Modeling of Web/TCP Transfer LatencyYujian Peter Li January 22, 2004

M. Sc. Committee: Dr. Carey WilliamsonDr. Wayne EberlyDr. Elena Braverman

Department of Computer Science, University of Calgary

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Outline

TCP Overview and Related Work

The Proposed TCP Transfer Latency Model

Model Validation by Simulation

Extending the Proposed Model to CATNIP TCP

Conclusions

Motivation and Objectives

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Motivation

Web response time Highly dominated by TCP performance

Understanding the sensitivity of TCP to network conditions helps to improve TCP performance

No work on modeling CATNIP TCP

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Objectives

To survey and compare existing TCP models

To develop an accurate model for short-lived TCP flows

To model CATNIP TCP

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SYNSYN/ACK

ACK

FIN FIN/ACKACK

TCP OverviewCharacteristics

Reliable, in-order byte stream

Flow control Connection-oriented Congestion Control

Web Browser Web Server DATA

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TCP OverviewCongestion Control

• When intermediate nodes (routers) become overloaded, the condition is called congestion.

• The mechanisms to solve the problem are called congestion control.

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TCP Overview – Congestion ControlSlow Start & Congestion Avoidance

Slow start: cwnd=cwnd+1 for every received ACK

Congestion avoidance: cwnd = cwnd + 1/cwnd

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Related Work

TCP Steady State Throughput Model [Padhye et al. 1998]

TCP Response Time Models Cardwell-00 Model [Cardwell et al. 2000] Padhye Model [Cardwell et al. 1998] Cardwell-98 Model [Cardwell et al. 1998] Sikdar Model [Sikdar et al. 2001]

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The Proposed TCP Response Time ModelAssumptions

Bernoulli packet loss model, i.e., packet is independently lost with fixed probability p Congestion avoidance algorithm ignored, i.e., cwnd always increases by one upon receiving one ACK (exponentially)

Packet loss can be via RTO or triple duplicate ACKs

The effect of delayed ACK, Tdelay, is added when necessary

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The Proposed Model (Cont’d)Congestion Window Evolution

iloss

iss

lastCycle

iTTET

][

1

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Simulation ExperimentsNetwork Topology

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Factor Levels

Transfer Size 1KB, 4KB, 8KB, 16KB, 32KB, 50KB, 64KB, 90KB, 110KB, 128KB, 160KB, 180KB, 200KB

Packet Loss Probability 1%, 3%, 5%, 8%, 10%

Simulation ExperimentsMetric & Experimental Factors

Performance Metric: Data Transfer Time, the time from when the sender sends the first packet until the time when the sender receives the ACK of the last data packet.

Experimental factors and levels

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Simulation ResultsShort-lived Flows

 ( p=3%) (p=10%)

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Data Transfer Size (Packets)

Tran

sfer

Tim

e (s

ec)

Simulated

Proposed

Sikdar

Cardwell-000

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Data Transfer Size (Packets)

Tran

sfer

Tim

e (s

ec)

Simulated

Proposed

Sikdar

Cardwell-00

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CATNIP TCP

  C. Williamson and Q. Wu : “A Case for Context-Aware TCP/IP”.

ACM Performance Evaluation Review, Vol. 29, No. 4, pp. 11-23, March 2002. Convey application-layer context information to TCP/IP Not all packet losses created equal

IP

TCP

HTTPDocument Size

Packet Loss Priority

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CATNIP TCP v.s. Partial CATNIP TCP

Packets First three

Last three

cwnd<3

CATNIP

Partial CATNIP

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70

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Transfer Time (sec)

Fre

qu

ency

(%

)

Partial CATNIP

CATNIP

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Transfer Time (sec)

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Partial CATNIP

CATNIP

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60

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1 2 3 4 5 6 7 8 9 10

Transfer Time (sec)

Fre

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ency

(%

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Partial CATNIP

CATNIP

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Transfer Time (sec)

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Partial CATNIP

CATNIP

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Transfer Time (sec)

Fre

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ency

(%

)

Partial CATNIP

CATNIP

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Transfer Time (sec)

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Partial CATNIP

CATNIP

CATNIP TCP v.s. Partial CATNIP TCP

p=3%

p=5%

p=10%

PDF CDF

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Modeling Partial CATNIP TCP Short-lived Flows

 ( p=3% p’=0%) (p=10% p’=0%)

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Data Transfer Size (Packets)

Tran

sfer

Tim

e (s

ec)

TCP Reno (Simulated)

Partial CATNIP (Simulated)

Partial CATNIP (Analytical)

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Data Transfer Size (Packets)

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sfer

Tim

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TCP Reno (Simulated)

Partial CATNIP (Simulated)

Partial CATNIP (Analytical)

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Conclusions

The proposed TCP latency model fits the simulation results better than earlier models.

The differences between Partial CATNIP and CATNIP are minimal when p<10%.

Partial CATNIP TCP model matches the simulation as well.

Partial CATNIP TCP improves TCP latency compared to TCP Reno. For short-lived flows, Partial CATNIP TCP is about 10% faster than TCP Reno in most cases.

CATNIP TCP is a suitable approach to improve TCP Performance.