XML-binary Optimized Packaging

1.1 Terminology

The following terms are used in this specification:

• Original XML Infoset - An XML Infoset to be optimized.
• Extracted Content - Optimized content which has been removed from the Infoset.
• XOP Infoset - The Original Infoset with any Extracted Content removed and replaced by xop:Include elements.
• XOP Document - A serialization of the XOP Infoset using XML 1.0.
• XOP Package - A package containing the XOP Document and any Extracted 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.

Figure 1: Architecture of the XOP framework

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 The base64-encoded content of the m:photo and m:sig elements have been replaced by a xop:Include element, while the binary octets have been serialized in separate MIME parts.

<soap:Envelope
    xmlns:soap='http://www.w3.org/2003/05/soap-envelope' 
    xmlns:xop='http://www.w3.org/2003/12/xop/include' 
    xmlns:xop-mime='http://www.w3.org/2003/12/xop/mime'>
  <soap:Body>
    <m:data xmlns:m='http://example.org/stuff'>
      <m:photo xop-mime:content-type='image/png'>
        /aWKKapGGyQ=
      </m:photo>
      <m:sig xop-mime:content-type='application/pkcs7-signature'>
        Faa7vROi2VQ=
      </m:sig>
    </m:data>
  </soap:Body>
</soap:Envelope>

MIME-Version: 1.0
Content-Type: Multipart/Related;boundary=MIME_boundary;
	      type=text/xml;start="<mymessage.xml@example.org>"
Content-Description: An XML document with my picture and signature in it

--MIME_boundary
Content-Type: text/xml; charset=UTF-8
Content-Transfer-Encoding: 8bit
Content-ID: <mymessage.xml@example.org>

<soap:Envelope
    xmlns:soap='http://www.w3.org/2003/05/soap-envelope'
    xmlns:xop='http://www.w3.org/2003/12/xop/include'
    xmlns:xop-mime='http://www.w3.org/2003/12/xop/mime'>
  <soap:Body>
    <m:data xmlns:m='http://example.org/stuff'>
      <m:photo xop-mime:content-type='image/png'>
        <xop:Include href='cid:http://example.org/me.png'/>
      </m:photo>
      <m:sig xop-mime:content-type='application/pkcs7-signature'>
        <xbinc:Include href='cid:http://example.org/my.hsh'/>
      </m:sig>
    </m:data>
  </soap:Body>
</soap:Envelope>

--MIME_boundary
Content-Type: image/png
Content-Transfer-Encoding: binary
Content-ID: <http://example.org/me.png>

// binary octets for png

--MIME_boundary
Content-Type: application/pkcs7-signature
Content-Transfer-Encoding: binary
Content-ID: <http://example.org/my.hsh>

// 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.

2.1 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 package, and MUST be an XML 1.0 serialization [XML 1.0] of the XOP Infoset, as defined below, and MUST be identified with the [ TBD ] media type.

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, if the URI used in the value of a xop:Include element's href attribute has a 'cid' scheme, the corresponding MIME part's Content-ID header field MUST have a corresponding field-value. Otherwise, the MIME part's Content-Location header field MUST have a field-value identical to the URI in the value of the href attribute.

Furthermore, if a xop-mime:content-type header is found (as described in 4. XOP's Processing Model), it SHOULD be reflected in the MIME Content-Type header's field-value.

3.1 xop:Include Element Information Item

The xop:Include Element property values are as follows:

• nanemspace name MUST be http://www.w3.org/2003/12/xop/include.
• localname MUST be Include.
• children MUST NOT contain any Information Item.
• There MAY be more than one Attributes comprising attributes. Among these MUST be the following:
  • href Attribute (see 3.2 href Attribute Information Item).
• Other properties such as base-uri, parent nad in-scope namespaces MUST be set according to the context.

3.2 href Attribute

The href Attribute has the following Infoset property values:

• namespace name MUST be empty.
• localname MUST be href.
• normalized value MUST be a representation of a URI referencing the part of the package containing the data logically included by the parent Element (i.e., the xop:Include Element).
• owner element MUST be the xop:Include Element which is parent of the Attribute.

3.3 xop-mime:content-type Attribute Information Item

The xop-mime:content-type Attribute has the following Infoset property values:

• namespace name MUST be http://www.w3.org/2003/12/xop/mime.
• localname MUST be content-type.
• normalized value MUST be the content-type of the binary data represented as base64 encoded data in the Element parent of this Attribute.
• owner element MUST be set according to the context.

4.1 Creating XOP Packages

To create a XOP Package from an Original XML Infoset:

1. Ensure that the Original Infoset contains no Element with a namespace name of http://www.w3.org/2003/12/xop/include and a localname of Include. As discussed in 3. XOP Infoset Constructs, Infosets with such Elements cannot be represented using XOP.
2. Create an empty package.
3. Identify within the Original Infoset the Elements to be optimized. Such Information Items' children MUST only be characters representing the canonical lexical form of base64Binary as described in Errata in XML Schema, E2-54.
4. Create a XOP Infoset which is a copy of the Original Infoset, but with the children of each Element identified in the previous step replaced by a xop:Include Element (see 3.1 xop:Include Element Information Item) constructed as follows:
   1. Transform the replaced characters into binary data by processing them as base64-encoded data.
   2. Serialize the binary data into a new part of the package, with appropriate metadata corresponding to the normalized value of the href Attribute of the xop:Include Element (see 3.2 href Attribute Information Item).
   3. If the Information Item being optimized (i.e., the parent of the newly inserted xop:Include Element) has a xop-mime:content-type Attribute, its value SHOULD be reflected appropriately in the part's metadata.
5. Serialize the resulting XOP Infoset into the package as XML 1.0 and identify it as the root part according to the packaging mechanism's convention.

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 Infoset, implementations SHOULD behave as if that portion of the Original Infoset was not nominated for optimization.

4.2 Interpreting XOP Packages

To create a Reconstituted Infoset from a XOP Package:

1. Parse the root part of the package as an XML 1.0 document to construct an XML Infoset (see [XML InfoSet]).
2. Using that Infoset, for each Element which has as its children a xop:Include Element (as defined in 3.1 xop:Include Element Information Item):
   1. Locate the part of the package corresponding to the URI in the xop:Include's href Attribute (i.e., corresponding to the URI encoded in the Attribute's normalized value).
   2. Replace the Element's children with characters containing the canonical base64 encoding of the entity body of the identified package part (i.e., effectively replace the xop:Include Element with the data reconstructed from the package part).