Network Working Group E. Hammer-Lahav
Internet-Draft Yahoo!
Intended status: Informational May 11, 2010
Expires: November 12, 2010
host-meta: Web Host Metadata
draft-hammer-hostmeta-08
Abstract
This memo describes a method for locating host metadata for Web-based
protocols.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on November 12, 2010.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 '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. host-meta XML Schema . . . . . . . . . . . . . . . . 9
Appendix B. Acknowledgments . . . . . . . . . . . . . . . . . . . 9
Appendix C. Document History . . . . . . . . . . . . . . . . . . 9
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 entity
controlling the collection of resources identified by Uniform
Resource Identifiers (URI) 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 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, 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 [RFC5785], 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
The following is 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 author information 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>example.com</hm:Host>
<hm:Host>www.example.com</hm:Host>
<Link rel='license'
href='http://example.com/license'>
<Title xml:lang='en-us'>Site License Policy</Title>
</Link>
<Link rel='author'
template='http://meta.example.com?uri={uri}'>
<Title xml:lang='en-us'>Author Profile</Title>
</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].
Examples in this specification uses the namespace prefix "hm:" for
the extension Namespace URI identified in Section 1.2. The "hm:"
namespace prefix is arbitrary and not is 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, where a 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], across all ports and schemes.
The scope MUST be expressed explicitly within the document using the
"hm:Host" element as described in 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. 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.
This memo defines the "hm:Host" element for declaring document scope.
The subject (or "context resource" as defined by
[I-D.nottingham-http-link-header]) of the XRD "Property" and "Link"
elements consists of the hosts included in the document scope.
However, the subject of "Link" elements with a "template" attribute
is the individual resources (included in the document scope) applied
to the link template as described 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 included in the document.
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 "href" attribute, conveys
a link relation between the 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' href='http://example.com/author' />
However, a "Link" element with a "template" attribute conveys
relations whose context are the individual resources within the host-
meta document scope, and whose target is constructed by applying each
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={uri}' />
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
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.
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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
Clients obtain 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
valid host-meta document, clients make an HTTP [RFC2616] GET request
to the host's port 80 for the "/.well-known/host-meta" path.
Servers MUST support at least one but SHOULD support both ports. If
both ports are supported, they MUST serve the same document. Clients
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, clients should 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 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 another host. Likewise, if the resource is
not available or does not exist (indicated respectively, by the 404
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and 410 response status codes) at both ports, the client should infer
that metadata is not available via this mechanism.
The scope declared within the host-meta document MUST match the
desired host. Clients MUST NOT use host-meta documents when the
desired host (used to obtain the document) is not listed in the
document.
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 another 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 [RFC5785].
URI suffix: host-meta
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Change controller: IETF
Specification document(s): [[ this document ]]
Related information: None
Appendix A. host-meta XML Schema
The following is the XML schema for the host-meta XRD extension
elements:
<?xml version="1.0" encoding="UTF-8"?>
<schema
xmlns="http://www.w3.org/2001/XMLSchema"
xmlns:hm="http://host-meta.net/ns/1.0"
targetNamespace="http://host-meta.net/ns/1.0"
elementFormDefault="unqualified"
attributeFormDefault="unqualified"
version="1.0">
<element name="Host" type="hm:HostType"/>
<complexType name="HostType">
<simpleContent>
<extension base="string">
<anyAttribute namespace="##other" processContents="lax"/>
</extension>
</simpleContent>
</complexType>
</schema>
Appendix B. 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 C. Document History
[[ to be removed by the RFC editor before publication as an RFC ]]
-08
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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
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.
[OASIS.XRD-1.0]
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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.
[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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