| Network Working Group | C. Lilley |
| Internet-Draft | W3C |
| Intended status: Standards Track | M. Murata |
| Expires: April 02, 2013 | International University of Japan |
| A. Melnikov | |
| Isode Ltd. | |
| H. S. Thompson | |
| University of Edinburgh | |
| October 2012 |
XML Media Types
draft-lilley-xml-mediatypes-00
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 a convention (using the suffix '+xml') for naming media types outside of these five types when those media types represent XML MIME entities. XML MIME entities are currently exchanged via the HyperText Transfer Protocol on the World Wide Web, are an integral part of the WebDAV protocol for remote web authoring, and are expected to have utility in many domains.
Major differences from [RFC3023] are alignment of charset handling for text/xml and text/xml-external-parsed-entity with application/xml, the addition of XPointer and XML Base as fragment identifiers and base URIs, respectively, mention of the XPointer Registry, and updating of many references.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http:/⁠/⁠datatracker.ietf.org/⁠drafts/⁠current/⁠.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 02, 2013.
Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http:/⁠/⁠trustee.ietf.org/⁠license-⁠info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
The World Wide Web Consortium has issued the Extensible Markup Language (XML) 1.0 specification. [XML]. 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 and two aliases -- text/xml and text/xml-external-parsed-entity, as well as a naming convention for identifying XML-based MIME media types (using +xml).
XML entities are currently exchanged on the World Wide Web, and XML is also used for property values and parameter marshalling by the WebDAV [RFC4918] protocol for remote web authoring. Thus, there is a need for a media type to properly label the exchange of XML network entities.
Although XML is a subset of the Standard Generalized Markup Language (SGML) ISO 8879 [SGML], which has been assigned the media types text/sgml and application/sgml, there are several reasons why use of text/sgml or application/sgml to label XML is inappropriate. First, there exist many applications that can process XML, but that cannot process SGML, due to SGML's larger feature set. Second, SGML applications cannot always process XML entities, because XML uses features of recent technical corrigenda to SGML. Third, the definition of text/sgml and application/sgml in [RFC1874] includes parameters for SGML bit combination transformation format (SGML-bctf), and SGML boot attribute (SGML-boot). Since XML does not use these parameters, it would be ambiguous if such parameters were given for an XML MIME entity. For these reasons, the best approach for labeling XML network entities has been to provide new media types for XML.
Since XML is an integral part of the WebDAV Distributed Authoring Protocol, and since World Wide Web Consortium Recommendations are assigned standards tree media types, and since similar media types (HTML, SGML) have been assigned standards tree media types, the XML media types were also placed in the standards tree [RFC3023].
Similarly, 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, media types based on XML, but that are not identified using application/xml (or text/xml), SHOULD be named using a suffix of '+xml' as described in Section 8. This will allow generic XML-based tools -- browsers, editors, search engines, and other processors -- to work with all XML-based media types.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this specification are to be interpreted as described in [RFC2119].
As defined in [RFC2781] (informative), the three charsets "utf-16", "utf-16le", and "utf-16be" are used to label UTF-16 text. In this specification, "the UTF-16 family" refers to those three charsets. By contrast, the phrases "utf-16" or UTF-16 in this specification refer specifically to the single charset "utf-16".
As sometimes happens between two communities, both MIME and XML have defined the term entity, with different meanings. Section 2.4 of [RFC2045] says:
Section 4 of [XML] says:
In this specification, "XML MIME entity" is defined as the latter (an XML entity) encapsulated in the former (a MIME entity).
This specification standardizes three media types related to XML MIME entities: application/xml (with text/xml as an alias), application/xml-external-parsed-entity (with text/xml-external-parsed-entity as an alias), and application/xml-dtd. Registration information for these media types is described in the sections below.
