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 use cases that do not fit into these categories. This section discusses these specialized cases and provide some additional details and design considerations for each use case: Specialized networking: describes running networking-oriented software that may involve reading packets directly from the wire or participating in routing protocols. Software-defined networking (SDN): describes both running an SDN controller from within OpenStack as well as participating in a software-defined network. Desktop-as-a-Service: describes running a virtualized desktop environment in a cloud (Desktop-as-a-Service). This applies to private and public clouds. OpenStack on OpenStack: describes building a multi-tiered cloud by running OpenStack on top of an OpenStack installation. Specialized hardware: describes the use of specialized hardware devices from within the OpenStack environment.