“On Designing Improved Controllers for AQM Routers Supporting TCP Flows” The PI Controller

1. “On Designing Improved Controllers for AQM Routers Supporting TCP Flows”The PI Controller Presented by Bob Kinicki

2. Outline • Introduction • Background • TCP Analytic Model • Brief Control Theory • Proportional Controller • Proportional Integral (PI) Controller • ns Simulation Results • Conclusions Advanced Computer Networks - PI Controller

3. Control Theoretic Analysis of RED Variable Definitions: Advanced Computer Networks - PI Controller

4. Block diagram of a TCP connection Control law (e.g. RED) congested queue __ 1 N __ TCP load factor Time Delay Rtt TCP window control Vishal Misra’s Slide Advanced Computer Networks - PI Controller

5. Linearized TCP Connection Advanced Computer Networks - PI Controller

6. AQM Feedback Control Advanced Computer Networks - PI Controller

7. Linearized AQM Advanced Computer Networks - PI Controller

8. RED Controller Advanced Computer Networks - PI Controller

9. Proportional Controller • The feedback signal is simply the regulated output, the queue length, multiplied by a gain factor. • In the RED context, this means using the instantaneous queue length instead of the average queue length. • Note – Control Theory emphasizes “stability” which is well-defined mathematically, but this significantly constrains the choices for RED parameters used in this paper. Advanced Computer Networks - PI Controller

10. Preliminary Simulation with Proportional Controller • 60 FTP flows, 180 HTTP sessions • C = bottleneck link = 15 Mbps • Propagation delays uniform between 160 and 240 ms. • Packet size = 500 bytes • Time varying dynamics • At t = 100, 20 FTP flows drop out. • At t = 140, the 20 FTP flows start again. Advanced Computer Networks - PI Controller

11. Comparison of RED and PC Time varying dynamics Proportional Controller has better response Advanced Computer Networks - PI Controller

12. Comparison of RED and PC Time varying dynamics When RTT’s are Doubled, RED has a large overshoot! Advanced Computer Networks - PI Controller

13. Limitations of PC • Under certain network conditions, the operating point p can be above the pmax imposed by buffer size limitations. • This leads to oscillations as seen in Figure 7. • Hence, PI, the Proportional Integral controller is used to clamp the queue size to qref regardless of the load. Advanced Computer Networks - PI Controller

14. PI Controller Advanced Computer Networks - PI Controller

15. PI Algorithm Executed once per sampling period: { p = a ( q – q_ref) – b (q_old – q_ref) + p_old p_old = p q_old = q } Advanced Computer Networks - PI Controller

16. ns Experimental Parameters • Sampling frequency = 160 Hz. • a = 1.822 (10)-5 ; b = 1.816 (10)-5 • qref = 200 packets • Buffer size = 800 packets • RED parameters (defined by stability): • pmax = 0.1 • minth = 150 • maxth = 700 • wq = 1.33 (10)-6 Advanced Computer Networks - PI Controller

17. Experiment 3 PI “relatively” insensitive to load Advanced Computer Networks - PI Controller

18. Experiment 4 Time varying dynamicsremoved PI Controller has faster response time. Advanced Computer Networks - PI Controller

19. Experiment 5 180 FTP flows and 360 HTTP flows PI Controller is more robust for higher loads. Advanced Computer Networks - PI Controller

20. Experiment 6 16 FTP flows and 180 HTTP flows Under lighter load, RED oscillates. Advanced Computer Networks - PI Controller

21. Experiment 7 400 FTP flows and 180 HTTP flows The heavy load has pushed the operating queue length beyond the buffer size for RED and PC. Advanced Computer Networks - PI Controller

22. Experiment 8 Time varying settings of Experiment1 with propagation delays reduced to 40 ms. Advanced Computer Networks - PI Controller

23. PI – Delay-Utilization Tradeoff Advanced Computer Networks - PI Controller

24. RED - Delay-Utilization Tradeoff Advanced Computer Networks - PI Controller

25. Conclusions • The authors introduced both the Proportional and the PI Controllers for AQM. • Both controllers respond faster than RED. • PI is better with respect to regulating the steady-state queue length. • PI controlled exhibited superior performance to RED in all the ns simulations shown. • Note – At the very end of the paper, the authors advocate the use of ECN (namely, marking) in all AQM’s. Advanced Computer Networks - PI Controller

26. Thanks!rek@cs.wpi.edu