RFC 8820

Internet Engineering Task Force (IETF)                     M. Nottingham
Request for Comments: 8820                                     June 2020
BCP: 190
Obsoletes: 7320                                                        
Updates: 3986                                                          
Category: Best Current Practice                                        
ISSN: 2070-1721

                        URI Design and Ownership


   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 substructure in URIs is often

   This document provides guidance on the specification of URI
   substructure in standards.

   This document obsoletes RFC 7320 and updates RFC 3986.

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 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2020 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Intended Audience
     1.2.  Notational Conventions
   2.  Best Current Practices for Standardizing Structured URIs
     2.1.  URI Schemes
     2.2.  URI Authorities
     2.3.  URI Paths
     2.4.  URI Queries
     2.5.  URI Fragment Identifiers
   3.  Alternatives to Specifying Structure in URIs
   4.  Security Considerations
   5.  IANA Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Appendix A.  Changes from RFC 7320

   Author's Address

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).

   Implementations can impose further constraints upon the structure of
   URIs; 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 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:

   *  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).

   *  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).

   *  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

   *  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 can preclude
      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.

   *  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 therefore sometimes
   problematic, both for these reasons and because the structure of a
   URI needs to be firmly under the control of its owner.

   This document explains some best current practices for establishing
   URI structures, conventions, and formats in standards.  It also
   offers strategies for specifications in Section 3.

1.1.  Intended Audience

   This document's guidelines and 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

   *  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, metadata for any URI, or a new format.

   *  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 any
   relevant community that 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",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Best Current Practices for Standardizing Structured URIs

   This section updates [RFC3986] by advising Specifications how they
   should define structure and semantics within URIs.  Best practices
   differ, depending on the URI component in question, as described

2.1.  URI Schemes

   Applications and Extensions can require the use of one or more
   specific URI schemes; for example, it is perfectly acceptable to
   require that an Application support "http" and "https" URIs.
   However, Applications ought not preclude the use of other URI schemes
   in the future, unless they are clearly only usable with the nominated

   A Specification that defines substructure for URI schemes overall
   (e.g., a prefix or suffix for URI scheme names) MUST do so by
   modifying [BCP35] (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 or the
   structures it relies upon (e.g., DNS name syntax, as defined in
   Section 3.5 of [RFC1034]).

   For example, an Extension or Application cannot say that the "foo"
   prefix in "https://foo_app.example.com" is meaningful or triggers
   special handling in URIs, unless they update either the "http" URI
   scheme or the DNS hostname syntax.

   Applications can 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, although these are often delegated to the
   Application(s) in a given deployment.

   To avoid collisions, rigidity, and erroneous client assumptions,
   Specifications MUST NOT define a fixed prefix for their URI paths --
   for example, "/myapp" -- unless allowed by the scheme definition.

   One such exception to this requirement is registered "well-known"
   URIs, as specified by [RFC8615].  See that document for a description
   of the applicability of that mechanism.

   Note that this does not apply to Applications defining a structure of
   a URI's path "under" a resource controlled by the server.  Because
   the prefix is under control of the party deploying the Application,
   collisions and rigidity are avoided, and the risk of erroneous client
   assumptions is reduced.

   For example, an Application might define "app_root" as a deployment-
   controlled URI prefix.  Application-defined resources might then be
   assumed to be present at "{app_root}/foo" and "{app_root}/bar".

   Extensions MUST NOT define a structure within individual URI
   components (e.g., a prefix or suffix), again to avoid collisions and
   erroneous client assumptions.

2.4.  URI Queries

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

   Applications can specify the syntax of queries for the resources
   under their control.  However, doing so 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, to
   avoid collisions and erroneous client assumptions.  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.

   Per the "Form submission" section of [HTML5], HTML constrains the
   syntax of query strings used in form submission.  New form languages
   are encouraged to allow creation of a broader variety of URIs (e.g.,
   by allowing the form to create new path components, and so forth).

2.5.  URI Fragment Identifiers

   Section 3.5 of [RFC3986] specifies fragment identifiers' syntax and
   semantics as being dependent upon the media type of a potentially
   retrieved resource.  As a result, other Specifications MUST NOT
   define structure within the fragment identifier, unless they are
   explicitly defining one for reuse by media types in their definitions
   (for example, as JSON Pointer [RFC6901] does).

   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 for which 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.

   [RFC8288] 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.

   [RFC8615] 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 described 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.  IANA Considerations

   This document has no IANA actions.

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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

6.2.  Informative References

   [BCP35]    Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
              and Registration Procedures for New URI Schemes", BCP 35,
              RFC 7595, June 2015,

   [HTML5]    WHATWG, "HTML - Living Standard", Section 4.10.21, June
              2020, <https://html.spec.whatwg.org/#form-submission>.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570,
              DOI 10.17487/RFC6570, March 2012,

   [RFC6901]  Bryan, P., Ed., Zyp, K., and M. Nottingham, Ed.,
              "JavaScript Object Notation (JSON) Pointer", RFC 6901,
              DOI 10.17487/RFC6901, April 2013,

   [RFC6943]  Thaler, D., Ed., "Issues in Identifier Comparison for
              Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May
              2013, <https://www.rfc-editor.org/info/rfc6943>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,

   [RFC8615]  Nottingham, M., "Well-Known Uniform Resource Identifiers
              (URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,

Appendix A.  Changes from RFC 7320

   Many of the requirements of RFC 7320 were removed, in the spirit of
   making this BCP guidance rather than rules.


   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:   https://www.mnot.net/