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Understanding TCP fairness over Wireless LAN

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Separately queue for TCP data and ACK packets at base station - Doesn't wok. Fake duplicate ACK packets or discard data packets to force TCP to reduce the ... – PowerPoint PPT presentation

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Title: Understanding TCP fairness over Wireless LAN


1
Understanding TCP fairness over Wireless LAN
  • Presented by Prashant gothi

2
Agenda
  • Introduction
  • Problem Overview
  • Our Solution
  • Related Work
  • Conclusion Discussion
  • References

3
Introduction A typical 802.11 installation
4
S2
R1
S3
S4
S1
R2
R3
R4
5
IntroductionScenarios (Assumption)
  • All senders or receivers Share bandwidth
    equally.
  • One sender and two receivers Sender get half BW,
    and receivers share other half.

6
Problem OverviewReal Experiment Setup
7
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

8
Problem OverviewResult
9
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

10
Problem OverviewFurther investigation with
sniffers
  • Upstream TCP window size reaches its maximum in
    all cases
  • Downstream TCP window size changes

11
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

12
Simulation Study Experiment 1 Observation
13
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)

14
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

15
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.

16
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))

17
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

18
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?

19
Our solutionA simulation for solution 3
20
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).

21
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.

22
Conclusion Discussion
  • Open discussion
  • Channel losses
  • TCP with different RTT
  • Providing higher share of the media to the base
    station
  • Interaction with IPSec

23
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
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