Internet Engineering Task Force (IETF)H. Thompson
Request for Comments: 7303University of Edinburgh
Obsoletes: 3023C. Lilley
Updates: 6839W3C
Category: Standards TrackJuly 2014
ISSN: 2070-1721

XML Media Types

Abstract

This specification standardizes three media types -- application/xml, application/xml-external-parsed-entity, and application/xml-dtd -- for use in exchanging network entities that are related to the Extensible Markup Language (XML) while defining text/xml and text/xml-external-parsed-entity as aliases for the respective application/ types. This specification also standardizes the '+xml' suffix for naming media types outside of these five types when those media types represent XML MIME entities.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7303.

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1. Introduction

The World Wide Web Consortium has issued the Extensible Markup Language (XML) 1.0 [XML] and Extensible Markup Language (XML) 1.1 [XML1.1] specifications. To enable the exchange of XML network entities, this specification standardizes three media types (application/xml, application/xml-external-parsed-entity, and application/xml-dtd), two aliases (text/xml and text/xml-external-parsed-entity), and a naming convention for identifying XML-based MIME media types (using '+xml').

XML has been used as a foundation for other media types, including types in every branch of the IETF media types tree. To facilitate the processing of such types, and in line with the recognition in [RFC6838] of structured syntax name suffixes, a suffix of '+xml' is registered in Section 9.6. This will allow generic XML-based tools -- browsers, editors, search engines, and other processors -- to work with all XML-based media types.

This specification replaces [RFC3023]. Major differences are in the areas of alignment of text/xml and text/xml-external-parsed-entity with application/xml and application/xml-external-parsed-entity respectively, the addition of XPointer and XML Base as fragment identifiers and base URIs, respectively, integration of the XPointer Registry and updating of many references.

2. Notational Conventions

2.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this specification are to be interpreted as described in [RFC2119].

2.2. Characters, Encodings, Charsets

Both XML (in an XML or Text declaration using the encoding pseudo- attribute) and MIME (in a Content-Type header field using the charset parameter) use a common set of labels [IANA-CHARSETS] to identify the MIME charset (mapping from byte stream to character sequence [RFC2978]).

In this specification, we will use the phrases "charset parameter" and "encoding declaration" to refer to whatever MIME charset is specified by a MIME charset parameter or XML encoding declaration, respectively. We reserve the phrase "character encoding" (or, when the context makes the intention clear, simply "encoding") for the MIME charset actually used in a particular XML MIME entity.

[UNICODE] defines three "encoding forms", namely UTF-8, UTF-16, and UTF-32. As UTF-8 can only be serialized in one way, the only possible label for UTF-8-encoded documents when serialised into MIME entities is "utf-8". UTF-16 XML documents, however, can be serialised into MIME entities in one of two ways: either big-endian, labelled (optionally) "utf-16" or "utf-16be", or little-endian, labelled (optionally) "utf-16" or "utf-16le". See Section 3.3 below for how a Byte Order Mark (BOM) is required when the "utf-16" serialization is used.

UTF-32 has four potential serializations, of which only two (UTF-32BE and UTF-32LE) are given names in [UNICODE]. Support for the various serializations varies widely, and security concerns about their use have been raised (for example, see [Sivonen]). The use of UTF-32 is NOT RECOMMENDED for XML MIME entities.

2.3. MIME Entities, XML Entities

As sometimes happens between two communities, both MIME and XML have defined the term entity, with different meanings. Section 2.4 of [RFC2045] says:

  • The term "entity", refers specifically to the MIME-defined header fields and contents of either a message or one of the parts in the body of a multipart entity.
  • An XML document may consist of one or many storage units. These are called entities; they all have content and are all (except for the document entity and the external DTD subset) identified by entity name.

In this specification, "XML MIME entity" is defined as the latter (an XML entity) encapsulated in the former (a MIME entity).

Furthermore, XML provides for the naming and referencing of entities for purposes of inclusion and/or substitution. In this specification, "XML-entity declaration/reference/..." is used to avoid confusion when referring to such cases.

3. Encoding Considerations

The registrations below all address issues around character encoding in the same way, by referencing this section.

As many as three distinct sources of information about character encoding may be present for an XML MIME entity: a charset parameter, a BOM (see Section 3.3 below), and an XML encoding declaration (see Section 4.3.3 of [XML]). Ensuring consistency among these sources requires coordination between entity authors and MIME agents (that is, processes that package, transfer, deliver, and/or receive MIME entities).

The use of UTF-8, without a BOM, is RECOMMENDED for all XML MIME entities.

Some MIME agents will be what we will call "XML-aware", that is, capable of processing XML MIME entities as XML and detecting the XML encoding declaration (or its absence). All three sources of information about encoding are available to them, and they can be expected to be aware of this specification.

Other MIME agents will not be XML-aware; thus, they cannot know anything about the XML encoding declaration. Not only do they lack one of the three sources of information about encoding, they are also less likely to be aware of or responsive to this specification.

Some MIME agents, such as proxies and transcoders, both consume and produce MIME entities.

This mixture of two kinds of agents handling XML MIME entities increases the complexity of the coordination task. The recommendations given below are intended to maximise interoperability in the face of this: on the one hand, by mandating consistent production and encouraging maximally robust forms of production and, on the other, by specifying recovery strategies to maximize the interoperability of consumers when the production rules are broken.

3.1. XML MIME Producers

XML-aware MIME producers SHOULD supply a charset parameter and/or an appropriate BOM with non-UTF-8-encoded XML MIME entities that lack an encoding declaration. Such producers SHOULD remove or correct an encoding declaration that is known to be incorrect (for example, as a result of transcoding).

XML-aware MIME producers MUST supply an XML text declaration at the beginning of non-UNICODE XML external parsed entities that would otherwise begin with the hexadecimal octet sequences 0xFE 0xFF, 0xFF 0xFE or 0xEF 0xBB 0xBF, in order to avoid the mistaken detection of a BOM.

XML-unaware MIME producers MUST NOT supply a charset parameter with an XML MIME entity unless the entity's character encoding is reliably known. Note that this is particularly relevant for central configuration of web servers, where configuring a default for the charset parameter will almost certainly violate this requirement.

XML MIME producers are RECOMMENDED to provide means for users to control what value, if any, is given to charset parameters for XML MIME entities, for example, by giving users control of the configuration of Web server filename-to-Content-Type-header mappings on a file-by-file or suffix basis.

3.2. XML MIME Consumers

For XML MIME consumers, the question of priority arises in cases when the available character encoding information is not consistent. Again, we must distinguish between XML-aware and XML-unaware agents.

