Copyright © 2005 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.
This document addresses the need to indicate the content-type associated with binary element content in an XML document and the need to specify, in XML Schema, the expected content-type(s) associated with binary element content. It is expected that the additional information about the content-type will be used for optimizing the handling of binary data that is part of a Web services message.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
This document is a W3C Working Group Note. This document includes the resolution of the comments received on the Last Call Working Draft previously published. The comments on this document and their resolution can be found in the Web Services Description Working Group's issues list.
There is no technical difference between this document and the 2 May 2005 version; the acknowledgement section has been updated to thank external contributors. A diff-marked version against the previous version of this document is available.
It has been produced jointly by the XML Protocol Working Group, and the Web Services Description Working Group, which are part of the Web Services Activity.
No further work on this topic is planned at this point. Errors in this document can be reported to the public [email protected] mailing list (public archive).
Publication as a Working Group Note does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
1 Introduction
1.1 Notational
Conventions
1.2 Requirements
2 Attributes for Declaring
Content-Type
2.1 xmime:contentType Attribute
2.2 xmime:expectedContentTypes
Attribute
3 Declaring Content-Type for Binary Data
3.1 Role of
xmime:expectedContentTypes Schema Annotation Attribute
4 Examples
4.1 Binary Data with
Known Media Type
4.2 Binary
Data with Preferred Media Type
5 Normative References
6 Informative
References
Data sent and received over the Web typically uses the MIME media type defined by [IETF RFC 2046], as the type system. For example, "image/jpeg", "application/pdf". There is a need to indicate the content-type of the XML element content, for example, in messages sent and received by Web services. There is also a need to express the content-type information using [XML Schema: Datatypes] and [XML Schema: Datatypes], which is the type system used by [WSDL 2.0 Part 1]. This would allow XML-based applications, such as Web services, to utilize the widely deployed and supported MIME media type infrastructure.
[XOP] and [MTOM] enables one to serialize binary content (element content that is in a canonical lexical representation of the xs:base64Binary type) in an optimized way using MIME packaging. There is a desire to specify the content-type information of such binary element content in a standard way in the [XML Information Set] and not just in the optimized serialization of that Infoset.
This document specifies:
An attribute (xmime:contentType) to indicate the content-type of an XML element content whose type is xs:base64Binary or xs:hexBinary. The value of the attribute is a valid content-type string (e.g., "text/xml; charset=utf-16"). This attribute specifies the content-type of the element content on which it occurs.
A XML Schema annotation attribute (xmime:expectedContentTypes) to indicate, in XML Schema, the expected content-type(s) for an element content whose type is xs:base64Binary or xs:hexBinary.
The XML Schema annotation, xmime:expectedContentTypes, specifies the expected range of values for the xmime:contentType attribute and the expected range of content-type for the binary element content.
Note that the use of this mechanism, in particular the xmime:contentType attribute, does not require the implementation, in whole or in part, of XML Schema. In the absence of XML Schema the type information (xs:base64Binary or xs:hexBinary) may have to be provided via other mechanisms; for example, using xsi:type.
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [IETF RFC 2119].
This specification uses properties from the XML Information Set (see [XML Information Set]). Such properties are denoted by square brackets, e.g. [namespace name].
This specification uses namespace prefixes that are listed in Table 1. Note that the choice of any namespace prefix is arbitrary and not semantically significant (see [XML Information Set]).
| Prefix | Namespace | Definition |
|---|---|---|
| xmime | http://www.w3.org/2005/05/xmlmime | Defined by this specification |
| xs | "http://www.w3.org/2001/XMLSchema" | Defined in the W3C XML Schema specifications [XML Schema: Structures], [XML Schema: Datatypes]. |
| xsi | "http://www.w3.org/2001/XMLSchema-instance" | Defined in the W3C XML Schema specification [XML Schema: Structures]. |
Namespace names of the general form "http://example.org/..." and "http://example.com/..." represent application or context-dependent URIs (see [IETF RFC 3986]).
All parts of this specification are normative, with the exception of examples and sections explicitly marked as "Non-Normative".
This section describes the set of requirements that this document addresses.
Define how to indicate the content-type of an XML element content whose type is xs:base64Binary or xs:hexBinary. This is meta-data that may be, but not required to, used by tools to infer the specific content-type of binary data.
Define how to indicate the expected content-type(s) of XML element content whose type is xs:base64Binary or xs:hexBinary in XML Schema. This information is needed to define the set of content-type that a binary data may have. For example, a Web services application may be willing to indicate that the binary data represents an image, but leaves it to a document instance to further specify whether it is "jpeg", or "gif". This meta-data is not required to be present.
Define the acceptable format of content-type values.
Define the relationship between the expected and the actual value of the content-type declared for binary data in XML documents.
This section defines two global attribute information items for declaring the content-type of binary data and expected content-type(s) of binary data in XML Schema to address requirements (1) and (2) above. Their usage is addressed in Section 3 Declaring Content-Type for Binary Data.
