22940589
Asynchronous voice: a personal account
Asynchronous voice is the interactive communication process of people leaving voice messages for other people and the other people responding with their voice messages. A primitive form of asynchronous voice is a kind of telephone tag in which people use voice mail to have an interactive conversation. The author gives a personal account of his work with asynchronous voice and asynchronous learning.
6259994
Hierarchical Peer-To-Peer Systems
Structured peer-to-peer (P2P) lookup services organize peers into a flat overlay network and offer distributed hash table (DHT) functionality. Data is associated with keys and each peer is responsible for a subset of the keys. In hierarchical DHTs, peers are organized into groups, and each group has its autonomous intra-group overlay network and lookup service. Groups are organized in a top-level overlay network. To find a peer that is responsible for a key, the top-level overlay first determines the group responsible for the key; the responsible group then uses its intra-group overlay to determine the specific peer that is responsible for the key. We provide a general framework for hierarchical DHTs with scalable overlay management. We specifically study a two-tier hierarchy that uses Chord for the top level. Our analysis shows that by using the most reliable peers in the top level, the hierarchical design significantly reduces the expected number of hops. We also present a method to construct hierarchical DHTs that map well to the Internet topology and achieve short intra-group communication delay. The results demonstrate the feasibility of locality-based peer groups, which allow P2P systems to take full advantage of the hierarchical design.
31310473
Packet multiplexers with adversarial regulated traffic
We consider a finite-buffer packet multiplexer to which traffic arrives from several independent sources. The traffic from each of the sources is regulated, i.e., the amount of traffic that can enter the multiplexer is constrained by known regulator constraints. The regulator constraints depend on, the source and are more general than those resulting from cascaded leaky buckets. We assume that the traffic is adversarial to the extent permitted by the regulators. For lossless multiplexing, we show that if the original multiplexer is lossless it is possible to allocate bandwidth and buffer to the sources so that the resulting segregated systems are lossless. For lossy multiplexing, we use our results for lossless multiplexing to estimate the loss probability of the multiplexer. Our estimate involves transforming the original system into two independent resource systems, and using adversarial sources for the two independent resources to obtain a bound on the loss probabilities for the transformed system. We show that the adversarial sources are not extremal on-off sources, even when the regulator consists of a peak rate controller in series with a leaky bucket. We explicitly characterize the form of the adversarial source for the transformed problem. We also provide numerical results for the case of the simple regulator.
14410544
Fluid modeling of pollution proliferation in P2P networks
P2P systems are highly vulnerable to pollution attacks in which attackers inject multiple versions of corrupted content into the system, which is then further proliferated by unsuspecting users. However, to our knowledge, there are no closed-form solutions that describe this phenomenon, nor are there models that describe how the injection of multiple versions of corrupted content impacts a clients' ability to receive a valid copy. In this paper we develop a suite of fluid models that model pollution proliferation in P2P systems. These fluid models lead to systems of non-linear differential equations. We obtain closed-form solutions for the differential equations; for the remaining models, we efficiently solve the differential equations numerically. The models capture a variety of user behaviors, including propensity for popular versions, abandonment after repeated failure to obtain a good version, freeloading, and local version blacklisting. Our analysis reveals intelligent strategies for attackers as well as strategies for clients seeking to recover non-polluted content within large-scale P2P networks.
7927284
Hierarchical Peer-to-Peer Systems
Structured peer-to-peer (P2P) lookup services organize peers into a flat overlay network and offer distributed hash table (DHT) functionality. Data is associated with keys and each peer is responsible for a subset of the keys. In hierarchical DHTs, peers are organized into groups, and each group has its autonomous intra-group overlay network and lookup service. Groups are organized in a top-level overlay network. To find a peer that is responsible for a key, the top-level overlay first determines the group responsible for the key; the responsible group then uses its intra-group overlay to determine the specific peer that is responsible for the key. We provide a general framework and a scalable hierarchical overlay management. We study a two-tier hierarchy using Chord for the top level. Our analysis shows that by using the most reliable peers in the top level, the hierarchical design significantly reduces the expected number of hops.
1377609
Computing approximate blocking probabilities for large loss networks with state-dependent routing
A reduced load approximation (also referred to as an Erlang fixed point approximation) for estimating point-to-point blocking probabilities in loss networks (e.g., circuit switched networks) with state-dependent routing is considered. In this approximation scheme, the idle capacity distribution for each link in the network is approximated, assuming that these distributions are independent from link to link. This leads to a set of nonlinear fixed-point equations which can be solved by repeated substitutions. The accuracy and the computational requirements of the approximation procedure for a particular routing scheme, namely least loaded routing, is examined. Numerical results for six-node and 36-node asymmetric networks are given. A novel reduced load approximation for multirate networks with state-dependent routing is also presented. >
5443410
Efficient Incremental Evaluation of Queries with Aggregation
We present a technique for eeciently evaluating queries on programs with mono-tonic aggregation, a class of programs deened by Ross and Sagiv. Our technique consists of the following components: incremental computation of aggregate functions , incremental xpoint evaluation of monotonic programs and Magic Sets transformation of monotonic programs. We also present a formalization of the notion of incremental computation of aggregate functions on a multiset, and upper and lower bounds for incremental computation of a variety of aggregate functions. We describe a proof-theoretic reformulation of the monotonic semantics in terms of computations, following the approach of Beeri et al.; this reformulation greatly simpliies the task of proving the correctness of our optimizations.
