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In closing, we would like to acknowledge the time and effort contributed by our assistants, Miss Yujing Wu of the University of Massachusetts, Amherst, and Mrs.


Guest Editorial Special Issue of the IEEE TRANSACTIONS ON AUTOMATIC CONTROL on Systems and Control Methods for Communication Networks


ECENT years have seen feedback control emerge as an indispensible conceptual, as well as mathematical, tool for regulation of traffic on communication networks. The celebrated TCP congestion control protocol is perhaps the most notable example corroborating this dictum. Researchers in the control community have long been active in shaping the advances in feedback control of communication networks. More recently, in 1996, the IEEE Control Systems Society organized an NSF-ARO-sponsored workshop on systems and control methods in communication networks at the Airlie House, Warrenton, VA. This workshop brought together approximately 60 researchers from both control and communication network communities to promote collaboration between the two communities. This successful workshop was followed by an equally successful 1998 Airlie House workshop on a similar topic and with a similar goal, and by a large number of special invited sessions on control methods for communication networks at the IEEE Conferences on Decision and Control over the past few years. These activities, along with other important efforts, have led to a very productive period of research in this area. Although Special Issues of a similar nature have been published in other journals in the past, we felt that it would still be appropriate and timely to review recent research accomplishments in a Special Issue of this flagship journal in control theory, so as to bring the readership up to date on current research in this still very active area. This Special Issue contains fourteen papers (ten regular papers and four technical notes) which cover a broad range of topics, all centered around the use of tools and methods from systems and control to address issues that arise in the context of comunication networks. To provide an overview of the issues covered in this Special Issue, we key them below to the individual papers included. The Special Issue opens with the paper by Kunniyur and Srikant, who consider Internet congestion control based on a deterministic fluid model of TCP traffic. Time-scale separation enables stability analysis of the control scheme. Massoulié analyzes the local stability of a distributed congestion control scheme for a fluid network model with inhomogeneous feedback delay. Boulogne, Altman, Kameda, and Pourtallier

Publisher Item Identifier S 0018-9286(02)05494-6.

look at the multiclass routing problem from a game theoretical point of view and establish existence and uniqueness for mixed Nash–Wardrop equilibrium under certain conditions. A combined routing and flow control problem in networks with a large number of users is considered by Altman, Bas¸ar, and Srikant, who obtain results on asymptotic behavior. Sarkar and Tassiulas discuss in their paper a general fairness concept in multicast networks. Hollot, Misra, Towsley, and Gong formulate active queue management (AQM) as a feedback control problem, and discuss the use of proportional and proportional-integral controllers in this context. Laberteaux, Rohrs, and Antsaklis develop an adaptive congestion controller for ABR service in ATM networks, with tradeoffs between performance and complexity. Wu, Chong, and Givan introduce a simulation-based congestion control scheme for network traffic modeled at the burst level. They demonstrate that their approach achieves better performance than the conventional proportional-derivative controller. Buche and Kushner consider various models of time varying mobile channels and associated control, and carry out a heavy traffic analysis. Gokbayrak and Cassandras use an online sensitivity estimation technique to develop admission control in circuit-switched networks to minimize blocking. The Special Issue also includes four technical notes covering the topics of packet marking schemes for Internet congestion control (Gibbens and Kelly) and pricing (Alvarez and Hajek), QoS routing based on local information (Nelakuditi, Varadarajan and Zhang), and application of constrained optimization for admission control (Wieselthier, Nguyen, Ephremides, and Barnhart). It should be evident from these papers that control theory, game theory, and optimization furnish the essential tools for communication network design and operation. This should come as no surprise since communication networks are complex dynamic systems. There are, in fact, several other related topics not covered in this Special Issue. For example, systems and control methodologies are important for the understanding of fundamental design principles in complex networking systems. A few examples in this vein include the following. • Economically sound Internet: The initial design of the Internet was not aimed for commercial usage, and as a result, the resource pricing structure was not setup according to the principles of economics. The explosive growth of

