Sieteng Soh

CAREL: computer aided reliability evaluator for distributed computing networks

An efficient method to compute the terminal reliability (the probability of communication between a pair of nodes) of a distributed computing system (DCS) is presented. It is assumed that the graph model G(V,E) for DCS is given and that the path and/or cut information for the network G(V,E) is available. Boolean algebraic concepts are used to define four operators: compare, reduce, combine, and generate. The proposed method, called CAREL, uses the four operators to generate exclusive and mutually disjoint events. CAREL has been implemented using bit vector representation on an Encore MULTIMAX 320 system. It is shown that CAREL solves large DCS networks (having a pathset on the order of 780 and a cutset on the order of 7300 or more) with a reasonable memory requirement. A comparison with other algorithms reveals the computational efficiency of the method. The proof of correctness of CAREL is included. >

Efficient Prefix Updates for IP Router Using Lexicographic Ordering and Updatable Address Set

Dynamic IP router table schemes, which have recently been proposed in the literature, perform an IP lookup or an online prefix update in O(log2|T|) memory accesses (MAs). In terms of lookup time, they are still slower than the full expansion/compression (FEC) scheme (compressed next-hop array/code word array (CNHA/CWA)), which requires exactly (at most) three MAs, irrespective of the number of prefixes |T| in a routing table T. The prefix updates in both FEC and CNHA/CWA have a drawback: Inefficient offline structure reconstruction is arguably the only viable solution. This paper solves the problem. We propose the use of lexicographic ordered prefixes to reduce the offline construction time of both schemes. Simulations on several real routing databases, run on the same platform, show that our approach constructs FEC (CNHA/CWA) tables in 2.68 to 7.54 (4.57 to 6) times faster than that from previous techniques. We also propose an online update scheme that, using an updatable address set and selectively decompressing the FEC and CNHA/CWA structures, modifies only the next hops of the addresses in the set. Recompressing the updated structures, the resulting forwarding tables are identical to those obtained by structure reconstructions, but are obtained at much lower computational cost. Our simulations show that the improved FEC and CNHA/CWA outperform the most recent O(log2|T|) schemes in terms of lookup time, update time, and memory requirement.

Experimental results on preprocessing of path/cut terms in sim of disjoint products technique

Researchers have proposed cardinality-, lexicographic-, and Hamming-distance-order methods to preprocess the path terms in sum of disjoint products (SDP) techniques for network reliability analysis. For cutsets, an ordering based on the node partition associated with each cut is suggested. Experimental results showing the number of disjoint products and computer time involved in generating SDP terms are presented. Nineteen benchmark networks containing paths varying from 4 to 780, and cuts from 4 to 7376, are considered. Several SDP techniques are generalized into three propositions to find their inherent merits and drawbacks. An efficient SDP technique is then used to run input files of paths/cuts preprocesses using cardinality-, lexicographic-, and Hamming-distance-ordering, and their combinations. The results are analyzed, showing that preprocessing based on cardinality or its combinations with lexicographic-, and/or Hamming-distance-ordering performs better. >

A computer approach for reliability evaluation of telecommunication networks with heterogeneous link-capacities

The authors present a computer approach to obtaining a survivability index called capacity related reliability (CRR) in large telecommunication networks where links have different capacities. The method is a two-step approach. The first step deals with composite path enumeration (CPE). A k-composite path is defined as the union of the set of edges in any k simple paths and relates link capacity and network connectivity. The CPE approach is an improvement over the algorithms proposed by the authors earlier (1991). In step two, k-composite paths information is manipulated to generate the CRR. The authors use CAREL (computer aided reliability evaluator) to solve this step. The technique is automated using C on the Encore Multimax System. The results on CRR for three networks with various values of minimum message capacity are presented. An exhaustive technique is used to verify these results. An informal proof of the CPE approach is also included. >

Cheating in networked computer games: a review

The increasing popularity of Massively Multiplayer Online Games (MMOG) -- games involving thousands of players participating simultaneously in a single virtual world - has highlighted the scalability bottlenecks present in centralised Client/Server (C/S) architectures. Researchers are proposing Peer-to-Peer (P2P) architectures as a scalable alternative to C/S; however, P2P is more vulnerable to cheating as it decentralises the game state and logic to un-trusted peer machines, rather than using trusted centralised servers. Cheating is a major concern for online games, as a minority of cheaters can potentially ruin the game for all players. In this paper we present a review and classification of known cheats, and provide real-world examples where possible. Further, we discuss counter measures used by C/S architectures to prevent cheating. Finally, we discuss several P2P architectures designed to prevent cheating, highlighting their strengths and weaknesses.

