Bruce Lowekamp

Topology discovery for large ethernet networks

Accurate network topology information is important for both network management and application performance prediction. Most topology discovery research has focused on wide-area networks and examined topology only at the IP router level, ignoring the need for LAN topology information. Recent work has demonstrated that bridged Ethernet topology can be determined using standard SNMP MIBs; however, these algorithms require each bridge to learn about all other bridges in the network. Our approach to Ethernet topology discovery can determine the connection between a pair of the bridges that share forwarding entries for only three hosts. This minimal knowledge requirement significantly expands the size of the network that can be discovered. We have implemented the new algorithm, and it has accurately determined the topology of several different networks using a variety of hardware and network configurations. Our implementation requires access to only one endpoint to perform the queries needed for topology discovery.

SOSIMPLE: A Serverless, Standards-based, P2P SIP Communication System

Voice over IP (VoIP) and instant messaging (IM) systems to date have either followed a client-server model or have required the use of clients that do not follow any VoIP or IM standard. We present SOSIMPLE - a fully decentralized, P2P, standards-based approach to communications. By building on the existing SIP/SIMPLE infrastructure for VoIP and IM, we support reuse of clients, network infrastructure, and open-source protocol stacks designed using current standards. By avoiding traditional centralized architectures, SOSIMPLE addresses corporate privacy concerns, eliminates dependency on constant Internet connectivity, and supports ad hoc groups. SOSIMPLE implements a DHT overlay based on Chord (Stoicha et al., 2001) using SIP messages, replicating location information for reliability. The DHT is used only for lookups, with actual communication passing directly between clients. We discuss important issues for security and authentication, as well as adaptations of conventional P2P routing for the social networks typical of personal communications. We are testing a prototype implementation of SOSIMPLE and anticipate a public release in the near future

A resource query interface for network-aware applications

Networked systems provide a cost-effective platform for parallel computing, but the applications have to deal with the changing availability of computation and communication resources. Network-awareness is a recent attempt to bridge the gap between the realities of networks and the demands of applications. Network-aware applications obtain information about their execution environment and dynamically adapt to enhance their performance. Adaptation is especially important for synchronous parallel applications because a single busy communication link can become the bottleneck and degrade overall performance dramatically. This paper presents Remos, a uniform API that allows applications to obtain relevant network information, and reports on the development of parallel applications in this environment. The challenges in defining a uniform interface include network heterogeneity, diversity and variability in network traffic, and resource sharing in the network and even inside an application. The first implementation of the Remos interface uses SNMP to monitor IP-based networks. This paper reports on our methodology for developing adaptive parallel applications for high-speed networks with Remos and presents experimental results using applications generated by the Fx parallelizing compiler. The results highlight the importance and effectiveness of adaptive parallel computing.

The architecture of the Remos system

Remos provides resource information to distributed applications. Its design goals of scalability, flexibility, and portability are achieved through an architecture that allows components to be positioned across the network, each collecting information about its local network. To collect information from different types of networks and from hosts on those networks, Remos provides several collectors that use different technologies, such as SNMP or benchmarking. By matching the appropriate collector to each particular network environment and by providing an architecture for distributing the output of these collectors across all querying environments, Remos collects appropriately detailed information at each site and distributes this information where needed in a scalable manner. Prediction services are integrated at the user-level, allowing history-based data collected across the network to be used to generate the predictions needed by a particular user. Remos has been implemented and tested in a variety of networks and is in use in a number of different environments.

ECO: Efficient Collective Operations for communication on heterogeneous networks

PVM and other distributed computing systems have enabled the use of networks of workstations for parallel computation, but their approach of treating all networks as collections of point-to-point connections does not promote efficient communication-particularly collective communication. The Efficient Collective Operations (ECO) package contains programs which solve this problem by analyzing the network and establishing efficient communication patterns. This paper describes ECO and gives performance results of using ECO to implement the collective communication in CHARMM, a widely used macromolecular dynamics package. ECO substantially improves the performance of CHARMM on a heterogeneous network. ECOs approach gives a programmer the ability to use the available networks to their full potential without acquiring any knowledge of the network structure.

