TCP in Wireless
Environments:
Problems and Solutions
Author: Ye Tian, Kai Xu, Nirwan Ansari
Reporter: 任菊梅
LOGO
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
13
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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