Experimental results on preprocessing of path/cut terms in sum of disjoint products technique

Experimental results are presented showing the number of disjoint products and computer time involved in generating sum of disjoint product (SDP) terms. To help obtain the results, the authors have considered 19 benchmark networks containing paths (cuts) varying from 4 (4) to 780 (7376). Several SDP techniques are reviewed and are generalized into three propositions to find their inherent merits and demerits. An efficient SDP technique is, then, utilized to run input files of paths/cuts preprocessed using (1) cardinality, (2) lexicographic, and (3) Hamming distance ordering methods and their combinations. The experimental evaluation has been performed on an FPS 500 system. Results are analyzed, and it is shown that the preprocessing based on cardinality or its combinations with (2) and/or (3) performs better.<<ETX>>

Enhanced mirrored servers for network games

The Mirrored Server (MS) architecture uses multiple mirrored servers across multiple locations to alleviate the bandwidth bottleneck in the Client/Server (C/S) architecture. Each mirror receives and multicasts player updates to the others, simulates the game, and disseminates the new game state to players. However, keeping the game state consistent between mirrors in the presence of network delay, and maintaining game responsiveness requires each server in MS to simulate the game multiple times for each game update, and additional times in the event of costly rollbacks. In this paper we propose the Enhanced Mirrored Server (EMS) architecture. Like in the Peer-to-Peer architecture, EMS allows peers to exchange updates directly, resulting in a higher tolerance to delay at the mirrors. We propose using bucket synchronization in the mirrors so that each server in EMS simulates the game only once for each update and does not require rollbacks. The server dissenates updates to clients only in the event of inconsistency, and thus its outgoing bandwidth is lower than in MS. Our EMS uses cryptographic techniques to provide security equivalent to C/S, and prevents the timestamp cheat possible in MS. Our analytical analysis and simulations show the advantages of EMS over MS.

Improved lower bounds on the reliability of hypercube architectures

The hypercube topology, also known as the Boolean n-cube, has recently been used for multiprocessing systems. The paper considers two structural-reliability models, namely, terminal reliability (TR) and network reliability (NR), for the hypercube. Terminal (network) reliability is defined as the probability that there exists a working path connecting two (all) nodes. There are no known polynomial time algorithms for exact computation of TR or NR for the hypercube. Thus, lower-bound computation is a better alternative, because it is more efficient computationally, and the system will be at least as reliable as the bound. The paper presents algorithms to compute lower bounds on TR and NR for the hypercube considering node and/or link failures. These algorithms provide tighter bounds for both TR and NR than known results and run in time polynomial in the cube dimension n, specifically, within time O(n/sup 2/). >

Efficient heuristics for energy-aware routing in networks with bundled links

Abstract Current networks are typically over-provisioned to ensure low delays, redundancy and reliability. These Quality of Service (QoS) guarantees are typically achieved using high end, high power network equipments. Their use, however, has led to concerns regarding green house gas emissions, which garnered a lot of attention recently and have resulted in a number of global initiatives aim at reducing the carbon footprint of Internet Service Providers (ISPs). These initiatives have motivated ISPs and researchers to design novel network algorithms and hardware that scale the usage or active time of a network according to traffic load. To this end, this paper considers the problem of shutting down a subset of bundled links during off-peak periods in order to minimize energy expenditure. Unfortunately, identifying the cables that minimize this objective is an NP-complete problem. Henceforth, we propose several practical heuristics based on Dijkstra’s algorithm and Yen’s k-shortest paths algorithm. We evaluated our heuristics on the Abilene network – with both real and synthetic traffic matrices and several larger random topologies with various loads. Our results show that the proposed heuristics to be effective and efficient. Moreover, our approaches could potentially reduce the energy usage of cables used in the Abilene network by up to 56.7%, assuming the traffic demands recorded on September 5, 2004.

Novel scheduling algorithms for concurrent transmit/receive wireless mesh networks

Recently, in an effort to increase the capacity of Wireless Mesh Networks (WMNs), researchers have begun equipping routers with multiple interfaces/radios, and connecting each one to a directional or smart antenna. A key feature of these routers is their ability to transmit or receive from multiple neighbors simultaneously. Hence, they have orders of magnitude higher capacity than their omni-directional counterparts. This significant capacity increase, however, is predicated upon a link scheduling algorithm that maximizes the number of active links at any given point in time. This paper proposes a number of link activation algorithms that derive maximal bipartite graphs from general topologies. These algorithms provide different trade-offs in terms of computation time and optimality. A key highlight is a greedy algorithm that has a time complexity of O(|V|^2), where V is the set of routers. Apart from that, we outline two algorithms that use an approximation to the well known maximum cut problem, and also a brute force algorithm, which is capable of deriving an optimal link activation schedule. The output from our algorithms can then be used by a spatial Time Division Multiple Access (TDMA) Medium Access Control (MAC) protocol to schedule concurrent transmitting and receiving links. We have verified our algorithms on various topologies with increasing node degrees as well as node numbers. From extensive simulation studies, we find that our algorithms have good performance in terms of number of links activated, superframe length, and end-to-end packet delay.