Network Working Group E. Hammer-Lahav
Internet-Draft Yahoo!
Intended status: Standards Track B. Cook
Expires: November 10, 2011 May 9, 2011
Web Host Metadata
draft-hammer-hostmeta-15
Abstract
This specification describes a method for locating host metadata as
well as information about individual resources controlled by the
host.
Editorial Note (to be removed by RFC Editor)
Please discuss this draft on the apps-discuss@ietf.org [1] mailing
list.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 10, 2011.
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
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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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.1. Processing Resource-Specific Information . . . . . . . 5
1.2. Notational Conventions . . . . . . . . . . . . . . . . . . 6
2. Obtaining host-meta Documents . . . . . . . . . . . . . . . . 6
3. The host-meta Document . . . . . . . . . . . . . . . . . . . . 7
3.1. XML Document format . . . . . . . . . . . . . . . . . . . 8
3.1.1. The 'Link' Element . . . . . . . . . . . . . . . . . . 8
4. Processing host-meta Documents . . . . . . . . . . . . . . . . 10
4.1. Host-Wide Information . . . . . . . . . . . . . . . . . . 10
4.2. Resource-Specific Information . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
6.1. The 'host-meta' Well-Known URI . . . . . . . . . . . . . . 12
6.2. The 'lrdd' Relation Type . . . . . . . . . . . . . . . . . 12
Appendix A. JRD Document Format . . . . . . . . . . . . . . . . . 13
Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . . 16
7. Normative References . . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
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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 (URI) with a common URI host [RFC3986].
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 specification registers the well-known URI suffix "host-meta" in
the Well-Known URI Registry established by [RFC5785], and specifies a
simple, general-purpose metadata document format for hosts, to be
used by multiple web-based protocols.
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:
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1.0
The host-wide information which applies to host in its entirety
provided by the document includes:
o A "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").
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1.1.1. Processing Resource-Specific Information
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:
The LRDD document for 'http://example.com/xy' is obtained using an
HTTP "GET" request:
http://example.com/xyred
Together, the information available about the individual resource
(presented as an XRD document for illustration purposes) is:
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http://example.com/xyred
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", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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
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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 which protocol is used to obtain the host-meta document
have 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 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.
The host-meta document SHOULD be served with the
"application/xrd+xml" media type. [[ media type registration pending
]]
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 XRD element not explicitly excluded.
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 a
"application/xrd+xml" representation, the server MUST respond using
the REQUIRED XRD 1.0 XML representation described in Section 3.1.
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The XRD 1.0 XML representation is the only canonical representation
for any XRD document. If there is any discrepancy between the
content of the XRD 1.0 XML representation and any other
representation for the same resource, the client MUST only use the
XRD 1.0 XML representation.
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) XRD 1.0 representation is
described in Appendix A.
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 resource" 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:
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:
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:
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
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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.
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.
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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.
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. or 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 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.
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
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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.
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): [[ this document ]]
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 'lrdd' Relation Type
This specification registers the "lrdd" relation type in the Link
Relation Type Registry defined by [RFC5988]:
Relation Name: lrdd
Description: "lrdd" (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 "Link"
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 "application/xrd+xml" representation
MUST be available, and this media type MAY appear in a link's
"type" attribute. Additional representations MAY be available
(using the HTTP "Accept" request header field), in which case the
link's "type" attribute SHOULD be omitted.
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Reference: [[ This specification ]]
Appendix A. JRD Document Format
The JRD document format - a general purpose XRD 1.0 represenation -
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 which is beyond the scope of this specification.
The client MAY requst a JRD representation using the HTTP "Accept"
request header field with value of "application/json". The server
MUST include the HTTP "Content-Type" response header field with value
of "application/json". Any other "Content-Type" value (or lack of)
indicates that the server does not support the JRD format.
XRD elements are serialized into a JSON structure as follows:
o The XML document declaration and "XRD" element are discarded.
o The "Subject" element is included as name/value pair with the name
'subject', and value included as a string.
o The "Expires" element is included as 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 'alias', and value a string array containing the values of
each element in order.
o "Property" elements are included as a single object 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.
o "Link" elements are included as a single name/value pair with the
name 'links' and with each element included as an object. Each
attribute is included as 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
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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 a "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 AuthorAuthor InformationeditorThe other guyThe other author
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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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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.
7. Normative References
[OASIS.XRD-1.0]
Hammer-Lahav, E. and W. Norris, "Extensible Resource
Descriptor (XRD) Version 1.0",
.
[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. 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.
[1]
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Authors' Addresses
Eran Hammer-Lahav
Yahoo!
Email: eran@hueniverse.com
URI: http://hueniverse.com
Blaine Cook
Email: romeda@gmail.com
URI: http://romeda.org
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