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Network Working Group S. Hollenbeck Request for Comments: 3470 VeriSign, Inc. BCP: 70 M. Rose Category: Best Current Practice Dover Beach Consulting, Inc.

                                                           L. Masinter
                                            Adobe Systems Incorporated
                                                          January 2003
     Guidelines for the Use of Extensible Markup Language (XML)
                       within IETF Protocols

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 Notice

 Copyright (C) The Internet Society (2003).  All Rights Reserved.


 The Extensible Markup Language (XML) is a framework for structuring
 data.  While it evolved from Standard Generalized Markup Language
 (SGML) -- a markup language primarily focused on structuring
 documents -- XML has evolved to be a widely-used mechanism for
 representing structured data.
 There are a wide variety of Internet protocols being developed; many
 have need for a representation for structured data relevant to their
 application.  There has been much interest in the use of XML as a
 representation method.  This document describes basic XML concepts,
 analyzes various alternatives in the use of XML, and provides
 guidelines for the use of XML within IETF standards-track protocols.

Table of Contents

 Conventions Used In This Document  . . . . . . . . . . . . . . . .  2
 1.    Introduction and Overview  . . . . . . . . . . . . . . . . .  2
       1.1   Intended Audience. . . . . . . . . . . . . . . . . . .  3
       1.2   Scope  . . . . . . . . . . . . . . . . . . . . . . . .  3
       1.3   XML Evolution  . . . . . . . . . . . . . . . . . . . .  3
       1.4   XML Users, Support Groups, and Additional
             Information. . . . . . . . . . . . . . . . . . . . . .  4
 2.    XML Selection Considerations . . . . . . . . . . . . . . . .  4
 3.    XML Alternatives . . . . . . . . . . . . . . . . . . . . . .  5

Hollenbeck, et al. Best Current Practice [Page 1] RFC 3470 XML within IETF Protocols January 2003

 4.    XML Use Considerations and Recommendations . . . . . . . . .  7
       4.1   XML Syntax and Well-Formedness . . . . . . . . . . . .  7
       4.2   XML Information Set  . . . . . . . . . . . . . . . . .  7
       4.3   Syntactic Restrictions . . . . . . . . . . . . . . . .  8
       4.4   XML Declarations . . . . . . . . . . . . . . . . . . .  9
       4.5   XML Processing Instructions  . . . . . . . . . . . . .  9
       4.6   XML Comments . . . . . . . . . . . . . . . . . . . . . 10
       4.7   Validity and Extensibility . . . . . . . . . . . . . . 10
       4.8   Semantics as Well as Syntax. . . . . . . . . . . . . . 12
       4.9   Namespaces . . . . . . . . . . . . . . . . . . . . . . 12
             4.9.1 Namespaces and Attributes. . . . . . . . . . . . 13
       4.10  Element and Attribute Design Considerations. . . . . . 14
       4.11  Binary Data and Text with Control Characters . . . . . 16
       4.12  Incremental Processing . . . . . . . . . . . . . . . . 16
       4.13  Entity Declarations and Entity References  . . . . . . 16
       4.14  External References  . . . . . . . . . . . . . . . . . 17
       4.15  URI Processing . . . . . . . . . . . . . . . . . . . . 17
       4.16  White Space  . . . . . . . . . . . . . . . . . . . . . 18
       4.17  Interaction with the IANA  . . . . . . . . . . . . . . 19
 5.    Internationalization Considerations  . . . . . . . . . . . . 19
       5.1   Character Sets and Encodings . . . . . . . . . . . . . 19
       5.2   Language Declaration . . . . . . . . . . . . . . . . . 20
       5.3   Other Internationalization Considerations  . . . . . . 20
 6.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 21
 7.    Security Considerations  . . . . . . . . . . . . . . . . . . 21
 8.    Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22
 9.    Normative References . . . . . . . . . . . . . . . . . . . . 22
 10.   Informative References . . . . . . . . . . . . . . . . . . . 23
 11.   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 27
 12.   Full Copyright Statement . . . . . . . . . . . . . . . . . . 28

Conventions Used In This Document

 This document recommends, as policy, what specifications for Internet
 protocols -- and, in particular, IETF standards track protocol
 documents -- should include as normative language within them.  The
 capitalized keywords "SHOULD", "MUST", "REQUIRED", etc. are used in
 the sense of how they would be used within other documents with the
 meanings as specified in BCP 14, RFC 2119 [1].

1. Introduction and Overview

 The Extensible Markup Language (XML, [8]) is a framework for
 structuring data.  While it evolved from the Standard Generalized
 Markup Language (SGML, [30]) -- a markup language primarily focused
 on structuring documents -- XML has evolved to be a widely-used
 mechanism for representing structured data in protocol exchanges.
 See "XML in 10 points" [47] for an introduction to XML.

Hollenbeck, et al. Best Current Practice [Page 2] RFC 3470 XML within IETF Protocols January 2003

1.1 Intended Audience

 Many Internet protocol designers are considering using XML and XML
 fragments within the context of existing and new Internet protocols.
 This document is intended as a guide to XML usage and as IETF policy
 for standards track documents.  Experienced XML practitioners will
 likely already be familiar with the background material here, but the
 guidelines are intended to be appropriate for those readers as well.

1.2 Scope

 This document is intended to give guidelines for the use of XML
 content within a larger protocol.  The goal is not to suggest that
 XML is the "best" or "preferred" way to represent data; rather, the
 goal is to lay out the context for the use of XML within a protocol
 once other factors point to XML as a possible data representation
 solution.  The Common Name Resolution Protocol (CNRP, [24]) is an
 example of a protocol that would be addressed by these guidelines if
 it were being newly defined.  This document does not address the use
 of protocols like SMTP or HTTP to send XML documents as ordinary
 email or web content.
 There are a number of protocol frameworks already in use or under
 development which focus entirely on "XML protocol" -- the exclusive
 use of XML as the data representation in the protocol.  For example,
 the World Wide Web Consortium (W3C) is developing an XML Protocol
 framework based on SOAP ([45] and [46]).  The applicability of such
 protocols is not part of the scope of this document.
 In addition, there are higher-level representation frameworks, based
 on XML, that have been designed as carriers of certain classes of
 information; for example, the Resource Description Framework (RDF,
 [38]) is an XML-based representation for logical assertions.  This
 document does not provide guidelines for the use of such frameworks.