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". The media types application/xml or text/xml MAY be used for "document entities", while application/xml-external-parsed-entity or text/xml-external-parsed-entity SHOULD be used for "external parsed entities". 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 charset handling which differed from common implementation practice. These media types are still commonly used, and this specification aligns the charset handling with industry practice. The media type application/xml-dtd SHOULD be used for "external DTD subsets" or "external parameter entities". application/xml and text/xml MUST NOT be used for "external parameter entities" or "external DTD subsets", and MUST NOT be used for "external parsed entities" unless they are also well-formed "document entities" and are referenced as such. Note that [RFC2376] (which is obsolete) allowed such usage, 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.
Application/xml and application/xml-external-parsed-entity are recommended. Compared to [RFC2376] or [RFC3023], this specification alters the charset handling of text/xml and text/xml-external-parsed-entity, treating them no differently from the respective application/ types. The reasons are as follows:
XML provides a general framework for defining sequences of structured data. In some cases, it may be desirable to define new media types that use XML but define a specific application of XML, perhaps due to domain-specific display, editing, security considerations or runtime information. Furthermore, such media types may allow UTF-8 or UTF-16 only and prohibit other charsets. This specification does not prohibit such media types and in fact expects them to proliferate. However, developers of such media types are STRONGLY RECOMMENDED to use this specification as a basis for their registration. In particular, the charset parameter, if used, MUST agree with the encoding of the XML entity, as described in Section 8.1, in order to enhance interoperability.
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 CSS stylesheet. Such a document might be handled by applications that would use this information to dispatch the document for appropriate processing.
text/xml is an alias for application/xml, as defined in Section 3.1 above.
text/xml-external-parsed-entity is an alias for application/xml-external-parsed-entity, as defined in Section 3.3 above.
Section 4.3.3 of [XML] specifies that XML MIME entities in the charset "utf-16" MUST begin with a byte order mark (BOM), which is a hexadecimal octet sequence 0xFE 0xFF (or 0xFF 0xFE, depending on endian). The XML Recommendation 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 a non-Unicode encoding MUST strip the BOM before conversion. Similarly, when converting from another encoding into "utf-16", the BOM MUST be added after conversion is complete.
In addition to the charset "utf-16", [RFC2781] introduces "utf-16le" (little endian) and "utf-16be" (big endian) as well. The BOM is prohibited for these charsets. When an XML MIME entity is encoded in "utf-16le" or "utf-16be", it MUST NOT begin with the BOM but SHOULD contain an encoding declaration. Conversion from "utf-16" to "utf-16be" or "utf-16le" and conversion in the other direction MUST strip or add the BOM, respectively.
Uniform Resource Identifiers (URIs) may contain fragment identifiers (see Section 3.5 of [RFC3986]). Likewise, Internationalized Resource Identifiers (IRIs) [RFC3987] may contain fragment identifiers.
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 IRI with a retrieved media type governed by this specification conforms to the syntax specified in [XPointerFramework], conformant applications MUST attempt to interpret such fragment identifiers as designating that part of the retrieved representation specified by [XPointerFramework] and whatever other specifications define any XPointer schemes used. Conformant 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. Unregistered schemes SHOULD NOT be used.
See Section 8.1 for additional rquirements which apply when an XML-based MIME 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 [ASCII] characters, as defined in [RFC3986]. When an IRI contains a fragment identifier, it is encoded by a much wider repertoire of characters. The conversion between IRI fragment identifiers and URI fragment identifiers is presented in Section 7 of [RFC3987].
Section 5.1 of [RFC3986] specifies that the semantics of a relative URI reference embedded in a MIME entity is dependent on the base URI. The base URI is either (1) the base URI embedded in context, (2) the base URI from the encapsulating entity, (3) the base URI from the Retrieval URI, or (4) the default base URI, where (1) has the highest precedence. [RFC3986] further specifies that the mechanism for embedding the base URI is dependent on the media type.
The media type dependent mechanism for embedding the base URI in a MIME entity of type application/xml, text/xml, application/xml-external-parsed-entity or text/xml-external-parsed-entity is to use the xml:base attribute described in detail in [XBase].
