Network Working Group E. Hammer-Lahav
Internet-Draft Yahoo!
Intended status: Informational May 25, 2010
Expires: November 26, 2010
Web Host Metadata
draft-hammer-hostmeta-10
Abstract
This memo 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
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and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on November 26, 2010.
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
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include Simplified BSD License text as described in Section 4.e of
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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 Format . . . . . . . . . . . . . . . . 7
3.1. The 'Link' Element . . . . . . . . . . . . . . . . . . . . 8
3.1.1. Template Syntax . . . . . . . . . . . . . . . . . . . 8
4. Processing host-meta Documents . . . . . . . . . . . . . . . . 9
4.1. Host-Wide Information . . . . . . . . . . . . . . . . . . 10
4.2. Resource-Specific Information . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
6.1. The 'host-meta' Well-Known URI . . . . . . . . . . . . . . 11
6.2. The 'lrdd' Relation Type . . . . . . . . . . . . . . . . . 12
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 12
Appendix B. Document History . . . . . . . . . . . . . . . . . . 12
7. Normative References . . . . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15
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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 memo 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 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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<?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>
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:
<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' is obtained using 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>
<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 XRD document for illustration purposes) is:
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<?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", "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 making
an HTTPS [RFC2818] GET request to the host's port 443 for the
"/.well-known/host-meta" path. If the request fails to produce a
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valid host-meta document, the client makes an HTTP [RFC2616] GET
request to the host's port 80 for the "/.well-known/host-meta" path.
The server MUST support at least one but SHOULD support both ports.
If both ports are supported, they MUST serve the same document. The
client MAY attempt to obtain the host-meta document from either port,
SHOULD attempt using port 443 first, and SHOULD attempt the other
port if the first fails.
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 a representation is successfully obtained, but is not in the
format described above, the client MUST infer that the path is being
used for other purposes, and not process the response 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.
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)
at both ports, the client should infer that metadata is not available
via this mechanism.
3. The host-meta Document Format
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 memo defines additional
processing rules needed to describe hosts. Documents MAY include any
XRD element not explicitly excluded.
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
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[I-D.nottingham-http-link-header]) 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.
3.1. 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.
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. Template Syntax
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.
Before substituting template variables, any value character other
than unreserved (as defined by [RFC3986]) MUST be percent-encoded per
[RFC3986].
This memo defines a single variable - "uri" - as the entire context
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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
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.
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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 "lrdd":
1. If the link's media type is "application/xrd+xml", or if the
link does not specify a media type:
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. The client MUST
only process the document if it was received with an HTTP
200 (OK) status code and is a valid XRD document per
[OASIS.XRD-1.0].
2. The client adds any link found in the LRDD document to
the resource descriptor in order, except for any link
using the "lrdd" relation type. When adding links, the
client SHOULD retain any extension attributes and child
elements if present (e.g. <Property> or <Title>
elements).
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).
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2. If the link media type is other than "application/xrd+xml",
the link MUST be ignored.
3. If the link's relation type is other than "lrdd", the client adds
the link to the resource descriptor in order.
A detailed example is provided in Section 1.1.1.
5. Security Considerations
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.
Protocols using host-meta templates SHOULD 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.
Protocols MAY restrict document retrieval to HTTPS based on their
security needs. Protocols utilizing host-meta documents obtained via
other methods not described in this memo SHOULD consider the security
and authority risks associated with such methods.
6. IANA Considerations
6.1. The 'host-meta' Well-Known URI
This memo 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: None
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6.2. The 'lrdd' Relation Type
This specification registers the "lrdd" relation type in the Link
Relation Type Registry defined by [I-D.nottingham-http-link-header]:
Relation Name: lrdd
Description: Used by the host-meta document processor to locate
resource-specific information about individual resources. When
used elsewhere (e.g. HTTP "Link" header fields or HTML <LINK>
elements), it operates as an include directive, identifying the
location of additional links and other metadata. If present, the
link's media type attribute MUST be set to "application/xrd+xml",
and an "application/xrd+xml" representation MUST be available.
However, additional representations using other media types MAY be
made available.
Reference: [[ This specification ]]
Appendix A. Acknowledgments
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.
Appendix B. Document History
[[ to be removed by the RFC editor before publication as an RFC ]]
-10
o Integrated LRDD into the memo, dropping the multiple sources and
using only host-meta for LRDD processing.
-09
o Removed the <hm:Host> element due to lack of use cases (protocols
with signature requirements can define their own way of declaring
the document's subject for this purpose).
o Minor editorial changes.
o Changed following redirections to MUST.
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o Updated references.
-08
o Fixed typo.
-07
o Minor editorial clarifications.
o Added XML schema for host-meta extension.
o Updated XRD reference to the latest draft (no normative changes).
-06
o Updated well-known reference to RFC 5785.
o Minor editorial changes.
o Made HTTPS a higher priority (SHOULD) over HTTP.
-05
o Adjusted syntax to the latest XRD schema.
o Added note about using a link template without variables.
-04
o Corrected the <hm:Host> example.
-03
o Changed scope to an entire host (per RFC 3986).
o Simplified template syntax to always percent-encode values and
vocabulary to a single 'uri' variable.
o Changed document retrieval to always use HTTP(S).
o Added security consideration about the use of templates.
o Explicitly defined the root element to be 'XRD'.
-02
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o Changed Scope element syntax from attributes to URI-like string
value.
-01
o Editorial rewrite.
o Redefined scope as a scheme-authority pair.
o Added document structure section.
-00
o Initial draft.
7. Normative References
[]
Nottingham, M., "Web Linking",
draft-nottingham-http-link-header-10 (work in progress),
May 2010.
[OASIS.XRD-1.0]
Hammer-Lahav, E. and W. Norris, "Extensible Resource
Descriptor (XRD) Version 1.0 (work in progress)", <http://
www.oasis-open.org/committees/download.php/37692/
xrd-1.0-wd16.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.
[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.
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[1] <https://www.ietf.org/mailman/listinfo/apps-discuss>
Author's Address
Eran Hammer-Lahav
Yahoo!
Email: eran@hueniverse.com
URI: http://hueniverse.com
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