RFC 8820: BCP 190: URI Design and Ownership

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:¶

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.¶

2.1. 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).¶