appsawg                                                    M. Nottingham
Internet-Draft                                          January 30, 2014
Updates: 3986 (if approved)
Intended status: BCP
Expires: August 3, 2014

                        URI Design and Ownership


   Sometimes, it is attractive to add features to protocols or
   applications by specifying a particular structure for URIs (or parts
   thereof).  However, publishing standards that mandate URI structure
   is inappropriate because the structure of a URI needs to be firmly
   under the control of its owner, and the IETF (as well as other
   organisations) should not usurp this ownership.

   This document is intended to prevent this practice (sometimes called
   "URI Squatting") in standards, but updating RFC3986 to indicate where
   it is acceptable.

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
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   This Internet-Draft will expire on August 3, 2014.

Copyright Notice

   Copyright (c) 2014 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
   ( in effect on the date of

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   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  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Who This Document Is For  . . . . . . . . . . . . . . . . . 4
     1.2.  Notational Conventions  . . . . . . . . . . . . . . . . . . 4
   2.  Best Current Practices for Standardising Structured URIs  . . . 5
     2.1.  URI Schemes . . . . . . . . . . . . . . . . . . . . . . . . 5
     2.2.  URI Authorities . . . . . . . . . . . . . . . . . . . . . . 5
     2.3.  URI Paths . . . . . . . . . . . . . . . . . . . . . . . . . 5
     2.4.  URI Queries . . . . . . . . . . . . . . . . . . . . . . . . 5
     2.5.  URI Fragment Identifiers  . . . . . . . . . . . . . . . . . 6
   3.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     5.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     5.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . . . 7
   Appendix B.  Alternatives to Specifying Structure in URIs . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   URIs [RFC3986] very often include structured application data.  This
   might include artifacts from filesystems (often occurring in the path
   component), and user information (often in the query component).  In
   some cases, there can even be application-specific data in the
   authority component (e.g., some applications are spread across
   several hostnames to enable a form of partitioning or dispatch).

   Furthermore, constraints upon the structure of URIs can be imposed by
   an implementation; for example, many Web servers use the filename
   extension of the last path segment to determine the media type of the
   response.  Likewise, pre-packaged applications often have highly
   structured URIs that can only be changed in limited ways (often, just
   the hostname and port they are deployed upon).

   Because the owner of the URI is choosing to use the server or the
   software, this can be seen as reasonable delegation of authority.
   When such conventions are mandated by standards, however, it can have
   several potentially detrimental effects:

   o  Collisions - As more conventions for URI structure become
      standardised, it becomes more likely that there will be collisions
      between such conventions (especially considering that servers,
      applications and individual deployments will have their own
   o  Dilution - When the information added to a URI is ephemeral, this
      dilutes its utility by reducing its stability (see [webarch]
      Section 3.5.1), and can cause several alternate forms of the URI
      to exist (see [webarch] Section 2.3.1).
   o  Rigidity - Fixed URI syntax often interferes with desired
      deployment patterns.  For example, if an authority wishes to offer
      several applications on a single hostname, it becomes difficult to
      impossible to do if their URIs do not allow the required
   o  Operational Difficulty - Supporting some URI conventions can be
      difficult in some implementations.  For example, specifying that a
      particular query parameter be used precludes the use of Web
      servers that serve the response from a filesystem.  Likewise, an
      application that fixes a base path for its operation (e.g., "/v1")
      makes it impossible to deploy other applications with the same
      prefix on the same host.
   o  Client Assumptions - When conventions are standardised, some
      clients will inevitably assume that the standards are in use when
      those conventions are seen.  This can lead to interoperability
      problems; for example, if a specification documents that the "sig"
      URI query parameter indicates that its payload is a cryptographic
      signature for the URI, it can lead to undesirable behaviour.

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   While it is not ideal when a server or a deployed application
   constrains URI structure (indeed, this is not recommended practice,
   but that discussion is out of scope for this document), publishing
   standards that mandate URI structure (beyond those allowed by
   [RFC3986]) is inappropriate because the structure of a URI needs to
   be firmly under the control of its owner, and the IETF (as well as
   other organisations) should not usurp this ownership; see [webarch]

   This document explains best current practices for establishing URI
   structures, conventions and formats in standards.  It also offers
   strategies for specifications to avoid violating these guidelines in
   Appendix B.

1.1.  Who This Document Is For

   This document's requirements specifically target a few different
   types of specifications:

   o  URI Scheme Definitions ("scheme definitions") - specifications
      that define and register URI schemes, as per [RFC4395].
   o  Protocol Extensions ("extensions") - specifications that offer new
      capabilities to potentially any identifier, or a large subset;
      e.g., a new signature mechanism for 'http' URIs, or metadata for
      any URI.
   o  Applications Using URIs ("applications") - specifications that use
      URIs to meet specific needs; e.g., a HTTP interface to particular
      information on a host.