Note that the base URI may be embedded in a different MIME entity, since the default value for the xml:base attribute may be specified in an external DTD subset or external parameter entity.
application/xml, application/xml-external-parsed-entity, and 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" may also be used, providing the content does indeed conform to [XML1.1].
The normative requirement of this specification upon XML 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. Therefore, this specification may be used with any version or edition of XML 1.0 or 1.1.
Specifications and recommendations based on or referring to this RFC SHOULD indicate any limitations on the particular versions of XML to be used. For example, a particular specification might indicate: "content MUST be represented using media-type application/xml, and the document must either (a) carry an xml declaration specifying version="1.0" or (b) omit the XML declaration, in which case per the XML recommendation the version defaults to 1.0"
This specification recommends the use of a naming convention (a suffix of '+xml') for identifying XML-based MIME media types, whatever their particular content may represent. 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.
Although the use of a suffix was not considered as part of the original MIME architecture, this choice is considered to provide the most functionality with the least potential for interoperability problems or lack of future extensibility. The alternatives to the '+xml' suffix and the reason for its selection are described in Appendix Appendix A.
As XML development continues, new XML document types are appearing rapidly. Many of these XML document types would benefit from the identification possibilities of a more specific MIME media type than text/xml or application/xml can provide, and it is likely that many new media types for XML-based document types will be registered in the near and ongoing future.
While the benefits of specific MIME types for particular types of XML documents are significant, all XML documents share common structures and syntax that make possible common processing.
Some areas where 'generic' processing is useful include:
When a new media type is introduced for an XML-based format, the name of the media type SHOULD end with '+xml'. 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 match for types that represent XML MIME entities by comparing the subtype to the pattern '*/*+xml'. (Of course, 4 of the 5 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 conforming to the '*/*+xml' pattern.)
Media types following the naming convention '+xml' SHOULD introduce the charset parameter for consistency, since XML-generic processing applies the same program for any such media type. However, there are some cases that the charset parameter need not be introduced. For example:
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 -- those registering media types SHOULD use the '+xml' convention unless they have a particularly compelling reason not to.
HST: This paragraph needs updating once some pending RFCs are out there The registration process for these media types is described in [RFC4288] and [RFC4289] . The registrar for the IETF tree will encourage new XML-based media type registrations in the IETF tree to follow this guideline. Registrars for other trees SHOULD follow this convention in order to ensure maximum interoperability of their XML-based documents. Similarly, media subtypes that do not represent XML MIME entities MUST NOT be allowed to register with a '+xml' suffix.
Registrations for new XML-based media types under top-level types 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 XXXX."
The use of the charset parameter is STRONGLY RECOMMENDED, since this information can be used by XML processors to determine authoritatively the charset of the XML MIME entity. 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 RFC XXXX plus any additional considerations specific to that media type.
These registrations SHOULD also make reference to RFC XXXX in specifying magic numbers, base URIs, and use of the BOM.
When these registrations use the '+xml' convention, they MUST also make reference to RFC XXXX 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 MAY add additional semantics for barename XPointers which, as provided for in Section 5, will only apply when this specification does not define an interpretation.
These registrations MAY reference the application/xml registration in RFC XXXX in specifying interoperability considerations, if these considerations are not overridden by issues specific to that media type.
The examples below give the value of the MIME Content-type header and the XML declaration (which includes the encoding declaration) inside the XML MIME entity. For UTF-16 examples, the Byte Order Mark character is denoted as "{BOM}", and the XML declaration is assumed to come at the beginning of the XML MIME entity, immediately following the BOM. Note that other MIME headers may be present, and the XML MIME entity may contain other data in addition to the XML declaration; the examples focus on the Content-type header and the encoding declaration for clarity.
Content-type: application/xml or text/xml
<?xml version="1.0" encoding="iso-8859-1"?>
Since the charset parameter is not provided in the Content-Type header, XML processors MUST treat the "iso-8859-1" encoding as authoritative. XML-unaware MIME processors SHOULD make no assumptions about the charset of the XML MIME entity.
