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Internet Engineering Task Force (IETF)                     M. NottinghamRequest for Comments: 7320BCP: 190                                                       July 2014Updates:3986Category: Best Current PracticeISSN: 2070-1721URI Design and OwnershipAbstractSection 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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7320.Nottingham                Best Current Practice                 [Page 1]

RFC 7320                  URI Design Ownership                 July 2014Copyright Notice   Copyright (c) 2014 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   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 Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .21.1.  Intended Audience . . . . . . . . . . . . . . . . . . . .41.2.  Notational Conventions  . . . . . . . . . . . . . . . . .42.  Best Current Practices for Standardizing Structured URIs  . .42.1.  URI Schemes . . . . . . . . . . . . . . . . . . . . . . .52.2.  URI Authorities . . . . . . . . . . . . . . . . . . . . .52.3.  URI Paths . . . . . . . . . . . . . . . . . . . . . . . .52.4.  URI Queries . . . . . . . . . . . . . . . . . . . . . . .62.5.  URI Fragment Identifiers  . . . . . . . . . . . . . . . .63.  Alternatives to Specifying Structure in URIs  . . . . . . . .74.  Security Considerations . . . . . . . . . . . . . . . . . . .75.  References  . . . . . . . . . . . . . . . . . . . . . . . . .85.1.  Normative References  . . . . . . . . . . . . . . . . . .85.2.  Informative References  . . . . . . . . . . . . . . . . .8Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .91.  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).Nottingham                Best Current Practice                 [Page 2]

RFC 7320                  URI Design Ownership                 July 2014   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 inSection 3.Nottingham                Best Current Practice                 [Page 3]

RFC 7320                  URI Design Ownership                 July 20141.1.  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.Nottingham                Best Current Practice                 [Page 4]

RFC 7320                  URI Design Ownership                 July 20142.1.  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   inSection 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).Nottingham                Best Current Practice                 [Page 5]

RFC 7320                  URI Design Ownership                 July 20142.4.  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).Nottingham                Best Current Practice                 [Page 6]

RFC 7320                  URI Design Ownership                 July 20143.  Alternatives to Specifying Structure in URIs   Given the issues described inSection 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 inSection 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].Nottingham                Best Current Practice                 [Page 7]

RFC 7320                  URI Design Ownership                 July 20145.  References5.1.  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,RFC3986, January 2005.   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type              Specifications and Registration Procedures",BCP 13,RFC6838, 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>.Nottingham                Best Current Practice                 [Page 8]

RFC 7320                  URI Design Ownership                 July 2014Appendix 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/Nottingham                Best Current Practice                 [Page 9]