Network Working Group T. Hansen
Request for Comments: 4395 AT&T Laboratories
Obsoletes: 2717, 2718 T. Hardie
BCP: 115 Qualcomm, Inc.
Category: Best Current Practice L. Masinter
Guidelines and Registration Procedures for New URI Schemes
Status of This Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2006).
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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Guidelines for Permanent URI Scheme Definitions . . . . . . . 4
2.1. Demonstratable, New, Long-Lived Utility . . . . . . . . . 4
2.2. Syntactic Compatibility . . . . . . . . . . . . . . . . . 5
2.3. Well-Defined . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Definition of Operations . . . . . . . . . . . . . . . . . 6
2.5. Context of Use . . . . . . . . . . . . . . . . . . . . . . 6
2.6. Internationalization and Character Encoding . . . . . . . 7
2.7. Clear Security Considerations . . . . . . . . . . . . . . 7
2.8. Scheme Name Considerations . . . . . . . . . . . . . . . . 7
3. Guidelines for Provisional URI Scheme Registration . . . . . . 8
4. Guidelines for Historical URI Scheme Registration . . . . . . 8
5. URI Scheme Registration Procedure . . . . . . . . . . . . . . 9
5.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.2. Registration Procedures . . . . . . . . . . . . . . . . . 9
5.3. Change Control . . . . . . . . . . . . . . . . . . . . . . 10
5.4. URI Scheme Registration Template . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Normative References . . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . . 13
The Uniform Resource Identifier (URI) protocol element and generic
syntax is defined by RFC 3986 . 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. This document provides guidelines
for the definition of new URI schemes, for consideration by those who
are defining, registering, or evaluating those definitions, as well
as a process and mechanism for registering URI 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 both RFCs 2717  and 2718 . 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 term "URI" for all
resource identifiers, avoiding the term "URL" and reserving the term
"URN" explicitly for those URIs using the "urn" scheme name (RFC 2141
). URN "namespaces" (RFC 3406 ) are specific to the "urn"
scheme and not covered explicitly by this document.
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 approved.
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:
o provide a central point of discovery for established URI scheme
names, and easy location of their defining documents;
o discourage use of the same URI scheme name for different purposes;
o help those proposing new URI scheme names to discern established
trends and conventions, and avoid names that might be confused
with existing ones;
o encourage registration by setting a low barrier for provisional
RFC 3987  introduced a new protocol element, the Internationalized
Resource Identifier (IRI), and defined a mapping between URIs and
IRIs. There is no separate registry or registration process for
IRIs. Those who wish to describe resource identifiers that are
useful as IRIs should define the corresponding URI syntax, and note
that the IRI usage follows the rules and transformations defined in
Within this document, the key words MUST, MAY, SHOULD, REQUIRED,
RECOMMENDED, and so forth are used within the general meanings
established in RFC 2119 , within the context that they are
requirements on future registration documents.
2. Guidelines for Permanent URI Scheme Definitions
This section gives considerations for new URI schemes. Meeting these
guidelines is REQUIRED for permanent URI scheme registration.
Meeting these guidelines is also RECOMMENDED for provisional
registration, as described in Section 3.
2.1. Demonstratable, New, Long-Lived Utility
The use and deployment of new URI schemes in the Internet
infrastructure is costly; some parts of URI processing may be
scheme-dependent, and deployed software already processes URIs of
well-known schemes. Introducing a new URI 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) . URI 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 schemes SHOULD have clear utility
to the broad Internet community, beyond that available with already
registered URI schemes.
2.2. Syntactic Compatibility
RFC 3986  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. All URI 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 Section 4.3 of RFC 3986.
New URI schemes SHOULD reuse the common URI components of RFC 3986
for the definition of hierarchical naming schemes. However, if there
is a strong reason for a URI scheme not to use the hierarchical
syntax, then the new scheme definition SHOULD follow the syntax of
previously registered schemes.
URI schemes that are not intended for use with relative URIs SHOULD
avoid use of the forward slash "/" character, which is used for
hierarchical delimiters, and the complete path segments "." and ".."
Avoid improper use of "//". The use of double slashes in the first
part of a URI is not an artistic indicator that what follows is a
URI: Double slashes are used ONLY when the syntax of the URI's
<scheme-specific-part> contains a hierarchical structure as described
in RFC 3986. In URIs from such schemes, the use of double slashes
indicates that what follows is the top hierarchical element for a
naming authority. (See Section 3.2 of RFC 3986 for more details.)
