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

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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

Advanced Computer Networks - PI Controller

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

Linearized TCP Connection

Advanced Computer Networks - PI Controller

AQM Feedback Control

Advanced Computer Networks - PI Controller

Linearized AQM

Advanced Computer Networks - PI Controller

RED Controller

Advanced Computer Networks - PI Controller

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

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

Comparison of RED and PC

Time varying dynamics

Proportional

Controller has

better response

Advanced Computer Networks - PI Controller

Comparison of RED and PC

Time varying dynamics

When RTT’s are

Doubled, RED has

a large overshoot!

Advanced Computer Networks - PI Controller

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

PI Controller

Advanced Computer Networks - PI Controller

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

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

Experiment 4

Time varying

dynamicsremoved

PI Controller has

faster response

time.

Advanced Computer Networks - PI Controller

Experiment 5

180 FTP flows and

360 HTTP flows

PI Controller is

more robust for

higher loads.

Advanced Computer Networks - PI Controller

Experiment 6

16 FTP flows and

180 HTTP flows

Under lighter load,

RED oscillates.

Advanced Computer Networks - PI Controller

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

Experiment 8

Time varying settings

of Experiment1 with

propagation delays

reduced to 40 ms.

Advanced Computer Networks - PI Controller

PI – Delay-Utilization Tradeoff

Advanced Computer Networks - PI Controller

RED - Delay-Utilization Tradeoff

Advanced Computer Networks - PI Controller

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

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