Capturing and sharing design knowledge such as architectural decisions is becoming increasingly important in professional Information Technology (IT) services firms. Methods, models, and tools supporting explicit knowledge management strategies have been proposed in recent years. In this paper, we extend previous work in the architectural knowledge management community to satisfy the requirements of an additional user group: the designers of IT infrastructure solutions that are outsourced from one company to another. Such strategic outsourcing solutions require complex, contractually relevant design decisions concerning many different resources such as IT infrastructures, people, and real estate. In this paper, we present a reference architecture and a decision process-oriented knowledge metamodel that we synthesized from the domain-specific functional requirements and quality attributes. We also present a tool implementation of these decision modeling concepts and discuss user feedback.