Network Working Group                                      M. Nottingham
Internet-Draft                                           August 29, 2013
Updates: 3986 (if approved)
Intended status: BCP
Expires: March 2, 2014

                    Standardising Structure in URIs


   Sometimes, it is attractive to add features to protocols or
   applications by specifying a particular structure for URIs (or parts
   thereof).  This document cautions against this practice in standards
   (sometimes called "URI Squatting").

Status of this Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on March 2, 2014.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Who This Document Is For  . . . . . . . . . . . . . . . . . 4
     1.2.  Notational Conventions  . . . . . . . . . . . . . . . . . . 5
   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.  Alternatives to Specifying Static URIs  . . . . . . . . . . . . 6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     6.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     6.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . . . 8
   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 - Adorning URIs with extra information to support new
      standard features dilutes their usefulness as identifiers when
      that information is ephemeral (as URIs ought to be stable; see
      [webarch] Section 3.5.1), or its inclusion causes several
      alternate forms of the URI to exist (see [webarch] Section 2.3.1).
   o  Brittleness - A standard that specifies a static URI cannot change
      its form in future revisions.
   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 false positives.

   While it is not ideal when a server or a deployed application

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   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 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] Section

   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
   Section 3.

1.1.  Who This Document Is For

   These guidelines are IETF Best Current Practice, and are therefore
   binding upon IETF standards-track documents, as well as submissions
   to the RFC Editor on the Independent and IRTF streams.  See [RFC2026]
   and [RFC4844] for more information.

   Other Open Standards organisations (in the sense of [RFC2026]) are
   encouraged to adopt them.  Questions as to their applicability ought
   to be handled through the liaison relationship, if present.

   Ad hoc efforts are also encouraged to adopt them, as this RFC
   reflects Best Current Practice.

   This document's requirements specifically targets 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 specify the semantics of a URI on the IANA.ORG Web
   site as part of the establishment of a registry.

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1.2.  Notational Conventions

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

2.  Best Current Practices for Standardising Structured URIs

   Different components of a URI have differing practices recommended.

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, to
   promote reuse, unless they are clearly specific to the nominated

   Specifications MUST NOT define substructure within URI schemes,
   unless they do so by modifying [RFC4395], or they are the
   registration document for the URI scheme(s) in question.

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 them.

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.

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.

   Applications SHOULD NOT directly specify the syntax of queries, as

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   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.  In
   particular, extensions MUST NOT assume that all HTTP(S) resources are
   capable of accepting queries in the format defined by [HTML4],
   Section 17.13.4.

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.  Alternatives to Specifying Static 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 run-time artifacts that are exchanged as part
   of the protocol, rather than statically specified syntax.

   For example, if a specific URI needs to be known to interact with an
   application, its "shape" can be determined by interacting with the
   application's more general interface (in Web terms, its "home page")
   to learn about that URI.

   [RFC5988] describes a framework for identifying the semantics of a
   link in a "link relation type" to aid this.  [RFC6570] provides a
   standard syntax for "link 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

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

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4.  Security Considerations

   This document does not introduce new protocol artifacts with security

5.  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.

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.

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

6.2.  Informative References

   [HTML4]    Jacobs, I., Le Hors, A., and D. Raggett, "HTML 4.01
              Specification", December 1999,

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, October 1996.

   [RFC4844]  Daigle, L. and Internet Architecture Board, "The RFC
              Series and RFC Editor", RFC 4844, July 2007.

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

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   [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, Anne van Kesteren and Erik Wilde for their
   suggestions and feedback.

Author's Address

   Mark Nottingham


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