This article presents an open and complete RF-digital interface that's particularly appropriate for software-defined radios (SDRs). The interface advanced here is packet-based; it includes data and metadata (control and context) packets. The control and context packets describe the entire RF front end. The proposed description has a hierarchical structure and is a hardware abstraction. The developed interface supports advanced architectures such as SDR clouds. The metadata packets can be represented using formal, computer-processable semantics.