Performance Enhancement of TFRC in Wireless Ad Hoc Networks

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Performance Enhancement of TFRC in Wireless Ad
Hoc Networks

• Mingzhe Li, Choong-Soo Lee, Emmanuel Agu,
• Mark Claypool and Bob Kinicki
• Computer Science Department
• Worcester Polytechnic Institute
• Worcester, Massachusetts

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Outline

• Introduction
• Background
• TFRC Performance over Wireless Networks
• RE-TFRC Algorithm
• Performance Evaluation
• Conclusions and Future Work

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Introduction

• The objective is improved support for streaming
  multimedia applications over wireless networks.
• The TCP Friendly Rate Control protocol (TFRC) was
  designed for wired networks. It can perform
  poorly over wireless networks.
• The 802.11 MAC layer wireless protocol uses
  Carrier Sense Multiple Access with Collision
  Avoidance (CSMA/CA) and Request-to-Send/Clear-to-S
  end (RTS/CTS) to avoid frame collisions.
• TFRC performance suffers from the contention
  delays and drops known as RTS/CTS jamming and
  RTS/CTS-induced congestion.

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Introduction

• This paper introduces a wireless extension to the
  TFRC protocol, Rate Estimation TFRC (RE-TFRC),
  that accounts for MAC layer saturation to select
  a sending rate that outperforms TFRC.
• The goal of RE-TFRC is to reduce MAC layer loss
  rates and collisions and thereby lower transport
  layer delays with minimal effect on throughput.

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Outline

• Introduction
• Background
• TFRC Performance over Wireless Networks
• RE-TFRC Algorithm
• Performance Evaluation
• Conclusions and Future Work

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TCP Friendly Rate Control (TFRC)

• TCP Friendly Rate Control (TFRC) Floyd00
• Designed for streaming media applications
• Uses rate-based congestion control and
• The TCP Friendly congestion response function
• TFRC is implemented in the Linux kernel as one of
  the congestion control options of the Datagram
  Congestion Control Protocol (DCCP).

X Transmission rate s packet size r round
trip time p lost event rate trto
Retransmission time out b num of packets in
each ack
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Hidden Terminal Problem

• Node 1 is hidden from Node 3
• Node 1 and node 3 cannot sense each others
  transmissions.
• If Node 1 and node 3 transmit at the same time to
  node 2, a collision occurs at node 2.
• Node 1 and node 3 back off and retransmit.

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Hidden Terminal Problem

• 802.11 Solution to the Hidden Terminal Problem
• Use a four-way handshake RTS-CTS-DATA-ACK where
  the RTS and CTS packets are significantly smaller
  than the average data packet.
• The maximum number of RTS retransmissions is set
  to 7.
• However, the 802.11 protocol will still have
  problems if the MAC layer becomes saturated!!

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MAC Layer Saturation

• MAC layer congestion
• The wireless network traffic load is increased
  above the MAC layer saturation point.
• Contention delays and drops are increased.
• The RTS/CTS jamming is hidden from upper layers.
• TFRC then computes an ineffective RTT (Round Trip
  Time) and loss event rate.
• This implies a TCP Friendly sending rate that is
  too high for optimal performance.

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Outline

• Introduction
• Background
• TFRC Performance over Wireless Networks
• RE-TFRC Algorithm
• Performance Evaluation
• Conclusions and Future Work

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TFRC Performance Investigation

• NS-2 simulations are used.
• Evaluate a single flow, 802.11b MAC layer
  protocol over a chain topology with a 2 Mbps
  wireless capacity.
• The throughput decreases as the number of hops
  increases.

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Rate Constrained TFRC

• A seven-hop chain network was simulated.
• The TFRC sending rate is manually constrained.
• The MAC layer saturates at 300Kbps.

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Rate Constrained TFRC

• The TFRC loss event rate and RTT increase sharply
  after a 300Kbps constrained sending rate.
• Thus, unconstrained TFRC runs in a sub-optimal
  state due to MAC layer congestion.

