The data layer of OSI is defined in the message specifications using the ProtoBuf IDL. It defines the data that can be transmitted using OSI, including the structure and the semantics of the data.
Additionally, it specifies the encoding to be used when OSI data is transmitted. Currently, ProtoBuf encoding is used, but other encodings are possible with the ProtoBuf IDL. FlatBuffer encoding has been implemented as an experimental feature.
The data layer does not directly define components and transmission routes. These are defined in the packaging layer of OSI. There may be different packaging layer implementations using the shared data layer definitions. The data that is exchanged remains compatible regardless of the packaging layer implementation. The use of a shared data layer ensures easy bridging between different packaging layer implementations.