1.3 XML Evolution

 XML 1.0 was originally published as a W3C recommendation in February
 1998 [35], and was revised in a 2nd edition [8] in October 2000.
 Several additional facilities have also been defined that layer on
 the base specification.  Although these additions are designed to be
 consistent with XML 1.0, they have varying levels of stability,
 consensus, and implementation.  Accordingly, this document identifies
 the major evolutionary features of XML and makes suggestions as to
 the circumstances in which each feature should be used.

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1.4 XML Users, Support Groups, and Additional Information

 There are many XML support groups, with some devoted to the entire
 XML industry [51], some devoted to developers [52], some devoted to
 the business applications of XML [53], and many, many groups devoted
 to the use of XML in a particular context.
 It is beyond the scope of this document to provide a comprehensive
 list of referrals.  Interested readers are directed to the three
 references above as starting points, as well as their favorite
 Internet search engine.

2. XML Selection Considerations

 XML is a tool that provides a means towards an end.  Choosing the
 right tool for a given task is an essential part of ensuring that the
 task can be completed in a satisfactory manner.  This section
 describes factors to be aware of when considering XML as a tool for
 use in IETF protocols:
 1.  XML is a meta-markup language that can be used to define markup
     languages for specific domains and problem spaces.
 2.  XML provides both logical structure and physical structure to
     describe data.  Data framing is built-in.
 3.  XML instances can be validated against the formal definition of a
     protocol specification.
 4.  XML supports internationalization.
 5.  XML is extensible.  Unlike some other markup languages (such as
     HTML), new tags (and thus new protocol elements) can be defined
     without requiring changes to XML itself.
 6.  XML is still evolving.  The formal specifications are still being
     influenced and updated as use experience is gained and applied.
 7.  XML does not provide native mechanisms to support detailed data
     typing.  Additional mechanisms  (such as those described in
     Section 4.7) are required to specify abstract protocol data
 8.  XML is text-based, so XML fragments are easily created, edited,
     and managed using common utilities.  Further, being text-based
     means it more readily supports incremental development,

Hollenbeck, et al. Best Current Practice [Page 4] RFC 3470 XML within IETF Protocols January 2003

     debugging, and logging.  A simple "canned" XML fragment can be
     embedded within a program as a string constant, rather than
     having to be constructed.
 9.  Binary data has to be encoded into a text-based form to be
     represented in XML.
 10. XML is verbose when compared with many other structured data
     representation languages.  A representation with element
     extensibility and human readability typically requires more bits
     when compared to one optimized for efficient machine processing.
 11. XML implementations are still relatively new.  As designers and
     implementers gain experience, it is not uncommon to find defects
     in early and current products.
 12. XML support is available in a large number of software
     development utilities, available in both open source and
     proprietary products.
 13. XML processing speed can be an issue in some environments.  XML
     processing can be slower because XML data streams may be larger
     than other representations, and the use of general purpose XML
     parsers will add a software layer with its own performance costs
     (though these costs can be reduced through consistent use of an
     optimized parser).  In some situations, processing XML requires
     examining every byte of the entire XML data stream, with higher
     overhead than with representations where uninteresting segments
     can be skipped.

3. XML Alternatives

 This document focuses on guidelines for the use of XML.  It is useful
 to consider why one might use XML as opposed to some other mechanism.
 This section considers some other commonly used representation
 mechanisms and compares XML to those alternatives.
 For many fundamental protocols, the extensibility requirements are
 modest, and the performance requirements are high enough that fixed
 binary data blocks are the appropriate representation; mechanisms
 such as XML merely add bloat.  RFC 3252 [23] describes a humorous
 example of XML as protocol bloat.
 In addition, there are other representation and extensibility
 frameworks that have been used successfully within communication
 protocols.  For example, Abstract Syntax Notation 1 (ASN.1) [28]
 along with the corresponding Basic Encoding Rules (BER, [29]) are
 part of the OSI communication protocol suite, and have been used in

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 many subsequent communications standards (e.g., the ANSI Information
 Retrieval protocol [27] and the Simple Network Management Protocol
 (SNMP, [13]).  The External Data Representation (XDR, [14]) and
 variations of it have been used in many other distributed network
 applications (e.g., the Network File System (NFS) protocol [22]).
 With some ASN.1 encoding types, data types are explicit in the
 representation, while with XDR, the data types of components are
 described externally as part of an interface specification.
 Many other protocols use data structures directly (without data
 encapsulation) by describing the data structure with Backus Normal
 Form (BNF, [25]); many IETF protocols use an Augmented Backus-Naur
 Form (ABNF, [16]).  The Simple Mail Transfer Protocol (SMTP, [21]) is
 an example of a protocol specified using ABNF.
 ASN.1, XDR, and BNF are described here as examples of alternatives to
 XML for use in IETF protocols.  There are other alternatives, but a
 complete enumeration of all possible alternatives is beyond the scope
 of this document.
 Other representation methods may differ from XML in several important
 Text Encoding and character sets: the character encoding used to
 represent a formal specification.  XML defines a consistent character
 model based on the Universal Character Set (UCS, [31] and [33]), and
 requires that XML parsers accept at least UTF-8 [4] and UTF-16 [20],
 and allows for other encodings.  While ASN.1 and XDR may carry
 strings in any encoding, there is no common mechanism for defining
 character encodings within them.  Typically, ABNF definitions tend to
 be defined in terms of octets or characters in ASCII.
 Data Encoding: XML is defined as a sequence of characters, rather
 than a sequence of bytes.  XML Schema [42] includes mechanisms for
 representing some data types (integer, date, array, etc.) but many
 binary data types are encoded in Base64 [15] or hexadecimal.  ASN.1
 and XDR have rich mechanisms for encoding a wide variety of data
 Extensibility: XML has a rich extensibility model such that XML
 specifications can frequently be versioned independently.
 Specifications can be extended by adding new element names and
 attributes (if done compatibly); other extensions can be added by
 defining new XML namespaces [9], though there is no standard
 mechanism in XML to indicating whether or not new extensions are
 mandatory to recognize.  Similarly, there are several techniques
 available to extend ASN.1 specifications.  XDR specifications tend to
 not be independently extensible by different parties because the

Hollenbeck, et al. Best Current Practice [Page 6] RFC 3470 XML within IETF Protocols January 2003

 framing and data types are implicit and not self-describing.  The
 extensibility of BNF-based protocol elements needs to be explicitly
 Legibility of protocol elements: As noted above, XML is text-based,
 and thus carries the advantages (and disadvantages) of text-based
 protocol elements.  Typically this is shared with (A)BNF-defined
 protocol elements.  ASN.1 and XDR use binary encodings which are not
 easily human readable.