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Outline

• Introduction
• Background
• TFRC Performance over Wireless Network
• RE-TFRC Algorithm
• Performance Evaluation
• Conclusions and Future Work

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Rate Estimation TFRC (RE-TRFC)

• RE-TFRC estimates the optimum sending rate based
  on
• The number of hops in the flow path
• The current loss event rate.
• The TFRC sending rate is adjusted depending on
  the estimate of the optimum sending rate.
• RE-TFRC preserves the ceiling imposed by the TCP
  Friendly sending rate.

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Rate Estimation

• Rate Estimate in TCP Westwood wang02
• Upon congestion, Westwood sets the TCP window
  size to
• W Bit-rateest rttmin
• rttmin is the smallest recorded rtt, i.e., an
  estimate of latency.
• RE-TRFC Rate Estimate Approach
• Estimate the optimum sending rate that will not
  saturate the MAC layer.
• Determine the MAC layer saturation rtt rttopt
• Control the sending rate on congestion.

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RE-TFRC Rate Estimation

• TCP Friendly equation
• Inverse TCP function
• X TCP Friendly rate
• p TFRC loss event rate

• Use R to estimate p
• Use p to estimate R

R TFRC estimated receiving rate p Adjusted
TFRC loss event rate R Estimated optimum
sending rate
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Round Trip Time Modeling

• Single hop delay model Carvalho03
• Multi-hop chain delay model
• Divide the N-hop chain into N-2 4-node networks
  and two 3-node networks.
• Sum the data/ack packet delay over the N hops.

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Round Trip Time Modeling
estimate of rttopt for N-hop chain topology
Single hop delay of Ack packet Single hop
delay of Data packet
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Rate Estimation TFRC Algorithm

• On receiving an ack
• Compute R (the original TCP Friendly rate) .
• Estimate rttopt. using the r(N) approximation.
  Assume N can be obtained from the routing
  protocol.
• Compute the adjusted loss event rate p using
  rttopt and R.
• Compute the estimated optimum send rate R.
• Use the original rate, R, if the new rate, R,
  is larger.
• If there is a rate change, make the change
  incrementally as TFRC does.

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Outline

• Introduction
• Background
• TFRC Performance over Wireless Network
• RE-TFRC Algorithm
• Performance Evaluation
• Conclusions and Future Work

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Simulation Details

• NS-2 was used to simulate and evaluate RE-TFRC
  performance.
• Wireless Multi-hop Chain Network
• N-hop network implies N1 nodes (n0 to nN).
• All simulated TRFC flows go from n0 to nN.
• The number of hops in the chain network was
  varied from 4 to 15.
• The bit err rate (BER) was varied from 10-6 to
  10-4.

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Seven-Hop Chain Topology
CDF of MAC layer retransmissions
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Loss Event Rate for Multi-Hop Chains
Average loss event rate versus number of hops
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Round Trip Times for Multi-Hop Chains
Average round trip time versus number of hops
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Throughput for Multi-Hop Chains
Average throughput versus number of hops
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Loss Event Rate for Multi-Flow Tests
Average loss event rate for various flow scenarios
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Round Trip Time for Multi-Flow Tests
Average round trip time for various flow scenarios
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Throughput for Multi-Flow Tests
Average throughput for various flow scenarios
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Bit Error Rate Test of RE-TFRC

• Single flow, seven-hop chain topology

BER 10-6 10-5 10-4
RTT Reduction 39 32 14
Loss Rate Reduction 55 45 29
Throughput Improvement 6.5 4.2 0.5
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Outline

• Introduction
• Background
• TFRC Performance over Wireless Networks
• RE-TFRC Algorithm
• Performance Evaluation
• Conclusions and Future Work

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Conclusions

• Rate Estimation TFRC (RE-TFRC)
• Estimates MAC layer saturation and controls the
  TFRC sending rate.
• Lowers the delay and loss rate and can even
  increase throughput in most cases
• Lowers round-trip time up to 40
• Lowers loss event rate up to 80
• Increases throughput up to 5.
• reduces MAC layer congestion.

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Future Work

• Extend Algorithm
• To other topologies cross, grid, and random
• Consider mobile nodes.
• Incorporate into applications
• Such as streaming multimedia
• Implement TFRC wireless extension in Linux.

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Thanks!rek_at_cs.wpi.edu