97 research outputs found
SSthreshless Start: A Sender-Side TCP Intelligence for Long Fat Network
Measurement shows that 85% of TCP flows in the internet are short-lived flows
that stay most of their operation in the TCP startup phase. However, many
previous studies indicate that the traditional TCP Slow Start algorithm does
not perform well, especially in long fat networks. Two obvious problems are
known to impact the Slow Start performance, which are the blind initial setting
of the Slow Start threshold and the aggressive increase of the probing rate
during the startup phase regardless of the buffer sizes along the path. Current
efforts focusing on tuning the Slow Start threshold and/or probing rate during
the startup phase have not been considered very effective, which has prompted
an investigation with a different approach. In this paper, we present a novel
TCP startup method, called threshold-less slow start or SSthreshless Start,
which does not need the Slow Start threshold to operate. Instead, SSthreshless
Start uses the backlog status at bottleneck buffer to adaptively adjust probing
rate which allows better seizing of the available bandwidth. Comparing to the
traditional and other major modified startup methods, our simulation results
show that SSthreshless Start achieves significant performance improvement
during the startup phase. Moreover, SSthreshless Start scales well with a wide
range of buffer size, propagation delay and network bandwidth. Besides, it
shows excellent friendliness when operating simultaneously with the currently
popular TCP NewReno connections.Comment: 25 pages, 10 figures, 7 table
Characterization and Analysis of Integrated Silicon Photonic Detectors for High-Speed Communications
As the digital age of rapidly expanding information systems and technology continue to grow and develop at an ever increasing rate, new fabrication media must be investigated in order to keep up with these trends. The modern age has been defined by the innovation and advancement of the semiconductor transistor specifically Silicon, however these days of exponential performance gain through gate minimization are coming to a close. One such field which shows great promise for meeting the challenges of the future is the integration of photonic and complementary metal oxide semiconductor components; leveraging the long standing fabrication history of Silicon devices. This document describes the characterization and analysis of integrated photodiodes for digital and analog applications. The photodiode is one small but necessary component for the integration of system-level photonic devices. A number of standard measurements were taken on the photodiodes to analyze their performance and potential application. Additionally, an anomalous detector behavior was investigated through both transient measurements to identify the driving mechanism of the abnormality. Through this testing the devices were found to perform with up to 30-GHz of bandwidth while maintaining dark currents below 5 nA. The non-linear behavior was observed under CW conditions and analyzed using the transient response of the photodiode. The transient response of the photodiode supported that the non-linear mechanism was photon-induced avalanche-like effect, however, further investigation is required. Additional work is described to further investigate this behavior, as well to identify potential effects on future application in system level communication designs
Stable and scalable congestion control for high-speed heterogeneous networks
For any congestion control mechanisms, the most fundamental design objectives
are stability and scalability. However, achieving both properties are very challenging
in such a heterogeneous environment as the Internet. From the end-users' perspective,
heterogeneity is due to the fact that different flows have different routing paths and
therefore different communication delays, which can significantly affect stability of the
entire system. In this work, we successfully address this problem by first proving a
sufficient and necessary condition for a system to be stable under arbitrary delay. Utilizing this result, we design a series of practical congestion control protocols (MKC
and JetMax) that achieve stability regardless of delay as well as many additional
appealing properties. From the routers' perspective, the system is heterogeneous because the incoming traffic is a mixture of short- and long-lived, TCP and non-TCP
flows. This imposes a severe challenge on traditional buffer sizing mechanisms, which
are derived using the simplistic model of a single or multiple synchronized long-lived
TCP flows. To overcome this problem, we take a control-theoretic approach and
design a new intelligent buffer sizing scheme called Adaptive Buffer Sizing (ABS),
which based on the current incoming traffic, dynamically sets the optimal buffer size
under the target performance constraints. Our extensive simulation results demonstrate that ABS exhibits quick responses to changes of traffic load, scalability to a
large number of incoming flows, and robustness to generic Internet traffic
Actas da 10ÂŞ ConferĂŞncia sobre Redes de Computadores
Universidade do MinhoCCTCCentro AlgoritmiCisco SystemsIEEE Portugal Sectio
Nonlinear continuous feedback controllers
Packet-switched communication networks such as today's Internet are built with several interconnected core and distribution packet forwarding routers and several sender and sink transport agents. In order to maintain stability and avoid congestion collapse in the network, the sources control their rate behavior and voluntarily adjust their sending rates to accommodate other sources in the network. In this thesis, we study one class of sender rate control that is modeled using continuous first-order differential equation of the sending rates. In order to adjust the rates appropriately, the network sends continuous packet-loss feedback to the sources. We study a form of closed-loop feedback congestion controllers whose rate adjustments exhibit a nonlinear form.
