This document updates the guidelines and recommendations for the definition of Uniform Resource Identifier (URI) schemes, and extends the registry and guidelines to apply when the schemes are used with Internationalized Resource Identifiers (IRIs). It also updates the process and IANA registry for URI/IRI schemes. It obsoletes RFC 4395.
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2. Conformance Guidelines
3. Guidelines for Permanent URI/IRI Scheme Definitions
3.1. Demonstratable, New, Long-Lived Utility
3.2. Syntactic Compatibility
3.4. Definition of Operations
3.5. Context of Use
3.6. Internationalization and Character Encoding
3.7. Clear Security Considerations
3.8. Scheme Name Considerations
4. Guidelines for Provisional URI/IRI Scheme Registration
5. Guidelines for Historical URI/IRI Scheme Registration
6. URI/IRI Scheme Registration Procedure
6.2. Registration Procedures
6.3. Change Control
6.4. URI/IRI Scheme Registration Template
7. The "example" Scheme
8. IANA Considerations
9. Security Considerations
Appendix A. Changes Since RFC 4395
11.1. Normative References
11.2. Informative References
§ Authors' Addresses
The Uniform Resource Identifier (URI) protocol element and generic syntax is defined by  (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.). Each URI begins with a scheme name, as defined by Section 3.1 of RFC 3986, that refers to a specification for identifiers within that scheme. The URI syntax provides a federated and extensible naming system, where each scheme's specification may further restrict the syntax and semantics of identifiers using that scheme. As originally defined, URIs only allowed a limited repertoire of characters chosen from US-ASCII. An Interationalized Resource Identifier (IRI) as defined by  (Duerst, M., Masinter, L., and M. Suignard, “Internationalized Resource Identifiers (IRIs),” September 2010.), extends the URI syntax to allow characters from a much greater repertoire, to accomodate resource identifiers from the world's languages. The same schemes used in URIs are used in IRIs. The term Resource Identifier (RI) is used as a shorthand for both URIs and IRIs.
This document extends the URI scheme registry to be a registry of URI/IRI schemes (i.e., applicable to both URIs and IRIs). This document also provides updated guidelines for the definition of new schemes, for consideration by those who are defining, registering, or evaluating those definitions, as well as a process and mechanism for registering URI/IRI schemes within the IANA URI scheme registry. The registry has two parts: 'provisional' and 'permanent', with different requirements. Guidelines and requirements for both parts are given.
This document obsoletes RFC 4395 (Hansen, T., Hardie, T., and L. Masinter, “Guidelines and Registration Procedures for New URI Schemes,” February 2006.) , which in turn obsoleted RFCs 2717 (Petke, R. and I. King, “Registration Procedures for URL Scheme Names,” November 1999.)  and 2718 (Masinter, L., Alvestrand, H., Zigmond, D., and R. Petke, “Guidelines for new URL Schemes,” November 1999.) . RFCs 2717 and 2718 drew a distinction between 'locators' (identifiers used for accessing resources available on the Internet) and 'names' (identifiers used for naming possibly abstract resources, independent of any mechanism for accessing them). The intent was to use the designation "URL" (Uniform Resource Locator) for those identifiers that were locators and "URN" (Uniform Resource Name) for those identifiers that were names. In practice, the line between 'locator' and 'name' has been difficult to draw: locators can be used as names, and names can be used as locators. As a result, recent documents have used the terms "URI"/"IRI" for all resource identifiers, avoiding the term "URL" and reserving the term "URN" explicitly for those URIs/IRIs using the "urn" scheme name ( (Moats, R., “URN Syntax,” May 1997.)). URN "namespaces" ( (Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom, “Uniform Resource Names (URN) Namespace Definition Mechanisms,” October 2002.)) are specific to the "urn" scheme and not covered explicitly by this specification.