The xmime:contentType attribute information
item has the following Infoset properties:
A [local name] of contentType.
A [namespace name] of "http://www.w3.org/2005/05/xmlmime".
The type of the xmime:contentType attribute
information item is xs:string with a minimum length
of three and all leading and trailing white space characters are
ignored.
The [normalized value] of the xmime:contentType
attribute information item MUST be a valid Content-Type
string, e.g., "image/png", "text/xml; charset=utf-16" as defined by
[IETF RFC 2045] and indicates the
content-type of the [owner element]. Note that [normalized value]
consists of normalized attribute value as defined by [XML Information Set] and does not mean that two
equivalent values of xmime:contentType will
necessarily be equal.
The xmime:contentType attribute information
item allows Web services applications to optimize the handling
of the binary data defined by a binary element information
item and should be considered as meta-data. The presence of
the xmime:contentType attribute does not changes the
value of the element content.
The xmime:expectedContentTypes attribute
information item has the following Infoset properties:
A [local name] of expectedContentTypes.
A [namespace name] of
http://www.w3.org/2005/05/xmlmime.
The type of the xmime:expectedContentTypes
attribute information item is xs:string.
The value and the meaning of the
xmime:expectedContentTypes attribute is similar to the
value allowed for the 'Accept' HTTP header defined by
HTTP 1.1 specification, Section 14.1 (see [IETF
RFC 2616]) and MUST follow the production rules defined in that
section except for the following:
The prefix "Accept:" MUST NOT be used.
The rule qdtext is changed from: qdtext = <any
TEXT except<">> to: qdtext = <any CHAR
except<">> This change is made to disallow
non-US-ASCII OCTETs.
The xmime:expectedContentTypes attribute
information item is intended to be used as part of XML Schema
annotation for a binary element information item
declaration (see 3 Declaring Content-Type for
Binary Data). This attribute information item is
meant to allow XML Schema authors to indicate the range of media
types and/or associated parameters that are acceptable for the
binary data. It serves as a static constrain on the
xmime:contentType. Users of this attribute
information item are urged to avoid using wild cards (for
example, "image/*") as it may lead to interoperability problems. If
the set of expected media types is not known, the use of
xmime:expectedContentTypes is NOT RECOMMENDED.
Documents that want to specify additional content-type information for binary data SHOULD denote this by using a binary element information item. A binary element information item is an element information item defined with the following additional constraints.
An OPTIONAL xmime:contentType attribute
information item as described above in 2.1 xmime:contentType Attribute .
The character information items comprising the [children] of the element information item MUST conform to the lexical constraints of xs:base64Binary or xs:hexBinary.
If the media type identified by the value of an
xmime:contentType attribute information item is a text
based media type then the value of the
xmime:contentType attribute information item SHOULD
include a charset parameter.
For authoring convenience, two types
xmime:base64Binary and xmime:hexBinary
are defined in B Schema
<?xml version="1.0" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:tns="http://example.com/ct-required"
xmlns:xmime="http://www.w3.org/2005/05/xmlmime"
targetNamespace="http://example.com/ct-required">
<xs:import namespace="http://www.w3.org/2005/05/xmlmime"
schemaLocation="http://www.w3.org/2005/05/xmlmime"/>
<!-- This element has binary content and requires the xmime:contentType
attribute that indicates the content-type of the binary element -->
<xs:element name="MyBinaryData">
<xs:complexType>
<xs:simpleContent>
<xs:extension base="xs:base64Binary" >
<xs:attribute ref="xmime:contentType" use="required"/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
</xs:schema>
The xmime:expectedContentTypes attribute is used
for annotating XML Schema to indicate the expected range of
content-type of the binary element content and the expected range
of values for xmime:contentType attribute.
The value of the xmime:contentType attribute, if
present, SHOULD be within the range specified by the
xmime:expectedContentTypes annotation attribute, if
specified in the schema. See Section 14.1 of [IETF RFC 2616] on how to interpret content-type
ranges that may be defined with respect to actual content. When the
xmime:expectedContentTypes annotation attribute
contains a wild card ("*") or a list of acceptable content-type
separated by commas (","), the schema SHOULD require the
xmime:contentType attribute to be present.
Applications that need to specify expected content-type SHOULD
use the schema annotation to declare the range of expected values.
xmime:expectedContentTypes annotation attribute MAY be
used in conjunction with the declaration of binary element
information items or with complex type definitions that are
derived from xs:base64Binary or xs:hexBinary in
XML Schema. If the xmime:expectedContentTypes
annotation attribute is used in both the binary element
information item declaration as well as definition of the
complex type which the binary element information item
belongs to, then the expected range of values defined for the
binary element information item MUST be a subset of the
expected range of values defined for the complex type.
The xmime:expectedContentTypes annotation can be
used in conjunction with either type or element declarations.
Certain data-binding frameworks which use static type mappings can
be more specific if the xmime:expectedContentTypes
annotation is applied to the complexType declarations instead of
the element declarations using those types. For this reason, the
use of expectedContentTypes on element declarations using named
complex types is not recommended. An example is provided in Example
6.