There are three dimensions to our work in this thesis. First, we study the network optimization problem in which sources choose utilities to maximize their underlying throughput. Each sender maximizes its utility proportional to the throughput achieved. In our model, sources choose a utility function to define their level of satisfaction of the underlying resource usages. The objective of this direction is to establish the properties of source utility functions using inequality constrained bounded sets and study the functional forms of utilities against a chosen rate differential equation.
Second, stability of the network and tolerance to perturbation are two essential factors that keep communication networks operational around the equilibrium point. Our objective in this part of the thesis is to analytically understand the existence of local asymptotic stability of delayed-feedback systems under homogeneous network delays.
Third, we propose a novel tangential controller for a generic maximization function and study its properties using nonlinear optimization techniques. We develop the necessary theoretical background and the properties of our controller to prove that it is a better rate adaptation algorithm for logarithmic utilities compared to the well-studied proportional controllers. We establish the asymptotic local stability of our controller with upper bounds on the increase / decrease gain parameters
Traffic Re-engineering: Extending Resource Pooling Through the Application of Re-feedback
Parallelism pervades the Internet, yet efficiently pooling this increasing path diversity has remained elusive. With no holistic solution for resource pooling, each layer of the Internet architecture attempts to balance traffic according to its own needs, potentially at the expense of others. From the edges, traffic is implicitly pooled over multiple paths by retrieving content from different sources. Within the network, traffic is explicitly balanced across multiple links through the use of traffic engineering. This work explores how the current architecture can be realigned to facilitate resource pooling at both network and transport layers, where tension between stakeholders is strongest. The central theme of this thesis is that traffic engineering can be performed more efficiently, flexibly and robustly through the use of re-feedback. A cross-layer architecture is proposed for sharing the responsibility for resource pooling across both hosts and network. Building on this framework, two novel forms of traffic management are evaluated. Efficient pooling of traffic across paths is achieved through the development of an in-network congestion balancer, which can function in the absence of multipath transport. Network and transport mechanisms are then designed and implemented to facilitate path fail-over, greatly improving resilience without requiring receiver side cooperation. These contributions are framed by a longitudinal measurement study which provides evidence for many of the design choices taken. A methodology for scalably recovering flow metrics from passive traces is developed which in turn is systematically applied to over five years of interdomain traffic data. The resulting findings challenge traditional assumptions on the preponderance of congestion control on resource sharing, with over half of all traffic being constrained by limits other than network capacity. All of the above represent concerted attempts to rethink and reassert traffic engineering in an Internet where competing solutions for resource pooling proliferate. By delegating responsibilities currently overloading the routing architecture towards hosts and re-engineering traffic management around the core strengths of the network, the proposed architectural changes allow the tussle surrounding resource pooling to be drawn out without compromising the scalability and evolvability of the Internet
Marginal productivity index policies for dynamic priority allocation in restless bandit models
Esta tesis estudia tres complejos problemas dinámicos y estocásticos de asignaciĂłn de recursos: (i) Enrutamiento y control de admisiĂłn con informaciĂłn retrasada, (ii) PromociĂłn dinámica de productos y el Problema de la mochila para artĂculos perecederos, y (iii) Control de congestiĂłn en “routers” con informaciĂłn del recorrido futuro. Debido a que la soluciĂłn Ăłptima de estos problemas no es asequible computacionalmente a gran y mediana escala, nos concentramos en cambio en diseñar polĂticas heurĂsticas de prioridad que sean computacionalmente tratables y cuyo rendimiento sea cuasi-Ăłptimo. Modelizamos los problemas arriba mencionados como problemas de “multi-armed restless bandit” en el marco de procesos de decisiĂłn Markovianos con estructura especial. Empleamos y enriquecemos resultados existentes en la literatura, que constituyen un principio unificador para el diseño de polĂticas de Ăndices de prioridad basadas en la relajaciĂłn Lagrangiana y la descomposiciĂłn de dichos problemas. Esta descomposiciĂłn permite considerar subproblemas de optimizaciĂłn paramĂ©trica, y en ciertos casos “indexables”, resolverlos de manera Ăłptima mediante el Ăndice de productividad marginal (MP). El Ăndice MP es usado como medida de prioridad dinámica para definir reglas heurĂsticas de prioridad para los problemas originales intratables. Para cada uno de los problemas bajo consideraciĂłn realizamos tal descomposiciĂłn, identificamos