14103668
Static priority scheduling for ATM networks
Static priority scheduling is popular for traffic scheduling in ATM switches because it is less costly than dynamic priority scheduling while being sensitive to the delay constraints of connections. We study delay computation and priority assignment problems in an ATM network with static priority scheduling. Given an ATM network with arbitrary topology, it is possible that the traffic on it may become unstable (i.e., packet delays become unbounded) due to the potential cyclic dependency of the traffic. An unstable network is definitely unacceptable for many delay sensitive applications. We start by formally deriving a simple condition under which the network is guaranteed to be stable. We then develop a numerical method to compute worst case end to end delays in an ATM network with arbitrary topology. Convergence of the method is formally proved and a closed form for the computing error is obtained. Despite its advantages, static priority scheduling remains sensitive to proper priority assignment. We describe two simple priority assignment methods, which we show to outperform other commonly used methods.
38045083
Guarantee protocols for communication in distributed hard real-time systems
A classification scheme is proposed which divides real-time messages into three different classes according to the impact on system performance when the deadline of a message is missed. A study is made of several multiaccess protocols that take into account the time constraints of messages as well as their classifications and that are able to given an early notice when an important message will miss its deadline. The performances of these protocols are evaluated and compared through simulation.<<ETX>>
35665703
Implementation of QoS-Provisioning system for voice over IP
In this paper, we address issues related to implementing Voice-over-IP (VoIP) services in packet switching networks. VoIP has been identified as a critical real-time application in the network QoS research community and has been implemented in commercial products. To provide competent quality of service for VoIP, the call admission control (CAC) mechanism has to be introduced to prevent packet losing and over-queuing. Several well-designed CAC mechanisms, such as the Site- Utilization-Based CAC and the link-utilization-based CAC mechanisms, are in place. However, the existing commercial VoIP systems have not been able to adequately apply and support these CAC mechanisms, and hence unable to provide QoS guarantees to VoIP. We have designed and implemented a QoS-Provisioning system that can be seamlessly integrated to the existing VoIP system to overcome its weakness in offering QoS guarantees. As a result, our system has been realized at Internet2 Voice Over IP Testbed in Texas A&M University.
16570884
FPGA-Based High-Order Finite Difference Algorithm for 2D Acoustic Wave Propagation Problems
Finite Difference Time Domain (FDTD) method is the most commonly used numerical algorithm for simulating linear wave propagation phenomena in geophysics, electromagnetics, and aeroor marineacoustics applications. Unfortunately, evaluating timeevolution for millions to trillions of discrete spatial points is computational intensive, especially when the geometrical size under study is much larger than the wavelength of excitations or long time integration is needed. In this paper, we proposed an FPGA-based hardware implementation to accelerate these simulation tasks. By adopting high-order temporal and spatial finite difference numerical algorithms along with efficient on-chip memory architecture, we alleviate the bandwidth bottleneck between the FPGA chip and onboard memories at the cost of increased computational burdens, which are absorbed effectively into the pipelined computing engine without speed penalty. The speed of our design implemented on Xilinx ML401 Virtex-4 evaluation platform is about 7~20 times faster than a pure software implementation of the same algorithm running on a 3.0GHz DELL workstation. Moreover, the desirable properties of simplicity and scalability make our design compatible with most commercial reconfigurable coprocessor platforms and correspondingly, the performance would be proportional to their available onboard memory bandwidth. 1. Motivation and related work Time domain numerical simulation can improve people’s understanding on dynamic behaviors of complex time-evolution problems, so plays an important role in scientific research and engineering design. In the past decades, efforts of simulating linear wave phenomena, including acoustic, electromagnetic, and elastic waves, have grown rapidly with the performance improvement of digital computers. The standard 2-order accurate FD method (including the famous Yee’s FDTD method on staggered grid in computational electromagnetic applications) is the most commonly used numerical algorithm because of its simplicity and flexibility. It is also well known that the propagation of waves over long distances requires long-time integration with minimal numerical dissipation and dispersion, and the performance of high-order FD schemes is superior to the standard one. However, their extraordinarily computational costs lead to relatively low algorithm efficiency, which limit their utilization in practice. Although pure software acceleration methods, from low-level instruction reordering to high-level process parallelism are all exhausted to speedup these applications, a typical 2D or 3D simulation task still costs hours to weeks using even the simplest 2-order FDTD method on High Performance Computing (HPC) environments. Recently, as the programmable hardware resources inside FPGA chips have increased greatly, people start showing their interests in accelerating time domain numerical simulations for wave-like equations on FPGA-based reconfigurable hardware platforms. Comparing with pure software procedures running on general-purpose computers or fully-customized VLSI hardware platforms, FPGA technology provide people a compromise between software’s best flexibility and the highest performance of hardware implementations. The idea of accelerating acoustic wave simulations using hardware platforms for geophysical applications