The example below consists of a type definition,
PictureType, and an element declaration,
Picture. The xmime:contentType attribute
is required to be present and specifies the content-type of the
binary content. The schema annotation attribute
xmime:expectedContentTypes specifies that the media
type of the binary content is 'image', and the subtype name is
either 'jpeg' or 'png'.
<?xml version="1.0" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:tns="http://example.com/wildcard"
xmlns:xmime="http://www.w3.org/2005/05/xmlmime"
targetNamespace="http://example.com/wildcard">
<xs:import namespace="http://www.w3.org/2005/05/xmlmime"
schemaLocation="http://www.w3.org/2005/05/xmlmime"/>
<xs:complexType name="PictureType">
<xs:simpleContent>
<xs:restriction base="xmime:base64Binary" >
<xs:attribute ref="xmime:contentType" use="required" />
</xs:restriction>
</xs:simpleContent>
</xs:complexType>
<!-- This element designates the range of values
that the element definition will accept -->
<xs:element name="Picture" type="tns:PictureType"
xmime:expectedContentTypes="image/jpeg, image/png"/>
</xs:schema>
The example document instance below conforms to the element
declaration of Picture and specifies that the binary
content is of type "image/png".
The examples in this section consists of a binary elements whose media type is known in advance to be "image/jpeg".
In Example 4, a fixed media type is
specified by declaring it with an annotation in conjunction with
the complex type definition. The attribute
xmime:contentType is not used as the media type of the
binary data is know in advance.
<?xml version="1.0" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:tns="http://example.com/know-type"
xmlns:xmime="http://www.w3.org/2005/05/xmlmime"
targetNamespace="http://example.com/know-type">
<xs:import namespace="http://www.w3.org/2005/05/xmlmime"
schemaLocation="http://www.w3.org/2005/05/xmlmime"/>
<xs:simpleType name="JPEGPictureType"
xmime:expectedContentTypes="image/jpeg">
<xs:restriction base="xs:base64Binary"/>
</xs:simpleType>
<xs:element name="JPEGPicture" type="tns:JPEGPictureType"/>
</xs:schema>
In Example 5, a fixed media
type is specified by declaring it with an annotation in conjunction
with the element declaration. The attribute
xmime:contentType is optionally used in document
instances to indicate the media type of the binary data.
<?xml version="1.0" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:tns="http://example.com/know-type"
xmlns:xmime="http://www.w3.org/2005/05/xmlmime"
targetNamespace="http://example.com/know-type">
<xs:import namespace="http://www.w3.org/2005/05/xmlmime"
schemaLocation="http://www.w3.org/2005/05/xmlmime"/>
<xs:element name="JPEGPicture" type="xmime:base64Binary"
xmime:expectedContentTypes="image/jpeg" />
</xs:schema>
This example illustrates that binary data with media type 'image/jpeg' is preferred but binary data with media type of 'image/tiff' is also allowed (with a lower preference).
<?xml version="1.0" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:tns="http://example.com/preferred-type"
xmlns:xmime="http://www.w3.org/2005/05/xmlmime"
targetNamespace="http://example.com/preferred-type">
<xs:import namespace="http://www.w3.org/2005/05/xmlmime"
schemaLocation="http://www.w3.org/2005/05/xmlmime"/>
<xs:complexType name="JPEGPreferredPictureType"
xmime:expectedContentTypes="image/jpeg;q=1.0, image/tiff;q=0.8">
<xs:simpleContent>
<xs:restriction base="xmime:base64Binary" >
<xs:attribute ref="xmime:contentType" use="required" />
</xs:restriction>
</xs:simpleContent>
</xs:complexType>
<xs:element name="JPEGPeferredPicture"
type="tns:JPEGPreferredPictureType"/>
</xs:schema>
This document is developed by the participants of the joint media types task force formed by Web Services Description and XML Protocol Working Groups. Participants of the taskforce, specifically Martin Gudgin, and Mark Nottingham are gratefully acknowledged.
The authors would also like to thank the commentors, in particular Joe Fialli, Larry Massinter, and Henry Thompson.
<?xml version="1.0" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:xmime="http://www.w3.org/2005/05/xmlmime"
targetNamespace="http://www.w3.org/2005/05/xmlmime" >
<xs:attribute name="contentType">
<xs:simpleType>
<xs:restriction base="xs:string" >
<xs:minLength value="3" />
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute name="expectedContentTypes" type="xs:string" />
<xs:complexType name="base64Binary" >
<xs:simpleContent>
<xs:extension base="xs:base64Binary" >
<xs:attribute ref="xmime:contentType" />
</xs:extension>
</xs:simpleContent>
</xs:complexType>
<xs:complexType name="hexBinary" >
<xs:simpleContent>
<xs:extension base="xs:hexBinary" >
<xs:attribute ref="xmime:contentType" />
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:schema>