RFC 2717 defined a set of registration trees in which URI schemes could be registered, one of which was called the IETF Tree, to be managed by IANA. RFC 2717 proposed that additional registration trees might be approved by the IESG. However, no such registration trees have been submitted. This document eliminates RFC 2717's distinction between different 'trees' for URI schemes; instead there is a single namespace for registered values. Within that namespace, there are values that are approved as meeting a set of criteria for URI schemes. Other scheme names may also be registered provisionally, without necessarily meeting those criteria. The intent of the registry is to:
RFC 3987 (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.)  introduced a new protocol element, the Internationalized Resource Identifier (IRI), by defining a mapping between URIs and IRIs.  (Duerst, M., Masinter, L., and M. Suignard, “Internationalized Resource Identifiers (IRIs),” September 2010.) updates this definition, allowing an IRI to be interpreted directly without translating into a URI. There is no separate, independent registry or registration process for IRIs: the URI Scheme Registry is to be used for both URIs and IRIs. Previously, those who wish to describe resource identifiers that are useful as IRIs were encouraged to define the corresponding URI syntax, and note that the IRI usage follows the rules and transformations defined in  (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.). This document changes that advice to encourage explicit definition of the scheme and allowable syntax elements within the larger character repertoire of IRIs, as defined by  (Duerst, M., Masinter, L., and M. Suignard, “Internationalized Resource Identifiers (IRIs),” September 2010.).
Within this document, the key words MUST, MAY, SHOULD, REQUIRED, RECOMMENDED, and so forth are used within the general meanings established in  (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.), within the context that they are requirements on future registration specifications.
This section gives considerations for new URI/IRI schemes. Meeting these guidelines is REQUIRED for permanent scheme registration. Meeting these guidelines is also RECOMMENDED for provisional registration, as described in Section 4 (Guidelines for Provisional URI/IRI Scheme Registration).
The use and deployment of new URI/IRI schemes in the Internet infrastructure is costly; some parts of URI/IRI processing may be scheme-dependent, and deployed software already processes URIs and IRIs of well-known schemes. Introducing a new scheme may require additional software, not only for client software and user agents but also in additional parts of the network infrastructure (gateways, proxies, caches)  (W3C Technical Architecture Group, “Architecture of the World Wide Web, Volume One,” December 2004.). URI/IRI schemes constitute a single, global namespace; it is desirable to avoid contention over use of short, mnemonic scheme names. For these reasons, the unbounded registration of new schemes is harmful. New URI/IRI schemes SHOULD have clear utility to the broad Internet community, beyond that available with already registered URI/IRI schemes.
 (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) defines the generic syntax for all URI schemes, along with the syntax of common URI components that are used by many URI schemes to define hierarchical identifiers.  (Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.) and  (Duerst, M., Masinter, L., and M. Suignard, “Internationalized Resource Identifiers (IRIs),” September 2010.) extended this generic syntax to cover IRIs. All URI/IRI scheme specifications MUST define their own syntax such that all strings matching their scheme-specific syntax will also match the <absolute‑URI> grammar described in  (Duerst, M., Masinter, L., and M. Suignard, “Internationalized Resource Identifiers (IRIs),” September 2010.).
New schemes SHOULD reuse the common components of  (Duerst, M., Masinter, L., and M. Suignard, “Internationalized Resource Identifiers (IRIs),” September 2010.) for the definition of hierarchical naming schemes. However, if there is a strong reason for a scheme not to use the hierarchical syntax, then the new scheme definition SHOULD follow the syntax of previously registered schemes.
Schemes that are not intended for use with relative URIs/IRIs SHOULD avoid use of the forward slash "/" character, which is used for hierarchical delimiters, and the complete path segments "." and ".." (dot-segments).
Avoid improper use of "//". The use of double slashes in the first part of a URI/IRI is not an artistic indicator that what follows is a URI/IRI: Double slashes are used ONLY when the syntax of the <scheme-specific-part> contains a hierarchical structure. In URIs and IRIs from such schemes, the use of double slashes indicates that what follows is the top hierarchical element for a naming authority. (Section 3.2 of RFC 3986 has more details.) Schemes that do not contain a conformant hierarchical structure in their <scheme-specific-part> SHOULD NOT use double slashes following the "<scheme>:" string.
New schemes SHOULD clearly define the role of  (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) reserved characters in URIs/IRIs of the scheme being defined. The syntax of the new scheme should be clear about which of the "reserved" set of characters are used as delimiters within the URIs/IRIs of the new scheme, and when those characters must be escaped, versus when they may be used without escaping.
While URIs/IRIs may or may not be defined as locators in practice, a scheme definition itself MUST be clear as to how it is expected to function. Schemes that are not intended to be used as locators SHOULD describe how the resource identified can be determined or accessed by software that obtains a URI/IRI of that scheme.
