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 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress.”

This Internet-Draft will expire on November 28, 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 Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must 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 Simplified BSD License.

Table of Contents

1.  Introduction
    1.1.  Example
        1.1.1.  Processing Resource-Specific Information
    1.2.  Notational Conventions
2.  Obtaining host-meta Documents
3.  The host-meta Document Format
    3.1.  The 'Link' Element
        3.1.1.  Template Syntax
4.  Processing host-meta Documents
    4.1.  Host-Wide Information
    4.2.  Resource-Specific Information
5.  Security Considerations
6.  IANA Considerations
    6.1.  The 'host-meta' Well-Known URI
    6.2.  The 'lrdd' Relation Type
Appendix A.  Acknowledgments
Appendix B.  Document History
7.  Normative References
§  Author's Address

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] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.).

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] (Nottingham, M. and E. Hammer-Lahav, “Defining Well-Known Uniform Resource Identifiers (URIs),” April 2010.), 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:

• 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.
• 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:

  <?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:

• A http://protocol.example.net/version host property with a value of 1.0.
• A link to the host's copyright policy (copyright).

The resource-specific information provided by the document includes:

• 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.
• A LRDD document link template (lrdd) for obtaining additional resource-specific information contained in a separate document for each individual resource.
• A link template for finding information about the author of individual resources (author).

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:

  <?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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

This document uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.). Additionally, the following rules are included from [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.): 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] (Rescorla, E., “HTTP Over TLS,” May 2000.) 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, the client makes an HTTP [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.) 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] (Hammer-Lahav, E. and W. Norris, “Extensible Resource Descriptor (XRD) Version 1.0 (work in progress),” .), 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 [I‑D.nottingham‑http‑link‑header] (Nottingham, M., “Web Linking,” May 2010.)) 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 (The 'Link' Element).

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] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.)) MUST be percent-encoded per [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.).

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.

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.

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] (Hammer-Lahav, E. and W. Norris, “Extensible Resource Descriptor (XRD) Version 1.0 (work in progress),” .).
      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).
   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 (Processing Resource-Specific Information).

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] (Nottingham, M. and E. Hammer-Lahav, “Defining Well-Known Uniform Resource Identifiers (URIs),” April 2010.).

URI suffix:

host-meta

Change controller:

IETF

Specification document(s):

[[ this document ]]

Related information:

None

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] (Nottingham, M., “Web Linking,” May 2010.):

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

• Integrated LRDD into the memo, dropping the multiple sources and using only host-meta for LRDD processing.

-09

• 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).
• Minor editorial changes.
• Changed following redirections to MUST.
• Updated references.

-08

• Fixed typo.

-07

• Minor editorial clarifications.
• Added XML schema for host-meta extension.
• Updated XRD reference to the latest draft (no normative changes).

-06

• Updated well-known reference to RFC 5785.
• Minor editorial changes.
• Made HTTPS a higher priority (SHOULD) over HTTP.

-05

• Adjusted syntax to the latest XRD schema.
• Added note about using a link template without variables.

-04

• Corrected the <hm:Host> example.

-03

• Changed scope to an entire host (per RFC 3986).
• Simplified template syntax to always percent-encode values and vocabulary to a single 'uri' variable.
• Changed document retrieval to always use HTTP(S).
• Added security consideration about the use of templates.
• Explicitly defined the root element to be 'XRD'.

-02

• Changed Scope element syntax from attributes to URI-like string value.

-01

• Editorial rewrite.
• Redefined scope as a scheme-authority pair.
• Added document structure section.

-00

• Initial draft.

7. Normative References

Author's Address