Search

In this paper,1 we investigate the throughput and delay in heterogeneous cognitive radio networks (HCRN), where the data source and the destination (S-D) is heterogeneously distributed following a rank based model and secondary users (SUs) provide relay service for primary users (PUs). We consider two scenarios: 1) PUs and SUs are both static; and 2) PUs are static and SUs are mobile. For scenario 1, we show that the primary network throughput is the same for different heterogeneous extents of S-D distribution owing to the flexible assistance of SUs, while the throughput of secondary networks is proven to be changing with the S-D heterogeneity exponent $\alpha$, which depicts the variation of different heterogeneous extents of S-D distribution. In addition, the delay of both primary and secondary networks are shown to be altering with $\alpha$. Further, we reveal that the number of SUs required to assist PUs can be dramatically reduced when considering the S-D heterogeneity, while achieving the same primary network throughput. For scenario 2, we utilize a modified uniform mobility (MUM) model to depict the motion of SUs and mainly focus on the analysis of throughput and delay for primary networks. It shows that the primary network throughput is also free of the heterogeneous extent of S-D distribution, while the delay changes with $\alpha$. Due to the mobility of SUs, a better delay-throughput tradeoff of primary networks is achieved compared with that in scenario 1. [ABSTRACT FROM AUTHOR]

Topic-based publish/subscribe is at the core of many distributed systems, ranging from application integration middleware to news dissemination. Therefore, much research was dedicated to publish/subscribe architectures and protocols, and in particular to the design of overlay networks for decentralized topic-based routing and efficient message dissemination. Nonetheless, existing systems fail to take full advantage of shared interests when disseminating information, hence suffering from high maintenance and traffic costs, or construct overlays that cope poorly with the scale and dynamism of large networks. In this paper, we present StaN, a decentralized protocol that optimizes the properties of gossip-based overlay networks for topic-based publish/subscribe by sharing a large number of physical connections without disrupting its logical properties. StaN relies only on local knowledge and operates by leveraging common interests among participants to improve global resource usage and promote topic and event scalability. The experimental evaluation under two real workloads, both via a real deployment and through simulation, shows that StaN provides an attractive infrastructure for scalable topic-based publish/subscribe. [ABSTRACT FROM PUBLISHER]

In this paper, we address attacks that exploit the timing behavior of TCP and other protocols and applications in low-latency anonymity networks. Mixes have been used in many anonymous communication systems and are supposed to provide countermeasures to defeat traffic analysis attacks. In this paper, we focus on a particular class of traffic analysis attacks, flowcorrelation attacks, by which an adversary attempts to analyze the network traffic and correlate the traffic of a flow over an input link with that over an output link. Two classes of correlation methods are considered, namely time-domain methods and frequency-domain methods. Based on our threat model and known strategies in existing mix networks, we perform extensive experiments to analyze the performance of mixes. We find that all but a few batching strategies fail against flow-correlation attacks, allowing the adversary to either identify ingress and egress points of a flow or to reconstruct the path used by the flow. Counter intuitively, some batching strategies are actually detrimental against attacks. The empirical results provided in this paper give an indication to designers of Mix networks about appropriate configurations and mechanisms to be used to counter flow-correlation attacks. [ABSTRACT FROM AUTHOR]

Various sensor types, e.g., temperature, humidity, and acoustic, sense physical phenomena in different ways, and thus, are expected to have different sensing models. Even for the same sensor type, the sensing model may need to be changed in different environments. Designing and testing a different coverage protocol for each sensing model is indeed a costly task. To address this challenging task, we propose a new probabilistic coverage protocol (denoted by PCP) that could employ different sensing models. We show that PCP works with the common disk sensing model as well as probabilistic sensing models, with minimal changes. We analyze the complexity of PCP and prove its correctness. In addition, we conduct an extensive simulation study of large-scale sensor networks to rigorously evaluate PCP and compare it against other deterministic and probabilistic protocols in the literature. Our simulation demonstrates that PCP is robust, and it can function correctly in presence of random node failures, inaccuracies in node locations, and imperfect time synchronization of nodes. Our comparisons with other protocols indicate that PCP outperforms them in several aspects, including number of activated sensors, total energy consumed, and network lifetime. [ABSTRACT FROM AUTHOR]

Gossip-based communication protocols are attractive in cases where absolute delivery guarantees are not required due to their scalability, low overhead, and probabilistically high reliability. In earlier work, a gossip-based protocol known as gravitational gossip was created that allows the selection of quality ratings within subgroups based on workload and information update frequency. This paper presents an improved protocol that adds an adaptive component that matches the actual subgroup communication rates with desired rates coping with network variations by modifying underlying gossip weights. The protocol is designed for use in environments where many information streams are being generated and interest levels vary between nodes in the system. The gossip-based protocol is able to allow subscribers to reduce their expected workload in return for a reduced information rate. The protocol is a good fit for applications such as military information systems, sensor networks, and rescue operations. Experiments were conducted in order to compare the merits of different adaptation mechanisms. Experimental results show promise for this approach. [ABSTRACT FROM AUTHOR]

