Mobility as a Service (MaaS) promises to dissolve the boundaries of today's transport network through the integration of transport modes. However, stakeholders such as the operators, public transport authorities, or end-user routing application developers, are facing interoperability barriers in the way of providing a seamless travel experience. To identify, understand, and devise a plan for integration, we must first be able to clearly define the interoperability requirements of MaaS. In this paper, we propose a framework consisting of 6 layers: (i) the Stakeholder layer, (ii) the Data layer, (iii) the Infrastructure layer, (iv) the Ownership layer, (v) the Auth layer, and (vi) the Connection layer. In each layer, we outline the opportunities for Semantic Web technologies. In addition, each layer is equipped with use-cases to illustrate the application of the framework. Through the Linked MaaS framework, we formulate a baseline that extends the research focus of semantics in transportation and builds a guideline for the development of an explicitly defined and understood MaaS