When a charset parameter is specified for an XML MIME entity, the normative component of the [XML] specification leaves the question open as to how to determine the encoding with which to attempt to process the entity. This is true independently of whether or not the entity contains in-band encoding information, that is, either a BOM (Section 3.3) or an XML encoding declaration, both, or neither. In particular, in the case where there is in-band information and it conflicts with the charset parameter, the [XML] specification does not specify which is authoritative. In its (non-normative) Appendix F, it defers to this specification:

  • [T]he preferred method of handling conflict should be specified as part of the higher-level protocol used to deliver XML. In particular, please refer to [IETF RFC 3023] or its successor...

Accordingly, to conform with deployed processors and content and to avoid conflicting with this or other normative specifications, this specification sets the priority as follows:

  • A BOM (Section 3.3) is authoritative if it is present in an XML MIME entity;
  • In the absence of a BOM (Section 3.3), the charset parameter is authoritative if it is present.

Whenever the above determines a source of encoding information as authoritative, consumers SHOULD process XML MIME entities based on that information.

When MIME producers conform to the requirements stated above (Section 3.1, Section 3) inconsistencies will not arise -- the above statement of priorities only has practical impact in the case of non-conforming XML MIME entities. In the face of inconsistencies, no uniform strategy can deliver the 'right' answer every time: the purpose of specifying one here is to encourage convergence over time, first on the part of consumers, then on the part of producers.

For XML-aware consumers, note that Section 4.3.3 of [XML] does not make it an error for the charset parameter and the XML encoding declaration (or the UTF-8 default in the absence of encoding declaration and BOM) to be inconsistent, although such consumers might choose to issue a warning in this case.

If an XML MIME entity is received where the charset parameter is omitted, no information is being provided about the character encoding by the MIME Content-Type header. XML-aware consumers MUST follow the requirements in Section 4.3.3 of [XML] that directly address this case. XML-unaware MIME consumers SHOULD NOT assume a default encoding in this case.

3.3. The BOM and Encoding Conversions

Section 4.3.3 of [XML] specifies that UTF-16 XML MIME entities not labelled as "utf-16le" or "utf-16be" MUST begin with a BOM, U+FEFF, which appears as the hexadecimal octet sequence 0xFE 0xFF (big-endian) or 0xFF 0xFE (little-endian). [XML] further states that the BOM is an encoding signature and is not part of either the markup or the character data of the XML document.

Due to the presence of the BOM, applications that convert XML from UTF-16 to an encoding other than UTF-8 MUST strip the BOM before conversion. Similarly, when converting from another encoding into UTF-16, either without a charset parameter or labelled "utf-16", the BOM MUST be added unless the original encoding was UTF-8 and a BOM was already present, in which case it MUST be transcoded into the appropriate UTF-16 BOM.

Section 4.3.3 of [XML] also allows for UTF-8 XML MIME entities to begin with a BOM, which appears as the hexadecimal octet sequence 0xEF 0xBB 0xBF. This is likewise defined to be an encoding signature, and not part of either the markup or the character data of the XML document.

Applications that convert XML from UTF-8 to an encoding other than UTF-16 MUST strip the BOM, if present, before conversion. Applications that convert XML into UTF-8 MAY add a BOM.

In addition to the MIME charset "utf-16", [RFC2781] introduces "utf‑16le" (little-endian) and "utf-16be" (big‑endian). When an XML MIME entity is encoded in "utf-16le" or "utf‑16be", it MUST NOT begin with the BOM but SHOULD contain an in-band XML encoding declaration. Conversion from UTF‑8 or UTF‑16 (unlabelled, or labelled with "utf‑16") to "utf-16be" or "utf‑16le" MUST strip a BOM if present. Conversion from UTF-16 labelled "utf-16le" or "utf‑16be" to UTF‑16 without a label or labelled "utf‑16" MUST add the appropriate BOM. Conversion from UTF-16 labelled "utf‑16le" or "utf‑16be" to UTF‑8 MAY add a UTF‑8 BOM, but this is NOT RECOMMENDED.

Appendix F of [XML] also implies that a UTF-32 BOM may be used in conjunction with UTF-32-encoded documents. As noted above, this specification recommends against the use of UTF-32. If it is used, the same considerations as UTF-16 apply with respect to its being a signature (not part of the document), transcoding into or out of it, and transcoding into or out of the MIME charsets "utf-32le" and "utf-32be". Consumers that do not support UTF-32 SHOULD nonetheless recognise UTF-32 signatures in order to give helpful error messages (instead of treating them as invalid UTF-16).

4. XML Media Types

4.1. XML MIME Entities

Within the XML specification, XML MIME entities can be classified into four types. In the XML terminology, they are called "document entities", "external DTD subsets", "external parsed entities", and "external parameter entities". Appropriate usage for the types registered below is as follows:

document entities:
The media types application/xml or text/xml, or a more specific media type (see Section 9.6), SHOULD be used.
external DTD subsets:
The media type application/xml-dtd SHOULD be used. The media types application/xml and text/xml MUST NOT be used.
external parsed entities:
The media types application/xml-external-parsed-entity or text/xml-external-parsed-entity SHOULD be used. The media types application/xml and text/xml MUST NOT be used unless the parsed entities are also well-formed "document entities".
external parameter entities:
The media type application/xml-dtd SHOULD be used. The media types application/xml and text/xml MUST NOT be used.

Note that [RFC3023] (which this specification obsoletes) recommended the use of text/xml and text/xml-external-parsed-entity for document entities and external parsed entities, respectively, but described handling of character encoding that differed from common implementation practice. These media types are still commonly used, and this specification aligns the handling of character encoding with industry practice.

Note that [RFC2376] (which is obsolete) allowed application/xml and text/xml to be used for any of the four types, although in practice it is likely to have been rare.

Neither external DTD subsets nor external parameter entities parse as XML documents, and while some XML document entities may be used as external parsed entities and vice versa, there are many cases where the two are not interchangeable. XML also has unparsed entities, internal parsed entities, and internal parameter entities, but they are not XML MIME entities.

Compared to [RFC2376] or [RFC3023], this specification alters the handling of character encoding of text/xml and text/xml-external-parsed-entity, treating them no differently from the respective application/ types. However, application/xml and application/xml-external-parsed-entity are still RECOMMENDED, to avoid possible confusion based on the earlier distinction. The former confusion around the question of default character sets for the two text/ types no longer arises because

See Section 3 for the now-unified approach to the charset parameter that results.

XML provides a general framework for defining sequences of structured data. It is often appropriate to define new media types that use XML but define a specific application of XML, due to domain-specific display, editing, security considerations, or runtime information. Furthermore, such media types may allow only UTF-8 and/or UTF-16 and prohibit other character sets. This specification does not prohibit such media types; in fact, they are expected to proliferate. However, developers of such media types are RECOMMENDED to use this specification as a basis for their registration. See Section 4.2 for more detailed recommendations on using the '+xml' suffix for registration of such media types.

