Seong-Ryong Kang

Delayed stability and performance of distributed congestion control

Recent research efforts to design better Internet transport protocols combined with scalable Active Queue Management (AQM) have led to significant advances in congestion control. One of the hottest topics in this area is the design of discrete congestion control algorithms that are asymptotically stable under heterogeneous feedback delay and whose control equations do not explicitly depend on the RTTs of end-flows. In this paper, we show that max-min fair congestion control methods with a stable symmetric Jacobian remain stable under arbitrary feedback delay (including heterogeneous directional delays) and that the stability condition of such methods does not involve any of the delays. To demonstrate the practicality of the obtained result, we change the original controller in Kellys work [14] to become robust under random feedback delay and fixed constants of the control equation. We call the resulting framework Max-min Kelly Control (MKC) and show that it offers smooth sending rate, exponential convergence to efficiency, and fast convergence to fairness, all of which make it appealing for future high-speed networks.

Packet-pair bandwidth estimation: stochastic analysis of a single congested node

We examine the problem of estimating the capacity of bottleneck links and available bandwidth of end-to-end paths under non-negligible cross-traffic conditions. We present a simple stochastic analysis of the problem in the context of a single congested node and derive several results that allow the construction of asymptotically-accurate bandwidth estimators. We first develop a generic queuing model of an Internet router and solve the estimation problem assuming renewal cross-traffic at the bottleneck link. Noticing that the renewal assumption on Internet flows is too strong, we investigate an alternative filtering solution that asymptotically converges to the desired values of the bottleneck capacity and available bandwidth under arbitrary (including non-stationary) cross-traffic. This is one of the first methods that simultaneously estimates both types of bandwidth and is provably accurate. We finish the paper by discussing the impossibility of a similar estimator for paths with two or more congested routers.

Modeling best-effort and FEC streaming of scalable video in lossy network channels

Video applications that transport delay-sensitive multimedia over best-effort networks usually require special mechanisms that can overcome packet loss without using retransmission. In response to this demand, forward-error correction (FEC) is often used in streaming applications to protect video and audio data in lossy network paths; however, studies in the literature report conflicting results on the benefits of FEC over best-effort streaming. To address this uncertainty, we start with a baseline case that examines the impact of packet loss on scalable (FGS-like) video in best-effort networks and derive a closed-form expression for the loss penalty imposed on embedded coding schemes under several simple loss models. Through this analysis, we find that the utility (i.e., usefulness to the user) of unprotected video converges to zero as streaming rates become high. We then study FEC-protected video streaming, re-derive the same utility metric, and show that for all values of loss rate inclusion of FEC overhead substantially improves the utility of video compared to the best-effort case. We finish the paper by constructing a dynamic controller on the amount of FEC that maximizes the utility of scalable video and show that the resulting system achieves a significantly better PSNR quality than alternative fixed-overhead methods

Delay-independent stability and performance of distributed congestion control

Recent research efforts to design better Internet transport protocols combined with scalable active queue management (AQM) have led to significant advances in congestion control. One of the hottest topics in this area is the design of discrete congestion control algorithms that are asymptotically stable under heterogeneous feedback delay and whose control equations do not explicitly depend on the RTTs of end-flows. In this paper, we first prove that single-link congestion control methods with a stable radial Jacobian remain stable under arbitrary feedback delay (including heterogeneous directional delays) and that the stability condition of such methods does not involve any of the delays. We then extend this result to generic networks with fixed consistent bottleneck assignments and max-min network feedback. To demonstrate the practicality of the obtained result, we change the original controller in Kellys work [ldquoRate Control for communication networks: Shadow prices, proportional fairness and stability,rdquo Journal of the Operational Research Society, vol. 49, no. 3, pp. 237-252, March 1998] to become robust under random feedback delay and fixed constants of the control equation. We call the resulting framework max-min Kelly control (MKC) and show that it offers smooth sending rate, exponential convergence to efficiency, and fast convergence to fairness, all of which make it appealing for future high-speed networks.

