Network Working Group E. Hammer-Lahav
Internet-Draft Yahoo!
Intended status: Informational March 3, 2010
Expires: September 4, 2010
LRDD: Link-based Resource Descriptor Discovery
draft-hammer-discovery-04
Abstract
LRDD (pronounced 'lard') provides a process for obtaining information
about a resource identified by a URI. The 'information about a
resource' - a resource descriptor - provides machine-readable
information that aims to increase interoperability and enhance
interactions with the resource. LRDD provides a narrow and well-
defined set of rules for obtaining and processing link-based
descriptors (found in multiple places such as HTTP headers, document
markup, and resource descriptors) which are often required for
security and consistent client behavior.
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
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Notational Conventions . . . . . . . . . . . . . . . . . . 6
2. Resource Descriptor . . . . . . . . . . . . . . . . . . . . . 6
3. Discovery Process . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Source Priority Order . . . . . . . . . . . . . . . . . . 7
3.2. Information Sources . . . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
5.1. The 'lrdd' Relation Type . . . . . . . . . . . . . . . . . 11
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 11
Appendix B. Document History . . . . . . . . . . . . . . . . . . 11
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.1. Normative References . . . . . . . . . . . . . . . . . . . 13
6.2. Informative References . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
LRDD defines a simple process for locating resource descriptors for
URI-identified resources. Resource descriptors are machine-readable
documents which provide information about resources (resource
metadata) for the purpose of promoting interoperability and assist in
interacting with unknown resources that support known interfaces.
For example, a web page about an upcoming meeting can provide in its
descriptor the location of the meeting organizer's free/busy
information to potentially negotiate a different time. A social
network profile page descriptor can identify the location of the
user's address book as well as accounts on other sites. A web
service implementing an API with optional components can advertise
which of these are supported.
Given the wide range of metadata needs, no single descriptor format
or retrieval method can adequately accommodate every use case. While
there are many methods for obtaining resource descriptors (e.g.,
links, HTTP headers, WebDAV's PROPFIND [RFC4918], HTTP OPTIONS,
URIQA's MGET [URIQA]), LRDD utilizes the Web Linking framework
defined in [I-D.nottingham-http-link-header]. LRDD defines a narrow
profile of Web Linking for obtaining and processing link-based
descriptors to accommodate the common discovery needs of many Web
protocols.
In LRDD, the resource descriptor is constructed by obtaining and
aggregating links and descriptor documents from multiple sources
(e.g. HTTP headers, document markup, site-meta). In most cases,
clients do not need to construct a complete resource descriptor, but
instead process the information in a specific order until the desired
information is found.
1.1. Example
An article published by a website allows readers to post comments and
provide the web address of their own blog. The article page includes
an avatar (a photo of the reader) when displaying comments, which is
obtained by performing discovery on the web address provided by the
reader (if supported by the reader's blog).
After receiving a comment from Jane which has her own blog at
"http://jane.example.com/blog", the website perform LRDD discovery to
try and obtain the Jane's photo.
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First, the website determines the source priority order for Jane's
blog by fetching its host-meta document from
"http://jane.example.com/.well-known/host-meta":
<?xml version='1.0' encoding='UTF-8'?>
<XRD xmlns='http://docs.oasis-open.org/ns/xri/xrd-1.0'
xmlns:hm='http://host-meta.net/ns/1.0'>
<hm:Host>jane.example.com</hm:Host>
<Property type='http://lrdd.net/priority/resource' />
<Link rel='lrdd' template='http://jane.example.com?lrdd={uri}'>
</XRD>
which indicates the blog is using Resource-priority (giving higher
priority to links provided by the resource itself over those defined
by the global policy). Since Jane's blog uses Resource-priority, the
website looks for "avatar" links in this order: <Link> elements, HTTP
Link headers, and then host-meta link templates.
To obtain a markup representation of Jane's blog, the website makes
an HTTP "GET" request to "http://jane.example.com/blog":
GET /blog HTTP/1.1
Host: jane.example.com
Accept: text/html
And receives back (HTML simplified for display purposes):
HTTP/1.1 200 OK
Content-Type: text/html; charset=UTF-8
Link: <http://jane.example.com/author>; rel="author"
<html>
<head>
<link href='http://jane.example.com/image' rel='avatar' />
</head>
<body>
<h1>Jane's Blog</h1>
</body>
</html>
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The HTML markup includes the desired link. The website can fetch
Jane's photo from "http://jane.example.com/image".