By combining an unstructured protocol with a DHT-based global index, hybrid peer-to-peer (P2P) improves search efficiency in terms of query recall and response time. The major challenge in hybrid search is how to estimate the number of peers that can answer a given query. Existing approaches assume that such a number can be directly obtained by computing item popularity. In this work, we show that such an assumption is not always valid, and previous designs cannot distinguish whether items related to a query are distributed in many peers or are in a few peers. To address this issue, we propose QRank, a difficulty-aware hybrid search, which ranks queries by weighting keywords based on term frequency. Using rank values, QRank selects proper search strategies for queries. We conduct comprehensive trace-driven simulations to evaluate this design. Results show that QRank significantly improves the search quality as well as reducing system traffic cost compared with existing approaches. [ABSTRACT FROM AUTHOR]

When employing a consensus algorithm for state machine replication, should one optimize for the case that all communication links are usually timely or for fewer timely links? Does optimizing a protocol for better message complexity hamper the time complexity? In this paper, we investigate these types of questions using mathematical analysis as well as experiments over PlanetLab (WAN) and a LAN. We present a new and efficient leader-based consensus protocol that has 0(n) stable-state message complexity (in a system with n processes) and requires only 0(n) links to be timely at stable times. We compare this protocol with several previously suggested protocols. Our results show that a protocol that requires fewer timely links can achieve better performance, even if it sends fewer messages. [ABSTRACT FROM AUTHOR]

We present Quiver, a system that coordinates service proxies placed at the "edge" of the Internet to serve distributed clients accessing a service involving mutable objects. Quiver enables these proxies to perform consistent accesses to shared objects by migrating the objects to proxies performing operations on those objects. These migrations dramatically improve performance when operations involving an object exhibit geographic locality, since migrating this object into the vicinity of proxies hosting these operations will benefit all such operations. Other workloads benefit from Quiver, dispersing the computation load across the proxies and saving the costs of sending operation parameters over the wide area when these are large. Quiver also supports optimizations for single-object reads that do not involve migrating the object. We detail the protocols for implementing object operations and for accommodating the addition, involuntary disconnection, and voluntary departure of proxies. We also evaluate Quiver through experiments on PlanetLab. Finally, we discuss the use of Quiver to build an e-commerce application and a distributed network traffic modeling service. [ABSTRACT FROM AUTHOR]

To construct an efficient overlay network, the information of underlay is important. We consider using end-to-end measurement tools such as traceroute to infer the underlay topology among a group of hosts. Previously, Max-Delta has been proposed to infer a highly accurate topology with a low number of traceroutes. However, Max-Delta relies on a central server to collect traceroute results and to select paths for hosts to traceroute. It is not scalable to large groups. In this paper, we investigate a distributed inference scheme to support scalable inference. In our scheme, each host joins an overlay tree before conducting traceroute. A host then independently selects paths for tracerouting and exchanges traceroute results with others through the overlay tree. As a result, each host can maintain a partially discovered topology. We have studied the key issue in the scheme, that is, how a low-diameter overlay tree can be constructed. Furthermore, we propose several techniques to reduce the measurement cost for topology inference. They include 1) integrating the Doubletree algorithm into our scheme to reduce measurement redundancy, 2) setting up a lookup table for routers to reduce traceroute size, and 3) conducting topology abstraction and reducing the computational frequency to reduce the computational overhead. As compared to the naive Max-Delta, our scheme is fully distributed and scalable. The computational loads for target selection are distributed to all the hosts instead of a single server. In addition, each host only communicates with a few other hosts. The consumption of edge bandwidth at a host is hence limited. We have done simulations on Internet-like topologies and conducted measurements on PlanetLab. The results show that the constructed tree has a low diameter and can support quick data exchange between hosts. Furthermore, the proposed improvements can efficiently reduce measurement redundancy, bandwidth consumption, and computational overhead. [ABSTRACT FROM AUTHOR]

In this comments paper, we revisit the network model introduced in. We discuss the inaccuracy of the model and, to correct the network model, we propose to apply directed capacity constraints for directed flows. Based on a comparison of numerical results, we show that the corrected model leads to better accuracy than the original model. [ABSTRACT FROM AUTHOR]

