Network Working Group E. Hammer-Lahav
Internet-Draft Yahoo!
Intended status: Informational October 23, 2009
Expires: April 26, 2010
Host-Meta: Web Host Metadata
draft-hammer-hostmeta-01
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Abstract
This memo describes a method for locating host metadata for Web-based
protocols.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Namespace and Version . . . . . . . . . . . . . . . . . . 4
1.3. Notational Conventions . . . . . . . . . . . . . . . . . . 4
2. Metadata Scope . . . . . . . . . . . . . . . . . . . . . . . . 5
3. The host-meta Document Format . . . . . . . . . . . . . . . . 5
3.1. The 'host-meta:Scope' Element . . . . . . . . . . . . . . 6
3.2. The 'Link' Element . . . . . . . . . . . . . . . . . . . . 6
4. Obtaining host-meta Documents . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6.1. The host-meta Well-Known URI . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 10
Appendix B. Document History . . . . . . . . . . . . . . . . . . 10
7. Normative References . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
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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 collection of
resources identified by URIs with a common scheme and authority as
defined by [RFC3986]. While these protocols have a wide range of
metadata needs, they often define metadata that is concise, has
simple syntax requirements, and can benefit from sharing its metadata
store with other related protocols.
Because there is no URI or a resource 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 usage of
the root HTTP resource as a placeholder for host metadata that is not
specific to the root resource, and often has nothing to do it.
This memo registers the "well-known" URI suffix 'host-meta' in the
Well-Known URI Registry established by [I-D.nottingham-site-meta],
and specifies a simple, general-purpose metadata document for hosts,
to be used by multiple Web-based protocols. The name 'host-meta' was
chosen based on the HTTP 'Host' header defined by [RFC2616].
Please discuss this draft on the apps-discuss@ietf.org [1] mailing
list.
1.1. Example
A simple host-meta document for the 'example.com' and
'www.example.com' authorities and 'http' scheme with a link providing
scope-wide copyright information and a link template providing a URI
for obtaining resource-specific metadata for each resource within the
host-meta document scope:
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<?xml version='1.0' encoding='UTF-8'?>
<XRD xmlns='http://docs.oasis-open.org/ns/xri/xrd-1.0'
xmlns:host-meta='http://host-meta.net/ns/1.0'>
<host-meta:Scope scheme='http' authority='example.com' />
<host-meta:Scope scheme='http' authority='www.example.com' />
<Link>
<Title xml:lang='en-us'>Site License Policy</Title>
<Rel>license</Rel>
<URI>http://example.com/license</URI>
</Link>
<Link>
<Title xml:lang='en-us'>Resource Descriptor</Title>
<Rel>describedby</Rel>
<URITemplate>http://meta.example.com?uri={uri}</URITemplate>
</Link>
</XRD>
1.2. Namespace and Version
The host-meta document uses the XRD 1.0 XML namespace URI
[W3C.REC-xml-names-19990114]:
http://docs.oasis-open.org/ns/xri/xrd-1.0
The XML namespace URI for the host-meta specific extension elements
defined in this specification is:
http://host-meta.net/ns/1.0
1.3. 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 specification uses the namespace prefix "host-meta:" for the
extension Namespace URI identified in Section 1.2. Note that the
choice of namespace prefix is arbitrary and not semantically
significant. Element names without a namespace prefix belong to the
XRD 1.0 XML namespace identified in Section 1.2.
This document uses the Augmented Backus-Naur Form (ABNF) notation of
[RFC5234]. Additionally, the following rules are included from
[RFC3986]: reserved and unreserved.
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2. Metadata Scope
The scope of each host-meta document is defined by one or more
scheme-authority pairs, each applies to the collection of resources
identified by URIs with the same scheme and authority as defined by
[RFC3986]. The scope MUST be expressed explicitly within the
document by using the one or more 'host-meta:Scope' elements
(Section 3.1).
