On Designing Improved Controllers for AQM Routers Supporting

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

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

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

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

Linearized TCP Connection Advanced Computer Networks - PI Controller 5

AQM Feedback Control Advanced Computer Networks - PI Controller 6

Linearized AQM Advanced Computer Networks - PI Controller 7

RED Controller Advanced Computer Networks - PI Controller 8

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 9

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 10

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

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

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 13

PI Controller Advanced Computer Networks - PI Controller 14

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

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 16

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

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

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

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

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 21

Experiment 8 Time varying settings of Experiment 1 with propagation delays reduced to 40 ms. Advanced Computer Networks - PI Controller 22

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

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

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 25

Thanks! rek@cs. wpi. edu