|Title||The IETF XML Registry
|Status:||BEST CURRENT PRACTICE
Network Working Group M. Mealling
Request for Comments: 3688 VeriSign, Inc.
BCP: 81 January 2004
Category: Best Current Practice
The IETF XML Registry
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 (2004). All Rights Reserved.
This document describes an IANA maintained registry for IETF
standards which use Extensible Markup Language (XML) related items
such as Namespaces, Document Type Declarations (DTDs), Schemas, and
Resource Description Framework (RDF) Schemas.
Over the past few years, the Extensible Markup Language (XML)
[W3C.REC-xml] has become a widely used method for data markup. There
have already been several IETF Working Groups that have produced
standards that define XML Document Type Definitions (DTDs), XML
Namespaces [W3C.REC-xml-names], and XML Schemas [W3C.REC-xmlschema-
1]. Each one of these technologies uses Uniform Resource Identifiers
(URIs) [RFC2396] and other standardized identifiers to identify
For example, while it has been the practice within some standards
that use Document Type Definitions (DTDs) to forego the use of the
PUBLIC identifiers in favor of 'well known' SYSTEM identifiers, it
has proven to be more trouble than its worth to attempt to
standardize SYSTEM identifiers. The result is that several IETF
standards that have simply created non-resolvable URIs in order to
simply identify but not resolve the DTD for some given XML document.
This document seeks to standardize and improve these practices by
creating an IANA maintained registry of XML element identifiers so
that document authors and implementors have a well maintained and
authoritative location for their XML elements. As part of this
standard, the IANA will maintain:
o the public representation of the document,
o the URI for the elements if one is provided at the time of
o a registry of Public Identifiers as URIs.
In the case where the registrant does not request a particular URI,
the IANA will assign it a Uniform Resource Name (URN) that follows
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 BCP 14, RFC 2119
3. Registerable Documents
3.1. The Assigned/Registered URI
All elements (except PUBLIC identifiers) in this registry will
require a URI in order to be registered. If the registrant wishes to
have a URI assigned, then a URN of the form
will be assigned where <class> is the type of the document being
registered (see below). <id> is a unique id generated by the IANA
based on any means the IANA deems necessary to maintain uniqueness
and persistence. NOTE: in order for a URN of this type to be
assigned, the item being registered MUST have been through the IETF
consensus process. Basically, this means that it must be documented
in a RFC. The RFC 3553 [RFC3553] URN registration template is found
in Section 6.
The IANA will also maintain a file server available via at least HTTP
and FTP that contains all of the registered elements in some publicly
accessible file space in the same way that all of the IANA's
registered elements are available via
http://www.iana.org/assignments/. While the directory structure of
this server is up to the IANA, it is suggested that the files be
organized by the <class> and the individual files have the <id> as
Implementors are warned that they should not programatically rely on
those resources being available or the directory structure remaining
static for any reason. It is explicitly recognized that some
software tools attempt to download DTDs, schema, etc., 'on the fly'
and that developers should understand when this is done and when to
not reference IANA network resources as a 'schema download
repository'. This is the reason that the IANA will not register or
provide SYSTEM identifiers.
3.2. Registerable Classes
The list of types of XML elements that can be registered with the
publicid -- An XML document that contains a DOCTYPE declaration or
any other external reference can identify that reference via both
a PUBLIC identifier and a SYSTEM identifier. The SYSTEM
identifier is system-specific information that enables the entity
manager of an XML system to locate the file, memory location, or
pointer within a file where the entity can be found. It should
also be noted that a system identifier could be an invocation of a
program that controls access to an entity that is being
identified. Thus, they are not registered items. In many cases,
SYSTEM identifiers are also URIs. However, in these cases, the
URI is still only used for system-specific information. In the
case where a PUBLIC Identifier is also a URI, it is possible for
the SYSTEM Identifier to contain the same URI but this behavior is
not recommended unless its side effects are well known and
understood to not cause any unacceptable harm.
A PUBLIC identifier is a name that is intended to be meaningful
across systems and different user environments. Typically, it
will be a name that has a registered owner associated with it, so
that public identifiers will be guaranteed unique and no two
entities will have the same public identifier. In practice,
PUBLIC identifiers are typically Formal Public Identifiers
[ISO.8879.1986] but they are not restricted to just that set. As
said in [RFC3151]:
"Any string which consists only of the public identifier
characters (defined by Production 13 of Extensible Markup
Language (XML) 1.0 Second Edition) is a legal public
Therefore, it is legal for a PUBLIC identifier to be a URN if it
adheres to the character set restrictions.
