R. Cohen

University of Haifa, H̱efa, Haifa District, Israel

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Publications (24)18.98 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: In recent years, hardware-based packet classification has became an essential component in many networking devices. It often relies on ternary content-addressable memories (TCAMs), which can compare in parallel the packet header against a large set of rules. Designers of TCAMs often have to deal with unpredictable sets of rules. These result in highly variable rule expansions, and can only rely on heuristic encoding algorithms with no reasonable guarantees. In this paper, given several types of rules, we provide new upper bounds on the TCAM worst case rule expansions. In particular, we prove that a W-bit range can be encoded in W TCAM entries, improving upon the previously known bound of 2W - 5. We further prove the optimality of this bound of W for prefix encoding, using new analytical tools based on independent sets and alternating paths. Next, we generalize these lower bounds to a new class of codes called hierarchical codes that includes both binary codes and Gray codes. Last, we propose a modified TCAM architecture that can use additional logic to significantly reduce the rule expansions, both in the worst case and using real-life classification databases.
    IEEE Transactions on Computers 01/2013; 62(6):1127-1140. · 1.38 Impact Factor
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    ABSTRACT: The recent growing popularity of cloud-based solutions and the variety of new applications present new challenges for cloud management and resource utilization. In this paper we concentrate on the networking aspect and consider the placement problem of virtual machines (VMs) of applications with intense bandwidth requirements. Optimizing the available network bandwidth is far more complex than optimizing resources like memory or CPU, since every network link may be used by many physical hosts and thus by the VMs residing in these hosts. We focus on maximizing the benefit from the overall communication sent by the VMs to a single designated point in the data center (called the root). This is the typical case when considering a storage area network of applications with intense storage requirements. We formulate a bandwidth-constrained VM placement optimization problem that models this setting. This problem is NP hard, and we present a polynomial-time constant approximation algorithm for its most general version, in which hosts are connected to the root by a general network graph. For more practical cases, in which the network topology is a tree and the revenue is a simple function of the allocated bandwidth, we present improved approximation algorithms that are more efficient in terms of running time. We evaluate the expected performance of our proposed algorithms through a simulation study over traces from a real production data center, providing strong indications to the superiority of our proposed solutions.
    INFOCOM, 2013 Proceedings IEEE; 01/2013
  • M. Aezladen, R. Cohen, D. Raz
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    ABSTRACT: This paper deals with efficient location-based decision-supporting content distribution to mobile groups. We consider the case where a set of information dissemination devices (IDDs) broadcast a limited amount of location-based information to passing mobile nodes that are moving along well-defined paths. We develop a novel model that captures the main aspects of the problem and define a new optimization problem we call Maximum Benefit Message Assignment Problem (MBMAP). We study several variants of this problem in the case where the IDDs are cooperative and in the case where they are not. We develop new approximation algorithms for these variants and then focus on the practical effects of using them in realistic networking scenarios.
    IEEE/ACM Transactions on Networking 01/2012; 20(5):1514-1526. · 2.01 Impact Factor
  • R. Cohen, D. Raz
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    ABSTRACT: Overlay routing in a very attractive scheme that allows improving certain properties of the routing without the need to change the standards of the current underlying routing. However, deploying overlay routing requires the placement and maintenance of overlay infrastructure. This gives rise to the following optimization problem: find a minimal set of overlay nodes such that the required routing properties are satisfied. In this paper we rigorously study this optimization problem. We show that it is NP hard and derive a non-trivial approximation algorithm for it, where the approximation ratio depends on specific properties of the problem at hand. We examine the practical aspects of the scheme by evaluating the gain one can get over two real scenarios. The first one is BGP routing, and we show, using up-to-date data reflecting the current BGP routing policy in the Internet, that a relative small number of less than 100 relay servers are sufficient to enable routing over shorter paths from a single source to all ASes, reducing the average path length of inflated paths by 40%. We also demonstrate that using the scheme for TCP performance improvement, results in an almost optimal placement of overlay nodes.
