Infrastructure as a Service (IaaS) is one of the most important layers of Cloud Computing. However, there is an evident deficiency of mechanisms for analysis, comparison and evaluation of IaaS cloud implementations, since no unified reference architecture is available. In this article, we propose a reference IaaS architectural framework. The reference framework is structured around seven layers: core service layer, support layer, value-added services, control layer, management layer, security layer and resource abstraction. We provide detailed description of each layer and define dependencies between the layers and components. Finally, we validate the proposed reference IaaS architectural framework on several real-world projects, while performing a comprehensive analysis of the most important commercial and open-source IaaS products. The evaluation results show notable distinction of feature support and capabilities between commercial and open-source IaaS platforms, significant deficiency of important architectural components in terms of fulfilling true promise of infrastructure clouds, and real-world usability of the proposed architectural framework.