An XML document labeled as application/xml or text/xml, or with a '+xml' media type, might contain namespace declarations, stylesheet-linking processing instructions (PIs), schema information, or other declarations that might be used to suggest how the document is to be processed. For example, a document might have the XHTML namespace and a reference to a Cascading Style Sheets (CSS) stylesheet. Such a document might be handled by applications that would use this information to dispatch the document for appropriate processing. Appendix B lists the core XML specifications that, taken together with [XML] itself, show how to determine an XML document's language-level semantics and suggest how information about its application-level semantics may be locatable.

4.2. Using '+xml' when Registering XML-Based Media Types

In Section 9.6, this specification updates the registration in [RFC6839] for XML-based MIME types (the '+xml' types).

When a new media type is introduced for an XML-based format, the name of the media type SHOULD end with '+xml' unless generic XML processing is in some way inappropriate for documents of the new type. This convention will allow applications that can process XML generically to detect that the MIME entity is supposed to be an XML document, verify this assumption by invoking some XML processor, and then process the XML document accordingly. Applications may check for types that represent XML MIME entities by comparing the last four characters of the subtype to the string '+xml'. (However, note that four of the five media types defined in this specification -- text/xml, application/xml, text/xml-external-parsed-entity, and application/xml-external-parsed-entity -- also represent XML MIME entities while not ending with '+xml'.)

  • NOTE: Section 5.3.2 of [RFC7231] does not support any form of Accept header that will match only '+xml' types. In particular, Accept headers of the form "Accept: */*+xml" are not allowed, and will not work for this purpose.

Media types following the naming convention '+xml' SHOULD define the charset parameter for consistency, since XML-generic processing by definition treats all XML MIME entities uniformly as regards character encoding information. However, there are some cases that the charset parameter need not be defined. For example:

  • When an XML-based media type is restricted to UTF-8, it is not necessary to define the charset parameter. UTF-8 is the default for XML.
  • When an XML-based media type is restricted to UTF-8 and UTF-16, it might not be unreasonable to omit the charset parameter. Neither UTF-8 nor UTF-16 require XML encoding declarations.

XML generic processing is not always appropriate for XML-based media types. For example, authors of some such media types may wish that the types remain entirely opaque except to applications that are specifically designed to deal with that media type. By NOT following the naming convention '+xml', such media types can avoid XML-generic processing. Since generic processing will be useful in many cases, however -- including in some situations that are difficult to predict ahead of time -- the '+xml' convention is to be preferred unless there is some particularly compelling reason not to use it.

The registration process for specific '+xml' media types is described in [RFC6838]. New XML-based media type registrations in the IETF must follow these guidelines. When other organisations register XML-based media types via the "Specification Required" IANA registration policy [RFC5226], the relevant Media Reviewer should ensure that they use the '+xml' convention, in order to ensure maximum interoperability of their XML-based documents. Only media subtypes that represent XML MIME entities are allowed to register with a '+xml' suffix.

In addition to the changes described above, the change controller has been changed to be the World Wide Web Consortium (W3C).

4.3. Registration Guidelines for XML-Based Media Types Not Using '+xml'

Registrations for new XML-based media types that do not use the '+xml' suffix SHOULD, in specifying the charset parameter and encoding considerations, define them as: "Same as [charset parameter / encoding considerations] of application/xml as specified in RFC 7303".

Defining the charset parameter is RECOMMENDED, since this information can be used by XML processors to determine authoritatively the character encoding of the XML MIME entity in the absence of a BOM. If there are some reasons not to follow this advice, they SHOULD be included as part of the registration. As shown above, two such reasons are "UTF-8 only" or "UTF-8 or UTF-16 only".

These registrations SHOULD specify that the XML-based media type being registered has all of the security considerations described in this specification plus any additional considerations specific to that media type.

These registrations SHOULD also make reference to this specification in specifying magic numbers, base URIs, and use of the BOM.

These registrations MAY reference the application/xml registration in this document in specifying interoperability and fragment identifier considerations, if these considerations are not overridden by issues specific to that media type.

5. Fragment Identifiers

Uniform Resource Identifiers (URIs) can contain fragment identifiers (see Section 3.5 of [RFC3986]). Specifying the syntax and semantics of fragment identifiers is devolved by [RFC3986] to the appropriate media type registration.

The syntax and semantics of fragment identifiers for the XML media types defined in this specification are based on the [XPointerFramework] W3C Recommendation. It allows simple names and more complex constructions based on named schemes. When the syntax of a fragment identifier part of any URI or Internationalized Resource Identifier (IRI) ([RFC3987]) with a retrieved media type governed by this specification conforms to the syntax specified in [XPointerFramework], conforming applications MUST interpret such fragment identifiers as designating whatever is specified by the [XPointerFramework] together with any other specifications governing the XPointer schemes used in those identifiers that the applications support. Conforming applications MUST support the 'element' scheme as defined in [XPointerElement], but need not support other schemes.

If an XPointer error is reported in the attempt to process the part, this specification does not define an interpretation for the part.

A registry of XPointer schemes [XPtrReg] is maintained at the W3C. Generic processors of XML MIME entities SHOULD NOT implement unregistered XPointer schemes ([XPtrRegPolicy] describes requirements and procedures for registering schemes).

See Section 4.2 for additional requirements that apply when an XML-based media type follows the naming convention '+xml'.

If [XPointerFramework] and [XPointerElement] are inappropriate for some XML-based media type, it SHOULD NOT follow the naming convention '+xml'.

When a URI has a fragment identifier, it is encoded by a limited subset of the repertoire of US-ASCII characters, see [XPointerFramework] for details.

6. The Base URI

An XML MIME entity of type application/xml, text/xml, application/xml-external-parsed-entity, or text/xml-external-parsed-entity MAY use the xml:base attribute, as described in [XMLBase], to embed a base URI in that entity for use in resolving relative URI references (see Section 5.1 of [RFC3986]).

Note that the base URI itself might be embedded in a different MIME entity, since the default value for the xml:base attribute can be specified in an external DTD subset or external parameter entity. Since conforming XML processors need not always read and process external entities, the effect of such an external default is uncertain; therefore, its use is NOT RECOMMENDED.

7. XML Versions

application/xml, application/xml-external-parsed-entity, application/xml-dtd, text/xml, and text/xml-external-parsed-entity are to be used with [XML]. In all examples herein where version="1.0" is shown, it is understood that version="1.1" might also appear, providing the content does indeed conform to [XML1.1].

