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General URI structure Sometimes, it is attractive to add features to protocols or applications by specifying a particular structure for URIs (or parts thereof). This document cautions against this practice in standards (sometimes called “URI Squatting”).

URIs very often include structure and application data. This might include artefacts from filesystems (often occuring 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 is choosing to use the server or the software, this can be seen as reasonable delegation of authority. When such conventions are mandated by standards, however, it can have several potentially detrimental effects: 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). 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 Section 3.5.1), or its inclusion causes several alternate forms of the URI to exist (see Section 2.3.1). Brittleness - A standard that specifies a static URI cannot change its form in future revisions. Operational Difficulty - Supporting some URI conventions can be difficult in some implementations. For example, specifying that a particular query parameter be used 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. Client Assumptions - When conventions are standardised, 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 false positives. While it is not ideal when a server or a deployed application constrains uri structure (indeed, this is not recommended practice, but that discussion is out of scope for this document), recommending standards that mandate URI structure 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 organisations) should not usurp this ownership; see Section 2.2.2.1. 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 .

These guidelines are IETF Best Current Practice, and are therefore binding upon IETF standards-track documents, as well as submissions to the RFC Editor on the Independent and IRTF streams. See and for more information. Other Open Standards organisations (in the sense of ) are encouraged to adopt them. Questions as to their applicability ought to be handled through the liaison relationship, if present. Ad hoc efforts are also encouraged to adopt them, as this RFC reflects Best Current Practice. This document’s requirements specifically targets a few different types of specifications: URI Scheme Definitions (“scheme definitions”) - specifications that define and register URI schemes, as per . 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. 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 specifications, including both those enumerated above 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 specify the semantics of a URI on the IANA.ORG Web site as part of the establishment of a registry.

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 .

Different components of a URI have differing practices recommended.

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 , or they are the registration document for the URI scheme(s) in question.

Scheme definitions define the presence, format and semantics of an authority component in URIs; all other specifications MUST NOT constrain, define structure or semantics for them.

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 any path component. The only exception to this requirement is registered “well-known” URIs, as specified by . See that document for a description of the applicability of that mechanism.

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 , Section 17.13.4.

Media type definitions (as per 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.

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. describes a framework for identifying the semantics of a link in a “link relation type” to aid this. 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. 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 docuement. For example, prefixing query parameters with “myapp_” does not help.

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

This document clarifies appropriate registry policy for new URI schemes, and potentially for the creation of new URI-related registries, if they attempt to mandate structure within URIs. There are no direct IANA actions specified in this document.

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General keyword In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. Authors who follow these guidelines should incorporate this phrase near the beginning of their document: 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 RFC 2119. Note that the force of these words is modified by the requirement level of the document in which they are used. World Wide Web Consortium

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Applications uniform resource identifier URI URL URN WWW resource A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. This document provides guidelines and recommendations for the definition of Uniform Resource Identifier (URI) schemes. It also updates the process and IANA registry for URI schemes. It obsoletes both RFC 2717 and RFC 2718. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document defines procedures for the specification and registration of media types for use in HTTP, MIME, and other Internet protocols. This memo documents an Internet Best Current Practice. Harvard University

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This memo documents the process used by the Internet community for the standardization of protocols and procedures. It defines the stages in the standardization process, the requirements for moving a document between stages and the types of documents used during this process. It also addresses the intellectual property rights and copyright issues associated with the standards process. Internet Architecture Board This document describes the framework for an RFC Series and an RFC Editor function that incorporate the principles of organized community involvement and accountability that has become necessary as the Internet technical community has grown, thereby enabling the RFC Series to continue to fulfill its mandate. This memo provides information for the Internet community. This memo defines a path prefix for "well-known locations", "/.well-known/", in selected Uniform Resource Identifier (URI) schemes. [STANDARDS-TRACK] This document specifies relation types for Web links, and defines a registry for them. It also defines the use of such links in HTTP headers with the Link header field. [STANDARDS-TRACK] A URI Template is a compact sequence of characters for describing a range of Uniform Resource Identifiers through variable expansion. This specification defines the URI Template syntax and the process for expanding a URI Template into a URI reference, along with guidelines for the use of URI Templates on the Internet. [STANDARDS-TRACK] W3C W3C

Thanks to David Booth, Anne van Kesteren and Erik Wilde for their suggestions and feedback