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Zegura,
Ellen W.
Zegura,
Ellen W.
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ItemOptimal Allocation of Clients to Replicated Multicast Servers(Georgia Institute of Technology, 1999) Ammar, Mostafa H. ; Zegura, Ellen W. ; Fei, ZongmingServer replication is an approach that is often used to improve the scalability of a service. One of the important factors in the efficient utilization of replicated servers is the ability to direct client requests to the `best" server, according to some optimality criteria. Recently, there have been several proposals for multicast services in which a server delivers information to multiple clients simultaneously. Such proposals include multicasting of web content, multicast-based video services (on-demand and pay-per-view style services), multicasting of database content and broadcast disks. The goal of many of these proposals is to use multicast to enhance the ability of the service to handle a large number of clients economically. Multicast servers may be replicated for several reasons: to distribute the load among on-demand multicast servers, to balance the `feedback" load on the servers or on entities along the multicast tree from the servers, or to select the server that is at the root of the `best" multicast routing tree. In this paper we first give a definition of the static multicast server selection problem, in which we assume a set of static clients and multicast servers and consider how one might produce an optimalallocation of the clients to the servers. We propose a transformation method for deriving multicast server selection algorithms from the traditional multicast routing algorithms. To investigate the dynamic behavior of client join and leave and the cost incurred during the process, we next define the dynamic multicast server selection problem, in which the potential clients join and leave the multicast session dynamically, and the goal is to produce an optimal allocation of clients to servers with an emphasis on how this allocation behaves over time. We formulate the problem as a Markovian Decision Process (MDP) and analyze the tradeoff between the cost of the multicast tree(s) and the transition cost of establishing and removing links from the tree(s). We also explore the effect of join/leave frequency on optimal policy. Our analysis leads to two heuristics which we use to propose a selection algorithm. The algorithm uses a very simple join and leave strategy yet still can generate low cost trees. Our simulation compares the performance of our proposed algorithm with various other multicast server selection algorithms.
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ItemInter-Receiver Fair Multicast Communication over the Internet(Georgia Institute of Technology, 1999) Zegura, Ellen W. ; Ammar, Mostafa H. ; Jiang, TianjiMulticast protocols target applications involving a large number of receivers with heterogeneous data reception capabilities.To accommodate heterogeneity, the sender may transmit at multiple rates, requiring mechanisms to determine the rates and allocate receivers. In this paper, we develop a protocol to control the rate of a multicast session, with the goal of maximizing the inter-receiver fairness, an intra-session measure that captures the collective "satisfaction" of the session receivers. Our target environment is the Internet, where fair sharing of bandwidth must be achieved via end-system mechanisms and fairness to TCP is important. We develop and evaluate protocols to maximize this measure by maintaining a fixed-rate base group and a variable-rate group. We show that our schemes offer improvement over single-rate sessions, while maintaining TCP-friendliness.
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ItemMultiple-Channel Multicast Scheduling for Scalable Bulk-data Transport(Georgia Institute of Technology, 1998) Ammar, Mostafa H. ; Zegura, Ellen W. ; Donahoo, Michael J.A key technique for allowing a server to handle a large volume of requests for file transfers is to multicast the data to the set of requesting clients. Typically, the paths from the server to the clients will be heterogeneous in bandwidth availability. Multiple-Channel Multicast (MCM) is an approach that can be used to handle this heterogeneity. In this approach the data is multicast over multiple channels, each addressed as a separate multicast group. Each receiver subscribes to a set of channels (i.e., joins the corresponding multicast groups) commensurate with its own rate capabilities. Of particular interest in the design of MCM schemes is the scheduling of data transmission across the multiple channels to accomodate asynchronous requests from clients. In this paper, we present and analyze a new multiple-channel multicast approach called Partition Organization (PO) scheduling. The scheme is designed to result in good reception efficiency when compared to existing proposals while improving on their performance when other measures of interest (which we introduce) are considered.
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ItemA Novel Server Selection Technique for Improving the Response Time of a Replicated Service(Georgia Institute of Technology, 1997) Bhattacharjee, Samrat ; Zegura, Ellen W. ; Ammar, Mostafa H. ; Fei, ZongmingServer replication is an approach often used to improve the ability of a service to handle a large number of clients. One of the important factors in the efficient utilization of replicated servers is the ability to direct client requests to the best server, according to some optimality criteria. In this paper we target an environment in which servers are distributed across the Internet, and clients identify servers using our application-layer anycasting service. Our goal is to allocate servers to clients in a way that minimizes a client's response time. To that end, we develop an approach for estimating the performance that a client would experience when accessing particular servers. Such information is maintained in a resolver that clients can query to obtain the identity of the server with the best response time. Our performance collection technique combines server push with client probes to estimate the expected response time. A set of experiments is used to demonstrate the properties of our performance determination approach and to show its advantages when used within the application-layer anycasting architecture.
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ItemApplication-Layer Anycasting(Georgia Institute of Technology, 1996) Ammar, Mostafa H. ; Zegura, Ellen W. ; Shah, Viren ; Fei, Zongming ; Bhattacharjee, SamratServer replication is a key approach for maintaining user-perceived quality of service within a geographically wide-spread network. The anycasting communication paradigm is designed to support server replication by allowing applications to easily select and communicate with the "best" server, according to some performance or policy criteria, in a group of content- equivalent servers. We examine the definition and support of the anycasting paradigm at the application layer, providing a service that maps anycast domain names into one or more IP addresses using anycast resolvers. In addition to being independent from network-layer support, our definition includes the notion of filters, functions that are applied to groups of addresses to affect the selection process. We consider both metric-based filters (e.g., server response time) and policy-based filters; we further allow filtering both at the anycast resolver and local to the anycast client. A key input to the filtering process is metric information describing the relative performance of replicated servers. We examine the use of various techniques for maintaining this information at anycast resolvers.
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ItemDistributed Laboratories: A Research Proposal(Georgia Institute of Technology, 1996) Schwan, Karsten ; Ahamad, Mustaque ; Hudson, Scott E. ; Limb, J. O. (John O.) ; Ammar, Mostafa H. ; Ezquerra, Norberto F. ; Mukherjee, Amarnath ; Potts, Colin ; Ramachandran, Umakishore ; Zegura, Ellen W. ; Fujimoto, Richard M.The continuing merger of computer and communication technologies is leading to a new computing/communications infrastructure of unprecedented magnitude, enabling new applications with broad economic and social impact. Yet, such applications pose major challenges to researchers in Computer Science and in application domains. The topic of the proposed research program is the realization of Distributed Laboratories, where individuals can interact with each other, and more importantly, with powerful, distributed computational tools as readily as if all were located in a single site. Our intent is to permit scientists, engineers, and managers at geographically distinct locations (including individuals 'tele-commuting' from home) to combine their expertise in solving shared problems, by allowing them to simultaneously view, interact with, and steer sophisticated computations executing on high performance distributed computing platforms.