The normative requirement of this specification upon XML documents and processors is to follow the requirements of [XML], Section 4.3.3. Except for minor clarifications, that section is substantially identical from the first edition to the current (5th) edition of XML 1.0, and for XML 1.1 first or second edition [XML1.1]. Therefore, references herein to [XML] may be interpreted as referencing any existing version or edition of XML, or any subsequent edition or version that makes no incompatible changes to that section.

Specifications and recommendations based on or referring to this RFC SHOULD indicate any limitations on the particular versions or editions of XML to be used.

8. Examples

This section is non-normative. In particular, note that all [RFC2119] language herein reproduces or summarizes the consequences of normative statements already made above, and has no independent normative force, and accordingly does not appear in uppercase.

The examples below give the MIME Content-Type header, including the charset parameter, if present and the XML declaration or Text declaration (which includes the encoding declaration) inside the XML MIME entity. For UTF-16 examples, the Byte Order Mark character appropriately UTF-16 encoded is denoted as "{BOM}", and the XML or Text declaration is assumed to come at the beginning of the XML MIME entity, immediately following the encoded BOM. Note that other MIME headers may be present, and the XML MIME entity will normally contain other data in addition to the XML declaration; the examples focus on the Content-Type header and the encoding declaration for clarity.

Although they show a content type of 'application/xml', all the examples below apply to all five media types declared below in Section 9, as well as to any media types declared using the '+xml' convention (with the exception of the examples involving the charset parameter for any such media types that do not enable its use). See the XML MIME entities table (Section 4.1, Paragraph 1) for discussion of which types are appropriate for which varieties of XML MIME entity.

8.1. UTF-8 Charset

Content-Type: application/xml; charset=utf-8

<?xml version="1.0" encoding="utf-8"?>

or

<?xml version="1.0"?>

UTF-8 is the recommended encoding for use with all the media types defined in this specification. Since the charset parameter is provided and there is no overriding BOM, conformant MIME and XML processors must treat the enclosed entity as UTF-8 encoded.

If sent using a 7-bit transport (e.g., SMTP), in general, a UTF-8 XML MIME entity must use a content-transfer-encoding of either quoted-printable or base64. For an 8-bit clean transport (e.g., 8BITMIME ESMTP or NNTP), or a binary clean transport (e.g., BINARY ESMTP or HTTP), no content-transfer-encoding is necessary (or even possible, in the case of HTTP).

8.2. UTF-16 Charset

Content-Type: application/xml; charset=utf-16

{BOM}<?xml version="1.0" encoding="utf-16"?>

or

{BOM}<?xml version="1.0"?>

For the three application/media types defined above, if sent using a 7-bit transport (e.g., SMTP) or an 8-bit clean transport (e.g., 8BITMIME ESMTP or NNTP), the XML MIME entity must be encoded in quoted-printable or base64; for a binary clean transport (e.g., BINARY ESMTP or HTTP), no content-transfer-encoding is necessary (or even possible, in the case of HTTP).

As described in [RFC2781], the UTF-16 family must not be used with media types under the top-level type "text" except over HTTP or HTTPS (see Section A.2 of HTTP for details). Hence, one of the two text/media types defined above can be used with this example only when the XML MIME entity is transmitted via HTTP or HTTPS, which use a MIME-like mechanism and are binary-clean protocols and hence do not perform CR and LF transformations and allow NUL octets. Since HTTP is binary clean, no content-transfer-encoding is necessary (or even possible).

8.3. Omitted Charset and 8-Bit MIME Entity

Content-Type: application/xml

<?xml version="1.0" encoding="iso-8859-1"?>

Since the charset parameter is not provided in the Content-Type header and there is no overriding BOM, conformant XML processors must treat the "iso-8859-1" encoding as authoritative. Conformant XML-unaware MIME processors should make no assumptions about the character encoding of the XML MIME entity.

8.4. Omitted Charset and 16-Bit MIME Entity

Content-Type: application/xml

{BOM}<?xml version="1.0" encoding="utf-16"?>

or

{BOM}<?xml version="1.0"?>

This example shows a 16-bit MIME entity with no charset parameter. However, since there is a BOM, conformant processors must treat the entity as UTF-16 encoded.

Omitting the charset parameter is not recommended in conjunction with media types under the top-level type "application" when used with transports other than HTTP or HTTPS. Media types under the top-level type "text" should not be used for 16-bit MIME with transports other than HTTP or HTTPS (see discussion above in Section 8.2, Paragraph 7).

8.5. Omitted Charset, No Internal Encoding Declaration

Content-Type: application/xml

<?xml version='1.0'?>

In this example, the charset parameter has been omitted, there is no internal encoding declaration, and there is no BOM. Since there is no BOM or charset parameter, the XML processor follows the requirements in Section 4.3.3, and optionally applies the mechanism described in Appendix F (which is non-normative) of [XML] to determine an encoding of UTF-8. Although the XML MIME entity does not contain an encoding declaration, provided the encoding actually is UTF-8, this is a conforming XML MIME entity.

A conformant XML-unaware MIME processor should make no assumptions about the character encoding of the XML MIME entity.

See Section 8.1 for transport-related issues for UTF-8 XML MIME entities.

8.6. UTF-16BE Charset

Content-Type: application/xml; charset=utf-16be

<?xml version='1.0' encoding='utf-16be'?>

Observe that, as required for this encoding, there is no BOM. Since the charset parameter is provided and there is no overriding BOM, conformant MIME and XML processors must treat the enclosed entity as UTF-16BE encoded.

See also the additional considerations in the UTF-16 example in (Section 8.2).

8.7. Non-UTF Charset

Content-Type: application/xml; charset=iso-2022-kr

<?xml version="1.0" encoding="iso-2022-kr"?>

This example shows the use of a non-UTF character encoding (in this case Hangul, but this example is intended to cover all non-UTF-family character encodings). Since the charset parameter is provided and there is no overriding BOM, conformant processors must treat the enclosed entity as encoded per RFC 1557.

Since ISO-2022-KR [RFC1557] has been defined to use only 7 bits of data, no content-transfer-encoding is necessary with any transport: for character sets needing 8 or more bits, considerations such as those discussed above (Sections 8.1 and 8.2) would apply.

8.8. INCONSISTENT EXAMPLE: Conflicting Charset and Internal Encoding Declaration

Content-Type: application/xml; charset=iso-8859-1

<?xml version="1.0" encoding="utf-8"?>

Although the charset parameter is provided in the Content-Type header and there is no BOM and the charset parameter differs from the XML encoding declaration, conformant MIME and XML processors will interoperate. Since the charset parameter is authoritative in the absence of a BOM, conformant processors will treat the enclosed entity as iso-8859-1 encoded. That is, the "UTF-8" encoding declaration will be ignored.

