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TCP in Wireless
Environments:
Problems and Solutions
Author: Ye Tian, Kai Xu, Nirwan Ansari
Reporter: 任菊梅
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Contents
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Abstract
2
Introduction
3
Challenges
4
Approaches
2
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
• TCP
RFC793, in 1981,end to end, connectionoriented, reliable and ordered delivery of
data
• TCP Reno
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Challenges in Heterogeneous
Network
• Heterogeneous Network
consists of wired and wireless network
• TCP exhibits weakness
degradation of throughput
inefficiency in network resource utilization
excessive interrupt of data transmissions
Why?
unique characteristics of wireless links
TCP’s design assumption
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Challenges in Heterogeneous Network
• 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!
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Challenges in Heterogeneous Network
• Unique characteristics of Wireless
links
• Limitations of radio coverage & user
mobility
• excessive interrupt
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Challenges in Heterogeneous Network
• 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
• 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
• 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
• Split Mode
• End-to-End Approach
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Reactive Approach
• TCP New Reno
• Partial ACK
• Modification: fast recovery
• Limitation: can’t distinguish the cause of
packet loss
• TCP SACK
•
•
•
•
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Selective ACK option for TCP
Exact number of packet losses
Modification: sender & receiver sides
Limitation: congestion avoidance
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Proactive Approach
• 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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