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.
Abstract
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
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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.
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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
types.
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,
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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
ways:
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
types.
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
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framing and data types are implicit and not self-describing. The
extensibility of BNF-based protocol elements needs to be explicitly
planned.
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
well-formed").
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.
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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,
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:
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
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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
languages,
o Character references are not recognized within a processing
instruction.
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
parser.
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:
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
protocols.
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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
URIs.
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
example:
Expense Report Summary
Total:
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
design.
Attributes are generally intended to contain meta-data that describes
the element, and as such they are subject to the following
restrictions:
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
processing
Consider the following example that describes an IP address using an
attribute to describe the address value:
10.1.2.3
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One might encode the same information using an element
instead of an "addrType" attribute:
ipv4
10.1.2.3
Another way of encoding the same information would be to use markup
for the "addrType":
10.1.2.3
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 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).
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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
invented.
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
implementers.
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.
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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: "&", "<", ">", "'", and """. 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.
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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:
10.1.2.3
This fragment contains an 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 and
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 element:
10.1.2.3
where white space is found on both sides of the IP address. XML
processors treat the white space surrounding "10.1.2.3" as an
integral part of the element. A failure to recognize this
behavior can lead to confusion and errors in both design and
implementation.
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.
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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
charsets.
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.
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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
text.
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
[8].
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"
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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.
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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
document:
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
Veillard.
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.
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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
2002.
[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, .
[33] Unicode Consortium, "The Unicode Standard, as it may from time
to time be revised or amended", March 2002, .
[34] Duerst, M. and A. Freytag, "Unicode in XML and other Markup
Languages", February 2002, .
[39] Suignard, M., Ishikawa, M., Duerst, M. and T. Texin, "Ruby
Annotation", W3C REC-RUBY, May 2001, .
[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, .
[48] OASIS Technical Committee: RELAX NG, "RELAX NG Specification",
December 2001, .
[49] Jelliffe, R., "The Schematron", November 2001,
RFC, FYI, BCP