Gen3: What's Up with DIS Version 8

摘要

The Distributed Interactive Simulation (DIS) development group is working on an overhaul of Protocol Data Unit (PDU) syntax to achieve significantly better capabilities for the upcoming Version 8 standard. The concept has been dubbed Gen3 because it will be the third generation of DIS PDU design. This design breaks compatibility with previous versions of DIS going back to the 1995 standard (Version 5), a decision not to be taken lightly. However, the potential advantages are many. Gen3 has been voted to move forward by the Product Support Group (PSG) for the DIS standard. This paper describes the rationale and design principles of Gen3 PDUs. The primary gain is better extensibility for every PDU. Increasing demands in simulation fidelity require an easier way to add customization and new capability. Every PDU can have a variable record section that allows additional information to be added when needed. This idea was considered for DIS Version 7 (2012) but was narrowly outvoted in favor of maintaining compatibility. Another advantage of a PDU redesign is simplicity and consistency in PDU format. PDUs for Version 7 and earlier were designed by multiple teams, where each found different and sometimes complex ways to add variability. A simpler PDU format will make it much easier to create a formal syntax description language. A prototype Extensible Markup Language (XML) schema has already been developed. The so-called machine-readable DIS syntax enables automated PDU processing. For example, it will be possible to create automated code generation of Wireshark dissectors and PDU marshalling code, the means for exercises to agree on and distribute custom records, and automated interoperability with the Real-time Platform Reference Federation Object Model (RPR FOM) and Web Live Virtual Constructive (LVC) object model. The list of other potential improvements includes: simpler timestamp, sequence numbers to detect dropped and out-of-order PDUs, use of geodetic coordinates, eliminate the ambiguous distinction between entities and objects, eliminate Endian conversion, and eliminate partially redundant PDUs that resulted from restrictions on how the 1998 standard was created. Not every one of these changes is necessary for the success of Gen3. Each will be developed and approved/disapproved separately. This list is by no means complete. More ideas will invariably come forth as developers consider the new possibilities. Breaking compatibility will result in the need for translation between Version 8 and older simulations. Any changes in syntax must be done in a way that makes translation practical and able to be performed in real time. This has been dubbed "translation compatibility" and is one of the highest priorities in the Gen3 design principles. Part of achieving translation compatibility is to avoid changes in semantics (rules of use) from previous versions of DIS. The rules are 70% of the DIS standard and would require an excessive effort to change. Therefore, the rules that already exist in Version 7 will be changed only as necessary for the syntax reformatting.