Understanding TCP fairness over Wireless LAN IEEE INFOCOM 2003 Saar Pilosof, Ramachandran Ramjee, Danny Raz, Yuval Shavitt, Prasun Sinha

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Understanding TCP fairness over Wireless LANIEEE
INFOCOM 2003Saar Pilosof, Ramachandran Ramjee,
Danny Raz, Yuval Shavitt, Prasun Sinha

• Presented by Yixin Hua

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Agenda

• Introduction
• Problem Overview
• Simulation Study
• Modeling TCP Access
• Our Solution
• Related Work
• Conclusion Discussion

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Introduction A typical 802.11 installation
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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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Problem OverviewFurther investigation with
sniffers
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Problem OverviewConcerns

• Wireless link interference
• Base station buffer size
• Implementation details of 802.11 MAC layer
• Difficult to vary and isolate parameters, and
  trace their impacts

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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 More observations
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Simulation Study Experiment 1 More observations

• RTT increases monotonically with base station
  buffer size w/o significant rate changes
• Data packet loss rate is always higher than ACK
  loss rate, not linear with base station buffer
  size

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Simulation Study Experiment 1 Further
investigation
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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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Simulation StudyExperiment 2 Multiple flows

• Case 1 One upstream and multiple downstream
  flows
• Case 2 Equal number of multiple upstream and
  downstream flows
• Base station buffer size is 100 packets
• 5 runs, each lasts 100 seconds

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Simulation Study Experiment 2 Observation
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Simulation Study Experiment 2 Observation

• Case 1
• Ratio is linear
• All downstream flows share bandwidth equally
• Total throughput stay stable
• Case 2
• Average ratio goes up to 800, since upstream
  flows ACK clutter base station buffer
• Upstream flows maintain maximum window size
• Downstream flows struggle with a window of 0-2
  packets

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Modeling TCP Access Scenario 1 One upstream
and one downstream flow

• Base station buffer size B
• TCP receiver window size w
• All packet loss due to buffer overflows at base
  station

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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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Modeling TCP Access Scenario 1 Further Analysis

• Using bounded size queuing system (M/M/1/K)
• Arrival rate Rd ?Ru , ? 1.
• The probability of K packets in a buffer in a
  stable state, Pk (1-?) ?k/(1- ?k1) (1)
• ? is ratio between arrival rate and service rate,
  
• ? (Rd ?Ru)/ Ru 1R, where R Rd/Ru (2)
• Drop rate approximate to p (1BR)/(B1) (3)
• Using Rd sqrt(3?/(2p))/RTTd, and Ruw/RTTu
• R RTTu/RTTdsqrt(3?/(2w2p)) (4)

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Modeling TCP Access Scenario 1 Further Analysis

• Using (3) and (4), We get (1BR)/(B1)
  3/(2w2R2)
• Finally
• Using 1B ? B and 1BR ? BR, R 1/10.56, it
  gives an approximation for region 6 to 42.

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Modeling TCP Access Scenario 1 Further Analysis
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Modeling TCP Access Scenario 1 Further Analysis

• When B gt ?w and only loss is due to buffer
  overflow, window size is composed from a fixed
  part B - ?w, and a part of interaction with
  acknowledgements in the BS buffer.
• Effective average window size is sqrt(3?/(2p))
  3(B - ?w)/4
• RRTTu/(wRTTd)sqrt(3?/(2p))3(B-?w)/4 (6)
• It gives an approximation to region 42 to 85.
• When w42, it matches with Eq. 4

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Modeling TCP Access Scenario 1 Validation

• TCP doesnt provide a nice arrival behavior like
  M/M/1/K

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Modeling TCP Access Scenario 2 Small Buffer

• Upstream flow with small buffer size, using
  discrete time Markov chain
• State i represents a state where TCP window size
  is 2i
• On state i, go to state 0 if a timeout occurs
  with probability p2i
• Otherwise double the window size and move into
  state i1 with probability 1- p2i
• With ns2, the upstream flow always end up with
  maximum window size

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Modeling TCP Access Scenario 3 Multiple Flows

• From eq. 1, ? (Rd nRu)/ Ru 1nR (7)
• Drop rate, p (1nBR)/(B1) (8)
• Rsqrt(3?/(2nw2p))3(B-?w)/(4nw) (9)
• It fits figure 6 very well!

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Modeling TCP Access Scenario 3 Multiple Flows
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Our solution

1. Separately queue for TCP data and ACK packets at
   base station - Doesnt wok
2. Fake duplicate ACK packets or discard data
   packets to force TCP to reduce the upstream
   window size Waste BW
3. 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?
• ? Web traffic, bursty flow, UDP
• ! An XCP way

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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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Related Work

• Deng and Chang 1 suggested to change the
  backoff period according to a station priority.
  The lower the priority the higher is the maximum
  backoff period a station can draw.
• Berry et al. 11 follower the line and use two
  distinct backoff periods for two priority
  classes.
• Vaidya et al. 4 suggests a distributed
  algorithm that calculates the backoff period for
  the stations that resulted access to the channel
  will closely match the Self-Clocked Fair Queueing
  scheduling.
• Ada and Castelluccia suggests three differential
  mechanism based on scaling of the congestion
  window, modifying the IFSs, and changing the
  maximum frame length.

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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.
• Using bounded size queuing system (M/M/1/K),
  authors explained TCPs behavior and interaction
  with MAC layer.
• The analysis identified four regions of
  unfairness that depend on the buffer availability
  at base station.
• 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