Network Working Group E. Hammer-Lahav
Internet-Draft Yahoo!
Intended status: Informational November 8, 2009
Expires: May 12, 2010
host-meta: Web Host Metadata
draft-hammer-hostmeta-03
Abstract
This memo describes a method for locating host metadata for Web-based
protocols.
Status of this Memo
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Copyright Notice
Copyright (c) 2009 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
the Trust Legal Provisions and are provided without warranty as
described in the BSD License.
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 'hm:Host' Element . . . . . . . . . . . . . . . . . . 5
3.2. The 'Link' Element . . . . . . . . . . . . . . . . . . . . 6
3.2.1. Template Syntax . . . . . . . . . . . . . . . . . . . 6
4. Obtaining host-meta Documents . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6.1. The host-meta Well-Known URI . . . . . . . . . . . . . . . 8
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 9
Appendix B. Document History . . . . . . . . . . . . . . . . . . 9
7. Normative References . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
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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 URIs with a
common host 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 storing
its metadata in a common location used by 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
overloading of the root HTTP resource (e.g. 'http://example.com/')
with host metadata that is not specific to the root resource (e.g. a
home page or web application), and which 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.
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' hosts with a link providing host-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:hm='http://host-meta.net/ns/1.0'>
<hm:Host>http://example.com</hm:Host>
<hm:Host>http://www.example.com</hm:Host>
<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 "hm:" 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, unreserved, and host.
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2. Metadata Scope
Each host-meta document describes one or more hosts. The scope MUST
be expressed explicitly within the document using the 'hm:Host'
elements (Section 3.1). The host-meta scope does not apply to any
other hostname (or sub-domain) not explicitly declared. For example,
'example.net', 'example.com', and 'www.example.com' all have
different and non-overlapping scopes.
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
elements and processing rules needed to describe hosts. XRD elements
not explicitly mentioned in this memo are permitted and used as
defined in [OASIS.XRD-1.0].
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.
This memo defines the 'hm:Host' 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 are the hosts included in the document scope, with the
exception of 'Link' elements with a 'URITemplate' child element for
which the subject are individual resources included in the document
scope as defined in Section 3.2.
3.1. The 'hm:Host' Element
The 'hm:Host" 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 'hm:
Host' elements (order does not matter). If a host-meta document
includes more than one 'hm:Host' element, it does not signify any
relationship between the individual hosts other than sharing the same
metadata.
The element value syntax ABNF:
Host-Element-Value = host
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3.2. The 'Link' Element
The XRD 'Link' element, when used with the 'URI' child element,
conveys a link relation between the host (or hosts) described by the
document 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>
In addition, 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 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</Rel>
<URITemplate>http://example.com?author={uri}</URITemplate>
</Link>
3.2.1. 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
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 (due to unknown
variable names, or unknown or incompatible syntax) the parent 'Link'
element SHOULD be ignored.
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The template syntax ABNF:
URI-Template = *( uri-char | variable )
variable = "{" var-name "}"
uri-char = ( reserved | unreserved )
var-name = "uri" | ( 1*var-char )
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. Obtaining host-meta Documents
The host-meta document is obtained by making an HTTP [RFC2616] GET
request to the host's port 80, or an HTTPS [RFC2818] GET request to
the host's port 443 for the '/.well-known/host-meta' path.
Servers MUST support at least one but SHOULD support both ports
unless restricted by other considerations. If both ports are
supported, they MUST serve the same document. Clients MAY attempt to
obtain the host-meta document from either port, and SHOULD attempt
the other port if the first fails, unless restricted by other
considerations.
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 codes), 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 a different host. Likewise, if the resource is
not available or exists (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 path is being
used 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"
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internet media type.
The scope declared within the host-meta document MUST match the
desired host.
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.
The host-meta scope is explicitly declared by the 'hm:Host' elements
listed in the document. Clients SHOULD evaluate the authority of a
host-meta document obtained from one host to describe any other host.
Protocols that change the scope from the one declared in the document
without careful consideration can incur security risks.
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 [I-D.nottingham-site-meta].
URI suffix: host-meta
Change controller: IETF
Specification document(s): [[ this document ]]
Related information: None
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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, James Manger, 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 ]]
-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
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-06 (work in progress),
July 2009.
[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.
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[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/34925/
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.
[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.
[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., Layman, A., and T. Bray, "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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