The paper presents a detailed discussion of various issues involved in designing a medium access control (MAC) protocol for multihop wireless networks with nodes employing multiple beam smart antennas. Multiple beam smart antennas can form several beams simultaneously and can initiate concurrent transmissions or receptions in them, thereby increasing the throughput of the bottleneck nodes. Traditional on-demand MAC protocols for omnidirectional and single beam directional antennas based on the IEEE 802.11 distributed coordination function (DCF) mechanism cannot take advantage of this unique capability of multiple beam antennas as they do not facilitate concurrent transmissions or receptions by a node. This paper introduces a novel protocol, hybrid MAC .(HMAC), which enables concurrent packet reception (CPR) and concurrent packet transmission (CPT) at a node equipped with multiple beam antennas and is backward compatible with IEEE 802.11 DCF. Simulation results show the superior performance of HMAC in most ad hoc scenarios. Moreover, in some sample topologies, the throughput of HMAC is close to the theoretical maximum. The paper also presents a wireless mesh network architecture with heterogeneous antenna technologies and illustrates the advantages of employing multiple beam smart antennas and HMAC in such networks. [ABSTRACT FROM AUTHOR]

The XNET XML content network was designed to implement efficient and reliable distribution of structured XML content to very large populations of consumers. For that purpose, our system integrates several technologies: the routing protocol XROUTE makes extensive use of subscription aggregation to limit the size of routing tables while ensuring perfect routing (that is, an event is forwarded to a link only if it leads to an interested consumer). The filtering engine XTRIE uses a sophisticated algorithm to match incoming XML documents against large populations of tree-structured subscriptions, whereas the XSEARCH subscription management algorithm enables the system to efficiently manage large and highly dynamic consumer populations. Finally, our XNET system integrates reliability mechanisms to guarantee that its state is consistent with the consumer population and implements several approaches to fault tolerance to recover from various types of router and link failures. We have analyzed the efficiency of our techniques with various simulations, and to assess the performance of our system in realistic settings and show that it is perfectly suitable for large-scale distributed environments, we have performed a large-scale experimental deployment on the PlanetLab testbed. [ABSTRACT FROM AUTHOR]

This paper proposes a new methodology to model the distribution of finite size content to a group of users connected through an overlay network. Our methodology describes the distribution process as a constrained stochastic graph process (CSGP), where the constraints dictated by the content distribution protocol and the characteristics of the overlay network define the interaction among nodes. A CSGP is a semi-Markov process whose state is described by the graph itself. CSGPs offer a powerful description technique that can be exploited by Monte Carlo integration methods to compute in a very efficient way not only the mean but also the full distribution of metrics such as the file download times or number of hops from the source to the receiving nodes. We model several distribution architectures based on trees and meshes as CSGPs and solve them numerically. We are able to study scenarios with a very large number of nodes and we can precisely quantify the performance differences between the tree and mesh-based distribution architectures. [ABSTRACT FROM AUTHOR]

Overlay networks are virtual networks residing over the IP network, consequently, overlay links may share hidden lower-level bottlenecks. Previous work have assumed an independent overlay model: a graph with independent link capacities. We introduce a model of overlays which incorporates correlated link capacities and linear capacity constraints (LCC) to formulate hidden shared bottlenecks; we refer to these as LCC-overlays. We define metrics to qualitatively measure overlay quality in terms of its accuracy (in representing the true network topology) and efficiency (i.e., performance). Through analysis and simulations,we show that LCC-overlay is perfectly accurate and hence enjoys much higher efficiency than the inaccurate independent overlay. We discover that even a highly restricted LCC class—node-based LCC—yields near-optimal accuracy and significantly higher efficiency. We study two network flow problems in the context of LCC-graphs: Widest-Path and Maximum-Flow. We prove that Widest-Path with LCC is NP-complete. We formulate Maximum-Flow with LCC as a linear program, and propose an efficient distributed algorithm to solve it. Based on the LCC model, we further study the problem of optimizing delay while still maintaining optimal or near-optimal bandwidth. We also outline a distributed algorithm to efficiently construct an overlay with node-based LCC. [ABSTRACT FROM AUTHOR]

With the advent of the new generation of Internet-based technology, in particular Web Services, the automation of business activities that are distributed across multiple enterprises becomes possible. Business activities are different from traditional transactions in that they are typically asynchronous, loosely-coupled and long-running. Therefore, extended transaction protocols are needed to coordinate business activities that span multiple enterprises. Existing extended transaction protocols typically rely on compensating transactions to handle exceptional conditions. In this paper we identify a number of issues with compensation-based extended transaction protocols, and describe a reservation-based extended transaction protocol that addresses those issues. Moreover, we define a set of properties, analogous to the ACID properties of traditional transactions, that are more appropriate for business activities that span multiple enterprises. In addition, we compare our reservation protocol with other extended transaction protocols for coordinating business activities, and present performance analyses and results. [ABSTRACT FROM AUTHOR]

