Internet Engineering Task Force (IETF) M. Nottingham

Internet Engineering Task Force (IETF) M. Nottingham
Request for Comments: 5785 E. Hammer-Lahav
Updates: 2616, 2818 April 2010
Category: Standards Track
ISSN: 2070-1721

Defining Well-Known Uniform Resource Identifiers (URIs)

Abstract

This memo defines a path prefix for "well-known locations",
"/.well-known/", in selected Uniform Resource Identifier (URI)
schemes.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5785.

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
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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 Simplified BSD License.

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RFC 5785 Defining Well-Known URIs April 2010

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Appropriate Use of Well-Known URIs . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
3. Well-Known URIs . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
5.1. The Well-Known URI Registry . . . . . . . . . . . . . . . . 4
5.1.1. Registration Template . . . . . . . . . . . . . . . . . 5
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Normative References . . . . . . . . . . . . . . . . . . . 5
6.2. Informative References . . . . . . . . . . . . . . . . . . 5
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 7
Appendix B. Frequently Asked Questions . . . . . . . . . . . . . . 7

1. Introduction

It is increasingly common for Web-based protocols to require the
discovery of policy or other information about a host ("site-wide
metadata") before making a request. For example, the Robots
Exclusion Protocol <http://www.robotstxt.org/> specifies a way for
automated processes to obtain permission to access resources;
likewise, the Platform for Privacy Preferences [W3C.REC-P3P-20020416]
tells user-agents how to discover privacy policy beforehand.

While there are several ways to access per-resource metadata (e.g.,
HTTP headers, WebDAV's PROPFIND [RFC4918]), the perceived overhead
(either in terms of client-perceived latency and/or deployment
difficulties) associated with them often precludes their use in these
scenarios.

When this happens, it is common to designate a "well-known location"
for such data, so that it can be easily located. However, this
approach has the drawback of risking collisions, both with other such
designated "well-known locations" and with pre-existing resources.

To address this, this memo defines a path prefix in HTTP(S) URIs for
these "well-known locations", "/.well-known/". Future specifications
that need to define a resource for such site-wide metadata can
register their use to avoid collisions and minimise impingement upon
sites' URI space.

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RFC 5785 Defining Well-Known URIs April 2010

1.1. Appropriate Use of Well-Known URIs

There are a number of possible ways that applications could use Well-
known URIs. However, in keeping with the Architecture of the World-
Wide Web [W3C.REC-webarch-20041215], well-known URIs are not intended
for general information retrieval or establishment of large URI
namespaces on the Web. Rather, they are designed to facilitate
discovery of information on a site when it isn't practical to use
other mechanisms; for example, when discovering policy that needs to
be evaluated before a resource is accessed, or when using multiple
round-trips is judged detrimental to performance.

As such, the well-known URI space was created with the expectation
that it will be used to make site-wide policy information and other
metadata available directly (if sufficiently concise), or provide
references to other URIs that provide such metadata.

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 RFC 2119 [RFC2119].

3. Well-Known URIs

A well-known URI is a URI [RFC3986] whose path component begins with
the characters "/.well-known/", and whose scheme is "HTTP", "HTTPS",
or another scheme that has explicitly been specified to use well-
known URIs.

Applications that wish to mint new well-known URIs MUST register
them, following the procedures in Section 5.1.

For example, if an application registers the name 'example', the
corresponding well-known URI on 'http://www.example.com/' would be
'http://www.example.com/.well-known/example'.

Registered names MUST conform to the segment-nz production in
[RFC3986].

Note that this specification defines neither how to determine the
authority to use for a particular context, nor the scope of the
metadata discovered by dereferencing the well-known URI; both should
be defined by the application itself.

Typically, a registration will reference a specification that defines
the format and associated media type to be obtained by dereferencing
the well-known URI.

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RFC 5785 Defining Well-Known URIs April 2010

It MAY also contain additional information, such as the syntax of
additional path components, query strings and/or fragment identifiers
to be appended to the well-known URI, or protocol-specific details
(e.g., HTTP [RFC2616] method handling).

