Internet Engineering Task Force (IETF)                     M. Nottingham
Request for Comments: 7320
BCP: 190                                                       July 2014
Updates: 3986
Category: Best Current Practice
ISSN: 2070-1721


                        URI Design and Ownership

Abstract

   Section 1.1.1 of RFC 3986 defines URI syntax as "a federated and
   extensible naming system wherein each scheme's specification may
   further restrict the syntax and semantics of identifiers using that
   scheme."  In other words, the structure of a URI is defined by its
   scheme.  While it is common for schemes to further delegate their
   substructure to the URI's owner, publishing independent standards
   that mandate particular forms of URI substructure is inappropriate,
   because that essentially usurps ownership.  This document further
   describes this problematic practice and provides some acceptable
   alternatives for use in standards.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   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
   BCPs 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/rfc7320.

Copyright Notice

   Copyright (c) 2014 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.  Intended Audience . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Notational Conventions  . . . . . . . . . . . . . . . . .   4
   2.  Best Current Practices for Standardizing Structured URIs  . .   4
     2.1.  URI Schemes . . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  URI Authorities . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  URI Paths . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.4.  URI Queries . . . . . . . . . . . . . . . . . . . . . . .   6
     2.5.  URI Fragment Identifiers  . . . . . . . . . . . . . . . .   6
   3.  Alternatives to Specifying Structure in URIs  . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .   9

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, prepackaged applications often have highly
   structured URIs that can only be changed in limited ways (often, just
   the hostname and port on which they are deployed).

   Because the owner of the URI (as defined in [webarch]
   Section 2.2.2.1) is choosing to use the server or the application,
   this can be seen as reasonable delegation of authority.  However,
   when such conventions are mandated by a party other than the owner,
   it can have several potentially detrimental effects:

   o  Collisions - As more ad hoc conventions for URI structure become
      standardized, it becomes more likely that there will be collisions
      between them (especially considering that servers, applications,
      and individual deployments will have their own conventions).

   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
      flexibility.

   o  Operational Difficulty - Supporting some URI conventions can be
      difficult in some implementations.  For example, specifying that a
      particular query parameter be used with "HTTP" URIs 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 standardized, 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 behavior.

   Publishing a standard that constrains an existing URI structure in
   ways that aren't explicitly allowed by [RFC3986] (usually, by
   updating the URI scheme definition) 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 organizations) should not usurp it.

   This document explains some 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.  Intended Audience

   This document's requirements target the authors of specifications
   that constrain the syntax or structure of URIs or parts of them.  Two
   classes of such specifications are called out specifically:

   o  Protocol Extensions ("extensions") - specifications that offer new
      capabilities that could apply to any identifier, or to a large
      subset of possible identifiers; 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., an HTTP interface to particular
      information on a host.

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

   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 IANA's Web site
   as part of the establishment of a registry.

   There may be existing IETF specifications that already deviate from
   the guidance in this document.  In these cases, it is up to the
   relevant communities (i.e., those of the URI scheme as well as that
   which produced the specification in question) to determine an
   appropriate outcome; e.g., updating the scheme definition, or
   changing the specification.

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].

2.  Best Current Practices for Standardizing Structured URIs

   This section updates [RFC3986] by setting limitations on how other
   specifications may define structure and semantics within URIs.  Best
   practices differ depending on the URI component, as described below.

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 only usable with the nominated schemes.

   A specification that defines substructure within a specific URI
   scheme MUST do so in the defining document for that URI scheme.  A
   specification that defines substructure for URI schemes overall MUST
   do so by modifying [BCP115] (an exceptional circumstance).

2.2.  URI Authorities

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

   For example, an extension or application ought not say that the "foo"
   prefix in "foo_app.example.com" is meaningful or triggers special
   handling in URIs.

   However, applications MAY nominate or constrain the port they use,
   when applicable.  For example, BarApp could run over port nnnn
   (provided that it is properly registered).

2.3.  URI Paths

   Scheme definitions define the presence, format, and semantics of a
   path component in URIs; all other specifications MUST NOT constrain,
   or define the structure or the 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 ought not 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 (even if "whatever" is discovered as suggested
   in Section 3); while doing so might prevent collisions, it does not
   avoid the potential for operational difficulties (for example, an
   implementation that prefers to use query processing instead, because
   of implementation constraints).

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 MUST 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.  For example, a site may wish
   to support an application using "static" files that do not support
   query parameters.

   Extensions MUST NOT constrain the format or semantics of queries.

   For example, an extension that indicates that all query parameters
   with the name "sig" indicate a cryptographic signature would collide
   with potentially preexisting query parameters on sites and lead
   clients to assume that any matching query parameter is a signature.

   HTML [W3C.REC-html401-19991224] constrains the syntax of query
   strings used in form submission.  New form languages SHOULD NOT
   emulate it, but instead allow creation of a broader variety of URIs
   (e.g., by allowing the form to create new path components, and so
   forth).

   Note that "well-known" URIs (see [RFC5785]) MAY constrain their own
   query syntax, since these name spaces are effectively delegated to
   the registering party.

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.

   For example, an application that defines common fragment identifiers
   across media types not controlled by it would engender
   interoperability problems with handlers for those media types
   (because the new, non-standard syntax is not expected).

3.  Alternatives to Specifying Structure in URIs

   Given the issues described in Section 1, 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
   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 an acceptable solution, and does not address the
   issues in Section 1.  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 "reserved").

4.  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).  See also [RFC6943].

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.

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

   [webarch]  Jacobs, I. and N. Walsh, "Architecture of the World Wide
              Web, Volume One", December 2004,
              <http://www.w3.org/TR/2004/REC-webarch-20041215>.

5.2.  Informative References

   [BCP115]   Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
              Registration Procedures for New URI Schemes", RFC 4395,
              BCP 115, February 2006.

   [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.

   [RFC6943]  Thaler, D., "Issues in Identifier Comparison for Security
              Purposes", RFC 6943, May 2013.

   [W3C.REC-html401-19991224]
              Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01
              Specification", World Wide Web Consortium Recommendation
              REC-html401-19991224, December 1999,
              <http://www.w3.org/TR/1999/REC-html401-19991224>.

Appendix A.  Acknowledgments

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

Author's Address

   Mark Nottingham

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