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“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.

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On designing improved controllers for aqm routers supporting tcp flows the pi controller

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

Presented by Bob Kinicki


Outline
Outline Supporting TCP Flows”

  • Introduction

  • Background

    • TCP Analytic Model

    • Brief Control Theory

  • Proportional Controller

  • Proportional Integral (PI) Controller

  • ns Simulation Results

  • Conclusions

Advanced Computer Networks - PI Controller


Control theoretic analysis of red
Control Theoretic Analysis of RED Supporting TCP Flows”

Variable Definitions:

Advanced Computer Networks - PI Controller


Block diagram of a tcp connection
Block diagram of a TCP connection Supporting TCP Flows”

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


Linearized tcp connection
Linearized TCP Connection Supporting TCP Flows”

Advanced Computer Networks - PI Controller


Aqm feedback control
AQM Feedback Control Supporting TCP Flows”

Advanced Computer Networks - PI Controller


Linearized aqm
Linearized AQM Supporting TCP Flows”

Advanced Computer Networks - PI Controller


Red controller
RED Controller Supporting TCP Flows”

Advanced Computer Networks - PI Controller


Proportional controller
Proportional Controller Supporting TCP Flows”

  • 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


Preliminary simulation with proportional controller
Preliminary Simulation with Proportional Controller Supporting TCP Flows”

  • 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


Comparison of red and pc
Comparison of RED and PC Supporting TCP Flows”

Time varying dynamics

Proportional

Controller has

better response

Advanced Computer Networks - PI Controller


Comparison of red and pc1
Comparison of RED and PC Supporting TCP Flows”

Time varying dynamics

When RTT’s are

Doubled, RED has

a large overshoot!

Advanced Computer Networks - PI Controller


Limitations of pc
Limitations of PC Supporting TCP Flows”

  • 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


Pi controller
PI Controller Supporting TCP Flows”

Advanced Computer Networks - PI Controller


Pi algorithm
PI Algorithm Supporting TCP Flows”

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


Ns experimental parameters
ns Experimental Parameters Supporting TCP Flows”

  • 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


Experiment 3
Experiment 3 Supporting TCP Flows”

PI “relatively”

insensitive to load

Advanced Computer Networks - PI Controller


Experiment 4
Experiment 4 Supporting TCP Flows”

Time varying

dynamicsremoved

PI Controller has

faster response

time.

Advanced Computer Networks - PI Controller


Experiment 5
Experiment 5 Supporting TCP Flows”

180 FTP flows and

360 HTTP flows

PI Controller is

more robust for

higher loads.

Advanced Computer Networks - PI Controller


Experiment 6
Experiment 6 Supporting TCP Flows”

16 FTP flows and

180 HTTP flows

Under lighter load,

RED oscillates.

Advanced Computer Networks - PI Controller


Experiment 7
Experiment 7 Supporting TCP Flows”

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


Experiment 8
Experiment 8 Supporting TCP Flows”

Time varying settings

of Experiment1 with

propagation delays

reduced to 40 ms.

Advanced Computer Networks - PI Controller


Pi delay utilization tradeoff
PI – Delay-Utilization Tradeoff Supporting TCP Flows”

Advanced Computer Networks - PI Controller


Red delay utilization tradeoff
RED - Delay-Utilization Tradeoff Supporting TCP Flows”

Advanced Computer Networks - PI Controller


Conclusions
Conclusions Supporting TCP Flows”

  • 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


Thanks rek@cs wpi edu

Thanks! Supporting TCP [email protected]


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