4. XML Use Considerations and Recommendations

 This section notes several aspects of XML and makes recommendations
 for use.  Since the 1998 publication of XML version 1 [35], an
 editorial second edition [8] was published in 2000; this section
 refers to the second edition.

4.1 XML Syntax and Well-Formedness

 XML [8] is defined in terms of a concrete syntax: a sequence of
 characters, using the characters "<", "=", "&", etc. as delimiters.
 An instance is XML if and only if it is well-formed, i.e., all
 character and markup data conforms to the structural rules defined in
 section 2.1 of [8].
 Character and markup data that is not well-formed is not XML; well-
 formedness is the basis for syntactic compatibility with XML.
 Without well-formedness, all of the advantages of using XML
 disappear.  For this reason, it is recommended that protocol
 specifications explicitly require XML well-formedness ("MUST be
 The IETF has a long-standing tradition of "be liberal in what you
 accept" that might seem to be at odds with this recommendation.
 Given that XML requires well-formedness, conforming XML parsers are
 intolerant of well-formedness errors.  When specifying the handing of
 erroneous XML protocol elements, a protocol design must never
 recommend attempting to partially interpret non-well-formed instances
 of an element which is required to be XML.  Reasonable behaviors in
 such a scenario could include attempting retransmission or aborting
 an in-progress session.

4.2 XML Information Set

 In addition to the concrete syntax of XML, there is an abstract model
 of XML content known as the "Information Set" (infoset) [37].  One
 might think of an XML parser as consuming the concrete syntax and
 producing an XML Information Set for further processing.

Hollenbeck, et al. Best Current Practice [Page 7] RFC 3470 XML within IETF Protocols January 2003

 In typical use of XML, the definition of allowable XML documents is
 often defined in terms of the Information Set of the XML and not the
 concrete syntax.  The notion is that any syntactic representation
 which yielded the same information set would be treated equivalently.
 It some cases, protocols have been defined solely in terms of the XML
 Information Set, or by allowing other concrete syntax
 representations.  However, since the context of XML embedded within
 other Internet protocols requires an unambiguous definition of the
 concrete syntax, defining an XML protocol element in terms of its XML
 Information Set alone and allowing other concrete syntax
 representations is out of scope for this document.

4.3 Syntactic Restrictions

 In some circumstances a protocol designer may be tempted to define an
 XML-based protocol element as "XML", but at the same time imposing
 additional restrictions beyond those imposed by the XML
 recommendation itself -- for example, restricting the document
 character encoding, or avoiding CDATA sections, character entity
 references, imposing additional restrictions on use of white space,
 etc.  The general category of restrictions addressed by this section
 are ones that would allow some but not other of the set of syntactic
 representations which have the same canonical representation
 according to canonical XML described in RFC 3076 [6].
 Making these kinds of restrictions in a protocol definition may have
 the disadvantage that an implementer of the protocol may not be able
 to use an otherwise conforming XML processor to parse the XML-based
 protocol elements.  In some cases, the motivation for subsetting XML
 is to allow implementers to build special-purpose processors that are
 lighter weight than a full-scale conforming XML processor.  There are
 a number of good, conforming XML parsers that are small, fast, and
 free, while special-purpose processors have frequently been known to
 fail to handle some cases of legal XML syntax.
 In general, such syntactic restrictions should be avoided.  In
 circumstances where restrictions on the variability of the syntactic
 representation of XML is necessary for one reason or another,
 designers should consider using "Canonical XML" [6] as the definition
 of the protocol element, since all such variability has been removed.
 Some specific issues are discussed in Section 4.4, Section 4.13, and
 Section 5.1 below.

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4.4 XML Declarations

 An XML declaration (defined in section 2.8 of [8]) is a small header
 at the beginning of an XML data stream that indicates the XML version
 and the character encoding used.  For example,
 <?xml version="1.0" encoding="UTF-8"?>
 specifies the use of XML version 1 and UTF-8 character encoding.
 In some uses of XML as an embedded protocol element, the XML used is
 a small fragment in a larger context, where the XML version is fixed
 at "1.0" and the character encoding is known to be "UTF-8".  In those
 cases, an XML declaration might add extra overhead.  In cases where
 the XML is a larger component which may find its way alone as an
 external entity body (transported as a MIME message, for example),
 the XML declaration is an important marker and is useful for
 reliability and extensibility.  The XML declaration is also an
 important marker for character set/encoding (see Section 5.1), if any
 encoding other than UTF-8 or UTF-16 is used.  Note that in the case
 of UTF-16, XML requires that the entity starts with a Byte Order Mark
 (BOM), which is not part of the character data.  Note that the XML
 Declaration itself is not part of the XML document's Information Set.
 Protocol specifications must be clear about use of XML declarations.
 XML [8] notes that "XML documents should begin with an XML
 declaration which specifies the version of XML being used."  In
 general, an XML declaration should be encouraged ("SHOULD be
 present") and must always be allowed ("MAY be sent").  An XML
 declaration should be required in cases where, if allowed, the
 character encoding is anything other than UTF-8 or UTF-16.