Content-type: application/xml or text/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. Since the charset parameter is not provided in the Content-Type header, in this case XML processors MUST treat the "utf-16" encoding and/or the BOM as authoritative. XML-unaware MIME processors SHOULD make no assumptions about the charset of the XML MIME entity.
Omitting the charset parameter is NOT RECOMMENDED for application/xml when used with transports other than HTTP or HTTPS---text/xml SHOULD NOT be used for 16-bit MIME with transports other than HTTP or HTTPS (see. Section 9.5).
Content-type: application/xml or text/xml; charset="utf-8"
<?xml version="1.0" encoding="utf-8"?>
This is the recommended encoding for use with all the media types defined in this specification. Since the charset parameter is provided, both MIME and XML processors MUST treat the enclosed entity as UTF-8 encoded.
If sent using a 7-bit transport (e.g. SMTP [RFC5321]), the 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., HTTP), no content-transfer-encoding is necessary.
Content-type: application/xml; charset="utf-16"
{BOM}<?xml version="1.0" encoding="utf-16"?>
or
{BOM}<?xml version="1.0"?>
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., HTTP), no content-transfer-encoding is necessary.
Content-type: text/xml; charset="utf-16"
{BOM}<?xml version='1.0' encoding='utf-16'?>
or
{BOM}<?xml version='1.0'?>
This is possible only when the XML MIME entity is transmitted via HTTP or HTTPS, which use a MIME-like mechanism and are binary-clean protocols, hence do not perform CR and LF transformations and allow NUL octets. 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 19.4.1 of [RFC2616] for details).
Since HTTP is binary clean, no content-transfer-encoding is necessary.
Content-type: application/xml; charset="utf-16be"
<?xml version='1.0' encoding='utf-16be'?>
Observe that the BOM does not exist. Since the charset parameter is provided, MIME and XML processors MUST treat the enclosed entity as UTF-16BE encoded.
Content-type: text/xml; charset="utf-16be"
<?xml version='1.0' encoding='utf-16be'?>
Observe that the BOM does not exist. As for UTF-16, this is possible only when the XML MIME entity is transmitted via HTTP.
Content-type: application/xml; charset="iso-2022-kr"
<?xml version="1.0" encoding="iso-2022-kr"?>
This example shows the use of a Korean charset (e.g., Hangul) encoded following the specification in [RFC1557]. Since the charset parameter is provided, MIME processors MUST treat the enclosed entity as encoded per RFC 1557. Since the XML MIME entity has an internal encoding declaration (this example does show such a declaration, which agrees with the charset parameter) XML processors MUST also treat the enclosed entity as encoded per RFC 1557. Thus, interoperability is assured.
Since ISO-2022-KR has been defined to use only 7 bits of data, no content-transfer-encoding is necessary with any transport.
Content-type: application/xml or text/xml
<?xml version='1.0'?>
In this example, the charset parameter has been omitted, the is no internal encoding declaration, and there is no BOM. Since there is no BOM, 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 the charset encoding of UTF-8. Although the XML MIME entity does not contain an encoding declaration, the encoding actually is UTF-8, so this is still a conforming XML MIME entity.
An XML-unaware MIME processor SHOULD make no assumptions about the charset of the XML MIME entity.
Content-type: application/xml or text/xml
<?xml version='1.0' encoding="iso-10646-ucs-4"?>
In this example, the charset parameter has been omitted, and there is no BOM. However, the XML MIME entity does have an encoding declaration inside the XML MIME entity that specifies the entity's charset. Following the requirements in section 4.3.3, and optionally applying the mechanism described in Appendix F (non-normative) of [XML], the XML processor determines the charset encoding of the XML MIME entity (in this example, UCS-4).
An XML-unaware MIME processor SHOULD make no assumptions about the charset of the XML MIME entity.
