Understanding TCP fairness over Wireless LAN

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Understanding TCP fairness over Wireless LAN

• Presented by Prashant gothi

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Agenda

• Introduction
• Problem Overview
• Our Solution
• Related Work
• Conclusion Discussion
• References

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Introduction A typical 802.11 installation
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S2
R1
S3
S4
S1
R2
R3
R4
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IntroductionScenarios (Assumption)

• All senders or receivers Share bandwidth
  equally.
• One sender and two receivers Sender get half BW,
  and receivers share other half.

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Problem OverviewReal Experiment Setup
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Problem OverviewReal Experiment Setup

• Ru Average TCP uplink throughput
• Rd Average TCP downlink throughput
• Ru/Rd Ratio
• MTU Maximum Transmission Unit Varied
• Background UDP to reduce buffer available to TCP
  flows varied by packet size and arrival interval

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Problem OverviewResult
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Problem OverviewResult

• In basic case, Ru/Rd 1.44
• Does commercial system give high priority to
  downstream? Since most applications involve
  download rather than upload.
• With UDP flows, ratio Ru/Rd increase
• With smaller MTU, ratio reaches 8

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Problem OverviewFurther investigation with
sniffers

• Upstream TCP window size reaches its maximum in
  all cases
• Downstream TCP window size changes

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Simulation StudyExperiment 1 1 TCP sender and 1
TCP receiver

• TCP receiver window size w 42
• MTU 1500
• Base station buffer size B 6 85 packets
• Number of ACK per data packet ? 1
• Data packet size 1024 bytes
• 5 runs, each lasts 100 second
• Nodes dont move

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Simulation Study Experiment 1 Observation
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Simulation Study Experiment 1 Observation

• Region 1 over 84, ratio is 1
• Region 2 42 to 84, ratio decreases from 10 to 1
• Region 3 6 to 42, ratio varies between 9 and 12
• Region 4 below 6, data points wide spread (too
  noisy)

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Simulation Study Experiment 1 Further
investigation

• When buffer size is smaller than 42, sharing
  result is 110
• When buffer size becomes larger, sharing ratio
  increases
• When buffer size is larger than 84, Base Packet
  is equal to the difference between Down ACK and
  the Up Packet

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Modeling TCP Access Scenario 1 Analysis

• Upstream flow window behavior
• When sender window is large, a loss of an ACK has
  no effect on the window size due to TCP
  cumulative acknowledgement nature.
• Sender window will reach w.

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Modeling TCP Access Scenario 1 Analysis

• Downstream flow window behavior
• It changes depending on B and w, since loss of
  data packet will cause sender half window size.
• If B ? (? 1)w, all packets have room in base
  station buffer, no drops.
• Assume BS buffer is full of ?w ACKs.(?) If B ?
  (?1)w, B - ?w buffer available for downstream.
  Sender window will vary between (B - ?w)/2 and B
  - ?w, average window size is 3(B - ?w)/4.
  (Simplified)
• Ratio R 4w/(3(B - ?w))

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Our solution

• Separately queue for TCP data and ACK packets at
  base station - Doesnt wok
• Fake duplicate ACK packets or discard data
  packets to force TCP to reduce the upstream
  window size Waste BW
• Using advertised receiver window field in the ACK
  packets towards TCP sender, BS manipulates the
  receiver window

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Our solution Solution 3

• Keep a counter for numbering current TCP flow in
  the system
• If n flows in system, BS set receiver window to
  ?B/n?

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Our solutionA simulation for solution 3
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Related Work

• Lu et al. 2 first identified the problem under
  a UDP model. They proposed a centralized
  scheduling algorithm performed at BS.
• Nandagopal et al. 3 suggests a fairness model
  that identify the different node fairness and
  flow fairness.
• Research 9 suggests employ BW reservation over
  MA channels to support QoS.
• Sobrinho and Krishnakumar 10 suggests
  blackburst to find the the real-time sender with
  longest waiting time( and thus the highest
  priority).

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Conclusion Discussion

• Buffer size at base station plays a key role in
  the observed unfairness.
• Based on simulation, the unfairness in TCP
  throughput ration could be as high as 800.
• Proposed solution using advertised window
  manipulated by base station alleviates the
  problem in simulation and testbed.

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Conclusion Discussion

• Open discussion
• Channel losses
• TCP with different RTT
• Providing higher share of the media to the base
  station
• Interaction with IPSec

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References

• Haito wu and youg peng performance of
  ReliableTransport protocol over IEEE 802.11
  Wireless LAN, 2002
• Saar pilosof, Ramachandran Ramjee Understanding
  TCP fairness over Wireless LAN, March 2003