETF

Specification document(s): RFC 6415

Related information: The "host-meta.json" documents obtained from
the same host using the HTTP and HTTPS protocols (using default
ports) MUST be identical.

6.3. The "lrdd" Relation Type

This specification registers the "lrdd" relation type in the Link
Relation Type Registry defined by [RFC5988]:

Relation Name: lrdd

Description: Refers to further information about the link's context,
expressed as a LRDD ("Link-based Resource Descriptor Document")
resource. See RFC 6415 for information about processing this
relation type in host-meta documents. When used elsewhere, it
refers to additional links and other metadata. Multiple instances
indicate additional LRDD resources. LRDD resources MUST have an
"application/xrd+xml" representation, and MAY have others.

Reference: RFC 6415

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Appendix A. JRD Document Format

The JRD document format -- a general-purpose XRD 1.0 representation
-- uses the JavaScript Object Notation (JSON) format defined in
[RFC4627]. JRD uses the same elements and processing rules described
in Section 3.1. The JRD format is designed to include the same base
functionality provided by the XML format, with the exception of
extensibility, as extensibility is beyond the scope of this
specification.

The client MAY request a JRD representation using the HTTP "Accept"
request header field with a value of "application/json". The server
MUST include the HTTP "Content-Type" response header field with a
value of "application/json". Any other "Content-Type" value (or lack
thereof) indicates that the server does not support the JRD format.

Alternatively, the client MAY request a JRD representation by
requesting the "host-meta.json" well-known document, by making a GET
request for "/.well-known/host-meta.json", following the same process
used for "/.well-known/host-meta". If the server does not support
serving a JRD representation at this location, the server MUST
respond with an HTTP 404 (Not Found) status code.

XRD elements are serialized into a JSON object as follows:

o The XML document declaration and "XRD" element are discarded.

o The "Subject" element is included as a name/value pair with the
name 'subject', and value included as a string.

o The "Expires" element is included as a name/value pair with the
name 'expires', and value included as a string.

o "Alias" elements are included as a single name/value pair with the
name 'aliases', and value a string array containing the values of
each element in order.

o "Property" elements are included as a single name/value pair with
the name 'properties', and value an object with each element
included as a name/value pair with the value of the "type"
attribute as name, and element value included as a string value.
The values of properties with empty values (i.e., using the
REQUIRED "xsi:nil='true'" attribute) are included as null. If
more than one "Property" element is present with the same "type"
attribute, only the last instance is included.

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o "Link" elements are included as a single name/value pair with the
name 'links', and value an array with each element included as an
object. Each attribute is included as a name/value pair with the
attribute name as name, and value included as a string.

o "Link" child "Property" elements are included using the same
method as XRD-level "Property" elements using a name/value pair
inside the link object.

o "Link" child "Title" elements are included as a single object with
the name 'titles', and value an object with each element included
as a name/value pair with the value of the "xml:lang" attribute as
name, and element value included as a string value. The names of
elements without an "xml:lang" attribute are added with the name
'default'. If more than one "Title" element is present with the
same (or no) "xml:lang" attribute, only the last instance is
included.

o The conversion of any other element is left undefined.

For example, the following XRD document...

<?xml version='1.0' encoding='UTF-8'?>
<XRD xmlns='http://docs.oasis-open.org/ns/xri/xrd-1.0'
xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'>

<Subject>http://blog.example.com/article/id/314</Subject>
<Expires>2010-01-30T09:30:00Z</Expires>

<Alias>http://blog.example.com/cool_new_thing</Alias>
<Alias>http://blog.example.com/steve/article/7</Alias>

<Property type='http://blgx.example.net/ns/version'>1.2</Property>
<Property type='http://blgx.example.net/ns/version'>1.3</Property>
<Property type='http://blgx.example.net/ns/ext' xsi:nil='true' />

<Link rel='author' type='text/html'
href='http://blog.example.com/author/steve'>
<Title>About the Author</Title>
<Title xml:lang='en-us'>Author Information</Title>
<Property type='http://example.com/role'>editor</Property>
</Link>

<Link rel='author' href='http://example.com/author/john'>
<Title>The other guy</Title>
<Title>The other author</Title>
</Link>

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<Link rel='copyright'
template='http://example.com/copyright?id={uri}' />
</XRD>

...is represented by the following JRD document:

{
"subject":"http://blog.example.com/article/id/314",
"expires":"2010-01-30T09:30:00Z",

"aliases":[
"http://blog.example.com/cool_new_thing",
"http://blog.example.com/steve/article/7"],

"properties":{
"http://blgx.example.net/ns/version":"1.3",
"http://blgx.example.net/ns/ext":null
},

"links":[
{
"rel":"author",
"type":"text/html",
"href":"http://blog.example.com/author/steve",
"titles":{
"default":"About the Author",
"en-us":"Author Information"
},
"properties":{
"http://example.com/role":"editor"
}
},
{
"rel":"author",
"href":"http://example.com/author/john",
"titles":{
"default":"The other author"
}
},
{
"rel":"copyright",
"template":"http://example.com/copyright?id={uri}"
}
]
}

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Note that the "Subject" and "Alias" elements are NOT RECOMMENDED in
the context of host-meta documents, and are included in the example
for completeness only.

Appendix B. Acknowledgments

The authors would like to acknowledge the contributions of everyone
who provided feedback and use cases for this specification -- in
particular, Dirk Balfanz, DeWitt Clinton, Eve Maler, Breno de
Medeiros, Brad Fitzpatrick, James Manger, Will Norris, Mark
Nottingham, John Panzer, Drummond Reed, and Peter Saint-Andre.

Normative References

[OASIS.XRD-1.0]
Hammer-Lahav, E., Ed., and W. Norris, Ed., "Extensible
Resource Descriptor (XRD) Version 1.0", November 2010,
<http://docs.oasis-open.org/xri/xrd/v1.0/xrd-1.0.html>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.

[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, July 2006.

[RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234,
January 2008.

[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785,
April 2010.

[RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010.

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Authors' Addresses

Eran Hammer-Lahav (editor)

EMail: [email protected]
URI: http://hueniverse.com

Blaine Cook

EMail: [email protected]
URI: http://romeda.org

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