can be traced back to 1990s [1]. The first effort of designing and implementing such a stand-alone system was described in [2]. In [3], a reconfigurable coprocessor platform using high density FPGA was proposed to speedup seismic migration problems. For computational electromagnetic applications, several authors proposed their FPGA-based solutions to accelerating Yee’s Finite-Difference TimeDomain (FDTD) algorithm from the early 1990s [4-5]. Recent work in this field can be found in [6-9]. While most previous implementations concentrated on integrating more hardware arithmetic units into FPGA so that the aggressive computational speed of their designs could exceed generic computers, we have to point out that the massive data volume is the fundamental hindrance of simulating wave propagation problems numerically in 2D or 3D space. Specifically, memory bandwidth available between the computing engine (FPGA) and onboard memory modules has been proven a bottleneck preventing people taking full advantage of FPGA’s computational potentials [3, 8, 9]. In this paper, we try to alleviate this bottleneck from two approaches: First, we adopt high-order spatial FD operators together with high-order time integration schemes to greatly reduce the number of discrete gird points inside the simulation area and the number of time-marching steps. Second, we propose a new on-chip memory architecture design for our high-order FD numerical algorithms. This new design can be mapped into most conventional FPGAbased reconfigurable coprocessor platforms efficiently and sustained high computational throughput can be achieved. The rest of this paper is organized as follow: In section 2, we introduce the high-order FD numerical algorithms we adopted in this work and show their advantages over the standard 2-order method using a simple one-dimensional acoustic wave propagation experiment. In section 3, we first propose our memory architecture design for the (2-4) FD scheme in 2D space, and then extend it to 4-order time-marching scheme to show its good scalability. Section 4 provides the performance comparisons between our hardware accelerator design implemented on Xilinx ML401 evaluation platform and its pure software counterpart on a P4 workstation for a simple 2D seismic modeling problem. Finally, conclusions and a discussion of future research direction are presented in section 5. 2. Acoustic wave equation and its High-order FD approximations 2.1 2-order accurate FD scheme Seismic modeling (forwarding) problems governed by acoustic or elastic wave equations in geophysics field are a class of numerical methods that simulate the scattering field arising when an impulsive source excites an underground region with known physical properties like density, velocity, anisotropy, elasticity, etc. We consider here the simplest 2D scalar acoustic case in the form of second-order hyperbolic Partial Differential Equation (PDE) as, ) , , ( ) , , ( ) , ( ) , , ( 2
11547845
Local synchronous capacity allocation schemes for guaranteeing message deadlines with the timed token protocol
The problem of guaranteeing synchronous message deadlines in communication networks using the timed token medium access control protocol is studied. To ensure the transmission of synchronous messages before their deadlines, synchronous capacities must be properly allocated to individual nodes. A class of local synchronous capacity allocation schemes, which allocate the synchronous capacity to a node without using information about messages on the other nodes, is developed, analyzed, and evaluated in terms of the ability of the schemes to guarantee message deadlines. It is shown that one of the local allocation schemes proposed can achieve the same performance as that of the best global allocation scheme known to date.<<ETX>>
16063472
On the confidential auditing of distributed computing systems
We propose a confidential logging and auditing service for distributed information systems. We propose a cluster-based TTP (trusted third party) architecture for the event log auditing services, so that no single TTP node can have the full knowledge of the logs, and thus no single node can misuse the log information without being detected. On the basis of a relaxed form of secure distributed computing paradigms, one can implement confidential auditing service so that the auditor can retrieve certain aggregated system information, e.g. the number of transactions, the total volume, the event traces, etc., without having to access the full log data. Similar to the peer relationship of routers to provide global network routing services, the mutually supported, mutually monitored cluster TTP architecture allows independent systems to collaborate in network-wide auditing without compromising their private information.
9870709
End-to-end latency of a fault-tolerant CORBA infrastructure
This paper presents an evaluation of the end-to-end latency of a fault-tolerant CORBA infrastructure that we have implemented. The fault-tolerant infrastructure replicates the server applications using active, passive and semi-active replication, and maintains strong replica consistency of the server replicas. By analyses and by measurements of the running fault-tolerant infrastructure, we characterize the end-to-end latency under fault-free conditions. The main determining factor of the run-time performance of the fault-tolerant infrastructure is the Totem group communication protocol, which contributes to the end-to-end latency primarily in two ways: the delay in sending messages and the processing cost of the rotating token.To reduce the delay in sending messages for passive and semi-active replication, the position of the primary server replica on the Totem ring, the token rotation time, the processing time at the client, and the processing time at the server must be considered. For active replication, the presence of duplicate messages adversely affects the performance. However, if an effective sending-side duplicate suppression mechanism is implemented, active replication is more advantageous than both passive and semi-active replication because of the automatic selection of the most favorable position of the server replica that sends the first non-duplicate reply.
