OOXML/Open Packaging Conventions (OPC)

Like ODF, OOXML documents are containers, stored physically as zip archives. Most of the files in the archive are in XML format, though binary files may be included in specific cases (e.g. for images).

Each OOXML document has certain files that serve a particular role, such as the document content, stylesheet, met data, and so forth. However, instead of simply using fixed path names within the archive (as ODF does), a part of the OOXML specification called Open Packaging Conventions (OPC) defines an additional layer of abstraction on top of the zip directory, which must be consulted to determine the actual paths at which certain files can be found.

Root relationships

Each file inside an OPC package is called a "part", and these are arrange as a directed graph - similar, but theoretically more flexible - than a traditional directory hierarchy. The graph's vertices are parts (files), and edges relationships. The set of outward relationships for a given part is stored as XML, in a file whose name is derived from the filename of the part.

To give a concrete example, let's consider the case of an empty document created in Word 2011 for mac. This document has the following structure, where each box (except for the root) represents a part:

The XML file containing the relationships for a given part is located in _rels/(filename).rels, relative to the directory containing that file. For the root relationships, this is simply /_rels/.rels. For another file, say /word/document.xml, the path is /word/_rels/document.xml.rels. Let's have a look at the root relationships file:

<Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships">
  <Relationship
    Id="rId3"
    Type="http://schemas.openxmlformats.org/package/2006/relationships/metadata/core-properties"
    Target="docProps/core.xml"/>
  <Relationship
    Id="rId4"
    Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/extended-properties"
    Target="docProps/app.xml"/
  <Relationship
    Id="rId1"
    Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/officeDocument"
    Target="word/document.xml"/>
  <Relationship
    Id="rId2"
    Type="http://schemas.openxmlformats.org/package/2006/relationships/metadata/thumbnail"
    Target="docProps/thumbnail.jpeg"/>
</Relationships>

As you can see from the file, each relationship has an id, a type, and a target. The target is simply the filename of the part; this is relative to the location of the source of the relationship (in this case, the root directory). The type indicates what the format or purpose of that part is. The id serves as a unique identifier to distinguish one relationship from another.

In some cases, you need to look up a relationship based on it's type; for example, to determine the name of the file containing the content of a word document, you would need to look through the relationships file to find one that has the appropriate namespace. In other cases, you would use the id; for example, where an image is included in a document, it is referenced by its relationship id (in the document part's relationships, not the root), and that id can then be used to find the path of the image.

Part relationships

Let's now have a look at the relationships file for the document content part. We know that in this particular case the filename happens to be word/document.xml, so we can infer that its relationships must be stored in word/_rels/document.xml.rels. Note that if a given part does not have any outgoing relationships, it does not need to have a relationships file.

<Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships">
  <Relationship
    Id="rId3"
    Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/settings"
    Target="settings.xml"/>
  <Relationship
    Id="rId4"
    Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/webSettings"
    Target="webSettings.xml"/>
  <Relationship
    Id="rId5"
    Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/fontTable"
    Target="fontTable.xml"/>
  <Relationship
    Id="rId6"
    Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/theme"
    Target="theme/theme1.xml"/>
  <Relationship
    Id="rId1"
    Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/styles"
    Target="styles.xml"/>
  <Relationship
    Id="rId2"
    Type="http://schemas.microsoft.com/office/2007/relationships/stylesWithEffects"
    Target="stylesWithEffects.xml"/>
</Relationships>

As you can see, this file is very similar to the root relationships file; the main difference is that it has a different set of relationships. The most important thing to note here is that all Target values are relative to the directory containing the source part's file. In this case, the file is /word/document.xml, so the styles part is located at /word/styles.xml, and the theme part is located at /word/theme/theme1.xml.

Just as when determining the filename of the document content part, you must always look at the relationships file to determine the location of other related parts, such as the styles part. Although Word saves in these locations, it's not guaranteed by the format, and in theory it would be possible for some other application to use a completely different set of physical path names. As long as you look up relations based on the required type, you'll always find the correct physical path.

External relationships

So far, all of the relationships we've seen are to other files in the same package. However, it is also possible to have external references, for example when hyperlinks are included in a document. WordProcessingML does not directly include URLs for hyperlinks, but instead stores a relationship for the link, which the content document refers to by its relationship id.

Here's an excerpt of a WordProcessingML file containing a hyperlink to http://www.openoffice.org

<w:p>
  <w:hyperlink r:id="rId5">
    <w:r>
      <w:rPr>
        <w:rStyle w:val="Hyperlink"/>
      </w:rPr>
      <w:t>Open Office</w:t>
    </w:r>
  </w:hyperlink>
</w:p>

Here, we can see that the hyperlink refers to relationship id "rId5". If we look in the relationships file for the content part, we'll see the following:

<Relationship
  Id="rId5"
  Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/hyperlink"
  Target="http://www.openoffice.org"
  TargetMode="External"/>

Note the TargetMode attribute here, which was not set in any relationships shown earlier. Every relationship has a target mode; the default value is Internal. Here, it is explicitly set to External, so that we know it's not referring to another file in the package.

The type of this relationship might initially seem sufficient to determine that the link is external, and this would be true if you had a list of all types that were meant to refer to external resources. However, it is possible that a piece of code that deals with OPC packages (such as a validation algorithm) may not have this information, and without knowing the target mode, might look for a file named http://www.openoffice.org, not find it, and report an error. Thus the TargetMode allows for arbitrary types (including any added in the future, or proprietary extensions) to be used for relationships, while still allowing a program examining the package to check that all internal references are valid.