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Design of Feedback Controls Supporting TCP based on Modern Control Theory

This work investigates how to design feedback controls supporting TCP (Transmission Control Protocol) based on modern control theories. The present paper studies a simplified version of this problem: the linearized version of the well-known AIMD (Additive Increase Multiplicative Decrease) dynamic model under the assumption that the network information is known. Since we formulate the feedback control design problem as state-space models without assuming its structure in advance, we get three interesting and important results that have not been observed in the previous studies on the congestion control problem: 1. In order to fully support TCP, we need a PD-type state-feedback control structure in terms of queue length (or RTT: Round Trip Time) which backs up the conjecture in the networking literature that the AQM RED is not enough to control TCP dynamic behavior, where RED can be classified as a P-type AQM (or as an output feedback control for the AIMD model); 2. In order to fully support TCP in the presence of delays, we derive delay-dependent feedback control structures to compensate for delays explicitly from the knowledge of RTT, capacity and number of sources, where all existing AQMs including RED, REM/PI and AVQ are delay-independent controls; 3. In order to analyze different AQM structures in a unified manner, rather than comparing them via simulations, we propose a PID-type mathematical framework using integral control action. As a performance index to measure the deviation of the closed-loop system from the equilibrium, we use a LQ (linear quadratic) cost of the transients on state and control variables such as queue length, aggregate rate, jitter in the aggregate rate and congestion measure. Stabilizing gains of the proposed feedback control structures are obtained minimizing the LQ cost. Then, the impact of the control structure on performance is discussed from the study of the stabilizing gain design for the proposed mathematical framework. All results are extended to the case of multiple links and heterogeneous delays.
Type of Seminar:
Public Seminar
Dr. Ki-Baek KIM
INRIA-ENS-DI Ecole Normale Supérieure Département Informatique 45 rue d'Ulm, 75230 PARIS CEDEX 05, FRANCE
Apr 28, 2004   17:15

ETH Zentrum, Gloriastrasse 35, 8006 Zurich, Building ETZ Room E6
Contact Person:

Prof. M.Morari
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Biographical Sketch:
Ki Baek Kim is working at INRIA-ENS, France since 2003. He received the B.S., M.S and Ph. D. degrees in Electrical Engineering (control) from Seoul National University in 1993, 1995, and 1999, respectively. After getting Ph.D., he worked at Control & Dynamical System Dep. of California Institute of Technology, Korea Univ, and Computer Science Dep. of California Institute of Technology. His research interests include network congestion control and predictive control theory.