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the Internet over the past 15 years makes implementation of consistent pricing difficult, and even infeasible. Furthermore, the emerging networking technologies challenge known economic models. It seems that systems and control methodologies would prove very relevant here. Network management: Although transport layer congestion control has received significant research attention in the control community, it is only a component of an overall congestion management strategy. Constrained optimization, game theory, and optimal control could be very useful here. Peer-to-peer (P2P) networks: P2P networking may revolutionize the way we model communication systems. Many issues need to be investigated in this context. Some P2P technologies have already caused serious campus network congestion. In P2P networks, each peer needs to balance its own request and supply, while the network must deal with bandwidth consumption. Control and game theoretical considerations seem to be important and highly relevant here. Network security: The majority of network security actions are based on signatures, which are, however, not effective against all types of attacks. Moreover, most tools are not aimed at attacks by insiders, which are typically more damaging. User behavior profiling has been suggested as a complementary approach. In this regard, it seems that pattern recognition, dynamic parameter estimation, and game theory could play a role. Furthermore, security issues can often be cast as a resource allocation issue, which calls for control theoretical treatment. Simulation modeling of complex networks: For high-speed complex networks, detailed simulation is difficult, or may

even be infeasible. The abstraction of coarse-grain modeling depends on the time scale at which traffic dynamics are most critical. Therefore, it is essential to understand the multi-time scale dynamic behavior of network traffic. System modeling methodologies familiar to the control community may very well be helpful in this area. Other important system theoretic topics, which have already received extensive attention by the control community, include connectivity of distributed sensor networks, power control in mobile networks, and power laws and their implications in networking. Current challenges and previous achievements point to a very fruitful future for system- and control-theoretic research in the area of communication networks. In closing, we would like to acknowledge the time and effort contributed by our assistants, Miss Yujing Wu of the University of Massachusetts, Amherst, and Mrs. Becky Lonberger of the University of Illinois at Urbana-Champaign. Their assistance has been crucial throughout the process of editing this Special Issue.

WEIBO GONG, Guest Editor Department of Electrical and Computer Engineering Department of Computer Science University of Massachusetts Amherst, MA 01003 USA TAMER BAS¸AR, Guest Editor Frederic G. and Elizabeth H. Nearing Professor of Electrical and Computer Engineering University of Illinois Urbana, IL 61801 USA

Weibo Gong (S’87–M’87–SM’97–F’99) received the Ph.D. degree from Harvard University, Cambridge, MA, in 1987. Since then, he has been with the Department of Electrical and Computer Engineering at the University of Massachusetts, Amherst, where he is also an Adjunct Professor in the Department of Computer Science. His research interests include modeling and control of networks and computer security. He has been on several editorial boards. Dr. Gong was a recipient of the IEEE TRANSACTIONS ON AUTOMATIC CONTROL George Axelby Outstanding Paper Award.



Tamer Bas¸ar (S’71–M’73–SM’79–F’83) received the B.S.E.E. degree from Robert College, Istanbul, Turkey, and the M.S., M.Phil., and Ph.D. degrees in engineering and applied science from Yale University, New Haven, CT. After stints at Harvard University, Cambridge, MA, and Marmara Research Institute, Gebze, Turkey, he joined the University of Illinois at Urbana-Champaign in 1981, where he is currently the Fredric G. and Elizabeth H. Nearing Professor of Electrical and Computer Engineering. He has published extensively in systems, control, communications, and dynamic games, and has current interests in robust nonlinear and adaptive control, modeling and control of communication networks, control over wireless links, resource management and pricing in networks, risk-sensitive estimation and control, and robust identification. He is currently the Deputy Editor-in-Chief of Automatica, Editor of the Birkhäuser Series (Boston, MA) on Systems and Control, Managing Editor of the Annals of the Society of Dynamic Games, and a member of editorial and advisory boards of several international journals. Dr. Bas¸ar has received several awards and recognitions over the years, among which are the Medal of Science of Turkey (1993), Distinguished Member Award of the IEEE Control Systems Society (CSS) (1993), and the Axelby Outstanding Paper Award of the same society (1995). He is a member of the National Academy of Engineering, and a Past President of CSS.