Content-type: text/xml-external-parsed-entity or application/xml-external-parsed-entity; charset="utf-8"
<?xml encoding="utf-8"?>
Since the charset parameter is provided, MIME and XML processors MUST treat the enclosed entity as UTF-8 encoded.
If sent using a 7-bit transport (e.g. SMTP), the 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., HTTP) no content-transfer-encoding is necessary.
Content-type: application/xml-external-parsed-entity; charset="utf-16"
{BOM}<?xml encoding="utf-16"?>
or
{BOM}<?xml?>
Since the charset parameter is provided, MIME and XML processors MUST treat the enclosed entity as UTF-16 encoded.
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., HTTP), no content-transfer-encoding is necessary.
Content-type: application/xml-external-parsed-entity; charset="utf-16be"
<?xml encoding="utf-16be"?>
Since the charset parameter is provided, MIME and XML processors MUST treat the enclosed entity as UTF-16BE encoded.
Content-type: application/xml-dtd; charset="utf-8"
<?xml encoding="utf-8"?>
Charset "utf-8" is a recommended charset value for use with application/xml-dtd. Since the charset parameter is provided, MIME and XML processors MUST treat the enclosed entity as UTF-8 encoded.
Content-type: application/mathml+xml
<?xml version="1.0" ?>
MathML documents are XML documents whose content describes mathematical information, as defined by [MathML]. As a format based on XML, MathML documents SHOULD follow the '+xml' suffix convention and use 'mathml+xml' in their MIME content-type identifier.This media type has been registered at IANA and is fully defined in [MathML].
Content-type: application/xslt+xml
<?xml version="1.0" ?>
Extensible Stylesheet Language (XSLT) documents are XML documents whose content describes stylesheets for other XML documents, as defined by [XSLT]. As a format based on XML, XSLT documents SHOULD follow the '+xml' suffix convention and use 'xslt+xml' in their MIME content-type identifier.This media type has been registered at IANA and is fully defined in [XSLT].
Content-type: application/rdf+xml
<?xml version="1.0" ?>
Resources identified using the application/rdf+xml media type are XML documents whose content describe RDF metadata. This media type has been registered at IANA and is fully defined in [RFC3870].
Content-type: image/svg+xml
<?xml version="1.0" ?>
Scalable Vector Graphics (SVG) documents are XML documents whose content describes graphical information, as defined by [SVG]. As a format based on XML, SVG documents SHOULD follow the '+xml' suffix convention and use 'svg+xml' in their MIME content-type identifier.The image/svg+xml media type has been registered at IANA and is fully defined in [SVG]. .
Content-type: model/x3d+xml
<?xml version="1.0" ?>
X3D is derived from VRML and is used for 3D models. Besides the XML representation, it may also be serialised in classic VRML syntax and using a fast infoset. Separate, but clearly related media types are used for these serialisations (model/x3d+vrml and model/x3d+fastinfoset respectively).
Content-type: text/xml; charset="utf-8"
<?xml version="1.0" encoding="iso-8859-1"?>
Since the charset parameter is provided in the Content-Type header and differs from the XML encoding declaration , MIME and XML processors will not interoperate. MIME processors will treat the enclosed entity as UTF-8 encoded. That is, the "iso-8859-1" encoding will be be ignored. XML processors on the other hand will ignore the charset parameter and treat the XML entity as encoded in iso-8859-1.
Processors generating XML MIME entities MUST NOT label conflicting charset information between the MIME Content-Type and the XML declaration. In particular, the addition of an explicit, site-wide charset without inspecting the XML entity has frequently lead to interoperability problems.
Content-type: application/soap+xml
<?xml version="1.0" ?>
Resources identified using the application/soap+xml media type are SOAP 1.2 message envelopes that have been serialized with XML 1.0. This media type has been registered at IANA and is fully defined in [RFC3902].
As described in Section 8, this specification updates the [RFC4288] and [RFC4289] registration process for XML-based MIME types.