Impact of FEC overhead on scalable video streaming

Forward-error correction (FEC) is used in many streaming applications for protecting multimedia data over lossy network paths. However, studies in the literature [1, 3, 4] report conflicting results on the benefits of FEC. To address this uncertainty, we study the performance of FEC-based streaming and provide additional insight into how FEC overhead rate affects the performance of scalable video streaming under dynamically changing network packet loss. Through analytical investigation, we derive the relationship between packet loss, FEC overhead, and utility of received video, and propose a simple control mechanism that adjusts the amount of FEC based on packet loss information. We find that our FEC control allows the application to maintain high end-user utility and achieve better quality of video at the receiver.

IMR-Pathload: robust available bandwidth estimation under end-host interrupt delay

Many paths in PlanetLab cannot be measured by Pathload. One of the main reasons for this is timing irregularities caused by interrupt moderation of network hardware, which delays generation of interrupts for a certain period of time to reduce per-packet CPU overhead. Motivated by this problem, we study Pathload in detail under various end-host interrupt delays and find that its trend detection mechanism becomes susceptible to non-negligible interrupt delays, making it unable to measure network paths under such conditions. To overcome this, we propose a new method called IMR-Pathload (Interrupt Moderation Resilient Pathload), which incorporates robust trend detection algorithms based on signal de-noising techniques and reliably estimates available bandwidth of network paths under a wide range of interrupt delays. Through experiments in Emulab and Internet, we find that IMR-Pathload substantially improves Pathloads measurement reliability and produces accurate bandwidth estimates under a variety of real-life conditions.

On Estimating Tight-Link Bandwidth Characteristics over Multi-Hop Paths

In this paper, we explore multi-hop bandwidth estimation assuming arbitrary cross-traffic at each node and develop a new probing method called Envelope that can asymptotically estimate not only the available bandwidth but also the raw capacity of the tight link. Envelope is based on a multi-link recursive extension of unbiased single-hop estimators proposed in the past (e.g., [14]) and a variation of the packet-cartouche technique [6]. Through extensive simulations, we evaluate Envelope in various network settings and cross-traffic conditions and find that it can measure tight-link bandwidth characteristics with accuracy that significantly surpasses that of the existing methods. We also find that Envelope can measure non-tight links in certain path and cross-traffic configurations.

Characterizing tight-link bandwidth of multi-hop paths using probing response curves

Bandwidth estimation plays an important role in characterizing Internet paths. Existing approaches can be classified into measurement tools [3], [6], [9], [10], [16], [17], [23], [29], which usually have extensive simulations, but no convergence analysis for general cross-traffic, and theoretical models [5], [14], [19], [20], [22], which usually have provable convergence, but no practical implementation. Another issue in related work is the unknown performance of certain proposed algorithms in real networks where delay measurements are not perfect due to various OS and hardware-related timing irregularities [26]. We address the former issue by developing a measurement tool PRC-MT that not only achieves asymptotic accuracy in multi-path networks with arbitrary cross-traffic, but also simultaneously measures the capacity and available bandwidth of the tight link. We address the latter issue by performing a comparison study of existing tools in Emulab and assessing their susceptibility to timing irregularities of end-hosts. Our results show that PRC-MT outperforms all existing tools in terms of accuracy, achieves similar convergence delay, and does not require any manual configuration. We also find that interrupt moderation may cause existing tools (such as Pathload [10], Pathchirp [27], and CapProbe [16]) to become quite inaccurate in certain network configurations and exhibit behavior completely different from that in ns2 [28].

SmartClick: An enabler for human-centered wireless Web service

Recently wireless Web services have been rapidly growing as a new way of information service. As the wireless Internet access grows, so do e-commerce opportunities through the wireless Web services. Wireless Internet access has inherent limitations since most handheld devices have much less display and means of operations. This imposes a particular problem to the design of mobile information services in terms of information and service navigation. In this paper, a novel approach called SmartClick is presented to achieve user-centered wireless Web services. SmartClick enables users to explicitly express their intentions or preferences with minimal clicks (user interaction). SmartClick with a middleware support provides users a way to get the desired Web contents more efficiently and easily.