Multiple time scale redundancy control for QoS-sensitive transport of real-time traffic
End-to-end QoS control over best-effort and differentiated service networks which exhibit variability in their exported service properties looms as an important challenge. In previous work, we have shown how packet-level adaptive FEC can be used in dynamic networks to facilitate invariant user-specified QoS in an end-to-end manner. This paper addresses two important problems-self-similar burstiness and performance degradation of reactive controls subject to long feedback loops-complementing the stability/optimality considerations studied earlier. First, for adaptive redundancy control to be effective, its susceptibility to correlated packet drops and queueing delays stemming from self-similar burstiness must be fortified. Second, to preserve FEC's viability over ARQ when transporting real-time traffic in WAN, preactivity must be injected to offset the performance degradation of reactive feedback controls when subject to long RTT. In this paper, we use the recently advanced multiple-time-scale congestion control framework-first investigated in the throughput maximization context-to endow adaptive redundancy control with both selective protection against self-similar burstiness as well as preactivity to feedback redundancy control. We analyze, implement, and benchmark our protocol-AFEC-MT-in the context of transporting periodic real-time traffic, in particular, MPEG video.

title: QoS-sensitive transport of real-time MPEG video using adaptive redundancy control
abstract: This paper presents an adaptive end-to-end protocol for quality of service (QoS)-sensitive transport of real-time MPEG video using packet-level forward error correction in dynamic networks. The objective is to facilitate a user-specified QoS end-to-end - i.e. without special network support - for real-time MPEG video whose timing constraints rule out the use of retransmission-based congestion control and QoS provisioning schemes. The degree of redundancy - overcode in forward error correction - injected into the network is adjusted as a function of network state, decreasing when the network is well-behaved to minimize unnecessary network resource consumption, and increasing when it is not to compensate for adverse network effects so as to maintain an invariant level of end-to-end QoS. We describe an adaptive packet-level forward error correction protocol called AFEC and analyze its properties with respect to optimality and stability. The optimal control problem is nontrivial due to the fact that increased redundancy, beyond a certain point, can backfire resulting in self-induced congestion which impedes the timely recovery of information - MPEG video frames - at the receiver. We experimentally evaluate the efficacy of our end-to-end QoS control by implementing and customizing AFEC to the transport of real-time MPEG video. We realize the system as a transport/application layer protocol running on UNIX workstations, and measure its performance over controlled network environments. We show that AFEC is able to effectively hide potentially adverse network effects such as packet drops and delays stemming from traffic burstiness and nonstationary structural changes using adaptive redundancy control, exporting a constant QoS service commensurate with user-specified QoS.

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: 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: Efficient Shaping of User-Specified QoS Using Aggregate-Flow Control
abstract: 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: Performance evaluation of multiple time scale TCP under self-similar traffic conditions
abstract: Measurements of network traffic have shown that self-similarity is a ubiquitous phenomenon spanning across diverse network environments. In previous work, we have explored the feasibility of exploiting long-range correlation structure in self-similar traffic for congestion control. We have advanced the framework of multiple time scale congestion control and shown its effectiveness at enhancing performance for rate-based feedback control. In this article, we extend the multiple time scale control framework to window-based congestion control, in particular, TCP. This is performed by interfacing TCP with a large time scale module that adjusts the aggressiveness of bandwidth consumpton behavior exhibited by TCP as a function of large time scale network state, that is, information that exceeds the time horizon of the feedback loop as determined by RTT. How to effectively utilize such information—due to its probabilistic nature, dispersion over multiple time scales, and realization on top of existing window-based congestion controls—is a nontrivial problem. First, we define a modular extension of TCP (a function call with a simple interface that applies to various flavors of TCP, e.g., Tahoe, Reno, and Vegas) and show that it significantly improves performance. Second, we show that multiple time scale TCP endows the underlying feedback control with proacativity by bridging the uncertainty gap associated with reactive controls which is exacerbated by the high delay-bandwidth product in broadband wide area networks. Third, we investigate the influence of three traffic control dimensions—tracking ability, connection duration, and fairness—on performance. Performance evaluation of multiple time scale TCP is facilitated by a simulation benchmark environment based on physical modeling of self-similar traffic. We explicate our methodology for disc

title: On the relationship between file sizes, transport protocols, and self-similar network traffic
abstract: Measurements of LAN and WAN traffic show that network traffic exhibits variability on different scales. We examine a mechanism that gives rise to self-similar network traffic and discuss performance. The mechanism we study is the transfer of files or messages whose size is drawn from a heavy-tailed distribution. In a realistic client/server network the degree to which file sizes are heavy-tailed can directly determine the degree of traffic self-similarity at the link level. This causal relationship is robust relative to changes in network resources, network topology, the influence of cross-traffic, and the distribution of interarrival times. Properties of the transport layer play an important role in preserving and modulating this relationship. The reliable transmission and flow control mechanisms of TCP serve to maintain the long-range dependency structure induced by heavy-tailed file size distributions. In contrast, if a non-flow-controlled and unreliable (UDP-based) transport protocol is used, the resulting traffic shows little self-similarity: although still bursty at short time scales, it has little long-range dependence. Performance implications of self-similarity are discussed as represented by various performance measures. Increased self-similarity as expected, results in degradation of performance. Queueing delay, in particular is discussed. Throughput-related measures such as packet loss and retransmission rate, however increase only gradually with increasing traffic self-similarity as long as reliable, flow-controlled transport protocol is used.

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.

title: A BEM Formulation for Transient Axisymmetric Thermoelastic Analysis Using Particular Integrals
abstract: A particular integral formulation is presented for the first time in a purely axisymmetric transient uncoupled thermoelastic analysis. The governing axisymmetric elastostatic equation is used as the complementary solution and the particular integrals for displacements and tractions are derived by integrating the three-dimensional formulation along the circumferential direction leading to elliptic integrals. The numerical results for three example problems are given and compared with their analytical solutions. Generally, the agreement among all of those results is satisfactory, if a few interior points are used to include the known values of temperature.

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: On the effectiveness of route-based packet filtering for distributed DoS attack prevention in power-law internets
abstract: Denial of service (DoS) attack on the Internet has become a pressing problem. In this paper, we describe and evaluate route-based distributed packet filtering (DPF), a novel approach to distributed DoS (DDoS) attack prevention. We show that DPF achieves proactiveness and scalability, and we show that there is an intimate relationship between the effectiveness of DPF at mitigating DDoS attack and power-law network topology.The salient features of this work are two-fold. First, we show that DPF is able to proactively filter out a significant fraction of spoofed packet flows and prevent attack packets from reaching their targets in the first place. The IP flows that cannot be proactively curtailed are extremely sparse so that their origin can be localized---i.e., IP traceback---to within a small, constant number of candidate sites. We show that the two proactive and reactive performance effects can be achieved by implementing route-based filtering on less than 20% of Internet autonomous system (AS) sites. Second, we show that the two complementary performance measures are dependent on the properties of the underlying AS graph. In particular, we show that the power-law structure of Internet AS topology leads to connectivity properties which are crucial in facilitating the observed performance effects.