las condiciones de indexabilidad, y obtenemos fĂłrmulas para los Ăndices MP o algoritmos computacionalmente tratables para su cálculo. Los Ăndices MP correspondientes a cada uno de estos tres problemas pueden ser interpretados en tĂ©rminos de prioridades como el nivel de: (i) la penalizaciĂłn de dirigir un trabajo a una cola particular, (ii) la necesidad de promocionar un cierto artĂculo perecedero, y (iii) la utilidad de una transmisiĂłn de flujo particular. Además de la contribuciĂłn práctica de la obtenciĂłn de reglas heurĂsticas de prioridad para los tres problemas analizados, las principales contribuciones teĂłricas son las siguientes: (i) un algoritmo lineal en el tiempo para el cĂłmputo de los Ăndices MP en el problema de control de admisiĂłn con informaciĂłn retrasada, igualando, por lo tanto, la complejidad del mejor algoritmo existente para el caso sin retrasos, (ii) un nuevo tipo de polĂtica de Ăndice de prioridad basada en la resoluciĂłn de un problema (determinista) de la mochila, y (iii) una nueva extensiĂłn del modelo existente de “multi-armed restless bandit” a travĂ©s de la incorporaciĂłn de las llegadas aleatorias de los “restless bandits”.This dissertation addresses three complex stochastic and dynamic resource allocation problems: (i) Admission Control and Routing with Delayed Information, (ii) Dynamic Product Promotion and Knapsack Problem for Perishable Items, and (iii) Congestion Control in Routers with Future-Path Information. Since these problems are intractable for finding an optimal solution at middle and large scale, we instead focus on designing tractable and well-performing heuristic priority rules. We model the above problems as the multi-armed restless bandit problems in the framework of Markov decision processes with special structure. We employ and enrich existing results in the literature, which identified a unifying principle to design dynamic priority index policies based on the Lagrangian relaxation and decomposition of such problems. This decomposition allows one to consider parametric-optimization subproblems and, in certain “indexable” cases, to solve them optimally via the marginal productivity (MP) index. The MP index is then used as a dynamic priority measure to define heuristic priority rules for the original intractable problems. For each of the problems considered we perform such a decomposition, identify indexability conditions, and obtain formulae for the MP indices or tractable algorithms for their computation. The MP indices admit the following priority interpretations in the three respective problems: (i) undesirability for routing a job to a particular queue, (ii) promotion necessity of a particular perishable product, and (iii) usefulness of a particular flow transmission. Apart from the practical contribution of deriving the heuristic priority rules for the three intractable problems considered, our main theoretical contributions are the following: (i) a linear-time algorithm for computing MP indices in the admission control problem with delayed information, matching thus the complexity of the best existing algorithm under no delays, (ii) a new type of priority index policy based on solving a (deterministic) knapsack problem, and (iii) a new extension of the existing multi-armed restless bandit model by incorporating random arrivals of restless bandits
Design of a low-voltage low-power dc-dc HF converter
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2008.Includes bibliographical references (p. 230-234).Many portable electronic applications could benefit from a power converter able to achieve high efficiency across wide input and output voltage ranges at a small size. However, it is difficult for many conventional power converter designs to provide wide operation range while maintaining high efficiency, especially if both up-and-down voltage conversion is to be achieved. Furthermore, the bulk energy storage required at contemporary switching frequencies of a few megahertz and below limits the degree of miniaturization that can be achieved and hampers fast transient response. Therefore, design methods that reduce energy storage requirements and expand efficient operation range are desirable. This thesis focuses on the development of a High Frequency (HF) dc-dc SEPIC converter exploiting resonant switching and gating with fixed frequency control techniques to achieve these goals. The proposed approach provides high efficiency over very wide input and output voltage ranges and power levels. It also provides up-and-down conversion, and requires little energy storage which allows for excellent transient response. The proposed design strategies are discussed in the context of a prototype converter operating over wide input voltage (3.6 - 7.2V), output voltage (3 - 9V) and power (0.3 - 3W) ranges. The 20MHz converter prototype, utilizing commercial vertical MOSFETs, takes advantage of a quasi-resonant SEPIC topology and resonant gating technique to provide good efficiency across the wide operating ranges required. The converter efficiency stays above 80% across the entire input voltage range at the nominal output voltage. The closed-loop performance is demonstrated via an implementation of a PWM on-off control scheme, illustrating the salient characteristics in terms of additional control circuitry power dissipation and transient response.by Jingying Hu.S.M
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