Now that it has Jane's photo, the website is looking for a short
description of Jane to include with her comment. It performs another
LRDD discovery, this time looking for an "about" link. Repeating the
same process, the website looks for qualified <Link> elements in
Jane's blog HTML markup and finds none. It then looks for a LRDD
document - a descriptor document containing additional information
about the resource - which uses the "lrdd" link relation. It finds
none in the HTML representation.
In Resource-priority, the next source is HTTP header. The HTTP
header (shown above with the HTML response) includes a qualified link
to Jane's author page.
Before it can display Jane's photo and description, the website needs
to find the copyright license used by Jane's blog. It performs
another LRDD discovery looking for a link with a "copyright" relation
type. It fails to find such link in the HTML markup as well as a
link to a LRDD document. It then tries and fails to find a
"copyright" link in the HTTP header or a LRDD document linked from
the header.
The website then proceeds to the host-meta source, looking for link
templates. It fails to find a link to a copyright statement, but it
does find a link to a LRDD document. It obtains the LRDD document
for Jane's blog by applying the blog's URI to the template:
http://jane.example.com?lrdd=http%3A%2F%2Fjane.example.com%2Fblog
and obtains the LRDD document for Jane's blog:
<?xml version='1.0' encoding='UTF-8'?>
<XRD xmlns='http://docs.oasis-open.org/ns/xri/xrd-1.0'>
<Subject>http://jane.example.com/blog</Subject>
<Link rel='copyright' href='http://jane.example.com/copyright'>
</XRD>
The LRDD document provided by the host-meta link template includes a
link to the copyright statement. The website now has all the
information it needs to display Jane's comment along with her photo
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and description on the article page.
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].
2. Resource Descriptor
The LRDD resource descriptor is the aggregation of links and
descriptor documents obtained from four sources (when applicable and
available):
o host-meta - links generated by applying the resource URI to the
link templates provided by the host's host-meta document as
defined in [I-D.hammer-hostmeta].
o HTTP Link headers - links included in the HTTP response header to
an HTTP [RFC2616] "HEAD" or "GET" request for the resource.
o <Link> elements - links included in the resource representation
markup.
o LRDD documents - links and other metadata contained in XRD
[OASIS.XRD-1.0] documents linked to from any of the previous three
link sources using the "lrdd" relation type.
LRDD clients usually look for a link with a specific relation type or
other well-typed metadata. In such cases, the client does not need
to construct the entire resource descriptor, but instead, process the
various sources in their prescribed order until the desired
information is found.
3. Discovery Process
The LRDD process begins with the URI of the resource being discovered
and the type of information being sought:
o Link of a given relation type - the client is looking for a link
with a specific relation type and other link attributes such as
media type or relation-specific attributes.
o Metadata contained within a LRDD-linked XRD document - the client
is seeking information other than a link to another resource, such
as resource properties or other structured metadata.
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The client MUST first determine the host source priority order as
described in Section 3.1. Once the order is determined, the client
process each of the sources listed in Section 3.2 as follows:
1. If the information being sought is a link: find a link matching
the desired criteria by processing the links in order. If a
qualified link is found, the process ends successfully.
2. Find the first link with the "lrdd" relation type. The link's
media type attribute MUST be set to "application/xrd+xml" if
present. The link's target is the location of the LRDD document.
Any "lrdd" links other than the first MUST be ignored. If no
qualified link is found, continue with the next source.
3. Obtain the LRDD document by following the scheme-specific rules
for the LRDD document's URI. If the document's URI scheme is
"http" or "https", the document is obtained via an HTTP "GET"
request to the identified URI. The client MUST obey HTTP
redirections (3xx). The document is considered valid only if
retrieved with a successful HTTP response status (2xx) and is a
valid XRD document per [OASIS.XRD-1.0]. If no valid document is
found, continue with the next source.
4. If the information being sought is a link: find a link matching
the desired criteria by processing the LRDD document as defined
by [OASIS.XRD-1.0] section 4. If a qualified link is found, the
process ends successfully.
5. If the information being sought is something other than a link:
parse the LRDD document as defined by [OASIS.XRD-1.0], looking
for the desired metadata. If the desired information is found,
the process ends successfully.
6. If the information being sought is not found, continue with the
next source. If all the sources have been exhausted, the process
ends unsuccessfully.
3.1. Source Priority Order
LRDD descriptors include information contained or linked to from
three sources: host-meta link templates, HTTP Link headers, and
<Link> elements. To ensure consistent client behavior and to enable
hosts to set their own security policy with regard to metadata
authority, LRDD provides two processing profiles:
o Host-priority - Priority is given to links set by the host policy
(via host-meta and HTTP Link headers) over those set by each
individual resources (via <Link> elements). Clients MUST process
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the three sources in the following order: host-meta link
templates, HTTP Link headers, and <Link> elements.
o Resource-priority - Priority is given to the individual resource
over the policies of the host. Clients MUST process the three
sources in the following order: <Link> elements, HTTP Link
headers, and host-meta link templates.