URI schemes that do not contain a conformant hierarchical structure
in their <scheme-specific-part> SHOULD NOT use double slashes
following the "<scheme>:" string.
New URI schemes SHOULD clearly define the role of RFC 3986 
reserved characters in URIs of the scheme being defined. The syntax
of the new scheme should be clear about which of the "reserved" set
of characters (as defined in RFC 3986) are used as delimiters within
the URIs of the new scheme, and when those characters must be
escaped, versus when they may be used without escaping.
While URIs may or may not be useful as locators in practice, a URI
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 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 URI scheme.
In many cases, new URI schemes are defined as ways to translate
between other namespaces or protocols and the general framework of
URIs. For example, the "ftp" URI scheme translates into the FTP
protocol, while the "mid" URI 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 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. In
particular, the mapping should describe the mechanisms for encoding
binary or character strings within valid character sequences in a URI
(See Section 2.6 for guidelines). If not all legal values or
protocol interactions of the base standard can be represented using
the URI scheme, the definition should be clear about which subset are
allowed, and why.
2.4. Definition of Operations
As part of the definition of how a URI identifies a resource, a URI
scheme definition SHOULD define the applicable set of operations that
may be performed on a resource using the URI 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 URI scheme definition should describe all
well-defined operations on the URI identifier, and what they are
supposed to do.
Some URI schemes don't fit into the "information access" paradigm of
URIs. 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 URI scheme should be documented.
Note: It is perfectly valid to say that "no operation apart from GET
is defined for this URI". It is also valid to say that "there's only
one operation defined for this URI, and it's not very GET-like". The
important point is that what is defined on this scheme is described.
2.5. Context of Use
In general, URIs are used within a broad range of protocols and
applications. Most commonly, URIs are used as references to
resources within directories or hypertext documents, as hyperlinks to
other resources. In some cases, a URI 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.
2.6. Internationalization and Character Encoding
When describing URI schemes in which (some of) the elements of the
URI 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 URI characters; see
RFC 3987  and Section 2.5 of RFC 3986  for guidelines. If URIs
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.
2.7. Clear Security Considerations
Definitions of URI schemes MUST be accompanied by a clear analysis of
the security implications for systems that use the URI scheme; this
follows the practice of Security Consideration sections within IANA
In particular, Section 7 of RFC 3986  describes general security
considerations for URI schemes. The definition of an individual URI
scheme should note which of these apply to the specified scheme.
2.8. Scheme Name Considerations
Section 3.1 of RFC 3986 defines the syntax of a URI scheme name. New
scheme registrations MUST comply. Note that although scheme names
are case insensitive, scheme names MUST be registered using lowercase
URI 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 .)
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 when the
described namespace is not.)
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
3. Guidelines for Provisional URI Scheme Registration
While the guidelines in Section 2 are REQUIRED for permanent
registration, they are RECOMMENDED for provisional registration. For
a provisional registration, the following are REQUIRED:
o The scheme name meets the syntactic requirements of Section 2.8.
o There is not already an entry with the same URI scheme name. (In
the unfortunate case that there are multiple, different uses of
the same scheme name, the IESG may approve a request to modify an
existing entry to note the separate use.)
o Contact information identifying the person supplying the
registration is included. Previously unregistered URI schemes
discovered in use may be registered by third parties on behalf of
those who created the URI scheme; in this case, both the
registering party and the scheme creator SHOULD be identified.
o If no permanent, citable specification for the URI scheme
definition is included, credible reasons for not providing it
should be given.
o A valid Security Considerations section, as required by Section 6
o If the scheme definition does not meet the guidelines laid out in
Section 2, the differences and reasons SHOULD be noted.
4. Guidelines for Historical URI Scheme Registration
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 URI 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.
5. URI Scheme Registration Procedure
The URI registration process is described in the terminology of .
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:
o Accept the URI scheme name for permanent registration.
o Suggest provisional registration instead.
o Request IETF review and IESG approval; in the meanwhile, suggest
URI 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.
5.2. Registration Procedures
Someone wishing to register a URI scheme SHOULD:
1. Check the IANA URI scheme registry to see whether or not there is
already an entry for the desired name. If there is already an
entry under the name, choose a different URI scheme name.