For schemes that function as locators, it is important that the mechanism of resource location be clearly defined. This might mean different things depending on the nature of the scheme.
In many cases, new schemes are defined as ways to translate between other namespaces or protocols and the general framework of URIs. For example, the "ftp" scheme translates into the FTP protocol, while the "mid" scheme translates into a Message-ID identifier of an email message. For such schemes, the description of the mapping must be complete, and in sufficient detail so that the mapping in both directions is clear: how to map from a URI/IRI into an identifier or set of protocol actions or name in the target namespace, and how legal values in the base namespace, or legal protocol interactions, might be represented in a valid URI or IRI. In particular, the mapping should describe the mechanisms for encoding binary or character strings within valid character sequences in a URI/IRI (See Section 3.6 (Internationalization and Character Encoding) for guidelines). If not all legal values or protocol interactions of the base standard can be represented using the scheme, the definition should be clear about which subset are allowed, and why.
As part of the definition of how a URI/IRI identifies a resource, a scheme definition SHOULD define the applicable set of operations that may be performed on a resource using the RI as its identifier. A model for this is HTTP; an HTTP resource can be operated on by GET, POST, PUT, and a number of other operations available through the HTTP protocol. The scheme definition should describe all well-defined operations on the resource identifier, and what they are supposed to do.
Some schemes don't fit into the "information access" paradigm of URIs/IRIs. For example, "telnet" provides location information for initiating a bi-directional data stream to a remote host; the only operation defined is to initiate the connection. In any case, the operations appropriate for a scheme should be documented.
Note: It is perfectly valid to say that "no operation apart from GET is defined for this RI". It is also valid to say that "there's only one operation defined for this RI, and it's not very GET-like". The important point is that what is defined on this scheme is described.
In general, URIs/IRIs are used within a broad range of protocols and applications. Most commonly, URIs/IRIs are used as references to resources within directories or hypertext documents, as hyperlinks to other resources. In some cases, a scheme is intended for use within a different, specific set of protocols or applications. If so, the scheme definition SHOULD describe the intended use and include references to documentation that define the applications and/or protocols cited.
When describing schemes in which (some of) the elements of the URI or IRI are actually representations of human-readable text, care should be taken not to introduce unnecessary variety in the ways in which characters are encoded into octets and then into characters; see  (Duerst, M., Masinter, L., and M. Suignard, “Internationalized Resource Identifiers (IRIs),” September 2010.) and Section 2.5 of RFC 3986 (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.)  for guidelines. If URIs/IRIs of a scheme contain any text fields, the scheme definition MUST describe the ways in which characters are encoded and any compatibility issues with IRIs of the scheme.
Specifications for IRIs schemes MUST be described in terms of processing an IRI as a sequence of Unicode codepoints, without reference to the encoding of those code points as a sequence of bytes, using UTF-8 or UTF-16. The scheme specification SHOULD be as restrictive as possible regarding what characters are allowed in the URI/IRI, because some characters can create several different security considerations (see for example  (Klensin, J., Faltstrom, P., Karp, C., and IAB, “Review and Recommendations for Internationalized Domain Names (IDNs),” September 2006.)).
Definitions of schemes MUST be accompanied by a clear analysis of the security implications for systems that use the scheme; this follows the practice of Security Consideration sections within IANA registrations  (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” October 1998.).
In particular, Section 7 of RFC 3986  (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) describes general security considerations for URIs, while Section ??? of  (Duerst, M., Masinter, L., and M. Suignard, “Internationalized Resource Identifiers (IRIs),” September 2010.) gives those for IRIs. The definition of an individual URI/IRI scheme should note which of these apply to the specified scheme.
Section 3.1 of RFC 3986 defines the syntax of a URI scheme name; this sytax remains the same for IRIs. New registered schemes registrations MUST follow this syntax, which only allows a limited repertoire of characters (taken from US-ASCII). Although the syntax for the scheme name in URI/IRIs is case insensitive, the scheme names itself MUST be registered using lowercase letters.
URI/IRI scheme names should be short, but also sufficiently descriptive and distinguished to avoid problems.
