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TCP-LP: A Distributed Algorithm for Low

Priority Data Transfer

Aleksandar Kuzmanovic, Edward W. Knightly

Department of Electrical and Computer Engineering

Rice UniversityIEEE INFOCOM 2003

Presented by Ryan

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Introduction

Service prioritization among different traffic classes E.g. better than best-effort (real-time service) Not easy to deploy in the current Internet

TCP-LP (Low Priority) An end-point protocol achieving two-class service

prioritization without any support from the network

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Introduction

Objective of TCP-LP Utilizing available bandwidth in a TCP transparent

fashion Fair sharing the excess bandwidth among

multiple TCP-LP flows (TCP-like fair share)

Application of TCP-LP Background file transfer Probing available bandwidth

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Reference Model

Two class hierarchical scheduling model High-priority VS Low-priority class Strict priority service

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TCP-LP Protocol

An end-point congestion control algorithm Early Congestion Indication Congestion Avoidance Policy

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TCP-LPEarly Congestion Indication

One-way packet delays as early indicators Smoothed one-way delay (weighted moving average)

Early congestion indication condition

d – measured one-way delay, γ- delay smoothing parameter, δ- delay threshold

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TCP-LPCongestion Avoidance Policy

Receipt of first early congestion indication halving the congestion window entering an inference phase

During the inference phase Without increasing the congestion window If receiving another indication

setting the congestion window to 1

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TCP-LPCongestion Avoidance Policy

After the expiration of the inference phase increasing the congestion window by 1 per

RTT (like TCP)

Early Congestion Induction

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Parameter Settings

Delay Smoothing, γ= 1/8 (typical value for computing the smoothed RTT for TCP)

Delay Threshold, δ= 0.15

Inference Phase Time-out, itt = 3*RTT

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Simulation

Run on NS2 (each run lasts 1000s)

Topology Bottleneck link – 1.5Mb/s or 10Mb/s with delay

20ms Other access links – 100Mb/s with delay 2ms

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Simulation Results

FTP and Reverse Background Traffic

First Row (excess capacity not available) 2 simultaneous FTP downloads

Second Row (excess capacity available) 2 simultaneous FTP downloads 10 TCP flows in the reverse direction

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Simulation Results

Square-wave Background Traffic 1 TCP/TCP-LP flow

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Simulation Results

10 TCP/TCP-LP flows

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Simulation Results

HTTP Background Traffic Web traffic between Node 0 and 1 FTP connection in the same direction

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Simulation

Multiple Bottlenecks Topology 1 Links 0-1, 1-2 and 2-3 with capacity of 1.5Mb/s Others with capacity of 100Mb/s

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Simulation Results

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Simulation

Multiple Bottleneck Topology 2 Links capacity – same as Topology 1

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Simulation Results

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Conclusion

TCP-LP achieves low-priority service without the support of the network

Simulations results support its functions Experiments on the Internet should be

performed to validate its performance