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TCP in Wireless Environments:

Problems and Solutions

Author: Ye Tian, Kai Xu, Nirwan AnsariReporter: 任菊梅

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Contents

Abstract

Introduction

Challenges

Approaches

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Abstract

• Dominance remarkable simplicity and reliability

• Design wired networks assumption: packet losses are signals of

network congestion.

• Wireless Environment performs poorly

high BER, unstable channel, user mobility

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Introduction

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• TCP RFC793, in 1981,end to end, connection-

oriented, reliable and ordered delivery of data

•TCP Reno

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Challenges in Heterogeneous Network

•Heterogeneous Networkconsists of wired and wireless network

•TCP exhibits weaknessdegradation of throughputinefficiency in network resource utilizationexcessive interrupt of data transmissions

Why?unique characteristics of wireless linksTCP’s design assumption

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Challenges in Heterogeneous Network

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• Unique characteristics of Wireless links• Transmission medium--open air

• Uncontrollable quality-affecting factors: weather condition, urban obstacles, mobility of wireless end devices

• High BER & multiple losses in one RTT• TCP sender reduce its sending rate

Unnecessarily!Unnecessarily!

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Challenges in Heterogeneous Network

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• Unique characteristics of Wireless links• Limitations of radio coverage & user

mobility• excessive interrupt

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Challenges in Heterogeneous Network

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• Unique characteristics of Wireless links• Link asymmetry

• Packet losses in reverse channel • TCP sender think it happens on forward

channel, and reduce sending rate• ACK compression effect• Break TCP’s self-clocking & possible

congestive packet losses in forward channel

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Approaches to Improve Wireless TCP Performance

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Application-specific approach

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• TCP-Peach• Satellite network long propagation delay• Dummy

• Copies of the last data packet• Low priority

• new mechanisms• Sudden start: 2RTTs(tradition: 7RTTs)• Congestion avoidance• Rapid recovery: improve throughput in

the presence of high link error rate• Fast retransmit

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Application-specific approach

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• ATCP• Ad hoc network: High error rate, frequent

rout changes and network partitions• Thin layer between TCP & IP• Packet losses:

• High BER: retransmits• Rout change: persist state• Congestion: normal control

• Freeze-TCP• Cellular network: handoff & high BER• Receiver: sets the advertised window size

to zero in ACK packets• Sender: persist mode

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Split Mode & End-to-End Approach

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• Split Mode

• End-to-End Approach

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Reactive Approach

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• TCP New Reno• Partial ACK• Modification: fast recovery• Limitation: can’t distinguish the cause of

packet loss

• TCP SACK• Selective ACK option for TCP• Exact number of packet losses• Modification: sender & receiver sides• Limitation: congestion avoidance

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Proactive Approach

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• TCP-Vegas• Minimal RTT as a reference• Stable congestion window

• TCP-Veno• Based on Vegas • differentiate the cause of the packet loss

• TCP-Westwood• Measuring the averaging the rate of

returning ACKs as reference

• TCP-jesery• ABE & CW as refrence

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