Thus, the identifier registered along with a DTD is its PUBLIC
identifier. The only restriction being that it must adhere to the
character set restrictions. In the case where the registrant does
not provide one, the IANA will assign one of the form
'urn:ietf:params:xml:pi:<id>'. Registrants are encouraged to
investigate RFC 3151 [RFC3151] as a recommended method for
minting a URN that can also be represented as an FPI.
ns -- XML Namespaces [W3C.REC-xml-names] are named by a URI. They
have no real, machine-parseable representation. Thus, the
registered document will be either the specification or a
reference to it. In the case where a URI is not provided by the
registrant, the IANA will assign a URN of the form
'urn:ietf:params:xml:ns:<id> which will be the XML Namespace's
schema -- XML Schemas [W3C.REC-xmlschema-1] are also identified by a
URI but their contents are machine parseable. The IANA registered
document will be the XML Schema file. The URN the IANA assigns
can be used as the URI for the schema and is of the form
rdfschema -- The Resource Description Format (RDF)
[W3C.CR-rdf-schema] is an XML serialization of a connected graph
based data model used for metadata expression. RDF makes use of
schemas for RDF that express grammars about relationships between
URIs. These grammars are identified by URIs. The URN assigned by
the IANA can be used as the identifying URI and is of the form
4. Registration Procedures
Until the IANA requests or implements an automated process for the
registration of these elements, any specifications must make that
request part of the IANA considerations section of their respective
documents. That request must be in the form of the following
The URI or PUBLIC identifier that identifies the XML component. If
the registrant is requesting that the IANA assign a URI then this
field should be specified as "please assign".
The individual/organization that is the registration contact for
the component being registered. Ideally, this will be the name
and pertinent physical and network contact information. In the
case of IETF developed standards, the Registrant will be the IESG.
The exact XML to be stored in the registry. Unless the beginning
and end of the file is obvious, the document should use the text
"BEGIN" to mark the beginning of the file and "END" to mark the
end of the file. The IANA will insert any text between those two
strings (minus any page breaks and RFC formatting inserted by the
RFC Editor) into the file kept in the repository.
5. Security Considerations
The information maintained by the IANA will be authoritative and will
be a target for attack. In some cases, such as XML Schema and DTDs,
the content maintained by the IANA may be directly input into
software. Thus, extra care should be taken by the IANA to maintain
the security precautions required for an important reference location
for the Internet.
Beyond this concern, there are no other security considerations not
already found with any other IANA registry.
6. IANA Considerations
This document seeks to create a rather large registry for which the
IANA (at the direction of the IESG) will be primarily responsible.
The amount of effort required to maintain this registry is not
insignificant and the policies and procedures surrounding any
approval process are non-trivial. The registry is on a First Come
First Served basis, but a Specification is Required. Once the IETF
has some experience with this registry, these policies may change.
RFC 3553 [RFC3553] specifies that any new registry requiring a name,
to be assigned below the 'urn:ietf:params' namespace and must specify
the structure of that space in template form. The IANA has created
and will maintain this new sub-namespace:
Specification: This document contains the registry specification.
The namespace is organized with one sub-namespace which is the
Repository: To be assigned according to the guidelines found above.
Index value: The class name
7. Normative References
[ISO.8879.1986] International Organization for Standardization,
"Information processing - Text and office
systems - Standard generalized markup language
(SGML)", ISO Standard 8879, 1986.
[RFC2119] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14, RFC 2119,
[RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter,
"Uniform Resource Identifiers (URI): Generic
Syntax", RFC 2396, August 1998.
[RFC3151] Walsh, N., Cowan, J. and P. Grosso, "A URN
Namespace for Public Identifiers", RFC 3151,
[RFC3553] Mealling, M., Masinter, L., Hardie, T. and G.
Klyne, "An IETF URN Sub-namespace for
Registered Protocol Parameters", BCP 73, RFC
3553, June 2003.
[W3C.CR-rdf-schema] Brickley, D. and R. Guha, "Resource Description
Framework (RDF) Schema Specification 1.0", W3C
CR-rdf-schema, March 2000,
[W3C.REC-xml] Bray, T., Paoli, J., Sperberg-McQueen, C. and
E. Maler, "Extensible Markup Language (XML) 1.0
(2nd ed)", W3C REC-xml, October 2000,
[W3C.REC-xml-names] Bray, T., Hollander, D. and A. Layman,
"Namespaces in XML", W3C REC-xml-names, January
[W3C.REC-xmlschema-1] Thompson, H., Beech, D., Maloney, M. and N.
Mendelsohn, "XML Schema Part 1: Structures",
W3C REC-xmlschema-1, May 2001,
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9. Author's Address
Mountain View, CA
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