ITR/SI(CISE):Optimal and Robust TCP Congestion Control

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Internet is undergoing an overhaul unprecedented in size, diversity, and reach, with profound im-pact in all aspects of our scientific, social, economic and political life through the integration ofnetworks of communication, transportation, entertainment, utilities, and finance. The stabilityand robustness of this vital infrastructure demands a rigorous theory to understand the currentprotocols and evolve them to meet emerging challenges. We propose to develop such a theory forTCP congestion control, and use it to drastically improve the stability, robustness and optimalityof the current protocols.A key insight is to view congestion control as a distributed asynchronous computation to maxi-mize aggregate source utility over the Internet; different TCP and active queue management (AQM)schemes correspond to different utility functions and different algorithms to maximize them.Our research hastwo components. First, we will develop a new theoretical model of TCPcongestion control based on duality in optimization and multivariate robust control. The theory willclarify the role of source algorithms, such as Tahoe, Reno and Vegas, and active queue management,such as DropTail, RED and REM, in the control of networks and establish performance limits ofthe current protocols; it will explain the effect on stability when delay, topology, capacity, and loadscale up; and it will provide conditions under which the feedback stability ofTCP/AQM algorithmsare invariant to these effects. Indeed, such a theory is already emerging from our recent works.Even in its currently preliminary stage, it already provides a fundamental understanding on somewidely observed performance and fairness behavior of the current protocols, and uncovers new andsurprising stability problems. For example, it shows that the current protocols become unstable andexhibit bifurcation when network capacity increases. Moreover, maintaining stability as capacityscales up arbitrarily imposes severe constraints on how sources adjust their rates (TCP) and whatcongestion information is fed back (AQM). The current protocol does not satisfy the conditionfor such stability invariance, and hence may be ill suited for future networks where, pulled byapplication demand and pushed by technological advances, the capacity will be large.The second component of our research is the design of practical TCP and AQM protocolsbased on the theory, and the development of prototypes and experiments to demonstrate theireffectiveness. We will use the theory to identify the sources of instability in the current protocolswhen delay, network size, capacity, and traffic load scale up. We will design both enhancements thatincrementally evolve the current protocols, and drastically new protocols that have the stronglyrobust stability property promised by theory. As a concrete application of our algorithms, we willapply them to improve TCP performance over wireless links, both because they are ubiquitous andbecause they are likely to remain the most important bottlenecks in future networks.