In this paper, we present a new multicast architecture and the corresponding multicast routing protocol for providing efficient and flexible multicast services over the Internet. Traditional multicast protocols construct and update the multicast tree in a distributed manner, which may cause two problems: first, since each node has only local or partial information on the network topology and group membership, it is difficult to build an efficient multicast tree and, second, due to the lack of complete information, broadcast is often used for sending control packets and data packets, which consumes a great deal of network bandwidth. In the newly proposed multicast architecture, a few powerful routers, called m-routers, collect multicast-related information and process multicast requests based on the information collected. The m-routers handle most of the multicast-related tasks, whereas other routers in the network only need to perform minimum functions for routing. The m-routers are designed to be able to handle simultaneous many-to-many communications efficiently. The new multicast routing protocol, called the Service-Centric Multicast Protocol (SCMP), builds a shared multicast tree rooted at the m-router for each group. The multicast tree is computed in the m-router by employing the Delay-Constrained Dynamic Multicast (DCDM) algorithm, which dynamically builds a delay-constrained multicast tree and minimizes the tree cost as well. The physical construction of the multicast tree over the Internet is performed by a special type of self-routing packets in order to minimize the protocol overhead. Our simulation results on ns-2 demonstrate that the new SCMP protocol outperforms other existing protocols and is a promising alternative for providing efficient and flexible multicast services over the Internet. [ABSTRACT FROM AUTHOR]

The bandwidth-constrained routing problem (BCRP) asks for a route that has sufficient bandwidth for data transmission. When BCRP is defined for wired networks, it can be solved in polynomial time. On the other hand, when it is defined for wireless ad hoc networks, it is NP-complete if the underlying MAC protocol is TDMA-based or CDMA-over-TDMA-based. In this paper, we show that BCRP is still NP-complete, even if CSMA-based or contention-based CDMA MAC protocols are used. Besides, we show that BCRP is polynomial-time solvable if the channel model is collision-free and the scheduling policy is FIFO. In wireless ad hoc networks, no MAC protocol was designed before, which would lead to a polynomial-time solution to BCIRP. The results of this paper suggest a design principle for MAC protocols that can support QoS routing well. [ABSTRACT FROM AUTHOR]

Distributed peer-to-peer systems rely on voluntary participation of peers to effectively manage a storage pool. In such systems, data is generally replicated for performance and availability. If the storage associated with replication is not monitored and provisioned, the underlying benefits may not be realized. Resource constraints, performance scalability, and availability present diverse considerations. Availability and performance scalability, in terms of response time, are improved by aggressive replication, whereas resource constraints limit total storage in the network. Identification and elimination of redundant data pose fundamental problems for such systems. In this paper, we present a novel and efficient solution that addresses availability and scalability with respect to management of redundant data. Specifically, we address the problem of duplicate elimination in the context of systems connected over an unstructured peer-to-peer network in which there is no a priori binding between an object and its location. We propose two randomized protocols to solve this problem in a scalable and decentralized fashion that does not compromise the availability requirements of the application. Performance results using both large-scale simulations and a prototype built on PlanetLab demonstrate that our protocols provide high probabilistic guarantees while incurring minimal administrative overheads. [ABSTRACT FROM AUTHOR]

Transmission Control Protocol (TCP) is the most popular transport layer protocol for the Internet. Due to various reasons, such as multipath routing, route fluttering, and retransmissions, packets belonging to the same flow may arrive out of order at a destination. Such packet reordering violates the design principles of some traffic control mechanisms in TCP and, thus, poses performance problems. In this paper, we provide a comprehensive and in-depth survey on recent research on packet reordering in TCP. The causes and problems for packet reordering are discussed. Various representative algorithms are examined and compared by computer simulations. The ported program codes and simulation scripts are available for download. Some open questions are discussed to stimulate further research in this area. [ABSTRACT FROM AUTHOR]

In this paper, we present uCast, a novel multicast protocol for energy efficient content distribution in sensor networks. We design uCast to support a large number of multicast sessions, especially when the number of destinations in a session is small. In uCast, we do not keep any state information relevant to ongoing multicast deliveries at intermediate nodes. Rather, we directly encode the multicast information in the packet headers and parse these headers at intermediate nodes using a scoreboard algorithm proposed in this paper. We demonstrate that 1) uCast is powerful enough to support multiple addressing and unicast routing schemes and 2) uCast is robust, efficient, and scalable in the face of changes in network topology, such as those introduced by energy conservation protocols. We systematically evaluate the performance of uCast through simulations, compare it with other state-of-the-art protocols, and collect preliminary data from a running system based on the Berkeley motes platform. [ABSTRACT FROM AUTHOR]