Conformant processors generating XML MIME entities must not label conflicting character encoding information between the MIME Content-Type and the XML declaration unless they have definitive information about the actual encoding, for example, as a result of systematic transcoding. In particular, the addition by servers of an explicit, site-wide charset parameter default has frequently lead to interoperability problems for XML documents.

8.9. INCONSISTENT EXAMPLE: Conflicting Charset and BOM

Content-Type: application/xml; charset=iso-8859-1

{BOM}<?xml version="1.0"?>

Although the charset parameter is provided in the Content-Type header, there is a BOM, so MIME and XML processors may not interoperate. Since the BOM parameter is authoritative for conformant XML processors, they will treat the enclosed entity as UTF-16 encoded. That is, the "iso-8859-1" charset parameter will be ignored. XML-unaware MIME processors on the other hand may be unaware of the BOM and so treat the entity as encoded in iso-8859-1.

Conformant processors generating XML MIME entities must not label conflicting character encoding information between the MIME Content-Type and an entity-initial BOM.

9. IANA Considerations

9.1. application/xml Registration

Type name:
application
Subtype name:
xml
Required parameters:
none
Optional parameters:
charset
Encoding considerations:
Depending on the character encoding used, XML MIME entities can consist of 7bit, 8bit, or binary data [RFC6838]. For 7-bit transports, 7bit data, for example, US-ASCII-encoded data, does not require content-transfer-encoding, but 8bit or binary data, for example, UTF-8 or UTF-16 data, MUST be content-transfer-encoded in quoted-printable or base64. For 8-bit clean transport (e.g., 8BITMIME ESMTP or NNTP), 7bit or 8bit data, for example, US-ASCII or UTF-8 data, does not require content-transfer-encoding, but binary data, for example, data with a UTF-16 encoding, MUST be content-transfer-encoded in base64. For binary clean transports (e.g., BINARY ESMTP or HTTP), no content-transfer-encoding is necessary (or even possible, in the case of HTTP) for 7bit, 8bit, or binary data.
Security considerations:
See Section 10.
Interoperability considerations:
XML has proven to be interoperable across both generic and task-specific applications and for import and export from multiple XML authoring and editing tools. Validating processors provide maximum interoperability, because they have to handle all aspects of XML. Although a non-validating processor may be more efficient, it might not handle all aspects. For further information, see Section 2.9 "Standalone Document Declaration" and Section 5 "Conformance" of [XML] .
In practice, character set issues have proved to be the biggest source of interoperability problems. The use of UTF-8, and careful attention to the guidelines set out in Section 3, are the best ways to avoid such problems.
Published specification:
Extensible Markup Language (XML) 1.0 (Fifth Edition) [XML] or subsequent editions or versions thereof.
Applications that use this media type:
XML is device, platform, and vendor neutral and is supported by generic and task-specific applications and a wide range of generic XML tools (editors, parsers, Web agents, ...).
Additional information:
 
Magic number(s):
None.
  • Although no byte sequences can be counted on to always be present, XML MIME entities in ASCII-compatible character sets (including UTF-8) often begin with hexadecimal 3C 3F 78 6D 6C ("<?xml"), and those in UTF-16 often begin with hexadecimal FE FF 00 3C 00 3F 00 78 00 6D 00 6C or FF FE 3C 00 3F 00 78 00 6D 00 6C 00 (the BOM followed by "<?xml"). For more information, see Appendix F of [XML].
 
File extension(s):
.xml
Macintosh File Type Code(s):
"TEXT"
 
Base URI:
See Section 6
Person and email address for further information:
See Authors' Addresses section
Intended usage:
COMMON
Author:
See Authors' Addresses section
Change controller:
The XML specification is a work product of the World Wide Web Consortium's XML Core Working Group. The W3C has change control over RFC 7303.

9.2. text/xml Registration

The registration information for text/xml is in all respects the same as that given for application/xml above (Section 9.1), except that the "Type name" is "text".

9.3. application/xml-external-parsed-entity Registration

Type name:
application
Subtype name:
xml-external-parsed-entity
Required parameters:
none
Optional parameters:
charset
Encoding considerations:
Same as for application/xml (Section 9.1).
Security considerations:
See Section 10.
Interoperability considerations:
XML external parsed entities are as interoperable as XML documents, though they have a less tightly constrained structure and therefore need to be referenced by XML documents for proper handling by XML processors. Similarly, XML documents cannot be reliably used as external parsed entities because external parsed entities are prohibited from having standalone document declarations or DTDs. Identifying XML external parsed entities with their own content type enhances interoperability of both XML documents and XML external parsed entities.
Published specification:
Same as for application/xml (Section 9.1).
Applications which use this media type:
Same as for application/xml (Section 9.1).
Additional information:
 
Magic number(s):
Same as for application/xml (Section 9.1).
File extension(s):
.xml or .ent
Macintosh File Type Code(s):
"TEXT"
 
Base URI:
See Section 6
Person and email address for further information:
See Authors' Addresses section.
Intended usage:
COMMON
Author:
See Authors' Addresses section.
Change controller:
The XML specification is a work product of the World Wide Web Consortium's XML Core Working Group. The W3C has change control over RFC 7303.

9.4. text/xml-external-parsed-entity Registration

The registration information for text/xml-external-parsed-entity is in all respects the same as that given for application/xml-external-parsed-entity above (Section 9.3), except that the "Type name" is "text".

9.5. application/xml-dtd Registration

Type name:
application
Subtype name:
xml-dtd
Required parameters:
none
Optional parameters:
charset
Encoding considerations:
Same as for application/xml (Section 9.1).
Security considerations:
See Section 10.
Interoperability considerations:
XML DTDs have proven to be interoperable by DTD authoring tools and XML validators, among others.
Published specification:
Same as for application/xml (Section 9.1).
Applications which use this media type:
DTD authoring tools handle external DTD subsets as well as external parameter entities. XML validators may also access external DTD subsets and external parameter entities.
Additional information:
 
Magic number(s):
Same as for application/xml (Section 9.1).
File extension(s):
.dtd or .mod
Macintosh File Type Code(s):
"TEXT"
Person and email address for further information:
See Authors' Addresses section.
Intended usage:
COMMON
Author:
See Authors' Addresses section.
Change controller:
The XML specification is a work product of the World Wide Web Consortium's XML Core Working Group. The W3C has change control over RFC 7303.

9.6. The '+xml' Naming Convention for XML-Based Media Types

This specification recommends the use of the '+xml' naming convention for identifying XML-based media types, in line with the recognition in [RFC6838] of structured syntax name suffixes. This allows the use of generic XML processors and technologies on a wide variety of different XML document types at a minimum cost, using existing frameworks for media type registration.

