Efficient Shaping of User-Specified QoS Using Aggregate-Flow Control
Architecting networks capable of providing scalable, efficient, and fair services to users with diverse QoS requirements is a pressing problem. The two principal issues are: design of "good" per-hop behavior and edge control. In previous work [2,3], we studied aggregate-flow QoS control from a noncooperative resource provisioning context. In [20], the framework was generalized by, one, solving an optimal aggregate-flow per-hop behavior problem, and two, showing how it can be used coupled with end-to-end label control to facilitate scalable and fair QoS when driven by selfish users and service providers. In this paper, we focus on optimal aggregate-flow per-hop control and complement analysis by experimental performance evaluation. We show that user-specified, diverse QoS is efficiently facilitated over the optimal per-hop behavior network substrate using adaptive label control end-to-end.

title: On the performance characteristics of WLANs: revisited
abstract: Wide-spread deployment of infrastructure WLANs has made Wi-Fi an integral part of today's Internet access technology. Despite its crucial role in affecting end-to-end performance, past research has focused on MAC protocol enhancement, analysis and simulation-based performance evaluation without sufficient consideration for modeling inaccuracies stemming from inter-layer dependencies, including physical layer diversity, that significantly impact performance. We take a fresh look at IEEE 802.11 WLANs, and using a combination of experiment, simulation, and analysis demonstrate its surprisingly agile performance traits. Our main findings are two-fold. First, contention-based MAC throughput degrades gracefully under congested conditions, enabled by physical layer channel diversity that reduces the effective level of MAC contention. In contrast, fairness and jitter significantly degrade at a critical offered load. This duality obviates the need for link layer flow control for throughput improvement but necessitates traffic control for fairness and QoS. Second, TCP-over-WLAN achieves high throughput commensurate with that of wireline TCP under saturated conditions, challenging the widely held perception that TCP throughput fares poorly over WLANs when subject to heavy contention. We show that TCP-over-WLAN prowess is facilitated by the self-regulating actions of DCF and TCP congestion control that jointly drive the shared physical channel at an effective load of 2--3 wireless stations, even when the number of active stations is very large. Our results highlight subtle inter-layer dependencies including the mitigating influence of TCP-over-WLAN on dynamic rate shifting.

title: A distributed protocol for multi-class QoS provision in noncooperative many-switch systems
abstract: This paper presents an architecture for multi-class quality of service (QoS) provision in wide area networks. Users or applications are assumed to be selfish and end-to-end QoS is determined by the service levels received by an application traffic flow at each router or switch along a path. In previous work, we have given a comprehensive analysis of the noncooperative multi-class QoS provision game for single-switch systems showing when Nash equilibria exist and under what conditions they are Pareto and/or system optimal. In this paper, we propose a specific network architecture for facilitating noncooperative and of provision in many-switch systems such as the Internet with emphasis on realizability. We shield the user from having to choose the service classes on the switches along a route-a hard combinatorial optimization problem even assuming perfect knowledge about network state-while preserving the basic premise of selfishness. This is achieved by employing a set of QoS agents installed at routers which act on behalf of an user's traffic flow. The QoS agent intercepts packets entering a switch implementing generalized processor sharing (GPS) packet scheduling-and using only constant space packet header overhead and zero per-connection state at the routers-determines which service class to assign the packet to to satisfy the user's end-to-end QoS requirement at minimum cost. We present simulation results which show that our architecture is able to provide stable, stratified services to application traffic with diverse QoS requirements.

title: Scalability and traffic control in IP networks

title: An architecture for noncooperative QoS provision in many-switch systems
abstract: With the proliferation of high-speed networks and networked services, provisioning differentiated services to a diverse user base with heterogeneous QoS requirements has become an important problem. The traditional approach of resource reservation and admission control provides both guarantees and graded services, however, at the cost of potentially underutilized resources and limited scalability. We describe a WAN QoS provision architecture that adaptively organizes best-effort bandwidth into stratified services with graded QoS properties such that the QoS needs of a diverse user base can be effectively met. This architecture-SBS (stratified best-effort service)-promotes a simple user/simple network realization where neither the user nor the network is burdened with complex computational responsibilities. SBS is scalable, efficient, and adaptive, and it complements the guaranteed service architecture, sharing a common network substrate comprised of GPS routers. It is also a functional complement, provisioning QoS efficiently commensurate with user needs, albeit at the cost of weaker protection. SBS is suited to noncooperative network environments where users behave selfishly and resource contention resolution is mediated by the principle of competitive interaction. A principal feature of SBS is the transformation of user-centric QoS provision mechanisms-a defining characteristic of competitive interaction entailing intimate user control of internal network resources-into network-centric mechanisms while preserving the former's resource allocation paradigm. End-to-end QoS control is facilitated by decentralized control based on Lagrangian optimization-achieve a target end-to-end QoS at minimum cost or resource usage-which, in turn, is amenable to distributed implementation. SBS achieves per-flow QoS control with zero per-flow state at routers and a packet header whose size is independent of hop count. SBS, in spite of foregoing both resource reservation and admission control, is able to provision stable, graded QoS.

