1 Introduction
This specification defines the XML-binary Optimized Packaging (XOP) convention, a means of more efficiently serializing XML Infosets (see [XML InfoSet]) that have certain types of content.
A XOP package is created by placing a serialization of the XML Infoset inside of an extensible packaging format (such a MIME Multipart/Related, see [RFC 2387]). Then, selected portions of its content that are base64-encoded binary data are extracted and re-encoded (i.e., the data is decoded from base64) and placed into the package. The locations of those selected portions are marked in the XML with a special element that links to the packaged data using URIs.
In a number of important XOP applications, binary data need never be encoded in base64 form. If the data to be included is already available as a binary octet stream, then either an application or other software acting on its behalf can directly copy that data into a XOP package, at the same time preparing suitable linking elements for use in the root part; when parsing a XOP package, the binary data can be made available directly to applications, or, if appropriate, the base64 binary character representation can be computed from the binary data.
However, at the conceptual level, this binary data can be thought of as
being base64-encoded in the XML Document. As this conceptual form might
be needed during some processing of the XML Document (e.g., for signing
the XML document), it is necessary to have a one to one correspondence
between XML Infosets and XOP Packages. Therefore, the conceptual
representation of such binary data is as if it were base64-encoded,
using the canonical lexical form of XML Schema base64Binary
datatype (see [XML Schema Part 2] 3.2.16
base64Binary). In the reverse direction, XOP is capable of
optimizing only base64-encoded Infoset data that is in the canonical
lexical form.
Only element content can be optimized; attributes,
non-base64-compatible character data, and data not in the canonical
representation of the base64Binary datatype cannot be
successfully optimized by XOP.
The remainder of this specification is organized in the following fashion:
Section 2 describes the XOP Infoset, which preserves the non-optimized content and structure of the original XML Infoset.
Section 3 specifies the XOP processing model.
Section 4 of this specification describes the form of the XOP Package.
Section 5 describes how XOP Documents are identified.
Section 6 explores the security considerations of using the XOP convention.
1.1 Terminology
This specification uses terminology from the XML Infoset (see [XML InfoSet]) when discussing XML content and structure. This is only a convention for clear specification of XOP behavior.
The following terms are used in this specification:
- Original XML Infoset - An XML Infoset to be optimized.
- Optimized Content - Content which has been removed from the XML Infoset.
- XOP Infoset - The Original Infoset with
any Optimized Content removed and replaced by
xop:Includeelement information items. - XOP Document - A serialization of the XOP Infoset using any W3C recommendation-level version of XML.
- XOP Package - A package containing the XOP Document and any Optimized Content. As a whole, the XOP Package is an alternate serialization of the Original Infoset.
- Reconstituted XML Infoset - An XML Infoset that has been constructed from the parts of a XOP Package.
1.2 Example
Example 1 shows an XML Infoset prior to XOP
processing. Example 2 shows the same
Infoset, serialized using the XOP format in a MIME Multipart/Related
package. The base64-encoded content of the m:photo and
m:sig elements have been replaced by a
xop:Include element, while
the binary octets have been serialized in separate MIME parts. Note
that those examples use [MediaTypes] to identify the
media type of the content of the m:photo and
m:sig elements. Note also that the sample base64 data is smaller
than would be typical and the binary octets are not shown; in practice, the
optimized form is likely to be much smaller than the original.
<soap:Envelope
xmlns:soap='http://www.w3.org/2003/05/soap-envelope'
xmlns:xmime='http://www.w3.org/2005/05/xmlmime'>
<soap:Body>
<m:data xmlns:m='http://example.org/stuff'>
<m:photo
xmime:contentType='image/png'>/aWKKapGGyQ=</m:photo>
<m:sig
xmime:contentType='application/pkcs7-signature'>Faa7vROi2VQ=</m:sig>
</m:data>
</soap:Body>
</soap:Envelope>MIME-Version: 1.0
Content-Type: Multipart/Related;boundary=MIME_boundary;
type="application/xop+xml";
start="<mymessage.xml@example.org>";
start-info="application/soap+xml; action=\"http://www.example.com/ProcessData\""
Content-Description: A SOAP message with my pic and sig in it
--MIME_boundary
Content-Type: application/xop+xml;
charset=UTF-8;
type="application/soap+xml; action=\"http://www.example.com/ProcessData\""
Content-Transfer-Encoding: 8bit
Content-ID: <mymessage.xml@example.org>
<soap:Envelope
xmlns:soap='http://www.w3.org/2003/05/soap-envelope'
xmlns:xmime='http://www.w3.org/2005/05/xmlmime'>
<soap:Body>
<m:data xmlns:m='http://example.org/stuff'>
<m:photo
xmime:contentType='image/png'><xop:Include
xmlns:xop='http://www.w3.org/2004/08/xop/include'
href='cid:mypicture.png@example.org'/></m:photo>
<m:sig
xmime:contentType='application/pkcs7-signature'><xop:Include
xmlns:xop='http://www.w3.org/2004/08/xop/include'
href='cid:mysignature.hsh@example.org'/></m:sig>
</m:data>
</soap:Body>
</soap:Envelope>
--MIME_boundary
Content-Type: image/png
Content-Transfer-Encoding: binary
Content-ID: <mypicture.png@example.org>
// binary octets for png
--MIME_boundary
Content-Type: application/pkcs7-signature
Content-Transfer-Encoding: binary
Content-ID: <mysignature.hsh@example.org>
// binary octets for signature
--MIME_boundary--
Example 3 shows an XML Infoset prior to XOP
processing. Example 4 shows the same
Infoset, serialized using the XOP format in a MIME Multipart/Related
package. The base64-encoded content of the m:photo and
m:sig elements have been replaced by a
xop:Include element, while
the binary octets have been serialized in separate MIME parts. Note also
that the sample base64 data is smaller than would be typical and the
binary octets are not shown; in practice, the optimized form is
likely to be much smaller than the original.
