XML-binary Optimized Packaging (XOP)

• An XML document has an information set if it is well-formed and satisfies the namespace constraints.
• XML-binary Optimized Packaging (XOP) convention, a means of more efficiently serializing XML Infosets.
• A XOP package is created by placing a serialization of the XML Infoset inside of an extensible packaging format (such a MIME Multipart)
• 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.
• necessary to have a one to one correspondence between XML Infosets and XOP Packages, hence binary data is base64-encoded

How it works?

Terminology:

• 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:Include element 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.

Reference:

W3C