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cs6235-acc2001b-6b

cs6235-acc2001b-6b - Modeling the Effect of Short-term Rate...

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Modeling the Effect of Short-term Rate Variations on TCP-Friendly Congestion Control Behavior Kang Li , Molly H. Shor Ψ , Jonathan Walpole , Calton Pu , David C. Steere Dept. of Computer Science and Engineering, Oregon Graduate Institute, Beaverton, Oregon 97921-1000 USA. This work was supported in part by DARPA/ITO under the Information Technology Expeditions, Ubiquitous Computing, Quorum, and PCES programs, and in part by NSF Grant CCR-9988440. Ψ Dept. of Electrical and Computer Engineering, Oregon State University, Corvallis, Oregon 97331-3211 USA. Research was supported in part by NSF Grant ECS-9988435 College of Computing, CCB Room 261, Georgia Institute of Technology, Atlanta, Georgia 30332-0280 USA Abstract: Transmission Control Protocol (TCP) is the dominant transport protocol in today’s Internet. To maintain stability of the Internet, flows other than TCP must be “friendly” to TCP flows, or share network bandwidth fairly with TCP traffic. Usually a flow is claimed to be TCP-friendly when its throughput is theoretically the same as the throughput of a TCP flow when they experience the same congestion signals. However, when flows compete for bandwidth, they may not have the same perception of congestion. Therefore, measured bandwidth shares of flows are not necessarily equal, even when all flows are theoretically designed to be TCP-friendly. To study the effect on bandwidth sharing of interactions among a set of competing TCP-friendly flows, we built a hybrid state-space-based model of TCP using differential equations and event-driven switches. We modified the TCP model, using TCP’s additive-increase multiplicative- decrease (AIMD) congestion avoidance algorithm with different increase and decrease parameters, to create theoretically TCP-friendly protocols with various short- term transmission rates. We prove that TCP-friendly flows result in a stable attractor if the backing off of flow transmission rates is synchronized. Experiments using our model and using ns simulator with unsynchronized backing off show unfairness among competing flows with different short-term behaviors. 1. Introduction One of the most common “protocols” used to control the transmission of data in a reliable fashion on the Internet is the Transmission Control Protocol (TCP) [1]. This protocol adjusts how much data may be sent for a particular connection between end applications over each interval of time. TCP is designed to "back off" the rate of transmission when network congestion occurs and to increase the rate over time when there is no congestion. The increase is TCP's mechanism to probe the network to determine if excess capacity is available. TCP congestion control can be viewed as a nonlinear feedback control system that dynamically adjusts its transmission rate according to the network’s congestion state. A significant amount of research work has been done on this system. For example, Jain and Chiu [2] proved that multiple TCPs converge to fair bandwidth share by analyzing TCP's additive-increase multiplicative- decrease (AIMD) algorithm. Padhye, et al., [3,
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