Standardising Structure in URIs
draft-nottingham-uri-get-off-my-lawn-00

Network Working Group                                      M. Nottingham
Internet-Draft                                            August 2, 2013
Updates: 3986 (if approved)
Intended status: BCP
Expires: February 3, 2014

                    Standardising Structure in URIs
                draft-nottingham-uri-get-off-my-lawn-00

Abstract

   It is sometimes attractive to specify a particular structure for URIs
   (or parts thereof) to add support for a new feature, application or
   facility.  This memo provides guidelines for such situations in
   standards documents.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 3, 2014.

Copyright Notice

   Copyright (c) 2013 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.

Nottingham              Expires February 3, 2014                [Page 1]
Internet-Draft           URI Structure Policies              August 2013

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . . . 4
   2.  Best Current Practices for Standarising Structured URIs . . . . 4
     2.1.  URI Schemes . . . . . . . . . . . . . . . . . . . . . . . . 4
     2.2.  URI Authorities . . . . . . . . . . . . . . . . . . . . . . 4
     2.3.  URI Paths . . . . . . . . . . . . . . . . . . . . . . . . . 5
     2.4.  URI Queries . . . . . . . . . . . . . . . . . . . . . . . . 5
     2.5.  URI Fragment Identifiers  . . . . . . . . . . . . . . . . . 5
   3.  Alternatives to Specifying Static URIs  . . . . . . . . . . . . 5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     5.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     5.2.  Informative References  . . . . . . . . . . . . . . . . . . 6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 7

Nottingham              Expires February 3, 2014                [Page 2]
Internet-Draft           URI Structure Policies              August 2013

1.  Introduction

   URIs [RFC3986] very often include structure and application data.
   This might include artefacts from filesystems (often occuring in the
   path component), user information (often in the query component) and
   application data throughout.  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).

   Such conventions for 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).

   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
      conventions).
   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  Operational Difficulty - Supporting some URI conventions can be
      difficult in some implementations.  For example, specifying that a
      particular query parameter be used preclude the use of Web servers
      that serve the response from a filesystem.
   o  Client Assumptions - When conventions are standardised, some
      clients will inevitably assume that the standards are in use when
      they are seen.  This can lead to interoperability problems.

   At a more philosophical level, the structure of a URI needs to be
   firmly under the control of a single party; its owner.  Standardising
   parts of a URI's structure usurps that control; see [webarch] Section
   2.2.2.1 for more information.

   This memo explains best current practices for establishing URI
   structures, conventions and formats in specifications; in particular,
   IETF specifications, although they are more broadly applicable.  It
   also offers strategies for specifications to avoid violating these
   guidelines in Section 3.

Nottingham              Expires February 3, 2014                [Page 3]
Internet-Draft           URI Structure Policies              August 2013

1.1.  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 Standarising Structured URIs

   These guidelines target 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 standards, including both those enumerated above above and
   others.  They are also applicable to non-standard RFC publications.

   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.

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
   schemes.

   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

Nottingham              Expires February 3, 2014                [Page 4]
Internet-Draft           URI Structure Policies              August 2013

   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 them.

   The only exception to this requirement is registered "well-known"
   URIs, as specified by [RFC5785].

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
   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 artefacts that are exchanged as part
   of the protocol, rather than staticly 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

Nottingham              Expires February 3, 2014                [Page 5]
Internet-Draft           URI Structure Policies              August 2013

   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
   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.

4.  Security Considerations

   This memo does not introduce new protocol artefacts with security
   considerations.

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.

   [RFC4395]  Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
              Registration Procedures for New URI Schemes", BCP 35,
              RFC 4395, February 2006.

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

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

5.2.  Informative References

   [HTML4]    Jacobs, I., Le Hors, A., and D. Raggett, "HTML 4.01
              Specification", December 1999,
              <http://www.w3.org/TR/REC-html40/>.

Nottingham              Expires February 3, 2014                [Page 6]
Internet-Draft           URI Structure Policies              August 2013

   [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,
              <http://www.w3.org/TR/2004/REC-webarch-20041215>.

Author's Address

   Mark Nottingham

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

Nottingham              Expires February 3, 2014                [Page 7]