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rfc:rfc7320

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.

Nottingham Best Current Practice [Page 1] RFC 7320 URI Design Ownership July 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
 (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 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).

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 in Section 3.

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

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.

Nottingham Best Current Practice [Page 4] RFC 7320 URI Design Ownership July 2014

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).

Nottingham Best Current Practice [Page 5] RFC 7320 URI Design Ownership July 2014

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).

Nottingham Best Current Practice [Page 6] RFC 7320 URI Design Ownership July 2014

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].

Nottingham Best Current Practice [Page 7] RFC 7320 URI Design Ownership July 2014

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>.

Nottingham Best Current Practice [Page 8] RFC 7320 URI Design Ownership July 2014

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/

Nottingham Best Current Practice [Page 9]

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