Person:

Ammar, Mostafa H.

Permanent Link

https://hdl.handle.net/1853/71056

Associated Organization(s)

Organizational Unit

School of Computer Science

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Publication Search Results

Now showing 1 - 9 of 9

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.
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.
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.
Network Aware Time Management and Event Distribution

(Georgia Institute of Technology, 2000-05) Riley, George F. ; Fujimoto, Richard M. ; Ammar, Mostafa H.

In this paper we discuss new synchronization algorithms for Parallel and Distributed Discrete Event Simulations (PDES) which exploit the capabilities and behavior of the underlying communications network. Previous work in this area has assumed the network to be a Black Box which provides a one-to-one, reliable and in-order message passing paradigm. In our work, we utilize the Broadcast capability of the ubiquitous Ethernet for synchronization computations, and both unreliable and reliable protocols for message passing, to achieve more efficient communications between the participating systems. We describe two new algorithms for computation of a distributed snapshot of global reduction operations on monotonically increasing values. The algorithms require O(N) messages (where N is the num- ber of systems participating in the snapshot) in the normal case. We specifically target the use of this algorithm for distributed discrete event simulations to determine a global lower bound on time-stamp (LBTS), but expect the algorithm has applicability outside the simulation community.
Stateless Routing in Network Simulations

(Georgia Institute of Technology, 2000) Riley, George F. ; Ammar, Mostafa H. ; Fujimoto, Richard M.

The memory resources required by network simulations can grow quadratically with size of the simulated network. In simulations that use routing tables at each node to perform per-hop packet forwarding, the storage required for the routing tables is O(N2), where N is the number of simulated network nodes in the topology. Additionally, the CPU time required in the simulation environment to compute and populate these routing tables can be excessive and can dominate the overall simulation tame. We propose a new routing technique, known as Neighbor-Index Vectors, or NIx-Vectors, which eliminates both the storage required for the routing tables and the CPU time required to compute them. We show experimental results using NIx- Vector routing in the popular network simulator ns. With our technique, we achieve a near order of magnitude increase in the maximum size of a simulated network running ns on a single workstation. Further, we demonstate an increase of two orders of magnitude in topology size (networks as large as 250,000 nodes) by using this technique and running the simulation in parallel on a network of workstations.
A Generic Framework for Parallelization of Network Simulations

(Georgia Institute of Technology, 1999-10) Riley, George F. ; Fujimoto, Richard M. ; Ammar, Mostafa H.

Discrete event simulation is widely used within the networking community for purposes such as demonstrating the validity of network protocols and architectures. Depending on the level of detail modeled within the simulation, the running time and memory requirements can be excessive. The goal of our research is to develop and demonstrate a practical, scalable approach to parallel and distributed simulation that will enable widespread reuse of sequential network simulation models and software. We focus on an approach to parallelization where an existing network simulator is used to build models of subnetworks that are composed to create simulations of larger networks. Changes to the original simulator care minimized, enabling the parallel simulator to easily track enhancements to the sequential version. We describe our lessons learned in applying this approach to the publicly available ns software package (McCanne and Floyd, 1997) and converting it to run in a parallel fashion on a network of workstations. This activity highlights a number of important problems, from the standpoint of how to parallelize an existing serial simulation model and achieving acceptable parallel performance
Distributed Laboratories: A Research Proposal

(Georgia Institute of Technology, 1996) Schwan, Karsten ; Ahamad, Mustaque ; Hudson, Scott E. ; Limb, J. O. (John O.) ; Ammar, Mostafa H. ; Ezquerra, Norberto F. ; Mukherjee, Amarnath ; Potts, Colin ; Ramachandran, Umakishore ; Zegura, Ellen W. ; Fujimoto, Richard M.

The continuing merger of computer and communication technologies is leading to a new computing/communications infrastructure of unprecedented magnitude, enabling new applications with broad economic and social impact. Yet, such applications pose major challenges to researchers in Computer Science and in application domains. The topic of the proposed research program is the realization of Distributed Laboratories, where individuals can interact with each other, and more importantly, with powerful, distributed computational tools as readily as if all were located in a single site. Our intent is to permit scientists, engineers, and managers at geographically distinct locations (including individuals 'tele-commuting' from home) to combine their expertise in solving shared problems, by allowing them to simultaneously view, interact with, and steer sophisticated computations executing on high performance distributed computing platforms.