The initialization problem, also known as naming, consists to give a unique identifier ranging from 1 to ii to a set of n indistinguishable nodes in a given network. We consider a network where n nodes (processors) are randomly deployed in a square (respectively, cube) X. We assume that the time is slotted and the network is synchronous, two nodes are able to communicate if they are within distance at most of r of each other (r is the transmitting/receiving range). Moreover, if two or more neighbors of a processor ~ transmit concurrently at the same time slot, then i would not receive either messages. We suppose also that the anonymous nodes know neither the topology of the network nor the number of nodes in the network. Under this extremal scenario, we first show how the transmitting range of the deployed processors affects the typical characteristics of the network. Then, by allowing the nodes to transmit at various ranges we design sublinear randomized initialization protocols: In the two, respectively, three, dimensional case, our randomized initialization algorithms run in O(n1/2log n1/2), respectively, O(n1/3 log n2/3, time slots. These latter protocols are based upon an optimal gossiping algorithm which is of independent interest. [ABSTRACT FROM AUTHOR]

Leader-based protocols rest on a primitive able to provide the processes with the same unique leader. Such protocols are very common in distributed computing to solve synchronization or coordination problems. Unfortunately, providing such a primitive is far from being trivial in asynchronous distributed systems prone to process crashes. (It is even impossible in fault-prone purely asynchronous systems.) To circumvent this difficulty, several protocols have been proposed that build a leader facility on top of an asynchronous distributed system enriched with additional assumptions. The protocols proposed so far consider either additional assumptions based on synchrony or additional assumptions on the pattern of the messages that are exchanged. Considering systems with n processes and up to ƒ process crashes, 1 ≤ ƒ < n, this paper investigates the combination of a time-free assumption on the message pattern with a synchrony assumption on process speed and message delay. It shows that both types of assumptions can be combined to obtain a hybrid eventual leader protocol benefiting from the best of both worlds. This combined assumption considers a star communication structure involving ƒ + 1 processes. Its noteworthy feature lies in the level of combination of both types of assumption that is ‘as fine as possible’ in the sense that each of the ƒ channels of the star has to satisfy a property independently of the property satisfied by each of the ƒ - 1 other channels (the ƒ channels do not have to satisfy the same assumption). More precisely, this combined assumption is the following: There is a correct process p (center of the star) and a set Q of ƒ processes q ϵ Q) such that, eventually, either 1) each time it broadcasts a query, q receives a response from p among the (n - ƒ) first responses to that query, or 2) the channel from p to q is timely. (The processes in the set Q can crash.) A surprisingly simple eventual leader protocol based on this fine grain hybrid assumption is proposed and proved correct. An improvement is also presented. [ABSTRACT FROM AUTHOR]

Epidemic-style (gossip-based) techniques have recently emerged as a class of scalable and reliable protocols for peer-to-peer multicast dissemination in large process groups. However, popular implementations of epidemic-style dissemination suffer from two major drawbacks: 1) Network overhead: When deployed on a WAN-wide or VPN-wide scale, they generate a large number of packets that transit across the boundaries of multiple network domains (e.g., LANs, subnets, ASs), causing an overload on core network elements such as bridges, routers, and associated links. 2) Lack of adaptivity: They impose the same load on process group members and the network even under reduced failure rates (viz., packet losses, process failures). In this paper, we describe two protocols to address these problems: 1 ) a hierarchical gossiping protocol and 2) an adaptive dissemination framework (for multicasts) that allows use of any gossiping primitive within it. These protocols work within a virtual peer-to-peer hierarchy called the Leaf Box Hierarchy. Processes can be allocated in a topologically aware manner to the leaf boxes of this structure, so that protocols 1 and 2 produce low traffic across domain boundaries in the network and induce minimal overhead when there are no failures. [ABSTRACT FROM AUTHOR]

In most existing localized topology control protocols for mobile ad hoc networks (MANETs), each node selects a few logical neighbors based on location information and uses a small transmission range to cover those logical neighbors. Transmission range reduction conserves energy and bandwidth consumption, while still maintaining network connectivity. However, the majority of these approaches assume a static network without mobility. In a mobile environment network connectivity can be compromised by two types of ‘bad’ location information: inconsistent information, which makes a node select too few logical neighbors, and outdated information, which makes a node use too small a transmission range. In this paper, we first show some issues in existing topology control. Then, we propose a mobility-sensitive topology control method that extends many existing mobility-insensitive protocols. Two mechanisms are introduced: consistent local views that avoid inconsistent information and delay and mobility management that tolerate outdated information. The effectiveness of the proposed approach is confirmed through an extensive simulation study. [ABSTRACT FROM AUTHOR]

This paper concerns the comparative performance evaluation of protocols for clustering and backbone formation in ad hoc networks characterized by a large number of resource-constrained nodes. Our aim is twofold: We provide the first simulation-based detailed investigation of techniques for clustering and backbone formation that are among the most representative of this area of ad hoc research. Second, we delve into the nature of the selected protocols to assess the effects of the "degree of localization" on their operations, i.e., how being able to execute the protocol based only on local information affects the overall protocol performance. Extensive ns2-based simulation results show that highly localized protocols are rewarded with good performance with respect to all metrics of interest which include protocol duration, energy consumption, message overhead, route length, and backbone size. [ABSTRACT FROM AUTHOR]