See Section 4.2 for guidance on when and how to register a media subtype that is '+xml' based, and Section 4.3 on registering a media subtype for XML but not using '+xml'.

9.6.1. The '+xml' Structured Syntax Suffix Registration

Name:
Extensible Markup Language (XML)
+suffix:
+xml
Reference:
RFC 7303
Encoding considerations:
Same as Section 9.1.
Fragment identifier considerations:
Registrations that use this '+xml' convention MUST also make reference to this document, specifically Section 5, in specifying fragment identifier syntax and semantics, and they MAY restrict the syntax to a specified subset of schemes, except that they MUST NOT disallow barenames or 'element' scheme pointers. They MAY further require support for other registered schemes. They also MAY add additional syntax (which MUST NOT overlap with [XPointerFramework] syntax) together with associated semantics, and they MAY add additional semantics for barename XPointers that, as provided for in Section 5, will only apply when this document does not define an interpretation.
  • In practice, these constraints imply that for a fragment identifier addressed to an instance of a specific "xxx/yyy+xml" type, there are three cases:
    • For fragment identifiers matching the syntax defined in [XPointerFramework], where the fragment identifier resolves per the rules specified there, then process as specified there;
    • For fragment identifiers matching the syntax defined in [XPointerFramework], where the fragment identifier does not resolve per the rules specified there, then process as specified in "xxx/yyy+xml";
    • For fragment identifiers not matching the syntax defined in [XPointerFramework], then process as specified in "xxx/yyy+xml". A fragment identifier of the form "xywh=160,120,320,240", as defined in [MediaFrags], which might be used in a URI for an XML-encoded image, would fall in this category.
Interoperability considerations:
Same as Section 9.1. See above, and also Section 3, for guidelines on the use of the 'charset' parameter.
Security considerations:
See Section 10.
Contact:
See Authors' Addresses section.
Author:
See Authors' Addresses section.
Change controller:
The XML specification is a work product of the World Wide Web Consortium's XML Core Working Group. The W3C has change control over RFC 7303.

10. Security Considerations

XML MIME entities contain information that may be parsed and further processed by the recipient. These entities may contain, and recipients may permit, explicit system level commands to be executed while processing the data. To the extent that a recipient application executes arbitrary command strings from within XML MIME entities, they may be at risk.

In general, any information stored outside of the direct control of the user -- including CSS style sheets, XSL transformations, XML-entity declarations, and DTDs -- can be a source of insecurity, by either obvious or subtle means. For example, a tiny "whiteout attack" modification made to a "master" style sheet could make words in critical locations disappear in user documents, without directly modifying the user document or the stylesheet it references. Thus, the security of any XML document is vitally dependent on all of the documents recursively referenced by that document.

The XML-entity lists and DTDs for XHTML 1.0 [XHTML], for instance, are likely to be a widely exploited set of resources. They will be used and trusted by many developers, few of whom will know much about the level of security on the W3C's servers, or on any similarly trusted repository.

The simplest attack involves adding declarations that break validation. Adding extraneous declarations to a list of character XML-entities can effectively "break the contract" used by documents. A tiny change that produces a fatal error in a DTD could halt XML processing on a large scale. Extraneous declarations are fairly obvious, but more sophisticated tricks, like changing attributes from being optional to required, can be difficult to track down. Perhaps the most dangerous option available to attackers, when external DTD subsets or external parameter entities or other externally specified defaulting is involved, is redefining default values for attributes: for example, if developers have relied on defaulted attributes for security, a relatively small change might expose enormous quantities of information.

Apart from the structural possibilities, another option, "XML-entity spoofing," can be used to insert text into documents, vandalizing and perhaps conveying an unintended message. Because XML permits multiple XML-entity declarations, and the first declaration takes precedence, it is possible to insert malicious content where an XML-entity reference is used, such as by inserting the full text of Winnie the Pooh in place of every occurrence of &mdash;.

Security considerations will vary by domain of use. For example, XML medical records will have much more stringent privacy and security considerations than XML library metadata. Similarly, use of XML as a parameter marshalling syntax necessitates a case by case security review.

XML may also have some of the same security concerns as plain text. Like plain text, XML can contain escape sequences that, when displayed, have the potential to change the display processor environment in ways that adversely affect subsequent operations. Possible effects include, but are not limited to, locking the keyboard, changing display parameters so subsequent displayed text is unreadable, or even changing display parameters to deliberately obscure or distort subsequent displayed material so that its meaning is lost or altered. Display processors SHOULD either filter such material from displayed text or else make sure to reset all important settings after a given display operation is complete.

With some terminal devices, sending particular character sequences to the display processor can change the output of subsequent key presses. If this is possible the display of a text object containing such character sequences could reprogram keys to perform some illicit or dangerous action when the key is subsequently pressed by the user. In some cases not only can keys be programmed, they can be triggered remotely, making it possible for a text display operation to directly perform some unwanted action. As such, the ability to program keys SHOULD be blocked either by filtering or by disabling the ability to program keys entirely.

Note that it is also possible to construct XML documents that make use of what XML terms "[XML-]entity references" to construct repeated expansions of text. Recursive expansions are prohibited by [XML] and XML processors are required to detect them. However, even non-recursive expansions may cause problems with the finite computing resources of computers, if they are performed many times. For example, consider the case where XML-entity A consists of 100 copies of XML-entity B, which in turn consists of 100 copies of XML-entity C, and so on.

