Anwar Walid

Multipath TCP: analysis, design, and implementation

Multipath TCP (MP-TCP) has the potential to greatly improve application performance by using multiple paths transparently. We propose a fluid model for a large class of MP-TCP algorithms and identify design criteria that guarantee the existence, uniqueness, and stability of system equilibrium. We clarify how algorithm parameters impact TCP-friendliness, responsiveness, and window oscillation and demonstrate an inevitable tradeoff among these properties. We discuss the implications of these properties on the behavior of existing algorithms and motivate our algorithm Balia (balanced linked adaptation), which generalizes existing algorithms and strikes a good balance among TCP-friendliness, responsiveness, and window oscillation. We have implemented Balia in the Linux kernel. We use our prototype to compare the new algorithm to existing MP-TCP algorithms.

Shadow-routing based dynamic algorithms for virtual machine placement in a network cloud

We consider a shadow routing based approach to the problem of real-time adaptive placement of virtual machines (VM) in large data centers (DC) within a network cloud. Such placement in particular has to respect vector packing constraints on the allocation of VMs to host physical machines (PM) within a DC, because each PM can potentially serve multiple VMs simultaneously. Shadow routing is attractive in that it allows a large variety of system objectives and/or constraints to be treated within a common framework (as long as the underlying optimization problem is convex). Perhaps even more attractive feature is that the corresponding algorithm is very simple to implement, it runs continuously, and adapts automatically to changes in the VM demand rates, changes in system parameters, etc., without the need to re-solve the underlying optimization problem “from scratch”. In this paper we focus on the minmax-DC-load problem. Namely, we propose a combined VM-toDC routing and VM-to-PM assignment algorithm, referred to as Shadow scheme, which minimizes the maximum of appropriately defined DC utilizations. We prove that the Shadow scheme is asymptotically optimal (as one of its parameters goes to 0). Simulation confirms good performance and high adaptivity of the algorithm. Favorable performance is also demonstrated in comparison with a baseline algorithm based on VMware implementation [7], [8]. We also propose a simplified - “more distributed” - version of the Shadow scheme, which performs almost as well in simulations.

Multipath TCP algorithms: theory and design

Multi-path TCP (MP-TCP) has the potential to greatly improve application performance by using multiple paths transparently. We propose a fluid model for a large class of MP-TCP algorithms and identify design criteria that guarantee the existence, uniqueness, and stability of system equilibrium. We characterize algorithm parameters for TCP-friendliness and prove an inevitable tradeoff between responsiveness and friendliness. We discuss the implications of these properties on the behavior of existing algorithms and motivate a new design that generalizes existing algorithms. We use ns2 simulations to evaluate the proposed algorithm and illustrate its superior overall performance.

Energy efficient multipath TCP for mobile devices

Most mobile devices today come with multiple access interfaces, \emph{e.g.}, 4G and WiFi. Multipath TCP (MP-TCP) can greatly improve network performance by exploiting the connection diversity of multiple access interfaces, at the expense of higher energy consumption. In this paper, we design MP-TCP algorithms for mobile devices by jointly considering the performance and energy consumption. We consider two main types of mobile applications: realtime applications that have a fixed duration and file transfer applications that have a fixed data size. For each type of applications, we propose a two-timescale algorithm with theoretical guarantee on the performance. We present simulation results that show that our algorithms can reduce energy consumption by up to 22

Outage-Storage Tradeoff in Frequency Regulation for Smart Grid With Renewables

\%

Scalable Routing in SDN-enabled Networks with Consolidated Middleboxes

without sacrificing throughput compared to a baseline MP-TCP algorithm.

Switch sizing for energy-efficient datacenter networks

Future power grid systems are envisioned to be integrated with many distributed renewable energy sources (DRES). Energy storage is a key technology to enable reliable and cost-effective renewable energy. Given the fact that a large-scale energy storage device is usually costly to install and operate, we are naturally led to the following question: How much storage is necessary to guarantee the stability of a power grid network with DRESs? This paper represents a first step in systematically exploring the tradeoff between the capacity of energy storage devices and the outage probability, i.e., the probability of the occurrence of imbalance between the supply and demand. We first propose a secure scheduling and dispatch (SSD) algorithm that is capable of maintaining the grid stability in the presence of volatility in the power generation. We then derive a closed-form bound to quantify the tradeoff between the storage capacity and the outage probability. Under mild assumptions, we show that the outage probability decreases exponentially with respect to the square of the storage capacity. This finding implies that energy storage is an effective and economically viable solution to maintain the stability of a smart grid network, even in the presence of many volatile and intermittent renewable energy sources. The impact of correlation in energy generation on the stability of a smart grid network is also investigated .

Energy-efficient scheduling in multi-core servers

Middleboxes are special network devices that perform various functions such as enabling security and efficiency. SDN-based routing approaches in networks with middleboxes need to address resource constraints, such as memory in the switches and processing power of middleboxes, and traversal constraint where a flow must visit the required middleboxes in a specific order. In this work we propose a solution based on MultiPoint-To-Point Trees (MPTPT) for routing traffic in SDN-enabled networks with consolidated middleboxes. We show both theoretically and via simulations that our solution significantly reduces the number of routing rules in the switches, while guaranteeing optimum throughput and meeting processing requirements. Additionally, the underlying algorithm has low complexity making it suitable in dynamic network environment.

Small versus large: Switch sizing in topology design of energy-efficient data centers

Saving power in datacenter networks has become a pressing issue. ElasticTree and CARPO fat-tree networks have recently been proposed to reduce power consumption by using sleep mode during the operation stage of the network. In this paper, we address the design stage where the right switch size is evaluated to maximize power saving during the expected operation of the network. Our findings reveal that deploying a large number of small switches is more power-efficient than a small number of large switches when the traffic demand is relatively moderate or when servers exchanging traffic are in close proximity. We also discuss the impact of sleep mode on performance such as packet delay and loss.

Energy-efficient congestion control

In this paper we develop techniques for analyzing and optimizing energy management in multi-core servers with speed scaling capabilities. Our framework incorporates the processors dynamic power, but it also accounts for other intricate and relevant power features such as the static (leakage) power and switching overhead between speed levels. Using stochastic fluid models to capture traffic burst dynamics, we propose and study different strategies for adapting the multi-core processor speeds based on the observable buffer content, so as to optimize objective functions that balance energy consumption and performance. The strategies can be non-hysteretic (i.e., the processor speed depends on current buffer level relative to the buffer thresholds) or hysteretic (i.e., it matters in which direction the buffer thresholds are crossed). It is shown that, under rather general conditions, strategies which use more threshold levels are more efficient with respect to power consumption; however, most of the efficiency gain is achieved with 1 or 2 thresholds only. In addition, the optimal power consumptions of the different strategies are only very mildly sensitive to perturbations in the input parameters, implying the highly advantageous property that the performance is robust to estimation errors in the systems input traffic parameters.