appsawg M. Nottingham
Internet-Draft
Updates: 3986 (if approved) April 7, 2014
Intended status: Best Current Practice
Expires: October 9, 2014

URI Design and Ownership
draft-ietf-appsawg-uri-get-off-my-lawn-03

Abstract

RFC3986 Section 1.1.1 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 the essentially usurps ownership. This document clarifies both this problematic practice and some acceptable alternatives in standards.

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Table of Contents

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, 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).

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

Publishing an independent standard that constrains an existing URI structure in ways which aren’t explicitly allowed by [RFC3986] (e.g., by defining it in the URI scheme) is usually 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 best current practices for establishing URI structures, conventions and formats in standards. It also offers strategies for specifications to avoid violating these guidelines in Appendix B.

1.1. Who This Document Is For

This document’s requirements primarily target a few different types of specifications:

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 the IANA.ORG Web site as part of the establishment of a registry.

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

Best practices differ depending on the URI component, as described in this section.

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 specific to the nominated schemes.

A specification that defines substructure within a URI scheme MUST do so in the defining document for that URI scheme, or by modifying [RFC4395].

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 cannot say that the “foo” prefix in “foo_app.example.com” is meaningful or triggers special handling in URIs.

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 cannot 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, since it “locks” the URIs in use; while doing so might prevent collisions, it does not avoid the other issues discussed.

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.

For example, an extension cannot be minted that indicates that all query parameters with the name “sig” indicate a cryptographic signature.

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. 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].

4. IANA Considerations

There are no direct IANA actions specified in this document.

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

5.2. Informative References

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

Appendix A. Acknowledgments

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

Appendix B. Alternatives to Specifying Structure in 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 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 adequate to conform to this document. 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”).

Author's Address

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