11. References

11.1. Normative References

[IANA-CHARSETS]
IANA, “Character Sets Registry”, 2013, <http://www.iana.org/assignments/character-sets/>.
[RFC2045]
Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies”, RFC 2045, November 1996, <https://www.rfc-editor.org/info/rfc2045>.
[RFC2046]
Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types”, RFC 2046, November 1996, <https://www.rfc-editor.org/info/rfc2046>.
[RFC2119]
Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.
[RFC2781]
Hoffman, P. and F. Yergeau, “UTF-16, an encoding of ISO 10646”, RFC 2781, February 2000, <https://www.rfc-editor.org/info/rfc2781>.
[RFC2978]
Freed, N. and J. Postel, “IANA Charset Registration Procedures”, BCP 19, RFC 2978, October 2000, <https://www.rfc-editor.org/info/rfc2978>.
[RFC3986]
Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax”, STD 66, RFC 3986, January 2005, <https://www.rfc-editor.org/info/rfc3986>.
[RFC3987]
Duerst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs)”, RFC 3987, January 2005, <https://www.rfc-editor.org/info/rfc3987>.
[RFC6657]
Melnikov, A. and J. Reschke, “Update to MIME regarding "charset" Parameter Handling in Textual Media Types”, RFC 6657, July 2012, <https://www.rfc-editor.org/info/rfc6657>.
[RFC6838]
Freed, N., Klensin, J., and T. Hansen, “Media Type Specifications and Registration Procedures”, BCP 13, RFC 6838, January 2013, <https://www.rfc-editor.org/info/rfc6838>.
[RFC6839]
Hansen, T. and A. Melnikov, “Additional Media Type Structured Syntax Suffixes”, RFC 6839, January 2013, <https://www.rfc-editor.org/info/rfc6839>.
[RFC7230]
Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing”, RFC 7230, June 2014, <https://www.rfc-editor.org/info/rfc7230>.
[RFC7231]
Fielding, R., Ed. and J. Reschke, Ed., “Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content”, RFC 7231, June 2014, <https://www.rfc-editor.org/info/rfc7231>.
[UNICODE]
The Unicode Consortium, “The Unicode Standard, Version 7.0.0”, (Mountain View, CA: The Unicode Consortium, 2014 ISBN 978-1-936213-09-2), <http://www.unicode.org/versions/Unicode7.0.0/>.
[XML]
Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and F. Yergeau, “Extensible Markup Language (XML) 1.0 (Fifth Edition)”, W3C Recommendation REC-xml, November 2008, <http://www.w3.org/TR/2008/REC-xml-20081126/>.
Latest version available at <http://www.w3.org/TR/xml>.
[XML1.1]
Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., Yergeau, F., and J. Cowan, “Extensible Markup Language (XML) 1.1 (Second Edition)”, W3C Recommendation REC-xml, September 2006, <http://www.w3.org/TR/2006/REC-xml11-20060816/>.
Latest version available at <http://www.w3.org/TR/xml11>.
[XMLBase]
Marsh, J. and R. Tobin, “XML Base (Second Edition)”, W3C Recommendation REC-xmlbase-20090128, January 2009, <http://www.w3.org/TR/2009/REC-xmlbase-20090128/>.
Latest version available at <http://www.w3.org/TR/xmlbase>.
[XPointerElement]
Grosso, P., Maler, E., Marsh, J., and N. Walsh, “XPointer element() Scheme”, W3C Recommendation REC-XPointer-Element, March 2003, <http://www.w3.org/TR/2003/REC-xptr-element-20030325/>.
Latest version available at <http://www.w3.org/TR/xptr-element>.
[XPointerFramework]
Grosso, P., Maler, E., Marsh, J., and N. Walsh, “XPointer Framework”, W3C Recommendation REC-XPointer-Framework, March 2003, <http://www.w3.org/TR/2003/REC-xptr-framework-20030325/>.
Latest version available at <http://www.w3.org/TR/xptr-framework>.
[XPtrReg]
Hazael-Massieux, D., “XPointer Registry”, 2005, <http://www.w3.org/2005/04/xpointer-schemes/>.
[XPtrRegPolicy]
Hazael-Massieux, D., “XPointer Scheme Name Registry Policy”, 2005, <http://www.w3.org/2005/04/xpointer-policy.html>.

11.2. Informative References

[ASCII]
American National Standards Institute, “Coded Character Set -- 7-bit American Standard Code for Information Interchange”, ANSI X3.4, 1986.
[AWWW]
Jacobs, I. and N. Walsh, “Architecture of the World Wide Web, Volume One”, W3C Recommendation REC-webarch-20041215, December 2004, <http://www.w3.org/TR/2004/REC-webarch-20041215/>.
Latest version available at <http://www.w3.org/TR/webarch>.
[FYN]
Mendelsohn, N., “The Self-Describing Web”, W3C TAG Finding selfDescribingDocuments-2009-02-07, February 2009, <http://www.w3.org/2001/tag/doc/selfDescribingDocuments-2009-02-07.html>.
Latest version available at <http://www.w3.org/2001/tag/doc/selfDescribingDocuments.html>
[Infoset]
Cowan, J. and R. Tobin, “XML Information Set (Second Edition)”, W3C Recommendation REC-xml-infoset-20040204, Febuary 2004, <http://www.w3.org/TR/2004/REC-xml-infoset-20040204/>.
Latest version available at <http://www.w3.org/TR/xml-infoset/>.
[MediaFrags]
Troncy, R., Mannens, E., Pfeiffer, S., and D. Van Deursen, “Media Fragments URI 1.0 (basic)”, W3C Recommendation media-frags, September 2012, <http://www.w3.org/TR/2012/REC-media-frags-20120925/>.
Latest version available at <http://www.w3.org/TR/media-frags>.
[RFC1557]
Choi, U., Chon, K., and H. Park, “Korean Character Encoding for Internet Messages”, RFC 1557, December 1993, <https://www.rfc-editor.org/info/rfc1557>.
[RFC2376]
Whitehead, E. and M. Murata, “XML Media Types”, RFC 2376, July 1998.
[RFC2616]
Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1”, RFC 2616, June 1999, <https://www.rfc-editor.org/info/rfc2616>.
[RFC3023]
Murata, M., St. Laurent, S., and D. Kohn, “XML Media Types”, RFC 3023, January 2001, <https://www.rfc-editor.org/info/rfc3023>.
[RFC3030]
Vaudreuil, G., “SMTP Service Extensions for Transmission of Large and Binary MIME Messages”, RFC 3030, December 2000, <https://www.rfc-editor.org/info/rfc3030>.
[RFC3977]
Feather, C., “Network News Transfer Protocol (NNTP)”, RFC 3977, October 2006, <https://www.rfc-editor.org/info/rfc3977>.
[RFC5226]
Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs”, RFC 5226, May 2008, <https://www.rfc-editor.org/info/rfc5226>.
[RFC5321]
Klensin, J., “Simple Mail Transfer Protocol”, RFC 5321, October 2008, <https://www.rfc-editor.org/info/rfc5321>.
[RFC6152]
Klensin, J., Freed, N., Rose, M., and D. Crocker, Ed., “SMTP Service Extension for 8-bit MIME Transport”, STD 71, RFC 6152, March 2011, <https://www.rfc-editor.org/info/rfc6152>.
[Sivonen]
Sivonen, H. and others, “Mozilla bug: Remove support for UTF-32 per HTML5 spec”, October 2011, <https://bugzilla.mozilla.org/show_bug.cgi?id=604317#c6>.
[TAGMIME]
Bray, T., Ed., “Internet Media Type registration, consistency of use”, April 2004, <http://www.w3.org/2001/tag/2004/0430-mime>.
[XHTML]
Pemberton, S. and et al, “XHTML 1.0: The Extensible HyperText Markup Language”, W3C Recommendation xhtml1, December 1999, <http://www.w3.org/TR/2000/REC-xhtml1-20000126/>.
Latest version available at <http://www.w3.org/TR/xhtml1>.
[XMLid]
Marsh, J., Veillard, D., and N. Walsh, “xml:id Version 1.0”, W3C Recommendation REC-xml-id-20050909, September 2005, <http://www.w3.org/TR/2005/REC-xml-id-20050909/>.
Latest version available at <http://www.w3.org/TR/xml-id>.
[XMLModel]
Grosso, P. and J. Kosek, “Associating Schemas with XML documents 1.0 (Third Edition)”, W3C Working Group Note NOTE-xml-model-20121009, October 2012, <http://www.w3.org/TR/2012/NOTE-xml-model-20121009/>.
Latest version available at <http://www.w3.org/TR/xml-model>.
[XMLNS10]
Bray, T., Hollander, D., Layman, A., Tobin, R., and H. Thompson, “Namespaces in XML 1.0 (Third Edition)”, W3C Recommendation REC-xml-names-20091208, December 2009, <http://www.w3.org/TR/2009/REC-xml-names-20091208/>.
Latest version available at <http://www.w3.org/TR/xml-names>.
[XMLNS11]
Bray, T., Hollander, D., Layman, A., and R. Tobin, “Namespaces in XML 1.1 (Second Edition)”, W3C Recommendation REC-xml-names11-20060816, August 2006, <http://www.w3.org/TR/2006/REC-xml-names11-20060816/>.
Latest version available at <http://www.w3.org/TR/xml-names11>.
[XMLSS]
Clark, J., Pieters, S., and H. Thompson, “Associating Style Sheets with XML documents 1.0 (Second Edition)”, W3C Recommendation REC-xml-stylesheet-20101028, October 2010, <http://www.w3.org/TR/2010/REC-xml-stylesheet-20101028/>.
Latest version available at <http://www.w3.org/TR/xml-stylesheet>.

