Given the task definition and input, reply with output. Generate a correct and concise answer for the question based on the words in the context.

Context : Service Modeling Language (SML) and Service Modeling Language Interchange Format (SML-IF) are a pair of XML-based specifications created by leading information technology companies that define a set of XML instance document extensions for expressing links between elements, a set of XML Schema extensions for constraining those links, and a way to associate Schematron rules with global element declarations, global complex type definitions, and/or model documents.
Question : The Service Modeling Language is based upon what?
XML