Avoid names or other symbols that might cause problems with rights to use the name in IETF specifications and Internet protocols. For example, be careful with trademark and service mark names. (See Section 7.4 of RFC 3978 (Bradner, S., “IETF Rights in Contributions,” March 2005.) .)
Avoid using names that are either very general purpose or associated in the community with some other application or protocol. Avoid scheme names that are overly general or grandiose in scope (e.g., that allude to their "universal" or "standard" nature.)
Organizations that desire a private name space for URI scheme names are encouraged to use a prefix based on their domain name, expressed in reverse order. For example, a URI scheme name of com-example-info might be registered by the vendor that owns the example.com domain name.
While the guidelines in Section 3 (Guidelines for Permanent URI/IRI Scheme Definitions) are REQUIRED for permanent registration, they are RECOMMENDED for provisional registration. For a provisional registration, the following are REQUIRED:
In some circumstances, it is appropriate to note a URI scheme that was once in use or registered but for whatever reason is no longer in common use or the use is not recommended. In this case, it is possible for an individual to request that the scheme be registered (newly, or as an update to an existing registration) as 'historical'. Any scheme that is no longer in common use MAY be designated as historical; the registration should contain some indication to where the scheme was previously defined or documented.
The URI/IRI registration process is described in the terminology of  (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” October 1998.). The registration process is an optional mailing list review, followed by "Expert Review". The registration request should note the desired status. The Designated Expert will evaluate the request against the criteria of the requested status. In the case of a permanent registration request, the Designated Expert may:
URI/IRI scheme definitions contained within other IETF documents (Informational, Experimental, or Standards-Track RFCs) must also undergo Expert Review; in the case of Standards-Track documents, permanent registration status approval is required.
Someone wishing to register a new URI/IRI scheme SHOULD:
Upon receipt of a URI/IRI scheme registration request, the following steps MUST be followed:
Either based on an explicit request or independently initiated, the Designated Expert or IESG may request the upgrade of a 'provisional' registration to a 'permanent' one. In such cases, IANA should move the corresponding entry from the provisional registry.
Registrations may be updated in each registry by the same mechanism as required for an initial registration. In cases where the original definition of the scheme is contained in an IESG-approved document, update of the specification also requires IESG approval.
Provisional registrations may be updated by the original registrant or anyone designated by the original registrant. In addition, the IESG may reassign responsibility for a provisional registration scheme, or may request specific changes to a scheme registration. This will enable changes to be made to schemes where the original registrant is out of contact, or unwilling or unable to make changes.
Transition from 'provisional' to 'permanent' status may be requested and approved in the same manner as a new 'permanent' registration. Transition from 'permanent' to 'historical' status requires IESG approval. Transition from 'provisional' to 'historical' may be requested by anyone authorized to update the provisional registration.
This template describes the fields that must be supplied in a URI/IRI scheme registration request:
- Resource Identifier (RI) Scheme name.
- See Section 3.8 (Scheme Name Considerations) for guidelines.
- This reflects the status requested, and should be one of 'permanent', 'provisional', or 'historical'.
- Scheme syntax.
- See Section 3.2 (Syntactic Compatibility) for guidelines.
- Scheme semantics.
- See Section 3.3 (Well-Defined) and Section 3.4 (Definition of Operations) for guidelines.
- Encoding considerations.
- See Section 3.3 (Well-Defined) and Section 3.6 (Internationalization and Character Encoding) for guidelines.
- Applications/protocols that use this scheme name.
- See Section 3.5 (Context of Use).
- Interoperability considerations.
- If the person or group registering the scheme is aware of any details regarding the scheme that might impact interoperability, identify them here. For example: proprietary or uncommon encoding methods; inability to support multibyte character sets; incompatibility with types or versions of any underlying protocol.
- Security considerations.
- See Section 3.7 (Clear Security Considerations) for guidelines.
- Person (including contact information) to contact for further information.
- Author/Change controller.
- Person (including contact information) authorized to change this, if a provisional registration.
- Include full citations for all referenced documents. Registration templates for provisional registration may be included in an Internet Draft; when the documents expire or are approved for publication as an RFC, the registration will be updated.
There is a need for a URI/IRI Scheme name that can be used for examples in documentation without fear of conflicts with current or future actual schemes. The URI/IRI Scheme "example" is hereby registered as a Permanent URI/IRI Scheme for that purpose.
