Person:

Fujimoto, Richard M.

Permanent Link

https://hdl.handle.net/1853/71271

Associated Organization(s)

Organizational Unit

School of Computational Science and Engineering

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Publication Search Results

Now showing 1 - 10 of 13

Agent-based simulations using human performance models for national airspace system risk assessment

(Georgia Institute of Technology, 2009-07-14) Goldsman, David ; Alexopoulos, Christos ; Fujimoto, Richard M. ; Loper, Margaret L. ; Pritchett, Amy R.
Space–Parallel Network Simulations Using Ghosts

(Georgia Institute of Technology, 2004-05) Riley, George F. ; Jaafar, Talal Mohamed ; Fujimoto, Richard M. ; Ammar, Mostafa H.

We discuss an approach for creating a federated network simulation that eases the burdens on the simulator user that typically arise from more traditional methods for defining space-parallel simulations. Previous approaches have difficulties that arise from the need for global topology knowledge when forwarding simulated packets between the federates. In all but the simplest cases, proper packet forwarding decisions between federates requires routing tables of size O(mn) (m is the number of nodes modeled in a particular simulator instance, and n is the total number of network nodes in the entire topology) in order to determine how packets should be routed between federates. Further, the benefits of the well-known NIx-Vector routing approach cannot be fully achieved without global knowledge of the overall topology. We seek to overcome these difficulties by utilizing a topology partitioning methodology that uses Ghost Nodes. A ghost node is a simulator object in a federate that represents a simulated network node that is spatially assigned to some other federate, and thus that other federate is responsible for maintaining all state associated with the node. However, ghost nodes do retain topology connectivity information with other nodes, allowing all federate in a space-parallel simulation to obtain a global picture of the network topology. We show with experimental results that the memory overhead associated with the ghosts is minimal relative to the overall memory footprint of the simulation.
Enabling Large-Scale Multicast Simulation by Reducing Memory Requirements

(Georgia Institute of Technology, 2003-06) Xu, Donghua ; Riley, George F. ; Ammar, Mostafa H. ; Fujimoto, Richard M.

The simulation of large–scale multicast networks often requires a significant amount of memory that can easily exceed the capacity of current computers, both because of the inherently large amount of state necessary to simulate message routing and because of design oversights in the multicast portion of existing simulators. In this paper we describe three approaches to substantially reduce the memory required by multicast simulations: 1) We introduce a novel technique called “negative forwarding table” to compress mutlicast routing state. 2) We aggregate the routing state objects from one replicator per router per group per source to one replicator per router. 3) We employ the NIx– Vector technique to replace the original unicast IP routing table. We implemented these techniques in the ns2 simulator to demonstrate their effectiveness. Our experiments show that these techniques enable packet level multicast simulations on a scale that was previously unachievable on modern workstations using ns2.
Scalable RTI-Based Parallel Simulation of Networks

(Georgia Institute of Technology, 2003-06) Perumalla, Kalyan S. ; Park, Alfred ; Fujimoto, Richard M. ; Riley, George F.

Federated simulation interfaces such as the High Level Architecture (HLA) were designed for interoperability, and as such are not traditionally associated with high performance computing. In this paper, we present results of a case study examining the use of federated simulations using runtime infrastructure (RTI) software to realize large-scale parallel network simulators. We examine the performance of two different federated network simulators, and describe RTI performance optimizations that were used to achieve efficient execution. We show that RTI-based parallel simulations can scale extremely well and achieve very high speedup. Our experiments yielded more than 80-fold scaled speedup in simulating large TCP/IP networks, demonstrating performance of up to 6 million simulated packet transmissions per second on a Linux cluster. Networks containing up to two million network nodes (routers and end systems) were simulated.
Exploiting the Predictability of TCP’s Steady-state Behavior to Speed Up Network Simulation

(Georgia Institute of Technology, 2002-10) He, Qi ; Ammar, Mostafa H. ; Riley, George F. ; Fujimoto, Richard M.

In discrete-event network simulation, a significant portion of resources and computation are dedicated to the creation and processing of packet transmission events. For large-scale network simulations with a large number of high-speed data flows, the processing of packet events is the most time consuming aspect of the simulation. In this work we develop a technique that saves on the processing of packet events for TCP flows using the well established results showing that the average behavior of a TCP flow is predictable given a steady-state path condition. We exploit this to predict the average behavior of a TCP flow over a future period of time where steady-state conditions hold, thus allowing for a reduction (or elimination) of the processing required for packet events during this period. We consider two approaches to predicting TCP’s steady-state behavior: using throughput formulas or by direct monitoring of a flow’s throughput in a simulation. We design a simulation framework that provides the flexibility to incorporate this method of simulating TCP packet flows. Our goal is 1) to accommodate different network configurations, on/off flow behaviors and interaction between predicted flows and packet-based flows; and 2) to preserve the statistical behavior of every entity in the system, from hosts to routers to links, so as to maintain the accuracy of the network simulation as a whole. In order to illustrate the promise of this idea we implement it in the context of the ns2 simulation system. A set of experiments illustrate the speedup and approximation of the simulation framework under different scenarios and for different network performance metrics.
Integrated Fluid and Packet Network Simulations

(Georgia Institute of Technology, 2002-10) Riley, George F. ; Jaafar, Talal Mohamed ; Fujimoto, Richard M.