4.5 XML Processing Instructions

 An XML processing instruction (defined in section 2.6 of [8]) is a
 component of an XML document that signals extra "out of band"
 information to the receiver; a common use of XML processing
 instructions are for document applications.  For example, the XML2RFC
 application used to generate this document and described in RFC 2629
 [19] supports a "table of contents" processing instruction:
 <?rfc toc="yes"?>
 As described in section 2.6 of [8], processing instructions are not
 part of the document's character data, but must be passed through to
 the application.  As a consequence, it is recommended that processing
 instructions be ignored when encountered in normal protocol
 processing.  It is thus also recommended that processing instructions

Hollenbeck, et al. Best Current Practice [Page 9] RFC 3470 XML within IETF Protocols January 2003

 not be used to define normative protocol data structures or
 extensions for the following reasons:
 o  Processing instructions are not namespace aware; there is no way
    to qualify a processing instruction target with a namespace.
 o  Processing instruction use can not be constrained by most schema
 o  Character references are not recognized within a processing
 o  Processing instructions don't have any XML-defined structure
    beyond the division between the target and everything else.  This
    means that applications typically have to parse the content of the
    processing instruction in a system-dependent way; if the content
    was provided within an element instead, the structure could be
    expressed in the XML and the parsing could be done by the XML

4.6 XML Comments

 An XML comment (defined in section 2.5 of [8]) is a component of an
 XML document that provides descriptive information that is not part
 of the document's character data.  XML comments, like comments used
 in programming languages, are often used to provide explanatory
 information in human-understandable terms.  An example:
 <!-- This is a example comment.  -->
 XML comments can be ignored by conformant processors.  As a
 consequence, it is strongly recommended that comments not be used to
 define normative protocol data structures or extensions.  It is thus
 also strongly recommended that comments be ignored if encountered in
 normal protocol processing.

4.7 Validity and Extensibility

 One important value of XML is that there are formal mechanisms for
 defining structural and data content constraints; these constrain the
 identity of elements or attributes or the values contained within
 them.  There is more than one such formalism:
 o  A "Document Type Definition" (DTD) is defined in section 2.8 of
    [8]; the concept came from a similar mechanism for SGML.  There is
    significant experience with using DTDs, including in IETF

Hollenbeck, et al. Best Current Practice [Page 10] RFC 3470 XML within IETF Protocols January 2003

 o  XML Schema (defined in [41] and [42]) provides additional features
    to allow a tighter and more precise specification of allowable
    protocol syntax and data type specifications.
 o  There are also a number of other mechanisms for describing XML
    instance validity; these include, for example, Schematron [49] and
    RELAX NG [48].  Part 2 of the ISO/IEC Document Schema Definition
    Language (DSDL, [32]) standard is based on RELAX NG.
 There is ongoing discussion (and controversy) within the XML
 community on the use and applicability of various validity constraint
 mechanisms.  The choice of tool depends on the needs for
 extensibility or for a formal language and mechanism for constraining
 permissible values and validating adherence to the constraints.
 There are cases where protocols have defined validity using one or
 another validity mechanism, but the protocol definitions have not
 insisted that all corresponding protocol elements be "valid".  The
 decision depends in part on the design for protocol extensibility.
 Each formalism has different ways of allowing for future extensions;
 in addition, a protocol design may have its own versioning mechanism,
 way of updating the schema, or pointing to a new one.  For example,
 the use of XML namespaces (Section 4.9) with XML Schema allows other
 kinds of extensibility without compromising schema validity.
 No matter what formalism is chosen, there are usually additional
 syntactic constraints, and inevitably additional semantic
 constraints, on the validity of XML elements that cannot be expressed
 in the formalism.
 This document makes the following recommendations for the definition
 of protocols using XML:
 o  Protocols should use an appropriate formalism for defining
    validity of XML protocol elements.
 o  Protocols may or may not insist that all corresponding protocol
    elements be valid, according to the validity mechanism chosen; in
    either case, the extensibility design should be clear.  What
    happens if the data is not valid?
 o  As described in Section 3 there is no standard mechanism in XML
    for indicating whether or not new extensions are mandatory to
    recognize.  XML-based protocol specifications should thus
    explicitly describe extension mechanisms and requirements to
    recognize or ignore extensions.

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 An idealized model for XML processing might first check for well-
 formedness; if OK, apply the primary formalism and, if the instances
 "passes", apply the other constraints so that the entire set (or as
 much as is machine processable) can be checked at the same time.
 However, it is reasonable to allow conforming implementations to
 avoid doing validation at run-time and rely instead on ad-hoc code to
 avoid the higher expense, for example, of schema validation,
 especially given that there will likely be additional hand-crafted
 semantic validation.

4.8 Semantics as Well as Syntax

 While the definition of an XML protocol element using a validity
 formalism is useful, it is not sufficient.  XML by itself does not
 supply semantics.  Any document defining a protocol element with XML
 MUST also have sufficient prose in the document describing the
 semantics of whatever XML the document has elected to define.

4.9 Namespaces

 XML namespaces, defined in [9], provide a means of assigning markup
 to a specific vocabulary.  If two elements or attributes from
 different vocabularies have the same name, they can be distinguished
 unambiguously if they belong to different namespaces.  Additionally,
 namespaces provide significant support for protocol extensibility as
 they can be defined, reused, and processed dynamically.
 Markup vocabulary collisions are very possible when namespaces are
 not used to separate and uniquely identify vocabularies.  Protocol
 definitions should use existing XML namespaces where appropriate.
 When a new namespace is needed, the "namespace name" is a URI that is
 used to identify the namespace; it's also useful for that URI to
 point to a description of the namespace.  Typically (and recommended
 practice in W3C) is to assign namespace names using persistent http
 In the case of namespaces in IETF standards-track documents, it would
 be useful if there were some permanent part of the IETF's own web
 space that could be used for this purpose.  In lieu of such, other
 permanent URIs can be used, e.g., URNs in the IETF URN namespace (see
 [11] and [12]).  Although there are instances of IETF specifications
 creating new URI schemes to define XML namespaces, this practice is
 strongly discouraged.