XML, as a subset of SGML, has all of the same security considerations as specified in [RFC1874], and likely more, due to its ubiquitous deployment.
To paraphrase section 3 of RFC 1874, XML MIME entities contain information to be parsed and processed by the recipient's XML system. These entities may contain and such systems may permit explicit system level commands to be executed while processing the data. To the extent that an XML system will execute arbitrary command strings, recipients of XML MIME entities may be a risk. In general, it may be possible to specify commands that perform unauthorized file operations or make changes to the display processor's environment that affect subsequent operations.
In general, any information stored outside of the direct control of the user -- including CSS style sheets, XSL transformations, 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 entity lists and DTDs for XHTML 1.0 [XHTML], for instance, are likely to be a commonly used set of information. Many developers will use and trust them, 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 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 crackers is redefining default values for attributes: e.g., 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, "entity spoofing," can be used to insert text into documents, vandalizing and perhaps conveying an unintended message. Because XML 1.0 permits multiple entity declarations, and the first declaration takes precedence, it's possible to insert malicious content where an entity is used, such as by inserting the full text of Winnie the Pooh in every occurrence of —.
Use of the digital signatures work currently underway by the xmldsig working group may eventually ameliorate the dangers of referencing external documents not under one's own control.
Use of XML is expected to be varied, and widespread. XML is under scrutiny by a wide range of communities for use as a common syntax for community-specific metadata. For example, the Dublin Core [RFC5013] group is using XML for document metadata, and a new effort has begun that is considering use of XML for medical information. Other groups view XML as a mechanism for marshalling parameters for remote procedure calls. More uses of XML will undoubtedly arise.
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.
Some terminal devices have keys whose output, when pressed, can be changed by sending the display processor a character sequence. 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 "entity references" (using the XML meaning of the term "entity" as described in Section 2), 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. (Entity A consists of 100 copies of entity B, which in turn consists of 100 copies of entity C, and so on)
Although the use of a suffix was not considered as part of the original MIME architecture, this choice is considered to provide the most functionality with the least potential for interoperability problems or lack of future extensibility. The alternatives to the '+xml' suffix and the reason for its selection are described below.
text/xml and application/xml remain useful in many situations, especially for document-oriented applications that involve combining XML with a stylesheet in order to present the data. However, XML is also used to define entirely new data types, and an XML-based format such as image/svg+xml fits the definition of a MIME media type exactly as well as image/png [PNG] does. (Note that image/svg+xml is not yet registered.) Although extra functionality is available for MIME processors that are also XML processors, XML-based media types -- even when treated as opaque, non-XML media types -- are just as useful as any other media type and should be treated as such.
Since MIME dispatchers work off of the MIME type, use of text/xml or application/xml to label discrete media types will hinder correct dispatching and general interoperability. Finally, many XML documents use neither DTDs nor namespaces, yet are perfectly legal XML.
The subtree under which a media type is registered -- IETF, vendor (*/vnd.*), or personal (*/prs.*); see [RFC4288] and [RFC4289] for details -- is completely orthogonal from whether the media type uses XML syntax or not. The suffix approach allows XML document types to be identified within any subtree. The vendor subtree, for example, is likely to include a large number of XML-based document types. By using a suffix, rather than setting up a separate subtree, those types may remain in the same location in the tree of MIME types that they would have occupied had they not been based on XML.
The top-level MIME type (e.g., model/* [RFC2077]) determines what kind of content the type is, not what syntax it uses. For example, agents using image/* to signal acceptance of any image format should certainly be given access to media type image/svg+xml, which is in all respects a standard image subtype. It just happens to use XML to describe its syntax. The two aspects of the media type are completely orthogonal.
XML-based data types will most likely be registered in ALL top-level categories. Potential, though currently unregistered, examples could include application/mathml+xml [MathML], model/uml+xml [UML], and image/svg+xml [SVG].