2635814
Querying Faceted Databases
Faceted classification allows one to model applications with complex classification hierarchies using orthogonal dimensions. Recent work has examined the use of faceted classification for browsing and search. In this paper, we go further by developing a general query language, called the entity algebra, for hierarchically classified data. The entity algebra is compositional, with query inputs and outputs being sets of entities. Our language has linear data complexity in terms of space and quadratic data complexity in terms of time. We compare the expressive power of the entity algebra with relational algebra. We also describe an end-to-end query system based on the language in the context of an archeological database.
2792911
The New Jersey Data Reduction Report
An arrangement is shown for controlling transmission of blocks of information to and from a plurality of major components of a digital computer system interconnected by common buses. The disclosed arrangement operates so that any component of the system may normally seize, on a nonpriority basis, one of the buses at the beginning of any time slot defined by two successive clock pulses generated by a single source and applied to all components simultaneously; however, if a special instruction is encountered during execution of a program, any component may retain a bus for more than one time slot. The disclosed arrangement also permits error checking of transmitted information from a given major component without interfering with transmission from any other major component and automatically causes retransmission of any block of information found to be improperly transmitted originally. Still further, the disclosed arrangement permits buses to be dedicated during execution of a program or, if desired, any complete major component to be replaced without affecting any other major component.
8569541
Partitioned optimization of complex queries
Performing complex analysis on top of massive data stores is essential to most modern enterprises and organizations and requires significant aggregation over different attribute sets (dimensions) of the participating relations. Such queries may take hours or days, a time period unacceptable in most cases. As a result, it is important to study these queries and identify special frequent cases that can be evaluated with specialized algorithms. Understanding complex aggregate queries leads to better execution plans and, consequently, performance. The idea of partitioning is fundamental and central in aggregate queries. This concept can be used to define a class of queries called group queries. The main characteristic of a group query is that it can be evaluated in a partitioned (or groupwise) fashion, i.e. the underlying relation(s) can be partitioned (based on a set of attributes) into disjoint groups and each group can be processed separately, possibly in parallel. For example, a query that performs a complex operation (e.g. joins and/or selections and/or aggregations) within each group is a group query. To express it in SQL, one has to join/correlate several views and/or subqueries on the grouping attributes. A naive plan (where the joins are executed) may be very expensive, even for relatively small base relations. On the other hand, a groupwise evaluation can lead to huge performance gains. We present a syntactic criterion to identify group queries in SQL and show that every group query can be expressed in a way that satisfies this criterion. This work is based on Chatziantoniou and Ross [Querying Multiple Features of Groups in Relational Databases. in: 22nd International Conference on Very Large Databases, VLDB, 1996, pp. 295-306]. The concept of group queries is useful not only in terms of evaluation, but also in terms of analyzing a complex decision support query that aggregates over different sets of attributes. In such a case the query may be decomposable to one or more query components, where each component is a group query. This observation allows parallel execution, multi-query processing and identification of special cases. We present in this paper two algorithms to decompose a complex aggregate query to its group query components. The value of groupwise processing has been recently recognized by the research community and implemented in at least a major commercial system. To be of use however in a relational system, partitioned evaluation has to be modeled as a relational operator. We review three different approaches for such an operator and propose a generalized groupwise operator. We also perform some experiments to show that naive optimization with the new operator incorporated without taking into consideration decompositions to group query components does not always lead to the most efficient plans. An extended syntax is another way to identify special frequent cases and apply efficient algorithms. Having specific operators for common operations contributes to the succinctness and optimizability of certain queries (e.g. datacubes). An extended syntax is presented with emphasis for multi-feature queries, a frequent and practical subclass of group queries that is amenable to specialized evaluation, involving (potentially repeated) selection, grouping and aggregation over the same groups.
970550
Realizing parallelism in database operations: insights from a massively multithreaded architecture
A new trend in processor design is increased on-chip support for multithreading in the form of both chip multiprocessors and simultaneous multithreading. Recent research in database systems has begun to explore increased thread-level parallelism made possible by these new multicore and multithreaded processors. The question of how best to use this new technology remains open, particularly as the number of cores per chip and threads per core increase. In this paper we use an existing massively multithreaded architecture, the Cray MTA-2, to explore the implications that a larger degree of on-chip multithreading may have for parallelism in database operations. We find that parallelism in database operations is easy to achieve on the MTA-2 and that, with little effort, parallelism can be made to scale linearly with the number of available processor cores.