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4.9.1 Namespaces and Attributes

 There is a frequently misunderstood aspect of the relationship
 between unprefixed attributes and the default XML namespace - the
 natural assumption is that an unprefixed attribute is qualified by
 the default namespace, but this is not true.  Rather, the unprefixed
 attribute belongs to no namespace at all.  Thus, in the following
 <ns1:fox a="xxx" ns1:b="qqq"
 <fox a="xxx" ns1:b="qqq"
  xmlns="" xmlns:ns1=""/>
 the attribute "a" is in no namespace, while "ns1:b" is the same
 namespace as the containing element.  A specific description of the
 relationship between default namespaces and attributes can be found
 in section 5.2 of [9].  The practical implication of the relationship
 between namespaces and attributes is that care must be taken to
 ensure that no element contains multiple attributes that have
 identical names or have qualified names with the same local part and
 with prefixes which have been bound to namespace names that are
 In XML applications, the choice between prefixed and non-prefixed
 attributes frequently is based on whether they always appear inside
 elements of the same namespace (in which case non-prefixed and
 thereby non-namespaced names are used) or whether it's required that
 they can be applied to elements in other arbitrary namespaces (in
 which case a prefixed name is used).  Both situations occur in the
 XSLT [43] language: while attributes are unprefixed when they occur
 inside elements in the XSLT namespace, such as:
 <xsl:value-of select="."/>

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 they are prefixed when they appear in non-XSLT elements, such as the
 "xsl:version" attribute when using "literal result element
 <html xsl:version="1.0"
     <title>Expense Report Summary</title>
     <p>Total: <xsl:value-of select="exp-rep/total"/></p>

4.10 Element and Attribute Design Considerations

 XML provides much flexibility in allowing a designer to use either
 elements, attributes, or element content to carry data.  This section
 gives a flavor of the design considerations; there is much written
 about this in the XML literature.  Consistent use of elements,
 attributes, and values is an important characteristic of a sound
 Attributes are generally intended to contain meta-data that describes
 the element, and as such they are subject to the following
 o  Attributes are unordered,
 o  There can be no more than one instance of a given attribute within
    a given element, though an attribute may contain several values
    separated by white space ([8], section 2.3 and 3.3.1),
 o  Attribute values can have no internal XML markup for providing
    internal structure, and
 o  Attribute values are normalized ([8], section 3.3) before
 Consider the following example that describes an IP address using an
 attribute to describe the address value:
 <address addrType="ipv4"></address>

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 One might encode the same information using an <addrType> element
 instead of an "addrType" attribute:
 Another way of encoding the same information would be to use markup
 for the "addrType":
 Choosing between these designs involves tradeoffs concerning, among
 other considerations, the likely extensibility patterns and the
 ability of the formalism to constrain the values appropriately.  In
 the first example, the attribute can be thought of as meta-data to
 the element which it modifies, and provides for a kind of "element
 extensibility".  The third example allows for a different kind of
 extensibility: the "ipv4" space can be extended using other
 namespaces, and the <ipv4> element can include additional markup.
 Many protocols include parameters that are selected from an
 enumerated set of values.  Such enumerated values can be encoded as
 elements, attributes, or strings within element values.  Any protocol
 design should consider how the set of enumerated values is to be
 extended: by revising the protocol, by including different values in
 different XML namespaces, or by establishing an IANA registry (as per
 RFC 2434 [18]).  In addition, a common practice in XML is to use a
 URI as an XML attribute value or content.
 Languages that describe syntactic validity (including XML Schema and
 DTDs) often provide a mechanism for specifying "default" values for
 an attribute.  If an element does not specify a value for the
 attribute, then the "default" value is used.  The use of default
 values for attributes is discouraged by this document.  Although the
 use of this feature can reduce both the size and clutter of XML
 documents, it has a negative impact on software which doesn't know
 the document's validity constraints (e.g., for packet tracing or
 digital signature).

Hollenbeck, et al. Best Current Practice [Page 15] RFC 3470 XML within IETF Protocols January 2003

4.11 Binary Data and Text with Control Characters

 XML is defined as a character stream rather than a stream of octets.
 There is no way to embed raw binary data directly within an XML data
 stream; all binary data must be encoded as characters.  There are a
 number of possible encodings; for example, XML Schema [42] defines
 encodings using decimal digits for integers, Base64 [15], or
 hexadecimal digits.  In addition, binary data might be transmitted
 using some other communication channel, and referenced within the XML
 data itself using a URI.
 Protocols that need a container that can hold both structural data
 and large quantities of binary data should consider carefully whether
 XML is appropriate, since the Base64 and hex encodings are
 inefficient.  Otherwise, protocols should use the mechanisms of XML
 Schema to represent binary data; the Base64 encoding is best for
 larger quantities of data.
 XML does not allow "control" characters (0x00-0x1F) except for TAB
 (0x09), CR (0x0A), and LF (0x0D).  They can not be specified even
 using character entity references.  There is currently no common way
 of encoding them within what is otherwise ordinary text.  This means
 that strings that might be considered "text" within an ABNF-defined
 protocol element may need to be treated as binary data within an XML
 representation, or some other encoding mechanism might need to be

4.12 Incremental Processing

 In some situations, it is possible to incrementally process an XML
 document as each tag is received; this is analogous to the process by
 which browsers incrementally render HTML pages as they are received.
 Note that incremental processing is difficult to implement if
 interspersed across multiple interactions.  In other words, if a
 protocol requires incremental processing across both directions of a
 bidirectional stream, then it may place an unusual burden on protocol

4.13 Entity Declarations and Entity References

 In addition to its role as a validity mechanism, an XML DTD provides
 a facility for "entity declarations" ([8], section 4.2).  An entity
 declaration defines, in the DTD, a kind of macro capability where an
 "entity reference" may be used to call up and include the content of
 the entity declaration.

Hollenbeck, et al. Best Current Practice [Page 16] RFC 3470 XML within IETF Protocols January 2003

 This feature adds complexity to XML processing, and seems more
 appropriate for use of XML in document processing than in data
 representation.  As such, this document recommends avoiding entity
 declarations in protocol specifications.
 On the other hand, there are five standard entity references built
 into XML: "&amp;", "&lt;", "&gt;", "&apos;", and "&quot;".  XML also
 has the ability to write character data using numeric entity
 references (using the Unicode [33] value for the character).  Entity
 references are normally expanded before the XML Information Set is
 computed.  Restricting the use of these entity references would
 introduce an additional syntactic restriction (see Section 4.3)
 unnecessarily; these entity references should be allowed.