- Scheme name
- Scheme syntax
- The entire range of allowable syntax for URI/IRI schemes specified in  (Duerst, M., Masinter, L., and M. Suignard, “Internationalized Resource Identifiers (IRIs),” September 2010.) is allowed for "example" URI/IRIs.
- Scheme semantics
- URI/IRIs in the "example" scheme should be used for documentation purposes only. The use of "example" URIs/IRIs must not be used as locators, identify any resources, or specify any particular set of operations.
- Encoding considerations
- See Section 2.5 of  (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) for guidelines.
- Applications/protocols that use this URI scheme name
- The "example" URI should be used for documentation purposes only. It MUST not be used for any protocol.
- Interoperability considerations
- Security considerations
- Author/Change controller
- This RFC XXXX. RFC Editor Note: Replace XXXX with this RFC's reference.
Previously, the former "URL Scheme" registry was replaced by the Uniform Resource Identifier scheme registry. The process was based on  (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” October 1998.) "Expert Review" with an initial (optional) mailing list review.
The updated template has an additional field for the status of the scheme, and the procedures for entering new name schemes have been augmented. Section 6 (URI/IRI Scheme Registration Procedure) establishes the process for new URI/IRI scheme registration.
The example URI scheme "example" is hereby registered. (See the template above for registration.)
All registered values are expected to contain accurate security consideration sections; 'permanent' registered scheme names are expected to contain complete definitions.
Information concerning possible security vulnerabilities of a protocol may change over time. Consequently, claims as to the security properties of a registered URI/IRI scheme may change as well. As new vulnerabilities are discovered, information about such vulnerabilities may need to be attached to existing documentation, so that users are not misled as to the true security properties of a registered URI scheme.
Many thanks to Patrick Faltstrom for his comments on this version.
Many thanks to Paul Hoffmann, Ira McDonald, Roy Fielding, Stu Weibel, Tony Hammond, Charles Lindsey, Mark Baker, and other members of the firstname.lastname@example.org mailing list for their comments on earlier versions.
Parts of this document are based on  (Petke, R. and I. King, “Registration Procedures for URL Scheme Names,” November 1999.),  (Masinter, L., Alvestrand, H., Zigmond, D., and R. Petke, “Guidelines for new URL Schemes,” November 1999.) and  (Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields,” September 2004.). Some of the ideas about use of URIs were taken from the "Architecture of the World Wide Web"  (W3C Technical Architecture Group, “Architecture of the World Wide Web, Volume One,” December 2004.).
|||Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).|
|||Moats, R., “URN Syntax,” RFC 2141, May 1997 (TXT, HTML, XML).|
|||Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” BCP 26, RFC 2434, October 1998 (TXT, HTML, XML).|
|||Bradner, S., “IETF Rights in Contributions,” RFC 3978, March 2005 (TXT).|
|||Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” STD 66, RFC 3986, January 2005 (TXT, HTML, XML).|
|||Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” RFC 3987, January 2005 (TXT).|
|||Duerst, M., Masinter, L., and M. Suignard, “Internationalized Resource Identifiers (IRIs),” September 2010.|
|||Petke, R. and I. King, “Registration Procedures for URL Scheme Names,” BCP 35, RFC 2717, November 1999 (TXT, HTML, XML).|
|||Masinter, L., Alvestrand, H., Zigmond, D., and R. Petke, “Guidelines for new URL Schemes,” RFC 2718, November 1999 (TXT, HTML, XML).|
|||Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom, “Uniform Resource Names (URN) Namespace Definition Mechanisms,” BCP 66, RFC 3406, October 2002 (TXT).|
|||Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields,” BCP 90, RFC 3864, September 2004 (TXT).|
|||Hansen, T., Hardie, T., and L. Masinter, “Guidelines and Registration Procedures for New URI Schemes,” BCP 35, RFC 4395, February 2006 (TXT).|
|||Klensin, J., Faltstrom, P., Karp, C., and IAB, “Review and Recommendations for Internationalized Domain Names (IDNs),” RFC 4690, September 2006 (TXT).|
|||W3C Technical Architecture Group, “Architecture of the World Wide Web, Volume One,” December 2004.|
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