    INFOCOM, 2011 Proceedings IEEE; 05/2011
  • R. Cohen, D. Raz
    [Show abstract] [Hide abstract]
    ABSTRACT: In recent years, hardware based packet classification has became an essential component in many networking devices. Ternary Content-Addressable Memories (TCAMs) are one of the most popular solutions in this domain, allowing to compare in parallel the packet header against a large set of rules, and to retrieve the first match. However, using TCAM to match a range of values is much more problematic and dramatically reduces the cost effectiveness of the solution. In this paper we study ways to use simple built-in TCAM mechanisms in order to increase the efficiency of range coverage. While current techniques have a worst expansion ratio of 2W-4, we present an efficient algorithm enabling to encode any range with at most W TCAM entries (where W in the number of bits), without using additional processing, extra bits, and without any external encoding. The same paradigm can be applied to multiple raging rules as well, resulting in significant improvement over current known techniques. Moreover, our simulation results indicate that these techniques can be used to reduce the actual TCAM size of hardware networking devices under realistic scenarios.
    INFOCOM, 2010 Proceedings IEEE; 04/2010
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    ABSTRACT: Monitoring is an essential part of any self-adaptive management loop. While providing the necessary information for making management decisions, monitoring itself incurs a cost in terms of the system and network resources committed to this management task. Thus, one can pose a generic question: what is the right amount of monitoring that maximizes its utility for management? This question turns out to be difficult to answer in general. In this paper we focus on quantifying the utility of monitoring for self-adaptive load sharing, where a stream of jobs arrives at a collection of n identical servers. We propose a novel model, that we dubbed an Extended Supermarket Model (ESM) to study the tradeoff between the usefulness of the monitoring information and the cost of obtaining it. We show that for each service request rate, there exists an optimal number of servers that should be monitored to obtain minimal average service time at an optimal cost. Using these findings, we present self-adaptive load-sharing algorithms both for centralized and fully distributed settings and evaluate these algorithms using simulations and a real testbed. Our results show that in realistic scenarios, where monitoring cost is not negligible, the self-adaptive load balancing is clearly superior to any cost-oblivious load-sharing mechanisms. We also demonstrate that in a fully distributed setting, where no dedicated monitoring component is employed, our self-adaptive heuristics perform very well with respect to the current common practice.
    IEEE Journal on Selected Areas in Communications 02/2010; · 3.12 Impact Factor
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    M. Aezladen, R. Cohen, D. Raz
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    ABSTRACT: The paper deals with efficient distribution of timely information to flows of mobile devices. We consider the case where a set of information dissemination devices (IDDs) broadcast a limited amount of information to passing mobile nodes that are moving along well-defined paths. This is the case, for example, in intelligent transportation systems. We develop a novel model that captures the main aspects of the problem, and define a new optimization problem we call MBMAP (maximum benefit message assignment problem). We study the computational complexity of this problem in the global and local cases, and provide new approximation algorithms.
    INFOCOM 2009, IEEE; 05/2009
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    R. Cohen, G. Nakibly
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    ABSTRACT: In this paper, we study the computational complexity and effectiveness of a concept we term ldquoN-hub shortest-path routingrdquo in IP networks. N-hub shortest-path routing allows the ingress node of a routing domain to determine up to N intermediate nodes (ldquohubsrdquo) through which a packet will pass before reaching its final destination. This facilitates better utilization of the network resources, while allowing the network routers to continue to employ the simple and well-known shortest-path routing paradigm. Although this concept has been proposed in the past, this paper is the first to investigate it in depth. We apply N-hub shortest-path routing to the problem of minimizing the maximum load in the network. We show that the resulting routing problem is NP-complete and hard to approximate. However, we propose efficient algorithms for solving it both in the online and the offline contexts. Our results show that N-hub shortest-path routing can increase network utilization significantly even for N = 1. Hence, this routing paradigm should be considered as a powerful mechanism for future datagram routing in the Internet.
    IEEE/ACM Transactions on Networking 07/2008; · 2.01 Impact Factor
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    R. Cohen, L. Katzir
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    ABSTRACT: OFDMA is one of the most important modulation and access methods for the future mobile networks. Before transmitting a frame on the downlink, an OFDMA base station has to invoke an algorithm that determines which of the pending packets will be transmitted, what modulation should be used for each of them, and how to construct the complex OFDMA frame matrix as a collection of rectangles that fit into a single matrix with fixed dimensions. We propose efficient, and theoretically best possible, algorithms that solves this intricate OFDMA scheduling problem by breaking it down into two sub-problems, referred to as macro and micro scheduling. We analyze the computational complexity of these sub-problems and develop efficient algorithms for solving them.