Host-priority is the default source priority order. Hosts that wish
to use Resource-priority MUST declare it by setting the LRDD priority
property in their host-meta document:
"http://lrdd.net/priority/resource". The priority property does not
have a value.
For example:
<?xml version='1.0' encoding='UTF-8'?>
<XRD xmlns='http://docs.oasis-open.org/ns/xri/xrd-1.0'
xmlns:hm='http://host-meta.net/ns/1.0'>
<hm:Host>example.com</hm:Host>
<Property type='http://lrdd.net/priority/resource' />
</XRD>
3.2. Information Sources
Each of the information sources supported by LRDD presents a
different set of requirements and benefits. The criteria used to
determine which sources a server MAY support are based on a
combination of factors:
o The ability to offer and obtain a representation of the resource
by dereferencing its URI.
o The availability of a representation supporting <Link> markup
compatible with [I-D.nottingham-http-link-header].
o The availability of an HTTP representation of the resource and the
ability to provide and access link information in its response
header.
3.2.1. host-meta Link Templates
The host-meta document source is available for any resource
identified by a URI with an authority that supports the host-meta
document as defined in [I-D.hammer-hostmeta]. This method does not
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require obtaining any representation of the resource, and operates
solely using the resource URI.
Links between the resource URI and other resources are expressed by
link templates as defined by [I-D.hammer-hostmeta] section 3.2. By
applying the host-wide templates to an individual resource URI, a
resource-specific link is constructed which can be used to express
links without the need to access or provide a representation for the
resource.
Clients MUST process the host-meta document, looking for link
templates and applying the resource URI to produce a list of links to
be used in the discovery process. Clients MUST retain the order of
the links as present in the host-meta document. Any links contained
in the host-meta document not using the "template" attribute MUST be
ignored and excluded from LRDD processing, but are still valid for
other purposes.
For example, the resource URI "http://example.com/x" and the
following host-meta link template:
<Link rel='author' template="http://example.com?author={uri}' />
generates an "author" link between "http://example.com/x" and
"http://example.com?author=http%3A%2F%2Fexample.com%2Fx".
3.2.2. HTTP Link Headers
The HTTP Link header source is limited to resources for which an HTTP
"GET" or "HEAD" request returns a 2xx, 3xx, or 4xx HTTP response per
[RFC2616]. This method uses the Link header defined in
[I-D.nottingham-http-link-header] and requires the retrieval of a
resource representation header.
For example:
Link: <http://example.com?author=http%3A%2F%2Fexample.com%2Fx>;
rel="author"
Clients obtain HTTP Link header links by making an HTTP (or HTTPS as
required) "GET" or "HEAD" request to the resource URI to obtain a
valid response header. If the HTTP response carries a status code
other than successful (2xx), redirection (3xx), or client error
(4xx), the method fails.
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Link headers can include multiple relation types in a single "rel"
attribute (for example "rel="license copyright""). Clients MUST
properly process such multiple relation "rel" attributes as defined
by [I-D.nottingham-http-link-header].
3.2.3. <Link> Elements
The <Link> element source is limited to resources with an available
markup representation that supports typed-relations using the <Link>
element, such as HTML [W3C.REC-html401-19991224], XHTML
[W3C.REC-xhtml1-20020801], and Atom [RFC4287]. Other markup formats
are permitted as long as the semantics of their <Link> elements are
fully compatible with the link framework defined in
[I-D.nottingham-http-link-header]. This source requires the
retrieval of a resource representation. While HTTP is the most
common transport for such documents, this source is transport
independent.
For example:
<LINK href='http://example.com?author=http%3A%2F%2Fexample.com%2Fx'
rel='author'>
Clients obtain <Link> element links by retrieving a representation of
the resource using the applicable transport for that resource URI.
If the markup document is obtained using HTTP, it MUST only be used
by the client if the document is a valid representation of the
resource identified by the HTTP request URI, typically received with
a successful (2xx) response code. If no such valid representation of
the request URI is found, the source MUST NOT be used.
The client MUST obey the document markup schema and ignore any
invalid elements (such as <Link> elements outside the <Head> section
of an HTML document). This is done to avoid unintentional markup
from other parts of the document to be used for discovery purposes,
which can have vast impact on usability and security.