title: Performance evaluation of optimal aggregate-flow scheduling: a simulation study
abstract: Providing scalable QoS-sensitive services to applications with varying degrees of elasticity using aggregate-flow scheduling is a challenging problem. In a previous paper , we advanced a theoretical framework where an optimal differentiated services provisioning problem is formulated and solved to yield solutions for optimal per-hop control. In this paper, we extend our previous work by investigating performance evaluation and implementation issues associated with the induced optimal differentiated services architecture. We design a system that realizes the optimal per-hop control coupled with end-to-end adaptive QoS control, and implement a practical enhancement by introducing a scaling function which is applied to the TOS field label value in the IP header at each router. The scaling function allows the service provider to configure the per-hop control so as to export customized QoS separation-essential when shaping end-to-end absolute QoS over per-hop relative QoS-commensurate with the QoS profiles of the service provider's user base. We use simulation to study the structural and dynamical properties of differentiated services as affected by optimal aggregate-flow per-hop control.

title: Securing Web Service by Automatic Robot Detection
abstract: Web sites are routinely visited by automated agents known as Web robots, that perform acts ranging from the beneficial, such as indexing for search engines, to the malicious, such as searching for vulnerabilities, attempting to crack passwords, or spamming bulletin boards. Previous work to identify malicious robots has relied on ad-hoc signature matching and has been performed on a per-site basis. As Web robots evolve and diversify, these techniques have not been scaling. 
 
We approach the problem as a special form of the Turing test and defend the system by inferring if the traffic source is human or robot. By extracting the implicit patterns of human Web browsing, we develop simple yet effective algorithms to detect human users. Our experiments with the CoDeeN content distribution network show that 95% of human users are detected within the first 57 requests, and 80% can be identified in only 20 requests, with a maximum false positive rate of 2.4%. In the time that this system has been deployed on CoDeeN, robot-related abuse complaints have dropped by a factor of 10.

title: Stochastic analysis of packet-pair probing for network bandwidth estimation
abstract: In this paper, we perform a stochastic analysis of the packet-pair technique, which is a widely used method for estimating the network bandwidth in an end-to-end manner. There has been no explicit delay model of the packet-pair technique primarily because the stochastic behavior of a packet pair has not been fully understood. Our analysis is based on a novel insight that the transient analysis of the G/D/1 system can accurately describe the behavior of a packet pair, providing an explicit stochastic model. We first investigate a single-hop case and derive an analytical relationship between the input and the output probing gaps of a packet pair. Using this single-hop model, we provide a multi-hop model under an assumption of a single tight link. Our model shows the following two important features of the packet-pair technique: (i) The difference between the proposed model and the previous fluid model becomes significant when the input probing gap is around the characteristic value.  We give a discussion on recent packet-pair models in relation to the proposed model and show that most of them can be regarded as special cases of the proposed model.

title: Robust playout mechanism for Internet audio applications
abstract: In Internet audio applications, delay and delay jitter affect mostly the applications' quality of service. Since packet delays are different and changing over time, the receiver needs to buffer some amount of packets before playout. Therefore, the amount of buffered packets and the timing of playout are very important for the performance of applications. We adopt an autoregressive (AR) model for estimation of packet delay and deploy a robust identification algorithm for adjustment of the parameters of the AR process. In our preliminary experiments, this robust algorithm leads to better performance when the noise is correlated and/or non-stationary, and also it is robust to model uncertainties.

title: Scale and Performance in the CoBlitz Large-File Distribution Service
abstract: Scalable distribution of large files has been the area of much research and commercial interest in the past few years. In this paper, we describe the CoBlitz system, which efficiently distributes large files using a content distribution network (CDN) designed for HTTP. As a result, CoBlitz is able to serve large files without requiring any modifications to standard Web servers and clients, making it an interesting option both for end users as well as infrastructure services. Over the 18 months that CoBlitz and its partner service, CoDeploy, have been running on PlanetLab, we have had the opportunity to observe its algorithms in practice, and to evolve its design. These changes stem not only from observations on its use, but also from a better understanding of their behavior in real-world conditions. This utilitarian approach has led us to better understand the effects of scale, peering policies, replication behavior, and congestion, giving us new insights into how to better improve their performance. With these changes, CoBlitz is able to deliver in excess of 1 Gbps on PlanetLab, and to outperform a range of systems, including research systems as well as the widely-used BitTorrent.

title: Reliability and Security in the CoDeeN Content Distribution Network
abstract: With the advent of large-scale, wide-area networking testbeds, researchers can deploy long-running distributed services that interact with other resources on the Web. The CoDeeN Content Distribution Network, deployed on PlanetLab, uses a network of caching Web proxy servers to intelligently distribute and cache requests from a potentially large client population. We have been running this system nearly continuously since June 2003, allowing open access from any client in the world. In that time, it has become the most heavily-used long-running service on PlanetLab, handling over four million accesses per day. In this paper, we discuss the design of our system, focusing on the reliability and security mechanisms that have kept the service in operation. 
 
Our reliability mechanisms assess node health, preventing failing nodes from disrupting the operation of the overall system. Our security mechanisms protect nodes from being exploited and from being implicated in malicious activities, problems that commonly plague other open proxies. We believe that future services, especially peer-to-peer systems, will require similar mechanisms as more services are deployed on non-dedicated distributed systems, and as their interaction with existing protocols and systems increases. Our experiences with CoDeeN and our data on its availability should serve as an important starting point for designers of future systems.