    INFOCOM 2008. The 27th Conference on Computer Communications. IEEE; 05/2008
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    R. Cohen, L. Katzir, R. Rizzi
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    ABSTRACT: In this paper, we define a new problem that has not been addressed in the past: the trade-off between energy efficiency and throughput for multicast services in 802.16e or similar mobile networks. In such networks, the mobile host can reduce its energy consumption by entering the sleep mode when it is not supposed to receive or transmit information. For unicast applications, the tradeoff between delay and energy efficiency has been extensively researched. However, for mobile hosts running multicast (usually push- based) applications, it is much more difficult to determine when data should be transmitted by the base station and when each host should enter the sleep mode. In order to maximize the channel throughput while limiting the energy consumption, a group of hosts needing similar data items should be active during the same time intervals. We define this as an optimization problem and present several algorithms for it. We show that the most efficient solution is the one that employs cross-layer optimization by dividing the hosts into groups according to the quality of their downlink physical (PHY) channels.
    IEEE Transactions on Mobile Computing 04/2008; · 2.40 Impact Factor
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    R.. Cohen, L.. Katzir
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    ABSTRACT: The scheduling logic at the base station of a shared wireless medium supports time-dependent (synchronous) applications by allocating timely transmission grants. To this end, it must take into account not only the deadlines of the pending packets, but also the channel conditions for each potential sender, the requirements of nonsynchronous applications, and the packet retransmission strategy. With respect to these factors, we identify three scheduling scenarios and show that the scheduler logic faces a different challenge in addressing each of them. We then present a generic scheduling algorithm that translates all the factors relevant to each scenario into a common profit parameter, and selects the most profitable transmission instances.
    IEEE/ACM Transactions on Networking 09/2007; · 2.01 Impact Factor
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    ABSTRACT: In this work we extend the aforementioned supermarket model by incorporating the management costs into it. In particular, we assume that when a server is polled about its load, it has to allocate resources in order to answer this query. We consider a system that consists of n identical servers. Each server processes its incoming service requests according to the FIFO
    Autonomic Computing, 2007. ICAC '07. Fourth International Conference on; 07/2007
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    R. Cohen, D. Raz
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    ABSTRACT: The Internet connectivity in the autonomous system (AS) level reflects the commercial relationship between ASes. A connection between two ASes could be of type customer-provider when one AS is a provider of the other AS, or of type peer-peer , if they are peering ASes. This commercial relationship induces a global hierarchical structure which is a key ingredient in the ability to understand the topological structure of the AS connectivity graph. Unfortunately, it is very difficult to collect data regarding the actual type of the relationships between ASes, and in general this information is not part of the collected AS connectivity data. The Type of Relationship ( ToR ) problem attempts to address this shortcoming, by inferring the type of relationship between connected ASes based on their routing policies. However, the approaches presented so far are local in nature and do not capture the global hierarchical structure. In this work we define a novel way to infer this type of relationship from the collected data, taking into consideration both local policies and global hierarchy constrains. We define the Acyclic Type of Relationship AToR problem that captures this global hierarchy and present an efficient algorithm that allows determining if there is a hierarchical assignment without invalid paths. We then show that the related general optimization problem is NP-complete and present a 2/3 approximation algorithm where the objective function is to minimize the total number of local policy mismatches. We support our approach by extensive experiments and simulation results showing that our algorithms classify the type of relationship between ASes much better than all previous algorithms.
    INFOCOM 2007. 26th IEEE International Conference on Computer Communications. IEEE; 06/2007
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    R. Cohen, D. Raz
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    ABSTRACT: The rapid growth of wireless and cellular networks, and the high availability of small communication devices, such as PDAs, brings us faster than ever to the point where context aware services (CASs) are becoming a commodity. In order to allow fast and efficient development, deployment, and management of such services, a global system that allows the services to gain access to the context information needs to be created, maintained, and managed. We study the requirements for such a context distribution system. We deal with the architectural decisions regarding the definition of context items and the way context information becomes available to the CASs, and also the algorithmic aspects of disseminating this information. We demonstrate the advantages of the architecture and the proposed information dissemination algorithms by conducting a simulation study under realistic practical assumptions. Our results indicate that a modular approach in which context information is provided in many network locations by brokers through an open simple API is both powerful enough to provide the needed context information, and simple enough to be easily implemented.
