Section: New Results

Application level

DEVS-based Modeling & Simulation

Participants : Olivier Dalle, Damian Vicino.

DEVS is a formalism for the specification of discrete-event simulation models, proposed by Zeigler in the 70's, that is still the subject of many research in the simulation community. Surprisingly, the problem of representing the time in this formalism has always been somehow neglected, and most DEVS simulators keep using Floating Point numbers for their arithmetics on time values, which leads to a range of systematic errors, including severe ones such as breaking the causal relations in the model.

In [15] we propose simulation algorithms, based on the Discrete Event System Specification (DEVS) formalism, that can be used to simulate and obtain every possible output and state trajecto-ries of simulations that receive input values with uncertainty quantification. Then, we present a subclass of DEVS models, called Finite Forkable DEVS (FF-DEVS), that can be simulated by the proposed algorithms. This subclass ensures that the simulation is forking only a finite number of processes for each simulation step. Finally, we discuss the simulation of a traffic light model and show the trajectories obtained when it is subject to input uncertainty.

We have also worked on improving the simulation of DEVS models in some particular situations[16] . Parallel Discrete Event System Specification (PDEVS), for example, is a well-known formalism used to model and simulate Discrete Event Systems. This formalism uses an abstract simulator that defines a set of abstract algorithms that are parallel by nature. To implement simulators using these abstract algorithms , several architectures were proposed. Most of these architectures follow distributed approaches that may not be appropriate for single core processors or microcontrollers. In order to reuse efficiently PDEVS models in this type of systems, we define a new architecture that provides a single threaded execution by passing messages in a call/return fashion to simplify the execution time analysis.

This work has also been presented and defended in the PhD Thesis of D. Vicino[5] .

Simulation of Software-Defined Networks

Participants : Olivier Dalle, Damian Vicino.

Software Defined Networks (SDN) is a new technology that has gained a lot of attention recently. It introduces programmatic ways to reorganize the network logical topology. To achieve this, the network interacts with a set of controllers, that can dynamically update the configuration of the network routing equipments based on the received events. As often with new network technologies, discrete-event simulation proves to be an invaluable tool for understanding and analzing the performance and behavior of the new systems. In [8] , we use such smulations for evaluating the impact of Software-Defined Networks' Reactive Routing on BitTorrent performance. Indeed, BitTorrent uses choking algorithms that continuously open and close connections to different peers. Software Defined Networks implementing Reactive Routing may be negatively affecting the performances of the system under specific conditions because of it lack of knowledge of BitTorrent strategies.