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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
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 substructure in URIs is often
problematic.
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
https://www.rfc-editor.org/info/rfc8820.
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
Provisions Relating to IETF Documents
(https://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
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
Acknowledgments
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 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:
* 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
flexibility.
* 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
specifically:
* 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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"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
below.
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
schemes.
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,
<https://www.rfc-editor.org/info/rfc2119>.
[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,
<https://www.rfc-editor.org/info/rfc3986>.
[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,
<https://www.w3.org/TR/2004/REC-webarch-20041215>.
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,
<https://www.rfc-editor.org/info/bcp35>.
[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,
<https://www.rfc-editor.org/info/rfc1034>.
[RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570,
DOI 10.17487/RFC6570, March 2012,
<https://www.rfc-editor.org/info/rfc6570>.
[RFC6901] Bryan, P., Ed., Zyp, K., and M. Nottingham, Ed.,
"JavaScript Object Notation (JSON) Pointer", RFC 6901,
DOI 10.17487/RFC6901, April 2013,
<https://www.rfc-editor.org/info/rfc6901>.
[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,
<https://www.rfc-editor.org/info/rfc8288>.
[RFC8615] Nottingham, M., "Well-Known Uniform Resource Identifiers
(URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
<https://www.rfc-editor.org/info/rfc8615>.
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.
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: https://www.mnot.net/