    Network Operations and Management Symposium, 2004. NOMS 2004. IEEE/IFIP; 05/2004
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    R. Cohen, L. Katzir
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    ABSTRACT: We present a scheme for allocating unsolicited grants to the end hosts of synchronous applications of a wireless access network, in accordance with the condition of the channel, the importance of each packet and the specific loss recovery mechanism employed in the channel. The proposed scheme is generic in the sense that it maximizes the effectiveness of the channel under various conditions and it can he used along with every FEC-based or retransmission-based error recovery strategy
    INFOCOM 2004. Twenty-third AnnualJoint Conference of the IEEE Computer and Communications Societies; 04/2004
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    R. Cohen, L. Katzir, D. Raz
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    ABSTRACT: Cache pre-filling is emerging as a new concept for increasing the availability of popular Web items in cache servers. According to this concept, Web items are sent by a "push-server" to the proxy cache servers, usually through a broadcast-based or a multicast-based distribution mechanism. One of the most difficult challenges is to design the scheduling algorithm of the push-server. This algorithm needs to determine the "broadcast scheduling map", namely which Web items to broadcast and when. In this paper we study the approach where every constant period of time each proxy cache analyzes the requests it has received in the past and determines which Web item it prefers to receive by broadcast and when. We formalize a related problem, called the "cache pre-filing push" (CPFP) problem, analyze its computational complexity, and describe efficient algorithms to solve it.
    INFOCOM 2002. Twenty-First Annual Joint Conference of the IEEE Computer and Communications Societies. Proceedings. IEEE; 02/2002
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    E. Bortnikov, R. Cohen
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    ABSTRACT: The paper addresses the problem of designing efficient scheduling policies for the transmission of control messages by hierarchical network protocols. Such protocols encounter a tradeoff between the desire to forward a control message across the tree as soon, as it is received, and the desire to reduce control traffic. Scheduling problems that arise in this context are defined and discussed. The paper mainly concentrates on minimizing the average extra delay encountered by the control messages under an upper bound on the number of outgoing messages a node can send during a fixed period of time. A polynomial-time algorithm is presented for the off-line version of the problem, and then several efficient on-line heuristics are presented and compared
    INFOCOM '98. Seventeenth Annual Joint Conference of the IEEE Computer and Communications Societies. Proceedings. IEEE; 04/2001
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    R. Cohen, G. Kaempfer
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    ABSTRACT: Network layer multicast is know as the most efficient way to support multicast sessions. However, for security, QoS and other considerations, most of the real-time application protocols can be better served by upper layer (transport or application) multicast. We propose a scheme called M-RTP for multicast RTP sessions. The idea behind this scheme is to set up the multicast RTP session over a set of unicast RTP sessions, established between the various participants (source and destinations) of the multicast session. We then address the issue of finding a set of paths with maximum bottleneck for an M-RTP session. We show that this problem is NP-complete, and propose several heuristics to solve it
    INFOCOM 2001. Twentieth Annual Joint Conference of the IEEE Computer and Communications Societies. Proceedings. IEEE; 02/2001
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    R. Cohen, G. Kaempfer
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    ABSTRACT: A virtual private network (VPN) is a private data network that uses a nonprivate data network to carry traffic between remote sites. An “Intranet VPN” establishes network layer connectivity between remote Intranet sites by creating an IP overlay network over the nonprivate network, using various tunneling mechanisms. There are two approaches for establishing such tunnels: a “CPE-based approach” and a “network-based approach.” In the first approach, tunnels are established only between the CPE devices, whereas in the second approach tunnels are also established between the routers of the core nonprivate network. In this paper we address the problem of determining a CPE-based and a network-based layout of VPN tunnels while taking into account two factors: the cost of the links over which the VPN tunnels are established and the cost of the core routers that serve as end points for the VPN. We define related graph algorithm problems, analyze their complexity, and present heuristics for solving these problems efficiently
    IEEE/ACM Transactions on Networking 01/2001; · 2.01 Impact Factor
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    R. Cohen, E. Felstaine, R. Emek
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    ABSTRACT: We propose a framework for the creation and maintenance of multicast trees in hierarchical ATM networks. This framework aims at coping with an inherent difficulty of topology aggregation in such networks. The main idea of the proposed framework is to distribute the tree topology information among a set of hierarchical multicast group servers (MGS) nominated for each multicast tree, while keeping regions that do not have a member in the multicast group unaware of the tree. The framework can be employed with every multicast routing algorithm designed for non-hierarchical networks
    INFOCOM 2000. Nineteenth Annual Joint Conference of the IEEE Computer and Communications Societies. Proceedings. IEEE; 04/2000