Freedom from deadlock is a key issue in Cut-Through, Wormhole, and Store and Forward networks, and such freedom is usually obtained through careful design of the routing algorithm. Most existing deadlock-free routing methods for irregular topologies do, however, impose severe limitations on the available routing paths. We present a method called Layered Routing, which gives rise to a series of routing algorithms, some of which perform considerably better than previous ones. Our method groups virtual channels into network layers and to each layer it assigns a limited set of source/destination address pairs. This separation of traffic yields a significant increase in routing efficiency. We show how the method can be used to improve the performance of irregular networks, both through load balancing and by guaranteeing shortest-path routing. The method is simple to implement, and its application does not require any features in the switches other than the existence of a modest number of virtual channels. The performance of the approach is evaluated through extensive experiments within three classes of technologies. These experiments reveal a need for virtual channels as well as an improvement in throughput for each technology class. [ABSTRACT FROM AUTHOR]

Network research generally requires a simulation or emulation environment to test protocol implementations, to evaluate the performance of a scheme or a system, and to study complicated and highly varying network operations. For large network simulation, simulators will consume a large amount of time and memory, and its result is largely based on some modeling assumptions that may not hold in the real world. Emulators are difficult to scale for large network emulation because of the high cost of equipment if a one-to-one mapping scheme is employed. Otherwise, the target network has to be abstracted to a single router modeled with some performance metrics. In this paper, we present a distributed IP network emulator cluster EMPOWER, which not only can be used to emulate a large network with a limited number of commodity computers, but also can generate user-defined arbitrary network conditions and traffic dynamics at packet level for specific test scenarios. EMPOWER is highly scalable in that each emulator node could be configured to emulate multiple network nodes, and the increment of the number of emulator nodes does not affect emulation validity. Some significant research issues such as network mapping and mobile wireless network emulation are discussed and addressed. Preliminary emulation results show that EMPOWER is capable of assisting the study of both wireline and wireless network protocols and applications. [ABSTRACT FROM AUTHOR]

Contributory group key agreement protocols generate group keys based on contributions of all group members. Particularly appropriate for relatively small collaborative peer groups, these protocols are resilient to many types of attacks. Unlike most group key distribution protocols, contributory group key agreement protocols offer strong security properties such as key independence and perfect forward secrecy. This paper presents the first robust contributory key agreement protocol resilient to any sequence of group changes. The protocol, based on the Group Diffie-Hellman contributory key agreement, uses the services of a group communication system supporting Virtual Synchrony semantics. We prove that it provides both Virtual Synchrony and the security properties of Group Diffie-Heliman, in the presence of any sequence of (potentially cascading) node failures, recoveries, network partitions, and heals. We implemented a secure group communication service, Secure Spread, based on our robust key agreement protocol and Spread group communication system. To illustrate its practicality, we compare the costs of establishing a secure group with the proposed protocol and a protocol based on centralized group key management, adapted to offer equivalent security properties. [ABSTRACT FROM AUTHOR]

This paper proposes a self-stabilizing asynchronous phase synchronization protocol for uniform unidirectional rings. Consider applications with phase bound K, i.e., the phases are phase 0, phase 1, …, phase K-1, phase 0, phase 1, etc. Under the protocol, when the ring is stabilized, it satisfies the following criterion: No node begins to execute phase (k + 1) mod K until all nodes have executed phase k, and after all nodes have executed their phase k, each node eventually executes phase (k + 1) mod K. Besides the variable used to denote the phase that a node is working on, each node maintains only one auxiliary variable with b states, where b can be any number greater than or equal to the ring size. Provided that K and b satisfy the limitation: K × b > n(b - 1), the proposed protocol is correct under the parallel model and takes at most 2(K × b) rounds to stabilize. [ABSTRACT FROM AUTHOR]

Many ubiquitous computing applications may be context-sensitive due to the ambient environments, mobile devices, and other detectable factors. A context-sensitive middleware provides the necessary support to context-sensitive application objects to participate in spontaneous and ad hoc communications with other applications in remote devices whenever suitable contexts exist. A context-sensitivity middleware can provide this support effectively if its communication subsystem, such as an Object Request Broker (ORB), can properly discover other objects In devices. This capability is usually known as object discovery protocol. An energy-efficient object discovery protocol is needed to help prolong a device's battery life because many devices in ubiquitous computing environments are battery-powered and, thus, have limited energy sources. In this paper, an energy-efficient object discovery protocol, RKS, for context-sensitive middleware for ubiquitous computing is presented. RKS reduces energy consumption by reducing the amount of information that needs to be sent to remote devices to discover objects. A novel feature of RKS is that it advertises its server-objects' availability only when it detects that these servers can be activated in the current context and when it finds that the neighbor devices have some potential clients that are willing to discover objects. Analytical comparisons of the energy-consumptions are given between RKS and two other protocols for object discovery in context-sensitive middleware. Furthermore, our experimental results, based on the implementations of these protocols and RKS on a context-sensitive middleware test bed, confirm our analytical results in that the RKS conserves more energy than the other two. [ABSTRACT FROM AUTHOR]