A structured group mobility model for the simulation of mobile ad hoc networks

Realistic models for node movement are essential in simulating mobile ad hoc networks. Many MANET scenarios are most realistically represented using group movement, but existing group movement models depict individual group members as independent actors moving randomly. For many scenarios however, group movement implies a common goal or orientation, and hence an inherent structure to the group. We show that this structure can be defined a-priori, and that knowledge of it will result in more accurate simulations. This paper presents the Structured Group Mobility Model~(SGMM), which parameterizes group structure and generates movement sequences for use in simulations. We define the model and demonstrate how such a model of node mobility may be used in creating simulations of several MANET scenarios. We compare simulations using the SGMM with other models using various routing algorithms. Our preliminary results indicate that accurate representation of group structure has a significant effect on the overall simulation, particularly in the area of link stability. The results also imply the need for routing algorithms that take group structure into account.

Automatic node selection for high performance applications on networks

A central problem in executing performance critical parallel and distributed applications on shared networks is the selection of computation nodes and communication paths for execution. Automatic selection of nodes is complex as the best choice depends on the application structure as well as the expected availability of computation and communication resources. This paper presents a solution to this problem for realistic application and network scenarios. A new algorithm to jointly analyze computation and communication resources for different application demands is introduced and a framework for automatic node selection is developed on top of Remos, which is a query interface to network information. The paper reports results from a set of applications, including Airshed pollution modeling and magnetic resonance imaging, executing on a high speed network testbed. The results demonstrate that node selection is effective in enhancing application performance in the presence of computation load as well as network traffic. Under the network conditions used for experiments, the increase in execution time due to compute loads and network congestion was reduced by half with node selection. The node selection algorithms developed in this research are also applicable to dynamic migration of long running jobs.

Design, Implementation, and Evaluation of the Remos Network Monitoring System ∗

Remos provides resource information to distributed applications. Its design goals of scalability, flexibility, and portability are achieved through an architecture that allows components to be positioned across the network, each collecting information about its local network. To collect information from different types of networks, Remos provides several Collectors that use different technologies, including SNMP and benchmarking. By matching the Collector to the particular network environment and by providing an architecture for distributing the output of these collectors across all querying environments, Remos collects appropriately detailed information at each site and distributes this information where needed in a scalable manner. Remos has been implemented and tested in a variety of networks and is in use in a number of different environments.

Combining active and passive network measurements to build scalable monitoring systems on the grid

Because the network provides the wires that connect a grid, understanding the performance provided by a network is crucial to achieving satisfactory performance from many grid applications. Monitoring the network to predict its performance for applications is an effective solution, but the costs and scalability challenges of actively injecting measurement traffic, as well as the information access and accuracy challenges of using passively collected measurements, complicate the problem of developing a monitoring solution for a global grid. This paper is a preliminary report on the Wren project, which is focused on developing scalable solutions for network performance monitoring. By combining active and passive monitoring techniques, Wren is able to reduce the need for invasive measurements of the network without sacrificing measurement accuracy on either the WAN or LAN levels. Specifically, we present topology-based steering, which dramatically reduces the number of measurements taken for a system by using passively acquired topology and utilization to select the bottleneck links that require active bandwidth probing. Furthermore, by using passive measurements while an application is running and active measurements when none is running, we preserve our ability to offer accurate, timely predictions of network performance, while eliminating additional invasive measurements.

Dome: parallel programming in a distributed computing environment

The Distributed object migration environment (Dome) addresses three major issues of distributed parallel programming: ease of use, load balancing, and fault tolerance. Dome provides process control, data distribution, communication, and synchronization for Dome programs running in a heterogeneous distributed computing environment. The parallel programmer writes a C++ program using Dome objects which are automatically partitioned and distributed over a network of computers. Dome incorporates a load balancing facility that automatically adjusts the mapping of objects to machines at runtime, exhibiting significant performance gains over standard message passing programs executing in an imbalanced system. Dome also provides checkpointing of program state in an architecture independent manner allowing Dome programs to be checkpointed on one architecture and restarted on another.