Rather than explicitly labeling XML-based media types, the processor could look inside each type and see whether or not it is XML. The processor could also cache a list of XML-based media types.
Although this method might work acceptably for some mail applications, it would fail completely in many other uses of MIME. For instance, an XML-based web crawler would have no way of determining whether a file is XML except to fetch it and check. The same issue applies in some IMAP4 [RFC3501] mail applications, where the client first fetches the MIME type as part of the message structure and then decides whether to fetch the MIME entity. Requiring these fetches just to determine whether the MIME type is XML could have significant bandwidth and latency disadvantages in many situations.
Sniffing XML also isn't as simple as it might seem. DOCTYPE declarations aren't required, and they can appear fairly deep into a document under certain unpreventable circumstances. (E.g., the XML declaration, comments, and processing instructions can occupy space before the DOCTYPE declaration.) Even sniffing the DOCTYPE isn't completely reliable, thanks to a variety of issues involving default values for namespaces within external DTDs and overrides inside the internal DTD. Finally, the variety in potential character encodings (something XML provides tools to deal with), also makes reliable sniffing less likely.
For example, one could use "Content-Type: application/iotp; alternate-type=text/xml" or "Content-Type: application/iotp; syntax=xml".
Section 5 of [RFC2045] says that "Parameters are modifiers of the media subtype, and as such do not fundamentally affect the nature of the content". However, all XML-based media types are by their nature always XML. Parameters, as they have been defined in the MIME architecture, are never invariant across all instantiations of a media type.
More practically, very few if any MIME dispatchers and other MIME agents support dispatching off of a parameter. While MIME agents on the receiving side will need to be updated in either case to support (or fall back to) generic XML processing, it has been suggested that it is easier to implement this functionality when acting off of the media type rather than a parameter. More important, sending agents require no update to properly tag an image as "image/svg+xml", but few if any sending agents currently support always tagging certain content types with a parameter.
This proposal fails under the simplest case, of a user with neither knowledge of XML nor an XML-capable MIME dispatcher. In that case, the user's MIME dispatcher is likely to dispatch the content to an XML processing application when the correct default behavior should be to dispatch the content to the application responsible for the content type (e.g., an ecommerce engine for application/iotp+xml [RFC2801], once this media type is registered).
Note that even if the user had already installed the appropriate application (e.g., the ecommerce engine), and that installation had updated the MIME registry, many operating system level MIME registries such as .mailcap in Unix and HKEY_CLASSES_ROOT in Windows do not currently support dispatching off a parameter, and cannot easily be upgraded to do so. And, even if the operating system were upgraded to support this, each MIME dispatcher would also separately need to be upgraded.
This combines the problems of Appendix Appendix A.5 and Appendix Appendix A.6.
If the sender attaches an image/svg+xml file to a message and includes the instructions "Please copy the French text on the road sign", someone with an XML-aware MIME client and an XML browser but no support for SVG can still probably open the file and copy the text. By contrast, with superclasses, the sender must add superclass support to her existing mailer AND the receiver must add superclass support to his before this transaction can work correctly.
If the receiver comes to rely on the superclass tag being present and applications are deployed relying on that tag (as always seems to happen), then only upgraded senders will be able to interoperate with those receiving applications.
This has nearly identical problems to Appendix Appendix A.7, in that it requires both senders and receivers to be upgraded, and few if any operating systems and MIME dispatchers support working off of anything other than the MIME type.
This is better than Appendix Appendix A.8, in that no extra functionality needs to be added to a MIME registry to support dispatching of information other than standard content types. However, it still requires both sender and receiver to be upgraded, and it will also fail in many cases (e.g., web hosting to an outsourced server), where the user can set MIME types (often through implicit mapping to file extensions), but has no way of adding arbitrary HTTP headers.
When the conneg protocol is fully defined, this may potentially be a reasonable thing to do. But given the limited current state of conneg [RFC2703] development, it is not a credible replacement for a MIME-based solution.
