The document proposes a cross-domain security reference architecture with the following key elements: - A framework to discuss the multi-faceted concerns of cross-domain security (CDS), including infrastructure, information, workflow, application, and policy aspects. - CDS participants like security domains, security monitors, mission applications, and security guards. Guards are associated with a single security domain. - Design decisions around guards being aware participants in workflows and trusting each other through mutual authentication. - Opportunities to standardize interactions through CDS protocols for application interfaces, inter-guard coordination, security monitor interfaces, and a CDS ontology.