A number of methods exist that can be used to create simulation models for measuring the performance of computer networks. The most commonly used method is packet level simulation, which models the detailed behavior of every packet in the network, and results in a highly accurate picture of overall network behavior. A less frequently used, but sometimes more computationally efficient, method is the fluid model approach. In this method, aggregations of flows are modeled as fluid flowing through pipes, and queues are modeled as fixed capacity buckets. The buckets are connected via pipes, where the maximum allowable flow rate of fluid in the pipes represents the bandwidth of the communication links being modeled. Fluid models generally result in a less accurate picture of the network’s behavior since they rely on aggregation of flows and ignore actions specific to individual flows. We introduce a new hybrid simulation environment that leverages the strong points of each of these two modeling methods. Our hybrid method uses fluid models to represent aggregations of flows for which less detail is required, and packet models to represent individual flows for which more detail is needed. The result is a computationally efficient simulation model that results in a high level of accuracy and detail in some of the flows, while abstracting away details of other flows. We show a computational speedup of more than twenty in some cases, with little reduction in accuracy of the simulation results.
Experiences Applying Parallel and Interoperable Network Simulation Techniques in On-line Simulations of Military Networks

(Georgia Institute of Technology, 2002) Perumalla, Kalyan S. ; Fujimoto, Richard M. ; McLean, Thom ; Riley, George F.

We present a case study in which we apply parallel simulation methods and interoperability techniques to network simulations for simulation-based on-line control of military communication networks. The on-line simulations model actual military networks, including wired shipboard sub-networks connected via satellite links, and wireless mobile devices. The modeled scenario depicts the communication requirements of an amphibious landing where a complex network connects troops ashore and naval vessels. The simulations use a heterogeneous set of tools, including ns2 models for shipboard wired networks, and GloMoSim models for the wireless devices. In this paper, we document the challenges we encountered in applying parallel and interoperable simulation methods, and describe our solutions. We describe our experiences in addressing the interoperability problems that naturally arose due to the heterogeneity of scenario models. We also present a preliminary study on the scalability of real-time performance of parallel network simulations, which is crucial for on-line simulations. Salient system characteristics of the subject military network scenarios are described for the benefit of exposure to the modeling and simulation research community. Our exercise not only highlights the relevance of parallel and distributed simulation techniques to an important real-life problem, but also demonstrates the feasibility of applying those techniques in a practical setting.
Experiences Parallelizing a Commercial Network Simulator

(Georgia Institute of Technology, 2001-12) Wu, Hao ; Fujimoto, Richard M. ; Riley, George F.

Most current approaches of parallel simulation focus on building new parallel simulation engines that require the development of new models and software. An alternate, emerging approach is to extend sequential simulators to execute on parallel computers. We describe a methodology for realizing parallel simulations in this manner. This work is specifically concerned with parallelization of commercial simulators where source code for some or all of the sequential simulator is not available. We describe our experiences in applying this methodology to realize a parallel version of the OPNET simulator for modeling computer networks. We show significant speedup can be readily obtained for some OPNET models if proper partitioning strategies are applied and the simulation attributes are tuned appropriately. However, we observe that substantial modifications to other OPNET models are needed to achieve efficient parallel execution because of their extensive use of global variables and “zero lookahead events”.
Distributed Network Simulations Using the Dynamic Simulation Backplane

(Georgia Institute of Technology, 2001-04) Riley, George F. ; Ammar, Mostafa H. ; Fujimoto, Richard M. ; Xu, Donghua ; Perumalla, Kalyan S.

Presents an approach for creating distributed, component-based simulations of communication networks by interconnecting models of sub-networks drawn from different network simulation packages. This approach supports the rapid construction of simulations for large networks by reusing existing models and software, and fast execution using parallel discrete event simulation techniques. A dynamic simulation backplane is proposed that provides a common format and protocol for message exchange, and services for transmitting data and synchronizing heterogeneous network simulation engines. In order to achieve plug-and-play interoperability, the backplane uses existing network communication standards and dynamically negotiates among the participant simulators to define a minimal subset of required information that each simulator must supply, as well as other optional information. The backplane then automatically creates a message format that can be understood by all participating simulators and dynamically creates the content of each message by using callbacks to the simulation engines. We describe our approach to interoperability as well as an implementation of the backplane. We present results that demonstrate the proper operation of the backplane by distributing a network simulation between two different simulation packages, ns2 and GloMoSim. Performance results show that the overhead for the creation of the dynamic messages is minimal. Although this work is specific to network simulations, we believe our methodology and approach can be used to achieve interoperability in other distributed computing applications as well.
Split Protocol Stack Network Simulations Using the Dynamic Simulation Backplane

(Georgia Institute of Technology, 2001) Xu, Donghua ; Riley, George F. ; Ammar, Mostafa H. ; Fujimoto, Richard M.

We introduce and discuss a methodology for heterogeneous simulations of computer networks using the dynamic simulation backplane. This methodology allows for exchanging of protocol information between simulators across layers of the protocol stack. For example, the simulationist may wish to construct a simulation using the rich set of TCP models found in the ns network simulator, and at the same time using the highly detailed wireless MAC models found in the GloMoSim simulator. The backplane provides an interface between heterogeneous simulators which allows these simulators to exchange meaningful information across layers of the protocol stack, without detailed knowledge of internal representation in the foreign simulator. With this method of heterogeneous simulation, new and experimental protocols can be validated and tested in conjunction with existing and accepted simulations of lower protocol layers. We discuss the particular problems presented by the split protocol stack model, and present our solutions. We give results of our implementation of the split protocol backplane, using the ns simulator for the higher protocol stack layers, and the GloMoSim simulator for the lower layers.