The host-meta scope does not apply to a subset, nor does it apply to
other combinations of scheme and authority (e.g., using another
scheme, port, or a different hostname in the same domain) not
explicitly declared. For example, resources with URIs beginning with
'http://example.com/', 'http://example.com:8080/',
'https://example.com/', and 'http://www.example.com/' all have
different and non-overlapping scopes.
The scope declared within the host-meta document MUST match the
scheme and authority used to obtain (Section 4) the document.
However, protocols MAY use a different scheme to obtain the document
than the one they are interested in, as long as both scheme and
authority combinations are declared. For example, a protocol can use
HTTP to obtain metadata about a scheme other than 'http' for which a
host-meta document cannot be obtained using the method described in
Section 4.
Protocols MAY place additional requirements on protocol-specific
metadata retrieved from multiple host-meta documents. For example,
protocols can require that protocol-specific metadata obtained for
the 'example.com:80' authority to be the same in both host-meta
documents for the 'http' and 'https' schemes.
Any changes in scope, the retrieval of metadata for one scheme using
another, and other requirements should only occur after a careful
consideration of their security implications and authoritativeness.
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
format for describing resources. Since the host-meta XRD document
does not describe a single resource but a collection of resources,
this memo defines additional XRD elements and processing rules. The
XRD elements not mentioned in this memo are allowed and used as
defined in [OASIS.XRD-1.0].
host-meta document SHOULD NOT include the 'Subject' or 'Alias' XRD
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elements since these elements require a valid URI to identify the
resource being described, which is not available for the host-meta
scope. The use of these elements in host-meta is undefined.
Instead, host-meta defines the 'host-meta:Scope' element
(Section 3.1) for declaring document scope.
The subject (or "context resource" as defined by
[I-D.nottingham-http-link-header]) of the XRD 'Type' and 'Link'
elements is the entire collection of resources included in the
document scope, with the exception of 'Link' elements with a
'URITemplate' child element as defined in Section 3.2.
3.1. The 'host-meta:Scope' Element
The 'host-meta:Scope" element is used to declare the scope of the
host-meta document and is defined as a child element of the root
'XRD' element. The parent 'XRD' element MUST include one but MAY
include more 'host-meta:Scope' elements (order does not matter). The
element includes two REQUIRED attributes: 'scheme' and 'authority'
and MUST NOT include a content.
The 'scheme' and 'authority' attributes use the same syntax and
semantics defined by [RFC3986], and MUST NOT contain empty values.
If the 'authority' attribute does not include a port number, the port
is implicitly set to the default port number for the given scheme,
and does not include any other port number.
For example, these two scope declarations are equivalent:
<host-meta:Scope scheme='http' authority='example.com' />
<host-meta:Scope scheme='http' authority='example.com:80' />
3.2. The 'Link' Element
The XRD 'Link' element, when used with the 'URI' child element,
conveys a link relation between the collection of resources included
in the host-meta document scope and a common target URI.
For example, the following link declares a common author for the
entire scope:
<Link>
<Rel>author</Rel>
<URI>http://example.com/author</URI>
</Link>
However, there are cases in which individual resources within the
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same scope do not share the same target URI, but follow a common
pattern in how the target URI is constructed. A 'Link' element with
a 'URITemplate' child element conveys relations whose context are
individual resources within the host-meta document scope, and whose
target is constructed by applying the context URI to a template.
For example, a blog with multiple authors can provide information
about each article's author by appending a suffix (such as ';by') 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</Rel>
<URITemplate>{+uri};by</URITemplate>
</Link>
3.2.1. Template Syntax
host-meta defines a simple template syntax and a vocabulary 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 reserved and unreserved (as defined by [RFC3986]) MUST be
percent-encoded per [RFC3986]. If a variable name is not prefixed by
a '+' character, any reserved character SHALL also be percent-
encoded.
To construct a URI using a template, the context URI is parsed into
its URI components and each component value assigned to a variable
name. The variable set is based on the URI vocabulary defined by
[RFC3986] section 3 and includes: 'scheme', 'authority', 'path',
'query', 'fragment', 'userinfo', 'host', and 'port'. In addition,
this memo defines the 'uri' variable as the entire context URI
excluding the fragment component and the '#' fragment separator.