<m:data xmlns:m='http://example.org/stuff'> <m:photo>/aWKKapGGyQ=</m:photo> <m:sig>Faa7vROi2VQ=</m:sig> </m:data>
MIME-Version: 1.0
Content-Type: Multipart/Related;boundary=MIME_boundary;
type="application/xop+xml";
start="<mymessage.xml@example.org>";
start-info="text/xml"
Content-Description: An XML document with my pic and sig in it
--MIME_boundary
Content-Type: application/xop+xml;
charset=UTF-8;
type="text/xml"
Content-Transfer-Encoding: 8bit
Content-ID: <mymessage.xml@example.org>
<m:data xmlns:m='http://example.org/stuff'>
<m:photo><xop:Include
xmlns:xop='http://www.w3.org/2004/08/xop/include'
href='cid:mypicture.png@example.org'/></m:photo>
<m:sig><xop:Include
xmlns:xop='http://www.w3.org/2004/08/xop/include'
href='cid:mysignature.hsh@example.org'/></m:sig>
</m:data>
--MIME_boundary
Content-Type: image/png
Content-Transfer-Encoding: binary
Content-ID: <mypicture.png@example.org>
// binary octets for png
--MIME_boundary
Content-Type: application/pkcs7-signature
Content-Transfer-Encoding: binary
Content-ID: <mysignature.hsh@example.org>
// binary octets for signature
--MIME_boundary--1.3 Notational Conventions
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 RFC 2119 [RFC 2119].
This specification uses a number of namespace prefixes throughout; they are listed below. Note that the choice of any namespace prefix is arbitrary and not semantically significant.
| Prefix | Namespace |
|---|---|
| Notes | |
| xop | "http://www.w3.org/2004/08/xop/include" |
| A non-normative XML Schema [XML Schema Part 1], [XML Schema Part 2] document for the "http://www.w3.org/2004/08/xop/include" namespace can be found at http://www.w3.org/2004/08/xop/include. Note that XML Schema currently provides only for validation of XML 1.0 Infosets; accordingly, the schema may not be usable with XOP Infosets corresponding to later versions of XML. | |
| xmime | "http://www.w3.org/2005/05/xmlmime" |
| The namespace for the content type attribute. | |
| soap | "http://www.w3.org/2003/05/soap-envelope" |
| The SOAP 1.2 namespace[SOAP Part 1]. | |
| xs | "http://www.w3.org/2001/XMLSchema" |
| The namespace of XML Schema data types [XML Schema Part 2]. |
2 XOP Infoset Constructs
XOP operates by extracting the Optimized Content from the Original
Infoset to create the XOP Infoset. In particular, the character
information item children of element information
items to be optimized are removed and replaced with an
element information item named xop:Include.
The xop:Include element information item
contains an attribute information item with a link to
the part of the XOP Package that carries a binary representation of the
data removed from the original element information item.
Details of the construction and processing of XOP serializations are
provided in 3 XOP Processing Model.
The Infoset used as input to XOP processing MUST NOT contain any
element information item with a [namespace name]
property of "http://www.w3.org/2004/08/xop/include"
and a [local name] property of Include. Infosets
containing such element information items cannot be
serialized using XOP. This is because during infoset reconstruction
a processor is unable to differentiate between
xop:Include element information items inserted during XOP package
construction and those that were part of the original infoset.
The following subsections provide formal definitions for allowable content in the element information item and attribute information items used to construct a XOP serialization; content not explicitly specified is disallowed. A non-normative XML Schema for [XML 1.0] serializations of those element information item and attribute information items can be found at http://www.w3.org/2004/08/xop/include.