Also, note that adding a content-type parameter doesn't work with conneg either, since conneg only deals with media types, not their parameters. This is another illustration of the limits of parameters for MIME dispatchers.
MIME explicitly defines two levels of content type, the top-level for the kind of content and the second-level for the specific media type. [RFC4288] and [RFC4289] extends this in an interoperable way by using prefixes to specify separate trees for IETF, vendor, and personal registrations. This specification also extends the two-level type by using the '+xml' suffix. In both cases, processors that are unaware of these later specifications treat them as opaque and continue to interoperate. By contrast, adding a third-level type would break the current MIME architecture and cause numerous interoperability failures.
As specified in Section 5.1 of [RFC2045], a tspecial can't be used:
It was thought that "." would not be a good choice since it is already used as an additional hierarchy delimiter. Also, "*" has a common wildcard meaning, and "-" and "_" are common word separators and easily confused. The characters %'`#& are frequently used for quoting or comments and so are not ideal.
That leaves: ~!$^+{}|
Note that "-" is used heavily in the current registry. "$" and "_" are used once each. The others are currently unused.
It was thought that '+' expressed the semantics that a MIME type can be treated (for example) as both scalable vector graphics AND ALSO as XML; it is both simultaneously.
MIME processors that are unaware of XML will treat the '+xml' suffix as completely opaque, so it is essential that no extra semantics be assigned to its presence. Therefore, application/foo and application/foo+xml SHOULD be treated as completely independent media types. Although, for example, text/calendar+xml could be an XML version of text/calendar [RFC2445], it is possible that this (hypothetical) new media type would include new semantics as well as new syntax, and in any case, there would be many applications that support text/calendar but had not yet been upgraded to support text/calendar+xml.
In the ten years that MIME has existed, XML is the first generic data format that has seemed to justify special treatment, so it is hoped that no further suffixes will be necessary. However, if some are later defined, and these documents were also XML, they would need to specify that the '+xml' suffix is always the outermost suffix (e.g., application/foo+ebml+xml not application/foo+xml+ebml). If they were not XML, then they would use a regular suffix (e.g., application/foo+ebml).
You don't have to, but unless you have a good reason to explicitly disallow generic XML processing, you should use the suffix so as not to curtail the options of future users and developers.
Whether the inventors of a media type, today, design it for dispatch to generic XML processing machinery (and most won't) is not the critical issue. The core notion is that the knowledge that some media type happens to use XML syntax opens the door to unanticipated kinds of processing beyond those envisioned by its inventors, and on this basis identifying such encoding is a good and useful thing.
Developers of new media types are often tightly focused on a particular type of processing that meets current needs. But there is no need to rule out generic processing as well, which could make your media type more valuable over time. It is believed that registering with the '+xml' suffix will cause no interoperability problems whatsoever, while it may enable significant new functionality and interoperability now and in the future. So, the conservative approach is to include the '+xml' suffix.
There are numerous and significant differences between this specification and [RFC3023], which it obsoletes. This appendix summarizes the major differences only.
First, XPointer ([XPointerFramework] and [XPointerElement] has been added as fragment identifier syntax for "application/xml", and the XPointer Registry ([XPtrReg]) mentioned. Second, [XBase] has been added as a mechanism for specifying base URIs. Third, the language regarding charsets was updated to correspond to the W3C TAG finding Internet Media Type registration, consistency of use [TAGMIME]. Fourth, many references are updated.
This specification reflects the input of numerous participants to the ietf-xml-mime@imc.org mailing list, though any errors are the responsibility of the authors. Special thanks to:
Mark Baker, James Clark, Dan Connolly, Martin Duerst, Ned Freed, Yaron Goland, Rick Jelliffe, Larry Masinter, David Megginson, Keith Moore, Chris Newman, Gavin Nicol, Marshall Rose, Jim Whitehead and participants of the XML activity and the TAG at the W3C.
Jim Whitehead and Simon St.Laurent are editors of [RFC2376] and [RFC3023], respectively.