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foo://william@example.com:8080/over/there?name=ferret#nose
\_/ \______________________/\_________/ \_________/ \__/
| | | | |
scheme authority path query fragment
foo://william@example.com:8080/over/there?name=ferret#nose
\_____/ \_________/ \__/
| | |
userinfo host port
foo://william@example.com:8080/over/there?name=ferret#nose
\___________________________________________________/
|
uri
Protocols MAY define additional variables and syntax rules, but
SHOULD only do so for protocol-specific relation types, and MUST NOT
change the meaning of the variables defined above. If a template
uses an unknown syntax or contains unknown variable names, the parent
'Link' element SHOULD be ignored.
The template syntax ABNF:
URI-Template = *( uri-char | variable )
variable = "{" [ "+" ] var-name "}"
uri-char = ( reserved | unreserved )
var-name = uri-var | ( 1*var-char )
uri-var = "scheme" | "authority" | "path" |
"query" | "fragment" | "userinfo" |
"host" | "port" | "uri"
var-char = ALPHA / DIGIT / "." / "_"
For example, given the input URI 'http://example.com/r/1?f=xml#top',
each of the following templates will produce the associated output
URI:
{+uri}&test -->
http://example.com/r/1?f=xml&test
http://example.org?q={uri} -->
http://example.org?q=http%3A%2F%2Fexample.com%2Fr%2F1%3Ff%3Dxml
http://meta.{host}:8080{+path}?{+query} -->
http://meta.example.com:8080/r/1?f=xml
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4. Obtaining host-meta Documents
The host-meta document for a given scheme-authority pair is
identified by a URI constructed using the desired scheme, authority,
and '/.well-known/host-meta' as the URI path. For example, given the
'http' scheme and 'www.example.com:80' authority, the host-meta URI
is:
http://www.example.com/.well-known/host-meta
The host-meta document is obtained by dereferencing the request host-
meta URI. The semantics of the protocol used to obtain the host-meta
document apply. Therefore, if the server indicates that the host-
meta resource is located elsewhere (in HTTP, 3xx response status
codes), the client MUST try to obtain the resource from the location
provided. This means that the host-meta document for one authority
MAY be retrieved from a different authority. Likewise, if the
resource is not available or exists (in HTTP, the 404 or 410 response
status codes), the client SHOULD infer that metadata is not available
via this mechanism.
If a representation is successfully obtained, but is not in the
format described above, clients SHOULD infer that the authority is
using this URI for other purposes, and not process it 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.
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.
Scoping metadata to a single scheme-authority pair is the default
host-meta scope. Protocols that change the scope without careful
consideration can incur security risks.
6. IANA Considerations
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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 [I-D.nottingham-site-meta].
URI suffix: host-meta
Change controller: IETF
Specification document(s): [[ this document ]]
Related information: None
Appendix A. Acknowledgments
This memo was initially based on [I-D.nottingham-site-meta].
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, Breno de Medeiros, Brad
Fitzpatrick, Will Norris, Mark Nottingham, John Panzer, and Drummond
Reed.
Appendix B. Document History
[[ to be removed by the RFC editor before publication as an RFC ]]
-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-06 (work in progress),
July 2009.
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[I-D.nottingham-site-meta]
Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
URIs", draft-nottingham-site-meta-03 (work in progress),
September 2009.
[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/34815/
xrd-1.0-cd01.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.
[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.
[W3C.REC-P3P-20020416]
Marchiori, M., "The Platform for Privacy Preferences 1.0
(P3P1.0) Specification", World Wide Web Consortium
Recommendation REC-P3P-20020416, April 2002,
<http://www.w3.org/TR/2002/REC-P3P-20020416>.
[W3C.REC-xml-names-19990114]
Hollander, D., Bray, T., and A. Layman, "Namespaces in
XML", World Wide Web Consortium FirstEdition REC-xml-
names-19990114, January 1999,
<http://www.w3.org/TR/1999/REC-xml-names-19990114>.
[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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