2. Prepare a URI scheme registration template, as specified in
Section 5.4. The URI scheme registration template may be
contained in an Internet Draft, alone or as part of some other
protocol specification. The template may also be submitted in
some other form (as part of another document or as a stand-alone
document), but the contents will be treated as an "IETF
Contribution" under the guidelines of RFC 3978 .
3. Send a copy of the template or a pointer to the containing
document (with specific reference to the section with the
template) to the mailing list firstname.lastname@example.org, requesting
review. In addition, request review on other mailing lists as
appropriate. For example, general discussion of URI syntactical
issues could be discussed on email@example.com; schemes for a network
protocol could be discussed on a mailing list for that protocol.
Allow a reasonable time for discussion and comments. Four weeks
is reasonable for a permanent registration requests.
4. Respond to review comments and make revisions to the proposed
registration as needed to bring it into line with the guidelines
given in this document.
5. Submit the (possibly updated) registration template (or pointer
to document containing it) to IANA at firstname.lastname@example.org, specifying
whether 'permanent' or 'provisional' registration is requested.
Upon receipt of a URI scheme registration request,
1. IANA checks the submission for completeness; if sections are
missing or citations are not correct, IANA rejects the
2. IANA checks the current registry for a entry with the same name;
if such a registry exists, IANA rejects the registration request.
3. IANA requests Expert Review of the registration request against
the corresponding guidelines.
4. The Designated Expert may request additional review or
discussion, as necessary.
5. If Expert Review recommends registration 'provisional' or
'permanent' registration, IANA adds the registration to the
6. Unless Expert Review has explicitly rejected the registration
request within two weeks, IANA should automatically add the
registration in the 'provisional' registry.
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.
5.3. Change Control
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
5.4. URI Scheme Registration Template
This template describes the fields that must be supplied in a URI
scheme registration request:
URI scheme name.
See Section 2.8 for guidelines.
This reflects the status requested, and should be one of
'permanent', 'provisional', or 'historical'.
URI scheme syntax.
See Section 2.2 for guidelines.
URI scheme semantics.
See Section 2.3 and Section 2.4 for guidelines.
See Section 2.3 and Section 2.6 for guidelines.
Applications/protocols that use this URI scheme name.
Applications and/or protocols that use this URI scheme name; see
If you are aware of any details regarding your scheme that might
impact interoperability, please identify them here. For example:
proprietary or uncommon encoding method; inability to support
multibyte character sets; incompatibility with types or versions
of any underlying protocol.
See Section 2.7 for guidelines.
Person (including contact information) to contact for further
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.
6. IANA Considerations
This document replaces the current "URL Scheme" registry with a new
Uniform Resource Identifier scheme registry, and establishes a new
registration template and a new process for registration. The
process is based on  "Expert Review" with an initial (optional)
mailing list review.
The template has an additional field for the status of the URI name
scheme, and the procedures for entering new name schemes have been
augmented. Section 5 establishes the process for new URI scheme
To transition to the new registry, all URL name schemes in the
existing table should be entered as URI schemes, with 'permanent'
7. Security Considerations
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 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 Paul Hoffmann, Ira McDonald, Roy Fielding, Stu Weibel,
Tony Hammond, Charles Lindsey, Mark Baker, and other members of the
email@example.com mailing list for their comments on earlier versions.
Parts of this document are based on ,  and . Some of the
ideas about use of URIs were taken from the "Architecture of the
World Wide Web" .
9.1. Normative References
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
 Moats, R., "URN Syntax", RFC 2141, May 1997.
 Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
 Bradner, S., "IETF Rights in Contributions", BCP 78, RFC 3978,
 Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
 Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
9.2. Informative References
 Petke, R. and I. King, "Registration Procedures for URL Scheme
Names", BCP 35, RFC 2717, November 1999.
 Masinter, L., Alvestrand, H., Zigmond, D., and R. Petke,
"Guidelines for new URL Schemes", RFC 2718, November 1999.
 Daigle, L., van Gulik, D., Iannella, R., and P. Faltstrom,
"Uniform Resource Names (URN) Namespace Definition Mechanisms",
BCP 66, RFC 3406, October 2002.
 Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
 W3C Technical Architecture Group, "Architecture of the World
Wide Web, Volume One", December 2004,
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