7097106
Hard real-time communication in multiple-access networks
With the increasing use of distributed hard real-time systems, the ability of computer networks to handle hard real-time message traffic is becoming more important. For traditional networks, maximizing the throughput or minimizing the average message delay is the most important performance criteria. In the hard real-time domain, however, concern focuses on satisfying the time constraints of individual messages. This paper examines recent developments in hard real-time communication in local area multiple-access networks. Two general strategies are used in hard real-time communication: the guarantee strategy and the best-effort strategy. In the former, messages are guaranteed to meet their deadlines during normal operation of the network. In the best-effort strategy, the network will attempt to send messages before their deadlines, but no guarantees are given. Real-time message traffic can be distinguished according to whether it is best suited for the guarantee strategy or the best-effort strategy. Although this paper concentrates on multiple-access networks, many of the concepts presented and lessons learned are also applicable to other types of networks.
14217231
Scheduling Tasks with Resource Requirements in Hard Real-Time Systems
This paper describes a heuristic approach for solving the problem of dynamically scheduling tasks in a real-time system where tasks have deadlines and general resource requirements. The crux of our approach lies in the heuristic function used to select the task to be scheduled next. The heuristic function is composed of three weighted factors. These factors explicitly consider information about real-time constraints of tasks and their utilization of resources. Simulation studies show that the weights for the various factors in the heuristic function have to be fine-tuned in order to obtain a degree of success in the range of 75-88 percent of that obtained via exhaustive search. However, modifying the approach to use limited backtracking improves the degree of success substantially to as high as 99.5 percent. This improvement is observed even when the initial set of weights are not tailored for a particular set of tasks. Simulation studies also show that in most cases the schedule determined by the heuristic algorithm is optimal or close to optimal.
3254470
Providing absolute differentiated services with statistical guarantees in static-priority scheduling networks
The authors propose and analyze a methodology for providing absolute differentiated services with statistical performance guarantees for real time applications in networks that use static priority schedulers. We extend previous work on delay analysis and develop a method that can be used to derive statistical delay guarantees in a flow-unaware fashion. While traditional delay analysis methods depend on flow population information, we have developed a method that relies on network (re-)configuration time analysis, and that allows us to analyze the delays without depending on the dynamic status of flow population. With this new method, we are able to reduce the runtime admission control to a simple utilization test. No explicit delay computation is necessary at admission time, making this approach scalable to large systems.
3436856
Distributed Privacy Preserving Information Sharing
In this paper, we address issues related to sharing information in a distributed system consisting of autonomous entities, each of which holds a private database. Semi-honest behavior has been widely adopted as the model for adversarial threats. However, it substantially underestimates the capability of adversaries in reality. In this paper, we consider a threat space containing more powerful adversaries that includes not only semi-honest but also those malicious adversaries. In particular, we classify malicious adversaries into two widely existing subclasses, called weakly malicious and strongly malicious adversaries, respectively. We define a measure of privacy leakage for information sharing systems and propose protocols that can effectively and efficiently protect privacy against different kinds of malicious adversaries.
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.
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>>
6109299
A message ferrying approach for data delivery in sparse mobile ad hoc networks
Mobile Ad Hoc Networks (MANETs) provide rapidly deployable and self-configuring network capacity required in many critical applications, e.g., battlefields, disaster relief and wide area sensing. In this paper we study the problem of efficient data delivery in sparse MANETs where network partitions can last for a significant period. Previous approaches rely on the use of either long range communication which leads to rapid draining of nodes' limited batteries, or existing node mobility which results in low data delivery rates and large delays. In this paper, we describe a Message Ferrying (MF) approach to address the problem. MF is a mobility-assisted approach which utilizes a set of special mobile nodes called message ferries (or ferries for short) to provide communication service for nodes in the deployment area. The main idea behind the MF approach is to introduce non-randomness in the movement of nodes and exploit such non-randomness to help deliver data. We study two variations of MF, depending on whether ferries or nodes initiate proactive movement. The MF design exploits mobility to improve data delivery performance and reduce energy consumption in nodes. We evaluate the performance of MF via extensive ns simulations which confirm the MF approach is efficient in both data delivery and energy consumption under a variety of network conditions.
318699
Design and implementation of a consistent time service for fault-tolerant distributed systems
Clock-related operations are one of the many sources of replica non-determinism and of replica inconsistency in fault-tolerant distributed systems. In passive replication, if the primary server crashes, the next clock value returned by the new primary server might have actually rolled back in time, which can lead to undesirable consequences for the replicated application. The same problem can happen for active replication when the result of the first replica to respond is taken as the next clock value. In this paper we describe the design and implementation of a Consistent Time Service for faulttolerant distributed systems. The Consistent Time Service introduces a group clock that is consistent across the replicas and that ensures the determinism of the replicas with respect to clock-related operations. The groupclock is monotonicallyincreasing, is transparent to the application, and is fault-tolerant. The Consistent Time Service guaranteesthe consistency of the groupclock evenwhen faults occur, when new replicas are added into the group, and when failed replicas recover.
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.
797656
Automated glue/wrapper code generation in integration of distributed and heterogeneous software components
UniFrame is a framework to help organizations to build interoperable distributed computing systems. Using UniFrame, a new system is built by assembling predeveloped heterogeneous and distributed software components. UniFrame solves the heterogeneity problem by explicitly modeling the domain knowledge of various technology domains (component model domains, programming language domains, operating system platform domains, etc.), from which the interoperation generative domain model (IGDM) straddling the technology domains can be constructed. The glue/wrapper code that realizes the interoperation among the distributed and heterogeneous software components can be generated from the IGDM. An informal implementation in Java of glue/wrapper code generator is given, followed by a discussion on a formalization of IGDM. The formalism comes from the fact that if the family of glue/wrapper code can be modeled formally, an instance glue/wrapper code can be generated automatically. In this formalization, the IGDM is formally modeled as a language definition using a grammar; the code that realizes the interoperation is a valid sentence derivable from the grammar, and will be generated automatically from the IGDM during the assembly time.
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.