4.14 External References

 When using XML in the context of a stateless protocol, be it the
 protocol itself (e.g., SOAP), or simply as content transferred by an
 existing protocol (e.g., XML/HTTP), care must be taken to not make
 the meaning of a message depend on information outside the message
 itself.  XML provides external entities (see Section 4.13), which are
 an easy way to make the meaning of a message depend on something
 external.  Using schema languages that can change the Infoset, like
 XML Schema, is another way.

4.15 URI Processing

 The XML Base specification [36] defines an attribute "xml:base" in
 the XML namespace that is intended to affect the "base" to be used
 for relative URI processing described in RFC 2396 [17].  The
 facilities of xml:base for controlling URI processing may be useful
 to protocol designers, but if xml:base is allowed the interaction
 with any other protocol facilities for establishing URI context must
 be specified clearly.  Note that use of relative URIs in namespace
 declarations has been deprecated by the W3C; some specific issues
 with relative URIs in namespace declarations and canonical XML can be
 found in section 1.3 of RFC 3076 [6].
 Note also that, in many cases, the term "URI" and the syntactic use
 of URIs within XML allows non-ASCII characters within URIs.  For
 example, the XML Schema "anyURI" datatype ([42] section 3.2.17)
 allows for direct encoding of characters outside of the US-ASCII
 range.  Most current IETF protocols and specifications do not allow
 this syntax.  Protocol specifications should be clear about the range
 of characters specified, e.g., by adding a restriction to the range
 of characters allowed in the anyURI schema datatype, or by specifying
 that characters outside the US-ASCII range should be escaped when
 passed to older protocols or APIs.

Hollenbeck, et al. Best Current Practice [Page 17] RFC 3470 XML within IETF Protocols January 2003

4.16 White Space

 XML's prescribed white space handling behavior can be a source of
 confusion between protocol designers and implementers.  In XML
 instances all white space is considered significant and is by default
 visible to processing applications.  Consider this example from
 Section 4.10:
 This fragment contains an <address> element and two child elements.
 It also contains white space for pretty-printing purposes:
 o  at least three line separators, which will be converted by the XML
    processor to newline (U+000A) characters (see section 2.11 of
    [8]), and
 o  one or more white space characters prefixing the <addrType> and
    <value> elements, which an XML processor will make visible to
    software reading the instance.
 Implementers might safely assume that they can ignore the white space
 in the example above, but white space used for pretty-printing can be
 a source of confusion in other situations.  Consider a minor change
 to the <value> element:
 where white space is found on both sides of the IP address.  XML
 processors treat the white space surrounding "" as an
 integral part of the <value> element.  A failure to recognize this
 behavior can lead to confusion and errors in both design and
 All white space is considered significant in XML instances.  As a
 consequence, it is recommended that protocol designers provide
 specific guidelines to address white space handling within protocols
 that use XML.

Hollenbeck, et al. Best Current Practice [Page 18] RFC 3470 XML within IETF Protocols January 2003

4.17 Interaction with the IANA

 When XML is used in an IETF protocol there are multiple factors that
 might require IANA action, including:
 o  XML media types.  A piece of XML in a protocol element is
    sometimes intrinsically bound to the protocol context in which it
    appears, and in particular might be directly derived from and/or
    input to protocol state-machine implementations.  In cases where
    the XML content has no relevant meaning outside it's original
    protocol context, there is no reason to register a MIME type.
    When it is possible that XML content can be interpreted outside of
    its original context (such as when that XML content is being
    stored in a file system or tunneled over another protocol), then a
    MIME type can be registered to specify the specific format for the
    data and to provide a hint as to how it might be processed.
    If MIME labeling is needed, then the advice of RFC 3023 [5]
    applies.  In particular, if the XML represents a new language or
    document type, a new MIME media type should be registered for the
    reasons described in RFC 3023 sections 7 and A.1.  In situations
    where XML is used to encode generic structured data (e.g., a
    document-oriented application that involves combining XML with a
    stylesheet), "application/xml" might be appropriate ("MAY be
    used").  The "text/xml" media type is not recommended ("SHOULD NOT
    be used") because of issues involving display behavior and default
 o  URI registration.  There is an ongoing effort ([11], [12]) to
    create a URN namespace explicitly for defining URIs for namespace
    names and other URI-designated protocol elements for use within
    IETF standards track documents; it might also establish IETF
    policy for such use.

5. Internationalization Considerations

 This section describes internationalization considerations for the
 use of XML to represent data in IETF protocols.  In addition to the
 recommendations here, IETF policy on the use of character sets and
 languages described in RFC 2277 [3] also applies.

5.1 Character Sets and Encodings

 IETF protocols frequently speak of the "character set" or "charset"
 of a string, which is used to denote both the character repertoire
 and the encoding used to represent sequences of characters as
 sequences of bytes.

Hollenbeck, et al. Best Current Practice [Page 19] RFC 3470 XML within IETF Protocols January 2003

 XML performs all character processing in terms of the Universal
 Character Set (UCS, [31] and [33]).  XML requires all XML processors
 to support both the UTF-8 [4] and UTF-16 [20] encodings of UCS,
 although other encodings (charsets) compatible with UCS may be
 allowed.  Documents and external parsed entities encoded in UTF-16
 are required to begin with a Byte Order Mark ([8] section 4.3.3).
 IETF policy [3] requires that the UTF-8 charset be allowed for all
 This document requires that IETF protocols using XML allow for the
 UTF-8 encoding of XML data.  Since conforming XML processors are
 mandated to also accept UTF-16 encoding, also allowing for UTF-16
 encoding (with the mandated Byte Order Mark) is recommended.  Some
 XML applications are using a Byte Order Mark with UTF-8 encoding, but
 this use should not be encouraged and isn't appropriate for XML
 embedded in other protocols.
 Restricting XML data to only be expressed in UTF-8 is an additional
 syntactic restriction (see Section 4.3) which, depending on
 circumstances, might add additional implementation complexity.  When
 encodings other than UTF-8 or UTF-16 are used, the encoding must be
 specified using an "encoding" attribute in the XML declaration (see
 Section 4.4), even if there might be other protocol mechanisms for
 designating the encoding.