Some <Link> elements allow multiple relation types in a single "rel"
attribute (for example "rel='license copyright'"). Clients MUST
properly process such multiple relation "rel" attributes as defined
by the format specification.
4. Security Considerations
The methods used to perform discovery are not secure, private or
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integrity-guaranteed, and due caution should be exercised when using
them. Applications that perform LRDD SHOULD consider the attack
vectors opened by automatically following, trusting, or otherwise
using links gathered from <Link> elements, HTTP Link headers, or
host-meta documents.
5. IANA Considerations
5.1. 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: Identifies a resource descriptor for the link's context
used by the LRDD protocol.
Reference: [[ This specification ]]
Appendix A. Acknowledgments
Inspiration for this memo derived from previous work on a descriptor
format called XRDS-Simple, which in turn derived from another
descriptor format, XRDS. Previous discovery workflows include Yadis
which is currently used by the OpenID community. While suffering
from significant shortcomings, Yadis was a breakthrough approach to
performing discovery using extremely restricted hosting environments,
and this memo has strived to preserve as much of that spirit as
possible.
The author wishes to thanks the OASIS XRI TC and WebFinger
communities for their support, encouragement, and enthusiasm for this
work. Special thanks go to Phil Archer, Lisa Dusseault, Joseph
Holsten, Mark Nottingham, John Panzer, Drummond Reed, and Jonathan
Rees for their invaluable feedback.
Appendix B. Document History
[[ to be removed by the RFC editor before publication as an RFC ]]
-04
o Changed focus to a narrow and well-defined discovery process.
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o Removed analysis appendix and discussion of discovery types and
removed informative references.
o Expanded the descriptor definition to include links as well as
LRDD documents, moving away from the single-document approach.
o Moved the Link-Pattern field and template syntax to new host-meta
draft.
o Updated references.
o Added example.
-03
o Added protocol name LRDD (pronounced 'lard').
o Fixed Link-Pattern examples to include missing semicolons.
-02
o Changed focus from an HTTP-based process to Link-based process.
o Completely revised and restructured document for better clarity.
o Realigned the methods to produce consistent results and changed
the way redirections and client-errors are handled.
o Updated to use newer version of site-meta, now called host-meta,
including a new plaintext-based format to replace the previous XML
format.
o Renamed Link-Template to Link-Pattern to avoid future conflict
with a previously proposed Link-Template HTTP header.
o Removed support for the "scheme" Link-Template parameter.
o Replaced restrictions with interoperability recommendations.
o Added IANA considerations per new host-meta registry requirements.
-01
o Rename 'resource discovery' to 'descriptor discovery'.
o Added informative reference to Metalink.
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o Clarified that the resource descriptor URI can use any URI scheme,
not just "http" or "https".
o Removed comment regarding redirects when using <Link> Elements.
o Clarified that HTTPS must be used with "https" URIs for both Link
headers and host-meta retrieval.
o Removed DNS verification step for host-meta with schemes other
then "http" and "https". Replaced with a general discussion of
authority and a security consideration comment.
o Organized host-meta section into another sub-section level.
o Enlarged the template vocabulary from a single "uri" variable to
include smaller URI components.
o Added informative reference to RFC 2295 in analysis appendix.
-00
o Initial draft.
6. References
6.1. Normative References
[I-D.hammer-hostmeta]
Hammer-Lahav, E., "host-meta: Web Host Metadata",
draft-hammer-hostmeta-05 (work in progress),
November 2009.
[I-D.nottingham-http-link-header]
Nottingham, M., "Web Linking",
draft-nottingham-http-link-header-08 (work in progress),
March 2010.
[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.
[RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
Syndication Format", RFC 4287, December 2005.
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[W3C.REC-html401-19991224]
Hors, A., Jacobs, I., and D. Raggett, "HTML 4.01
Specification", World Wide Web Consortium
Recommendation REC-html401-19991224, December 1999,
<http://www.w3.org/TR/1999/REC-html401-19991224>.
[W3C.REC-xhtml1-20020801]
Pemberton, S., "XHTML[TM] 1.0 The Extensible HyperText
Markup Language (Second Edition)", World Wide Web
Consortium Recommendation REC-xhtml1-20020801,
August 2002,
<http://www.w3.org/TR/2002/REC-xhtml1-20020801>.
6.2. Informative References
[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/36542/
xrd-1.0-wd15.html>.
[RFC4918] Dusseault, L., "HTTP Extensions for Web Distributed
Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
[URIQA] Nokia, "The URI Query Agent Model",
<http://sw.nokia.com/uriqa/URIQA.html>.
URIs
[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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