17761375
Comparative performance analysis of versions of TCP in a local network with a lossy link
We use a stochastic model to study the throughput performance of various transport control protocol (TCP) versions (Tahoe (including its older version that we call OldTahoe), Reno, and NewReno) in the presence of random losses on a wireless link in a local network. We model the cyclic evolution of TCP, each cycle starting at the epoch at which recovery starts from the losses in the previous cycle. TCP throughput is computed as the reward rate in a certain Markov renewal-reward process. Our model allows us to study the performance implications of various protocol features, such as fast retransmit and fast recovery. We show the impact of coarse timeouts. In the local network environment the key issue is to avoid a coarse timeout after a loss occurs. We show the effect of reducing the number of duplicate acknowledgements (ACKs) for triggering a fast retransmit. A large coarse timeout granularity seriously affects the performance of TCP, and the various protocol versions differ in their ability to avoid a coarse timeout when random loss occurs; we quantify these differences. We show that, for large packet-loss probabilities, TCP-Reno performs no better, or worse, than TCP-Tahoe. TCP-NewReno is a considerable improvement over TCP-Tahoe, and reducing the fast-retransmit threshold from three to one yields a large gain in throughput; this is similar to one of the modifications in the TCP-Vegas proposal. We explain some of these observations in terms of the variation of fast-recovery probabilities with packet-loss probability. The results of our analysis compare well with a simulation that uses actual TCP code.
2247483
Saturation throughput analysis of a system of interfering IEEE 802.11 WLANs
IEEE 802.11 wireless local area networks that span large buildings or campuses must comprise multiple cells, several of which must necessarily be cochannel cells. It can be shown that in such multicell networks, typically, the radio ranges of cochannel cells overlap. The paper studies the saturation throughput performance of two cochannel cells with critical overlap, i.e., their interference ranges completely overlap but no node in either cell can decode any transmissions from the other cell. We identify that the difference between the two MAC parameters, EIFS (extended interframe space) and DIFS (DCF interframe space) is a key issue and incorporate this difference as a parameter into an analytical model. The model yields a fixed point equation that yields an approximation to the saturation throughputs of the two cells. The results from the analysis are validated against ns-2 simulations. We find that with EIFS>DIFS there is substantial temporal unfairness in the channel access between the two cells, but, because of fewer collisions, the critical overlap configuration has higher per cell throughput than if the cells' decoding ranges overlapped.
5368108
A stochastic approximation approach for max-min fair adaptive rate control of ABR sessions with MCRs
The available bit rate (ABR) sessions in an ATM network share the bandwidth left over after guaranteeing service to constant bit rate (CBR) and variable bit rate (VBR) traffic. Hence the bandwidth available to ABR sessions is randomly varying. This bandwidth must be shared by the sessions in a max-min fair fashion. Our point of departure in this paper is to formulate the problem of determining the max-min fair session rates as the problem of finding the root of a certain nonlinear vector equation; the same formulation also arises with our notion of max-min fairness with positive minimum cell rates (MCRs). This formulation allows us to use a stochastic approximation algorithm for online distributed computation of the max-min fair rates. We use the well known ordinary differential equation technique to prove convergence of the algorithm in the synchronous update case. We provide simulation results using the NIST simulator to show that the algorithm is able to track the max-min fair rates for slowly varying random available link bandwidths.
47268289
Optimal configuration of OSPF aggregates
Open Shortest Path First (OSPF) is a popular protocol for routing within an autonomous system (AS) domain. In order to scale for large networks containing hundreds and thousands of subnets, OSPF supports a two-level hierarchical routing scheme through the use of OSPF areas. Subnet addresses within an area are aggregated, and this aggregation is a crucial requirement for scaling OSPF to large AS domains, as it results in significant reductions in routing table sizes, smaller link-state databases, and less network traffic to synchronize the router link-state databases. On the other hand, address aggregation also implies loss of information about the length of the shortest path to each subnet, which in turn, can lead to suboptimal routing.In this paper, we address the important practical problem of configuring OSPF aggregates to minimize the error in OSPF shortest-path computations due to subnet aggregation. We first develop an optimal dynamic programming algorithm that, given an upper bound k on the number of aggregates to be advertised and a weight assignment function for the aggregates, computes the k aggregates that result in the minimum cumulative error in the shortest-path computations for all source-destination subnet pairs. Subsequently, we tackle the problem of assigning weights to OSPF aggregates such that the cumulative error in the computed shortest paths is minimized. We demonstrate that, while for certain special cases (e.g., unweighted cumulative error) efficient optimal algorithms for the weight assignment problem can be devised, the general problem itself is NP-hard. Consequently, we have to rely on search heuristics to solve the weight assignment problem. To the best of our knowledge, our work is the first to address the algorithmic issues underlying the configuration of OSPF aggregates and to propose efficient configuration algorithms that are provably optimal for many practical scenarios.
39839954
A randomized algorithm for distributed consensus
We describe a randomized, fully distributed algorithm for distributed consensus and evaluate its performance assuming probabilistically bounded message delay. Each node randomly contacts a few other nodes and incorporates their values into its own value. All the nodes are able to reach consensus in this manner after a few rounds. The results show that the randomized algorithm is flexible, efficient and robust, and offers several advantages over a deterministic algorithm.
8461087
Articulatory class based spectral envelope representation for voice fonts
Voice fonts are used to represent and transform the personality of speech. The spectral envelope is one of the main descriptors of voice fonts. We present a new technique to represent the spectral envelope in voice fonts which makes it possible to create a universal voice font for a given speaker which can be used for multiple languages. It enables voice fonts to be used for translingual personality transformation of speech. We evaluate the performance of the proposed approach through monolingual and translingual personality transformation experiments. We also compare the performance with the phoneme class based spectral envelope representation in voice fonts through various objective and subjective tests.
39824506
An analysis of borrowing policies for escrow transactions in a replicated data environment
Three issues in the context of escrow-based methods, are addressed. It is shown that full serializability can be guaranteed for escrow transactions by means of repeat semantics, that is, a transaction which fails is rerun in a special mode. The author describes a crash recovery protocol consistent with the concurrency control scheme used. An analysis is made, by means of a simulation model, of several different borrowing policies for escrow transactions and the best one is identified.<<ETX>>
38260906
An Efficient SuperGrid Protocol for High Availability and Load Balancing
Recently there has been a resurgence of interest in quorum-based methods for maintaining consistency of a group of replicated servers as a way to achieve load balancing in addition to high availability. We describe a SuperGrid protocol that performs well for small number of items (less than 100) and evaluate its performance analytically and through simulation experiments. Although it has been shown that it is possible to construct quorum systems where the load is O(1//spl radic/N) and the availability is exp(-/spl Omega/(/spl radic/N)), these asymptotic results do not perform very well when the number of items is relatively small. In such cases, our results show that the Supergrid protocol outperforms the recent Crumbling Walls protocol and the Paths system. It is also simple and easy to implement.