Analysis of Unfairness between TCP Uplink and Downlink Flows in WiFi Hot Spots

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Analysis of Unfairness between TCP Uplink and
Downlink Flows in Wi-Fi Hot Spots
IEEE Globecom 2006 Nov. 30, 2006 Eun-Chan Park
Chong-Ho Choi Telecommunication RD Center,
SAMSUNG Electronics Co. LTD. School of Elec.
Eng. and Computer Science, Seoul National
University
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Outline

• 1. Introduction
• 2. TCP-induced unfairness
• 3. MAC-induced unfairness
• 4. Effect of packet loss
• 5. Interaction between TCP and MAC
• 6. Conclusion

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1. Introduction

• Wide deployment of Wi-Fi hotspots
• Explosive growth of Wi-Fi-enabled portable
  devices
• Laptop, PDA, digital camera/camcoder
• Portable multimedia player, portable game device

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Contributions

• Identify unfair bandwidth sharing between UP/DN
  STAs
• Analyze cause of unfairness
• Asymmetry of TCP congestion control
• Asymmetry of MAC contention control
• Interaction between TCP and MAC
• Derive counter-intuitive results
• (i)Even though a station has a sufficiently
  large amount of traffic to send, it cannot always
  participate in MAC-layer contention.
• (ii)Both TCP and MAC-induced unfairness
  problems are mitigated if packet loss due to
  buffer overflow in the AP does not occur.

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2. TCP-induced unfairness

• Bias toward UP STA
• Compare UDP and TCP cases
• Nup Ndn 1

• UDP little difference between UP STA and DN STA
• TCP UP STA (2.26Mb/s) achieves higher
  throughput than DN STA (1.28Mb/s) by more than 75

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Cause Asymmetric behavior of TCP

• Packet (data/ACK) loss in AP buffer

• Response to data/ACK loss
• - DN STA Sender (wired node) reduces cwnd or
  time-out occurs
• - UP STA Sender (wireless node) is unresponsive
  to an ACK loss due to cumulative ACK

Asymmetric behavior in response to packet loss
in the AP buffer results in a bias toward UP STA.
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Cause Burstness of TCP

• Exacerbate unfairness
• Compare TCP window size of UP/DN STAs

Increase up to RWND, Wmax
time-out
As the number of STAs increases, multiple data
packet loss due to burstness of TCP significantly
reduces throughput of DN STA.
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3. MAC-induced unfairness

• Contention asymmetry of MAC in Wi-Fi hot spot
• Individual UP STA takes part in contention
• AP contends on behalf of all DN STAs
• Predict throughput ratio (?est) avg(th. of UP
  STA)/avg(th. of DN STA)
• Assumption1 All contending stations (UP STAs and
  AP) have equal channel access opportunities under
  802.11 DCF (CSMA)
• Assumption2 All stations always participate in
  contention

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• Comparison of ?est for TCP/UDP traffic

• UDP traffic agrees with estimated results
• TCP traffic severe unfairness as the number of
  STAs increases
• ? Assumptions does not hold for TCP traffic

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4. Effect of packet loss on fairness and
utilization

• What happens if no packet loss occurs?
• By restricting Wmax or making buffer size, B,
  large
• Wmax 1080 packets, B 50 packets

no packet loss
no unfairness

• No packet loss assures fairness at the cost of
  utilization
• ? Tradeoff between fairness and utilization

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Effect of packet loss on MAC-induced unfairness

• Reduce packet loss by setting B to a large value
• Observe throughput ratio w.r.t loss rate

no unfairness
As long as packet loss does not occur, fairness
is maintained, i.e., both TCP and MAC-induced
asymmetries are removed.
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5. Interaction between TCP MAC

• TCP MAC Analogous to rules of card game
• MAC a fair gaming rule assuring equal winning
  prob. among active gamers (AP and UP STAs, not
  all gamers)
• TCP another rule determining whether a gamer
  joins or not
• TCP window size a credit for betting games

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TCP/MAC Cause of unfairness

• Limited credit (TCP window size) for each STA
• If credit is exhausted, no more join to the
  MAC-layer contention
• Depending on loss, credit is adjusted and
  regenerated every RTT
• Asymmetry of TCP congestion control
• DN STA is responsive to packet loss
• Less credit (multiplicative decrease of window
  size)
• Less opportunity of winning game
• UP STA is unresponsive to (ACK) loss
• More credit (additive increase of window size)
• More opportunity of winning game

Since TCP controls the opportunity for a STA to
participate in the MAC-layer contention, TCP
asymmetry due to cumulative ACK exacerbates
unfairness.
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TCP/MAC Possible solution for fairness

• Assume no packet loss
• DN STA and UP STA have the same credit (
  advertised window size)
• Credit of AP of DN STAs credit of a DN STA
• ? AP gets more chance to occupy channel compared
  to an UP STA by Ndn times

If packet loss is prevented, MAC-induced
unfairness is resolved, as well as TCP-induced
unfairness.
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6. Conclusion

• Identify the unfairness problem in Wi-Fi hotspot
• Service is biased toward UP STA
• Analyze the cause of unfair capacity sharing
• TCP-induced asymmetry
• MAC-induced asymmetry
• Interaction between TCP/MAC
• Future work
• Devise a cross-layer (TCP/MAC) approach for
  per-station fairness