   Requirements that target the generic class "Specifications" apply to
   all specifications, including both those enumerated above above and

   Note that this specification ought not be interpreted as preventing
   the allocation of control of URIs by parties that legitimately own
   them, or have delegated that ownership; for example, a specification
   might legitimately define the semantics of a URI on the IANA.ORG Web
   site as part of the establishment of a registry.

1.2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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2.  Best Current Practices for Standardising Structured URIs

   Best practices differ depending on the URI component.

2.1.  URI Schemes

   Applications and extensions MAY require use of specific URI
   scheme(s); for example, it is perfectly acceptable to require that an
   application support 'http' and 'https' URIs.  However, applications
   SHOULD NOT preclude the use of other URI schemes in the future,
   unless they are clearly specific to the nominated schemes.

   A specification that defines substructure within a URI scheme MUST do
   so in a registration document for the URI scheme in question, or by
   modifying [RFC4395].

2.2.  URI Authorities

   Scheme definitions define the presence, format and semantics of an
   authority component in URIs; all other specifications MUST NOT
   constrain, define structure or semantics for URI authorities.

   For example, an extension or application cannot say that the "foo"
   prefix in "" is meaningful or triggers special

2.3.  URI Paths

   Scheme definitions define the presence, format, and semantics of a
   path component in URIs; all other specifications MUST NOT constrain,
   define structure or semantics for any path component.

   The only exception to this requirement is registered "well-known"
   URIs, as specified by [RFC5785].  See that document for a description
   of the applicability of that mechanism.

   For example, an application cannot specify a fixed URI path "/myapp",
   since this usurps the host's control of that space.  Specifying a
   fixed path relative to another (e.g., {whatever}/myapp) is also bad
   practice, since it "locks" the URIs in use; while doing so might
   prevent collisions, it does not avoid the other issues discussed.

2.4.  URI Queries

   The presence, format and semantics of the query component of URIs is
   dependent upon many factors, and MAY be constrained by a scheme
   definition.  Often, they are determined by the implementation of a
   resource itself.

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   Applications SHOULD NOT directly specify the syntax of queries, as
   this can cause operational difficulties for deployments that do not
   support a particular form of a query.

   Extensions MUST NOT specify the format or semantics of queries.

   For example, an extension cannot be minted that indicates that all
   query parameters with the name "sig" indicate a cryptographic

2.5.  URI Fragment Identifiers

   Media type definitions (as per [RFC6838] SHOULD specify the fragment
   identifier syntax(es) to be used with them; other specifications MUST
   NOT define structure within the fragment identifier, unless they are
   explicitly defining one for reuse by media type definitions.

3.  Security Considerations

   This document does not introduce new protocol artifacts with security
   considerations.  It prohibits some practices that might lead to
   vulnerabilities; for example, if a security-sensitive mechanism is
   introduced by assuming that a URI path component or query string has
   a particular meaning, false positives might be encountered (due to
   sites that already use the chosen string).

4.  IANA Considerations

   This document clarifies appropriate registry policy for new URI
   schemes, and potentially for the creation of new URI-related
   registries, if they attempt to mandate structure within URIs.  There
   are no direct IANA actions specified in this document.

5.  References

5.1.  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.

   [RFC4395]  Hansen, T., Hardie, T., and L. Masinter, "Guidelines and

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              Registration Procedures for New URI Schemes", BCP 35,
              RFC 4395, February 2006.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, January 2013.

5.2.  Informative References

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              April 2010.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988, October 2010.

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570, March 2012.

   [webarch]  Jacobs, I. and N. Walsh, "Architecture of the World Wide
              Web, Volume One", December 2004,

Appendix A.  Acknowledgments

   Thanks to David Booth, Dave Crocker, Tim Bray, Anne van Kesteren,
   Martin Thomson and Erik Wilde for their suggestions and feedback.

Appendix B.  Alternatives to Specifying Structure in URIs

   Given the issues above, the most successful strategy for applications
   and extensions that wish to use URIs is to use them in the fashion
   they were designed; as links that are exchanged as part of the
   protocol, rather than statically specified syntax.  Several existing
   specifications can aid in this.

   [RFC5988] specifies relation types for Web links.  By providing a
   framework for linking on the Web, where every link has a relation
   type, context and target, it allows applications to define a link's
   semantics and connectivity.

   [RFC6570] provides a standard syntax for URI Templates that can be
   used to dynamically insert application-specific variables into a URI
   to enable such applications while avoiding impinging upon URI owners'
   control of them.

   [RFC5785] allows specific paths to be 'reserved' for standard use on

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   URI schemes that opt into that mechanism ('http' and 'https' by
   default).  Note, however, that this is not a general "escape valve"
   for applications that need structured URIs; see that specification
   for more information.

   Specifying more elaborate structures in an attempt to avoid
   collisions is not adequate to conform to this document.  For example,
   prefixing query parameters with "myapp_" does not help, because the
   prefix itself is subject to the risk of collision (since it is not

Author's Address

   Mark Nottingham


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