5.2 Language Declaration

 Text encapsulated in XML can be represented in many different human
 languages, and it is often useful to explicitly identify the language
 used to present the text.  XML defines a special attribute in the
 "xml" namespace, xml:lang, that can be used to specify the language
 used to represent data in an XML document.  The xml:lang attribute
 (which has to be explicitly declared for use within a DTD or XML
 Schema) and the values it can assume are defined in section 2.12 of
 It is strongly recommended that protocols representing data in a
 human language mandate use of an xml:lang attribute if the XML
 instance might be interpreted in language-dependent contexts.

5.3 Other Internationalization Considerations

 There are standard mechanisms in the typography of some human
 languages that can be difficult to represent using merely XML
 character string data types.  For example, pronunciation clues can be
 provided using Ruby annotation [39], and embedding controls (such as
 those described in section 3.4 of [34]) or an XHTML [40] "dir"

Hollenbeck, et al. Best Current Practice [Page 20] RFC 3470 XML within IETF Protocols January 2003

 attribute can be used to note the proper display direction for
 bidirectional text.
 There are a number of tricky issues that can arise when using
 extended character sets with XML document formats.  For example:
 o  There are different ways of representing characters consisting of
    combining characters, and
 o  There has been some debate about whether URIs should be
    represented using a restricted US-ASCII subset or arbitrary
    Unicode (e.g., "URI character sequence" vs "original character
    sequence" in RFC 2396 [17]).
 Some of these issues are discussed, with recommendations, in the
 W3C's "Character Model for the World Wide Web" document [44].
 It is strongly recommended that protocols representing data in a
 human language reuse existing mechanisms as needed to ensure proper
 display of human-legible text.

6. IANA Considerations

 This memo, per se, has no impact on the IANA.  Section 4.17 notes
 some factors that might require IANA action when protocols using XML
 are defined.

7. Security Considerations

 Network protocols face many different kinds of threats, including
 unintended disclosure, modification, and replay.  Passive attacks,
 such as packet sniffing, allow an attacker to capture and view
 information intended for someone else.  Captured data can be modified
 and replayed to the original intended recipient, with the recipient
 having no way to know that the information has been compromised,
 detect modifications, be assured of the sender's identity, or to
 confirm which protocol instance is legitimate.
 Several security service options for XML are available to help
 mitigate these risks.  Though XML does not include any built-in
 security services, other protocols and protocol layers provide
 services that can be used to protect XML protocols.  XML encryption
 [10] provides privacy services to prevent unintended disclosure.
 Canonical XML [6] and XML digital signatures [7] provide integrity
 services to detect modification and authentication services to
 confirm the identity of the data source.  Other IETF security
 protocols (e.g., the Transport Layer Security (TLS) protocol [2]) are
 also available to protect data and service endpoints as appropriate.

Hollenbeck, et al. Best Current Practice [Page 21] RFC 3470 XML within IETF Protocols January 2003

 Given the lack of security services in XML, it is imperative that
 protocol specifications mandate additional security services to
 counter common threats and attacks; the specific required services
 will depend on the protocol's threat model.
 Experience has shown that code that parses network traffic is often a
 "soft target" for blackhats.  Accordingly, implementers MUST take
 great care to ensure that their XML handling code is robust with
 respect to malformed XML, buffer overruns, misuse of entity
 declarations, and so on.
 XML mechanisms that follow external references (Section 4.14) may
 also expose an implementation to various threats by causing the
 implementation to access external resources automatically.  It is
 important to disallow arbitrary access to such external references
 within XML data from untrusted sources.  Many XML grammars define
 constructs using URIs for external references; in such cases, the
 same precautions must be taken.

8. Acknowledgements

 The authors would like to thank the following people who have
 provided significant contributions to the development of this
 Mark Baker, Tim Berners-Lee, Tim Bray, James Clark, Josh Cohen, John
 Cowan, Alan Crouch, Martin Duerst, Jun Fujisawa, Christian Geuer-
 Pollmann, Yaron Goland, Graham Klyne, Dan Kohn, Rick Jeliffe, Chris
 Lilley, Murata Makoto, Michael Mealling, Jean-Jacques Moreau, Andrew
 Newton, Julian Reschke, Jonathan Rosenberg, Miles Sabin, Rich Salz,
 Peter Saint-Andre, Simon St Laurent, Margaret Wasserman, and Daniel

9. Normative References

 [1]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.
 [2]   Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
       2246, January 1999.
 [3]   Alvestrand, H., "IETF Policy on Character Sets and Languages",
       BCP 18, RFC 2277, January 1998.
 [4]   Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
       2279, January 1998.

Hollenbeck, et al. Best Current Practice [Page 22] RFC 3470 XML within IETF Protocols January 2003

 [5]   Murata, M., St. Laurent, S. and D. Kohn, "XML Media Types", RFC
       3023, January 2001.
 [6]   Boyer, J., "Canonical XML Version 1.0", RFC 3076, March 2001.
 [7]   Eastlake, D., Reagle, J. and D. Solo, "(Extensible Markup
       Language) XML-Signature Syntax and Processing", RFC 3275, March
 [8]   Bray, T., Paoli, J., Sperberg-McQueen, C. and E. Maler,
       "Extensible Markup Language (XML) 1.0 (2nd ed)", W3C REC-xml,
       October 2000, <>.
 [9]   Bray, T., Hollander, D. and A. Layman, "Namespaces in XML", W3C
       REC-xml-names, January 1999, <
 [10]  Imamura, T., Dillaway, B., Schaad, J. and E. Simon, "XML
       Encryption Syntax and Processing", W3C REC-xmlenc-core, October
       2001, <>.

