Although most OpenStack architecture designs fall into one of the seven major scenarios outlined in other sections (compute focused, network focused, storage focused, general purpose, multi-site, hybrid cloud, and massively scalable), there are a few other use cases that are unique enough they can't be neatly categorized into one of the other major sections. This section discusses some of these unique use cases with some additional details and design considerations for each use case: Specialized Networking: This describes running networking-oriented software that may involve reading packets directly from the wire or participating in routing protocols. Software-defined networking (SDN): This use case details both running an SDN controller from within OpenStack as well as participating in a software-defined network. Desktop-as-a-Service: This is for organizations that want to run a virtualized desktop environment on a cloud. This can apply to private or public clouds. OpenStack on OpenStack: Some organizations are finding that it makes technical sense to build a multi-tiered cloud by running OpenStack on top of an OpenStack installation. Specialized hardware: Some highly specialized situations will require the use of specialized hardware devices from within the OpenStack environment.