Note that this specification does not define a format or media-type
for the resource located at "/.well-known/" and clients should not
expect a resource to exist at that location.

4. Security Considerations

This memo does not specify the scope of applicability of metadata or
policy obtained from a well-known URI, and does not specify how to
discover a well-known URI for a particular application. Individual
applications using this mechanism must define both aspects.

Applications minting new well-known URIs, as well as administrators
deploying them, will need to consider several security-related
issues, including (but not limited to) exposure of sensitive data,
denial-of-service attacks (in addition to normal load issues), server
and client authentication, vulnerability to DNS rebinding attacks,
and attacks where limited access to a server grants the ability to
affect how well-known URIs are served.

5. IANA Considerations

5.1. The Well-Known URI Registry

This document establishes the well-known URI registry.

Well-known URIs are registered on the advice of one or more
Designated Experts (appointed by the IESG or their delegate), with a
Specification Required (using terminology from [RFC5226]). However,
to allow for the allocation of values prior to publication, the
Designated Expert(s) may approve registration once they are satisfied
that such a specification will be published.

Registration requests should be sent to the
[email protected] mailing list for review and comment,
with an appropriate subject (e.g., "Request for well-known URI:
example").

Before a period of 14 days has passed, the Designated Expert(s) will
either approve or deny the registration request, communicating this
decision both to the review list and to IANA. Denials should include
an explanation and, if applicable, suggestions as to how to make the

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RFC 5785 Defining Well-Known URIs April 2010

request successful. Registration requests that are undetermined for
a period longer than 21 days can be brought to the IESG's attention
(using the [email protected] mailing list) for resolution.

5.1.1. Registration Template

URI suffix: The name requested for the well-known URI, relative to
"/.well-known/"; e.g., "example".

Change controller: For Standards-Track RFCs, state "IETF". For
others, give the name of the responsible party. Other details
(e.g., postal address, e-mail address, home page URI) may also be
included.

Specification document(s): Reference to the document that specifies
the field, preferably including a URI that can be used to retrieve
a copy of the document. An indication of the relevant sections
may also be included, but is not required.

Related information: Optionally, citations to additional documents
containing further relevant information.

6. References

6.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.

[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.

6.2. Informative References

[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.

[RFC4918] Dusseault, L., "HTTP Extensions for Web Distributed
Authoring and Versioning (WebDAV)", RFC 4918, June 2007.

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RFC 5785 Defining Well-Known URIs 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-webarch-20041215]
Jacobs, I. and N. Walsh, "Architecture of the World Wide
Web, Volume One", World Wide Web Consortium
Recommendation REC- webarch-20041215, December 2004,
<http:// www.w3.org/TR/2004/REC-webarch-20041215>.

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Appendix A. Acknowledgements

We would like to acknowledge the contributions of everyone who
provided feedback and use cases for this document; in particular,
Phil Archer, Dirk Balfanz, Adam Barth, Tim Bray, Brian Eaton, Brad
Fitzpatrick, Joe Gregorio, Paul Hoffman, Barry Leiba, Ashok Malhotra,
Breno de Medeiros, John Panzer, and Drummond Reed. However, they are
not responsible for errors and omissions.

Appendix B. Frequently Asked Questions

1. Aren't well-known locations bad for the Web?

They are, but for various reasons -- both technical and social --
they are commonly used and their use is increasing. This memo
defines a "sandbox" for them, to reduce the risks of collision and
to minimise the impact upon pre-existing URIs on sites.

2. Why /.well-known?

It's short, descriptive, and according to search indices, not
widely used.

3. What impact does this have on existing mechanisms, such as P3P and
robots.txt?

None, until they choose to use this mechanism.

4. Why aren't per-directory well-known locations defined?

Allowing every URI path segment to have a well-known location
(e.g., "/images/.well-known/") would increase the risks of
colliding with a pre-existing URI on a site, and generally these
solutions are found not to scale well, because they're too
"chatty".

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Authors' Addresses

Mark Nottingham

EMail: [email protected]
URI: http://www.mnot.net/

Eran Hammer-Lahav

EMail: [email protected]
URI: http://hueniverse.com/

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