10. Informative References

 [11]  Masinter, L., Mealling, M., Klyne, G. and T. Hardie, "An IETF
       URN Sub-namespace for Registered Protocol Parameters", Work in
 [12]  Mealling, M., "The IETF XML Registry", Work in Progress.
 [13]  Case, J., Fedor, M., Schoffstall, M. and C. Davin, "Simple
       Network Management Protocol (SNMP)", STD 15, RFC 1157, May
 [14]  Srinivasan, R., "XDR: External Data Representation Standard",
       RFC 1832, August 1995.
 [15]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
       Extensions (MIME) Part One: Format of Internet Message Bodies",
       RFC 2045, November 1996.
 [16]  Crocker, D. (Ed.) and P. Overell, "Augmented BNF for Syntax
       Specifications: ABNF", RFC 2234, November 1997.
 [17]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
       Resource Identifiers (URI): Generic Syntax", RFC 2396, August

Hollenbeck, et al. Best Current Practice [Page 23] RFC 3470 XML within IETF Protocols January 2003

 [18]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
       Considerations Section in RFCs", BCP 26, RFC 2434, October
 [19]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629, June
 [20]  Hoffman, P. and F. Yergeau, "UTF-16, an encoding of ISO 10646",
       RFC 2781, February 2000.
 [21]  Klensin, J. (Ed.), "Simple Mail Transfer Protocol", RFC 2821,
       April 2001.
 [22]  Shepler, S., Callaghan, B., Robinson, D., Thurlow, R., Beame,
       C., Eisler, M. and D. Noveck, "NFS version 4 Protocol", RFC
       3010, December 2000.
 [23]  Kennedy, H., "Binary Lexical Octet Ad-hoc Transport", RFC 3252,
       April 2002.
 [24]  Popp, N., Mealling, M. and M. Moseley, "Common Name Resolution
       Protocol (CNRP)", RFC 3367, August 2002.
 [25]  Backus, J., "The syntax and semantics of the proposed
       international algebraic language of the Zurich ACM-GAMM
       conference", June 1959.
 [26]  American National Standards Institute, "Code Extension
       Techniques for Use with the 7-bit Coded Character Set of
       American National Standard Code (ASCII) for Information
       Interchange", ANSI X3.41, FIPS PUB 35, 1974.
 [27]  American National Standards Institute, "Information Retrieval:
       Application Service Definition and Protocol Specification",
       ANSI Z39.50, ISO Standard 23950, 1995.
 [28]  International Organization for Standardization, "Information
       Processing Systems - Open Systems Interconnection -
       Specification of Abstract Syntax Notation One (ASN.1)", ISO
       Standard 8824, December 1990.
 [29]  International Organization for Standardization, "Information
       Processing Systems - Open Systems Interconnection -
       Specification of Basic Encoding Rules for Abstract Syntax
       Notation One (ASN.1)", ISO Standard 8825, December 1990.

Hollenbeck, et al. Best Current Practice [Page 24] RFC 3470 XML within IETF Protocols January 2003

 [30]  International Organization for Standardization, "Information
       processing - Text and office systems - Standard Generalized
       Markup Language (SGML)", ISO Standard 8879, 1988.
 [31]  International Organization for Standardization, "Information
       Technology - Universal Multiple-octet coded Character Set (UCS)
       - Part 1: Architecture and Basic Multilingual Plane", ISO
       Standard 10646-1, May 1993.
 [32]  International Organization for Standardization, "DSDL Part 0 -
       Overview", December 2001, <
 [33]  Unicode Consortium, "The Unicode Standard, as it may from time
       to time be revised or amended", March 2002, <http://>.
 [34]  Duerst, M. and A. Freytag, "Unicode in XML and other Markup
       Languages", February 2002, <>.
 [35]  Bray, T., Paoli, J. and C. Sperberg-McQueen, "Extensible Markup
       Language (XML) 1.0", W3C REC-xml-1998, February 1998, <http://>.
 [36]  Marsh, J., "XML Base", W3C REC-xmlbase, June 2001, <http://>.
 [37]  Cowan, J. and R. Tobin, "XML Information Set", W3C REC-infoset,
       October 2001, <>.
 [38]  Lassila, O. and R. Swick, "Resource Description Framework (RDF)
       Model and Syntax Specification", W3C REC-rdf-syntax, February
       1999, <>.
 [39]  Suignard, M., Ishikawa, M., Duerst, M. and T. Texin, "Ruby
       Annotation", W3C REC-RUBY, May 2001, <
 [40]  Pemberton, S., "XHTML 1.0: The Extensible HyperText Markup
       Language", W3C REC-XHTML, January 2000, <
 [41]  Thompson, H., Beech, D., Maloney, M. and N. Mendelsohn, "XML
       Schema Part 1: Structures", W3C REC-xmlschema-1, May 2001,
 [42]  Biron, P. and A. Malhotra, "XML Schema Part 2: Datatypes", W3C
       REC-xmlschema-2, May 2001, <>.

Hollenbeck, et al. Best Current Practice [Page 25] RFC 3470 XML within IETF Protocols January 2003

 [43]  Clark, J., "XSL Transformations (XSLT) Version 1.0", W3C REC-
       xslt, November 1999, <>.
 [44]  Duerst, M., Yergeau, F., Ishida, R., Wolf, M., Freytag, A. and
       T. Texin, "Character Model for the World Wide Web 1.0", April
       2002, <>.
 [45]  Gudgin, M., Hadley, M., Moreau, JJ. and H. Nielsen, "SOAP
       Version 1.2 Part 1: Messaging Framework", June 2002,
 [46]  Gudgin, M., Hadley, M., Moreau, JJ. and H. Nielsen, "SOAP
       Version 1.2 Part 2: Adjuncts", June 2002,
 [47]  W3C Communications Team, "XML in 10 points", November 2001,
 [48]  OASIS Technical Committee: RELAX NG, "RELAX NG Specification",
       December 2001, <
 [49]  Jelliffe, R., "The Schematron", November 2001, <http://>.


 [50]  <>
 [51]  <>
 [52]  <>
 [53]  <>

Hollenbeck, et al. Best Current Practice [Page 26] RFC 3470 XML within IETF Protocols January 2003

11. Authors' Addresses

 Scott Hollenbeck
 VeriSign, Inc.
 21345 Ridgetop Circle
 Dulles, VA  20166-6503
 Phone: +1 703 948 3257
 Marshall T. Rose
 Dover Beach Consulting, Inc.
 POB 255268
 Sacramento, CA  95865-5268
 Phone: +1 916 483 8878
 Larry Masinter
 Adobe Systems Incorporated
 Mail Stop W14
 345 Park Ave.
 San Jose, CA  95110
 Phone: +1 408 536 3024

Hollenbeck, et al. Best Current Practice [Page 27] RFC 3470 XML within IETF Protocols January 2003

12. Full Copyright Statement

 Copyright (C) The Internet Society (2003).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an


 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Hollenbeck, et al. Best Current Practice [Page 28]

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