446110
Controlling the mobility of multiple data transport ferries in a delay-tolerant network
As technology rapidly progresses, more devices will combine both communication and mobility capabilities. With mobility in devices, we envision a new class of proactive networks that are able to adapt themselves, via physical movement, to meet the needs of applications. To fully realize these opportunities, effective control of device mobility and the interaction between devices is needed. In this paper, we consider the message ferrying (MF) scheme which exploits controlled mobility to transport data in delay-tolerant networks, where end-to-end paths may not exist between nodes. In the MF scheme, a set of special mobile nodes called message ferries are responsible for carrying data for nodes in the network. We study the use of multiple ferries in such networks, which may be necessary to address performance and robustness concerns. We focus on the design of ferry routes. With the possibilities of interaction between ferries, the route design problem is challenging. We present algorithms to calculate routes such that the traffic demand is met and the data delivery delay is minimized. We evaluate these algorithms under a variety of network conditions via simulations. Our goal is to guide the design of MF systems and understand the tradeoff between the incurred cost of multiple ferries and the improved performance. We show that the performance scales well with the number of ferries in terms of throughput, delay and resource requirements in both ferries and nodes.
5158621
Unification of replication and transaction processing in three-tier architectures
In this paper we describe a software infrastructure that unifies replication and transaction processing in three-tier architectures and, thus, provides high availability and fault tolerance for enterprise applications. The infrastructure is based on the Fault Tolerant CORBA and CORBA Object Transaction Service standards, and works with commercial-off-the-shelf application servers and database systems. The infrastructure replicates the application servers to protect the business logic processing. In addition, it replicates the transaction coordinator which renders the two-phase commit protocol non-blocking and, thus, avoids potentially long service disruptions caused by coordinator failure. The infrastructure handles the interactions between the application servers and the database servers through replicated gateways that prevent duplicate requests from reaching the database servers. The infrastructure implements client-side automatic failover mechanisms, which guarantees that clients know the outcome of the requests that they have made. The infrastructure starts the transactions at the application servers, and retries aborted transactions, caused by process or communication failures, automatically on the behalf of the clients.
45502875
DotSlash - Creating Content Distribution Networks on Demand
Traditional content distribution networks, such as Akamai, are well-suited for static web services that routinely experience large traffic volumes. They are unsuited for active content, i.e., content generated by scripts from databases, and web sites that are unlikely to receive significant number of requests. However, a few such sites will invariably experience their "fifteen minutes of fame", typically by being mentioned on a high-volume news site such as SlashDot or CNN. Such flashcrowds or "slashdot effect" will routinely cause single-server websites to collapse. We have designed and prototyped an autonomic web replication system, called DotSlash, that drafts rescue servers fully automatically, without user intervention. The system discovers suitable rescue servers via wide-area service location, either among peer servers or from a dedicated pool of rescue servers, allocates them for temporary use and redirects requests to them. The system is completely transparent to clients and does not require URL rewriting or other client modifications. We have designed two versions. The first, an Apache extension, deals only with static content, e.g., HTML pages or media objects. The second version can also replicate and execute scripts remotely. We have prototyped the system for the common LAMP (Linux, Apache, MySQL and PHP) configuration and shown that a common benchmark for bulletin boards can be replicated without code changes, yielding capacity increases bounded only by the database server. Since many such systems, including most blogs, are bottlenecked by the web server, our system can significantly increase capacity and works even for extremely rapid load increases. We are currently investigating how such systems can be further extended by increasing the database capacity of read-mostly systems with loose consistency constraints.
17989665
Message ferrying: proactive routing in highly-partitioned wireless ad hoc networks
An ad hoc network allows devices with wireless interfaces to communicate with each other without any pre-installed infrastructure. Due to node mobility, limited radio power, node failure and wide deployment area, ad hoc networks are often vulnerable to network partitioning. A number of examples are in battlefield, disaster recovery and wide area surveillance. Unfortunately, most existing ad hoc routing protocols will fail to deliver messages under these circumstances since no route to the destination exists. In this work we propose the Message Ferrying or MF scheme that provides efficient data delivery in disconnected ad hoc networks. In the MF scheme, nodes move proactively to send or receive messages. By introducing non-randomness in a node's proactive movement and exploiting such nonrandomness to deliver messages, the MF scheme improves data delivery performance in a disconnected network. In this paper, we propose the basic design of the MF scheme and develop a general framework to classify variations of MF systems. We also study ferry route design problem in stationary node case which is shown to be NP-hard and provide an efficient algorithm to compute ferry route.