2.1 xop:Include element information item
The xop:Include element information item
has:
-
A [local name] of
Include. - A [namespace name] of "http://www.w3.org/2004/08/xop/include".
-
One or more attribute information items
amongst its [attributes] property as follows:
-
A mandatory
hrefattribute information item (see 2.2 href attribute information item). -
Zero or more additional namespace qualified attribute information
items. Any such attribute information items MUST NOT have a [namespace name] of
"http://www.w3.org/2004/08/xop/include", MUST NOT change
the semantics of processing the
xop:Includeelement information item and MUST be ignored if not recognized.
-
A mandatory
-
Zero or more namespace qualified element
information items in its [children] property. Any such
element information items MUST NOT have a [namespace name]
of "http://www.w3.org/2004/08/xop/include", MUST
NOT change the semantics of processing the
xop:Includeelement information item and MUST be ignored if not recognized.
2.2 href attribute information item
The href attribute information item has:
-
A [local name] of
href. - An empty [namespace name].
-
A [normalized value] which is a representation of a URI (see
[RFC 3986])
referencing the part of the package containing the data logically
included by the [owner element] (i.e., the
xop:Includeelement information item). The [normalized value] MUST be a valid URI per the cid: URI scheme (see [RFC 2392]). In addition, the [normalized value] MUST be a valid lexical form of the XML Schemaxs:anyURIdatatype (see [XML Schema Part 2]3.2.17 anyURI). -
An [owner element] which is the
xop:Includeelement information item containing the attribute information item.
3 XOP Processing Model
This section describes the processing model for creating XOP Packages and interpreting XOP Packages. Unless otherwise stated, the result of such processing MUST be semantically equivalent to performing the specified steps separately, and in the order given.
3.1 Creating XOP Packages
To create a XOP Package from an Original XML Infoset:
-
Ensure that the Original XML Infoset contains no element
information item with a [namespace name] of
"http://www.w3.org/2004/08/xop/include" and a [local
name] of
Include. As discussed in 2 XOP Infoset Constructs, XML Infosets with such element information items cannot be represented using XOP. - Create an empty package.
-
Identify within the Original XML Infoset the element
information items to be optimized. To be optimized, the
characters comprising the [children] of the element
information item MUST be in the canonical form of
xs:base64Binary(see [XML Schema Part 2]3.2.16 base64Binary) and MUST NOT contain any whitespace characters, preceding, inline with or following the non-whitespace content. Note that this rule requires that the [children] of the element information item to be optimized contains only character information items. -
Create a XOP Infoset which is a copy of the Original XML Infoset,
but with the [children] of each element
information item
identified in the previous step replaced by a
xop:Includeelement information item (see 2.1 xop:Include element information item) constructed as follows:- Transform the replaced characters into binary data by processing them as base64-encoded data.
-
Serialize the binary data into a new part of the package, with
appropriate metadata corresponding to the [normalized value] of
the
hrefattribute information item of thexop:Includeelement information item (see 2.2 href attribute information item). -
If the element information item being optimized
(i.e., the [parent] of the newly inserted
xop:Includeelement information item) has axmime:contentTypeattribute information item, its value SHOULD be reflected appropriately in the metadata for the part.
- Serialize the resulting XOP Infoset into the package using any W3C recommendation-level version of XML (e.g., [XML 1.0], [XML 1.1]) and identify it as the root part according to the packaging mechanism's convention, labeling it with the application/xop+xml media type, as described in 5 Identifying XOP Documents.
Additional parts MAY be added to the package to satisfy application specific requirements. Other content-specific metadata MAY be reflected in the packaging metadata as appropriate.
If content cannot be successfully encoded into the XOP package, implementations SHOULD behave as if that portion of the Original XML Infoset was not nominated for optimization.
3.2 Interpreting XOP Packages
This section specifies the means by which the Original XML Infoset can be reconstructed from a XOP Package that has been prepared according to the rules of 3.1 Creating XOP Packages.
Note: conventions or error reporting mechanisms to be used in processing packages that incorrectly purport to be XOP Packages are beyond the scope of this specification.
To create a Reconstituted XML Infoset from a XOP Package:
- Construct an XML Infoset by parsing the root part of the package as an XML document. The document MUST be parsed according to the level of the XML Recommendation identified by the XML declaration of that document. If no XML declaration is present, then the document MUST be parsed per [XML 1.0].
-
Using that XML Infoset, for each element information
item, E, which has, as the sole member of its [children] property, a
xop:Includeelement information item (as defined in 2.1 xop:Include element information item):-
Locate the part of the package corresponding to the URI in the
hrefattribute information item of thexop:Includeelement information item (i.e., corresponding to the URI encoded in the attribute information item's [normalized value]). -
Replace the
xop:Includeelement information item that appears in the [children] property of E with character information items representing the canonical base64 encoding of the entity body of the identified package part (i.e., effectively replace thexop:Includeelement information item with the data reconstructed from the package part).
