Well-Known URIs: Defining Well-Known Uniform Resource Identifiers (URIs)

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.

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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.

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.

   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
   wellknown-uri-review@ietf.org 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
   request successful.  Registration requests that are undetermined for
   a period longer than 21 days can be brought to the IESG's attention
   (using the iesg@iesg.org 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.

   [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>.

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".

Authors' Addresses

   Mark Nottingham

   EMail: mnot@mnot.net
   URI:   http://www.mnot.net/


   Eran Hammer-Lahav

   EMail: eran@hueniverse.com
   URI:   http://hueniverse.com/