Rollback-Dependency Trackability (RDT) is a property stating that all rollback dependencies between local checkpoints are online trackable by using a transitive dependency vector. The most crucial RDT characterizations introduced in the literature can be represented as certain types of RDT-PXCM-paths. Here, let the U-path and V-path be any two types of RDT-PXCM-paths. In this paper, we investigate several properties of communication-induced checkpointing protocols that ensure the RDT property. First, we prove that if an online RDT protocol encounters a U-path at a point of a checkpoint and communication pattern associated with a distributed computation, it also encounters a V-path there. Moreover, if this encountered U-path is invisibly doubled, the corresponding encountered V-path is invisibly doubled as well. Therefore, we can conclude that breaking all invisibly doubled U-paths is equivalent to breaking all invisibly doubled V-paths for an online RDT protocol. Next, we continue to demonstrate that a visibly doubled U-path must contain a doubled U-cycle in the causal past. These results can further deduce that some different checkpointing protocols actually have the same behavior for all possible patterns. Finally, we present a commendatory systematic technique for comparing the performance of online RDT protocols. [ABSTRACT FROM AUTHOR]

This paper proposes to use a logical hypercube structure for detecting message stability in distributed systems. In particular, a stability detection protocol thai uses such a superimposed logical structure is presented, and its scalability is being compared with other known stability detection protocols. The main benefits of the logical hypercube approach are scalability, faulttolerance, and refraining from overloading a single node or link in the system. These benefits become evident both by an analytical comparison and by simulations. Another important feature of the logical hypercube approach is that the performance of the protocol is in general not sensitive to the topology of the underlying physical network. [ABSTRACT FROM AUTHOR]

Efficient and effective full-text retrieval in unstructured peer-to-peer networks remains a challenge in the research community. First, it is difficult, if not impossible, for unstructured P2P systems to effectively locate items with guaranteed recall. Second, existing schemes to improve search success rate often rely on replicating a large number of item replicas across the wide area network, incurring a large amount of communication and storage costs. In this paper, we propose BloomCast, an efficient and effective full-text retrieval scheme, in unstructured P2P networks. By leveraging a hybrid P2P protocol, BloomCast replicates the items uniformly at random across the P2P networks, achieving a guaranteed recall at a communication cost of O(\sqrtN), where N is the size of the network. Furthermore, by casting Bloom Filters instead of the raw documents across the network, BloomCast significantly reduces the communication and storage costs for replication. We demonstrate the power of BloomCast design through both mathematical proof and comprehensive simulations based on the query logs from a major commercial search engine and NIST TREC WT10G data collection. Results show that BloomCast achieves an average query recall of 91 percent, which outperforms the existing WP algorithm by 18 percent, while BloomCast greatly reduces the search latency for query processing by 57 percent. [ABSTRACT FROM PUBLISHER]

Token Coherence is a cache coherence protocol that simultaneously captures the best attributes of the traditional approximations to coherence: direct communication between processors (like snooping-based protocols) and no reliance on bus-like interconnects (like directory-based protocols). This is possible thanks to a class of unordered requests that usually succeed in resolving the cache misses. The problem of the unordered requests is that they can cause protocol races, which prevent some misses from being resolved. To eliminate races and ensure the completion of the unresolved misses, Token Coherence uses a starvation prevention mechanism named persistent requests. This mechanism is extremely inefficient and, besides, it endangers the scalability of Token Coherence since it requires storage structures (at each node) whose size grows proportionally to the system size. While multiprocessors continue including an increasingly number of nodes, both the performance and scalability of cache coherence protocols will continue to be key aspects. In this work, we propose an alternative starvation prevention mechanism, named priority requests, that outperforms the persistent request one. This mechanism is able to reduce the application runtime more than 20 percent (on average) in a 64-processor system. Furthermore, thanks to the flexibility shown by priority requests, it is possible to drastically minimize its storage requirements, thereby improving the whole scalability of Token Coherence. Although this is achieved at the expense of a slight performance degradation, priority requests still outperform persistent requests significantly. [ABSTRACT FROM AUTHOR]

Counting the number of RFID tags (cardinality) is a fundamental problem for large-scale RFID systems. Not only does it satisfy some real application requirements, it also acts as an important aid for RFID identification. Due to the extremely long processing time, slotted ALOHA-based or tree-based arbitration protocols are often impractical for many applications, because tags are usually attached to moving objects and they may have left the readers interrogation region before being counted. Recently, estimation schemes have been proposed to count the approximate number of tags. Most of them, however, suffer from two scalability problems: time inefficiency and multiple-reading. Without resolving these problems, large-scale RFID systems cannot easily apply the estimation scheme as well as the corresponding identification. In this paper, we present the Lottery Frame (LoF) estimation scheme, which can achieve high accuracy, low latency, and scalability. LoF estimates the tag numbers by utilizing the collision information. We show the significant advantages, e.g., high accuracy, short processing time, and low overhead, of the proposed LoF scheme through analysis and simulations. [ABSTRACT FROM AUTHOR]

