SCION Association SCION Association Anapaya Systems This document describes the data plane of the path-aware, inter- domain network architecture SCION (Scalability, Control, and Isolation On Next-generation networks). One of the basic characteristics of SCION is that it gives path control to endpoints. The SCION control plane is responsible for discovering these paths and making them available as path segments to the endpoints. The role of the SCION data plane is to combine the path segments into end-to-end paths, and forward data between endpoints according to the specified path. The SCION data plane fundamentally differs from today's IP-based data plane in that it is _path-aware_: In SCION, interdomain forwarding directives are embedded in the packet header. This document first provides a detailed specification of the SCION data packet format as well as the structure of the SCION header. SCION also supports extension headers - these are described, too. The document then shows the life cycle of a SCION packet while traversing the SCION Internet. This is followed by a specification of the SCION path authorization mechanisms and the packet processing at routers.