Appendix A. Why Use the '+xml' Suffix for XML-Based MIME Types?

[RFC3023] contains a detailed discussion of the (at the time) novel use of a suffix, a practice that has since become widespread. Those interested in a historical perspective on this topic are referred to [RFC3023], Appendix A.

The registration process for new '+xml' media types is described in [RFC6838].

Appendix B. Core XML Specifications

The following specifications each articulate key aspects of XML document semantics:

The W3C Technical Architecture group has produced two documents that are also relevant:

Appendix C. Operational Considerations

This section provides an informal summary of the major operational considerations that arise when exchanging XML MIME entities over a network.

C.1. General Considerations

The existence of both XML-aware and XML-unaware agents handling XML MIME entities can compromise introperability. Generic transcoding proxies pose a particular risk in this regard. Detailed advice about the handling of BOMs when transcoding can be found in Section 3.3.

This specification requires XML consumers to treat BOMs as authoritative: this is in principle a backwards-incompatibility. In practice, serious interoperability issues already exist when BOMs are used. Making BOMs authoritative, in conjunction with the deprecation of the UTF-32 encoding form and the requirement to include an XML encoding declaration in certain cases (Section 3.1), is intended to improve in-practice interoperability as much as possible over time.

This specification establishes Section 5 as the basis for interpreting URIs for XML MIME entities that include fragment identifiers, mandates support only for shorthand ("simple name") and 'element'-scheme fragments and deprecates support for unregistered XPointer schemes by XML MIME entity processors. Accordingly, URIs will interoperate best if they use only simple names and 'element'-scheme fragment identifiers, with registered schemes varying widely in the degree of support to be found in generic tools. XPointer scheme authors can only expect generic tool support if they register their schemes.

C.2. Considerations for Producers

Interoperability for all XML MIME entities is maximized by the use of UTF-8, without a BOM. When UTF-8 is not used, a charset parameter and/or a BOM improve interoperability, particularly when XML-unaware consumers may be involved.

In the very rare case where the substantive content of a non-UNICODE XML external parsed entity begins with the hexadecimal octet sequences 0xFE 0xFF, 0xFF 0xFE or 0xEF 0xBB 0xBF, including an XML text declaration will forestall the mistaken detection of a BOM.

The use of UTF-32 for XML MIME entities puts interoperability at very high risk.

Web-server configurations that supply default charset parameters risk misrepresenting XML MIME entities. Allowing users to control the value of charset parameters improves interoperability.

Supplying a mistaken charset parameter is worse than supplying none at all. In particular, generic processors such as transcoders, when processing based on a mistaken charset parameter, if they do not fail altogether are likely to produce arbitrarily bogus results from which the original is not recoverable.

C.3. Considerations for Consumers

Consumers of XML MIME entities can maximize interoperability by

  1. Taking a BOM as authoritative if it is present in an XML MIME entity;
  2. In the absence of a BOM, taking a charset parameter as authoritative if it is present.

Assuming a default character encoding in the absence of a charset parameter harms interoperability.

Although support for UTF-32 is not required by [XML] itself, and this specification deprecates its use, consumers that check for UTF-32 BOMs can thereby avoid mistakenly processing UTF-32 entities as (invalid) UTF-16 entities.

Appendix D. Changes from RFC 3023

There are numerous and significant differences between this specification and [RFC3023], which it obsoletes. This appendix summarizes the major differences only.

Appendix E. Acknowledgements

MURATA Makoto (FAMILY Given) and Alexey Melnikov made early and important contributions to the effort to revise [RFC3023].

This specification reflects the input of numerous participants to the ietf-xml-mime@imc.org, xml-mime@ietf.org, and apps-discuss@ietf.org mailing lists, though any errors are the responsibility of the authors. Special thanks to:

Mark Baker, James Clark, Dan Connolly, Martin Duerst, Ned Freed, Yaron Goland, Bjoern Hoehrmann, Rick Jelliffe, Murray S. Kucherawy, Larry Masinter, David Megginson, S. Moonesamy, Keith Moore, Chris Newman, Gavin Nicol, Julian Reschke, Marshall Rose, Jim Whitehead, Erik Wilde, and participants of the XML activity and the TAG at the W3C.

Jim Whitehead and Simon St. Laurent were editors of [RFC2376] and [RFC3023], respectively.

Authors' Addresses

Henry S. Thompson
University of Edinburgh
Email: ht@inf.ed.ac.uk
URI: http://www.ltg.ed.ac.uk/~ht/
Chris Lilley
World Wide Web Consortium
2004, Route des Lucioles - B.P. 93 06902
Sophia Antipolis Cedex
France
Email: chris@w3.org
URI: http://www.w3.org/People/chris/