Massive data dissemination is often disrupted by frequent join and departure or failure of client nodes in a peer-to-peer (P2P) network. We propose a new churn-resilient protocol (CRP) to assure alternating path and data proximity to accelerate the data dissemination process under network churn. The CRP enables the construction of proximity-aware P2P content delivery systems. We present new data dissemination algorithms using this proximity-aware overlay design. We simulated P2P networks up to 20,000 nodes to validate the claimed advantages. Specifically, we make four technical contributions: 1). The CRP scheme promotes proximity awareness, dynamic load balancing, and resilience to node failures and network anomalies. 2). The proximity-aware overlay network has a 28-50 percent speed gain in massive data dissemination, compared with the use of scope-flooding or epidemic tree schemes in unstructured P2P networks. 3). The CRP-enabled network requires only 1/3 of the control messages used in a large CAM-Chord network. 4) Even with 40 percent of node failures, the CRP network guarantees atomic broadcast of all data items. These results clearly demonstrate the scalability and robustness of CRP networks under churn conditions. The scheme appeals especially to web-scale applications in digital content delivery, network worm containment, and consumer relationship management over hundreds of datacenters in cloud computing services. [ABSTRACT FROM AUTHOR]

Intercontact time between moving vehicles is one of the key metrics in vehicular ad hoc networks (VANETs) and central to forwarding algorithms and the end-to-end delay. Due to prohibitive costs, little work has conducted experimental study on intercontact time in urban vehicular environments. In this paper, we carry out an extensive experiment involving thousands of operational taxies in Shanghai city. Studying the taxi trace data on the frequency and duration of transfer opportunities between taxies, we observe that the tail distribution of the intercontact time, that is, the time gap separating two contacts of the same pair of taxies, exhibits an exponential decay, over a large range of timescale. This observation is in sharp contrast to recent empirical data studies based on human mobility, in which the distribution of the intercontact time obeys a power law. By analyzing a simplified mobility model that captures the effect of hot areas in the city, we rigorously prove that common traffic influxes, where large volume of traffic converges, play a major role in generating the exponential tail of the intercontact time. Our results thus provide fundamental guidelines on design of new vehicular mobility models in urban scenarios, new data forwarding protocols and their performance analysis. [ABSTRACT FROM AUTHOR]

A number of energy-aware routing protocols were proposed to seek the energy efficiency of routes in multihop wireless networks. Among them, several geographical localized routing protocols were proposed to help making smarter routing decision using only local information and reduce the routing overhead. However, all proposed localized routing methods cannot guarantee the energy efficiency of their routes. In this paper, we first give a simple localized routing algorithm, called Localized Energy-Aware Restricted Neighborhood routing (LEARN), which can guarantee the energy efficiency of its route if it can find the route successfully. We then theoretically study its critical transmission radius in random networks which can guarantee that LEARN routing finds a route for any source and destination pairs asymptotically almost surely. We also extend the proposed routing into three-dimensional (3D) networks and derive its critical transmission radius in 3D random networks. Simulation results confirm our theoretical analysis of LEARN routing and demonstrate its energy efficiency in large scale random networks. [ABSTRACT FROM AUTHOR]

In an n-way broadcast application, each one of n overlay nodes wants to push its own distinct large data file to all other n - 1 destinations as well as download their respective data files. BitTorrent-like swarming protocols are ideal choices for handling such massive data volume transfers. The original BitTorrent targets one-to-many broadcasts of a single file to a very large number of receivers, and thus, by necessity, employs a suboptimized overlay topology. n-way broadcast applications, on the other hand, owing to their inherent complexity, are realizable only in small to medium scale networks. In this paper, we show that we can leverage this scale constraint to construct optimized overlay topologies that take into consideration the end-to-end characteristics of the network and as a consequence deliver far superior performance compared to random and myopic (greedy) approaches. We present the Max-Min and Max-Sum peer-selection policies used by individual nodes to select their neighbors. The first one strives to maximize the available bandwidth to the slowest destination, while the second maximizes the aggregate output rate. We design a swarming protocol suitable for n-way broadcast and operate it on top of overlay graphs formed by nodes that employ Max-Min or Max-Sum policies. Using measurements from a PlanetLab prototype implementation and trace-driven simulations, we demonstrate that the performance of swarming protocols on top of our constructed topologies is far superior to the performance of random and myopic overlays. [ABSTRACT FROM AUTHOR]