-
Locate the part of the package corresponding to the URI in the
4 XOP Packages
XOP is capable of using a variety of underlying packaging mechanisms. Such packaging mechanisms MUST be able to represent, with full fidelity all the parts created according to 3 XOP Processing Model (see 3.1 Creating XOP Packages), and MUST be used in a manner that provides a means of designating a distinguished root (main, primary etc.) part.
The subsection below specifies normatively how a particular packaging mechanism, MIME Multipart/Related, is used, but does not preclude the use of other packaging mechanisms with the XOP convention.
4.1 MIME Multipart/Related XOP Packages
This section describes how MIME Multipart/Related packaging (as specified in [RFC 2387]) is used with XOP.
The root MIME part is the root part of the XOP package, MUST be a serialization of the XOP Infoset using any W3C recommendation-level version of XML (e.g., [XML 1.0], [XML 1.1]), and MUST be identified with a media type of "application/xop+xml" (as defined below). The "start-info" parameter of the package's media type MUST contain the content type associated with the content's XML serialization. (i.e. it will contain the same value as the "type" parameter of the root part).
Except for purposes of determining the root MIME part, as specified by [RFC 2387], ordering of MIME parts MUST NOT be considered significant to XOP processing or to the construction of the XOP Infoset.
Part metadata is reflected in MIME header fields. Specifically,
the URI used in the value of an href attribute
information item on a xop:Include element
information item contains a URI that uses the 'cid:' scheme
(see [RFC 2392]), so the corresponding MIME
part MUST have a Content-ID header field (see [RFC 2387]
with a corresponding field-value.
Furthermore, if a xmime:contentType attribute
information item is found (as described in 3 XOP Processing Model), it SHOULD be reflected in the field value
of the MIME Content-Type header.
5 Identifying XOP Documents
XOP Documents, when used in MIME-like systems, are identified with the "application/xop+xml" media type, with the required "type" parameter conveying the original XML serialisation's associated content type. Note that when the type parameter contains reserved characters, it needs to be appropriately quoted and escaped.
For example, a XOP package using MIME multipart/related packaging to serialize a SOAP 1.2 message [SOAP Part 1] with an action parameter of "http://www.example.net/foo" would label the package itself with the "multipart/related" media type, and the root part with the "application/xop+xml" media type along with a type parameter containing "application/soap+xml;action=\"http://www.example.net/foo\"".
5.1 Registration
- MIME media type name:
application
- MIME subtype name:
xop+xml
- Required parameters:
- type
This parameter conveys the content type associated with the XML serialization of the XOP infoset, including parameters as appropriate.
- Optional parameters:
- charset
This parameter has identical semantics to the charset parameter of the "application/xml" media type as specified in RFC 3023 [RFC 3023].
- Encoding considerations:
Identical to those of "application/xml" as described in RFC 3023 [RFC 3023], section 3.2.
- Security considerations:
In addition to application-specific considerations, XOP has the same security considerations described in RFC3023 [RFC 3023], section 10.
- Interoperability considerations:
There are no known interoperability issues.
- Published specification:
This document
- Applications which use this media type:
No known applications currently use this media type.
- Additional information:
- File extension:
XOP
- Fragment identifiers:
Identical to that of "application/xml" as described in RFC 3023 [RFC 3023], section 5.
- Base URI:
As specified in RFC 3023 [RFC 3023], section 6.
- Macintosh File Type code:
TEXT
- Person and email address to contact for further information:
Mark Nottingham <mnot@pobox.com>
- Intended usage:
COMMON
- Author/Change controller:
The XOP specification is a work product of the World Wide Web Consortium's XML Protocol Working Group. The W3C has change control over this specification.
6 Security Considerations
6.1 XOP Package Integrity
The integrity of Infosets optimized using XOP may need to be ensured. As XOP packages can be transformed to recover such Infosets (see 3.2 Interpreting XOP Packages), existing XML Digital Signature techniques can be used to protect them. Note, however, that a signature over the Infoset does not necessarily protect against modifications of other aspects of the XOP packaging; for example, an Infoset signature check might not protect against re-ordering of non-root parts.
In the future a transform algorithm for use with XML Signature could provide a more efficient processing model where the raw octets are digested directly.
6.2 XOP Package Confidentiality
The confidentiality of XOP Packages may need to be ensured. As such
packages can be transformed to an XML Information Set, existing XML
Encryption (see [XML Encryption Syntax and Processing]) techniques can be used to
protect such packages. Any part of a package can be encrypted,
whether it includes base64 characters or not. The resulting
CipherData element information item can then
be optimized because the content of such an element information
item is base64 characters.
In the future a transform algorithm for use with XML Encryption could provide a more efficient processing model where the raw octets are encrypted directly.