Performance Enhancement of TFRC in Wireless Ad Hoc Networks
Travis Grant – [email protected]
Mingzhe Li, Choong-Soo Lee, Emmanuel Agu, Mark Claypool, and Robert Kinicki
WPI
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Outline
• Introduction & Background
• RE-TFRC
• Evaluation
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TFRC - Background
• TCP Friendly Rate Control (RFC 3488)• CC Mechanism that reduces variation of throughput• Ideally suited for Applications sensitive to “jitter” as
opposed to overall transfer times
ReceiverSender
(1) Rcvr measures
loss event rate (p) & passes info back to sender
(2) Sender uses info to
calculate RTT (r)
(4) Sender adjusts transmit rate to match calculated
rate X
(3) Sender passes loss event rate & RTT into TFRC Equation
Calculated
Dynamic
Constant
X=T = Transmit Rate
p = loss event rateR=r = RTT
trto =TCP RTOs =packet sizeb = #of packets acked by single TCP ack
FLOYD00
X s2bp
3r
Trto3bp
83 p1 32p2
RFC 3488
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Wireless Challenges
• 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
• MAC Layer Congestion can increase in Chained Ad-Hoc Topologies
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MAC Layer Saturation
• First Simulation is a 7-hop NS-2 with a constrained sending rate• Wireless traffic load is increased above MAC Layer saturation point• lower layer contention, RTS/CTS Jamming, can be hidden
– Could still see a successful ack at the transport layer after 6 MAC Layer retransmission occurred
• TFRC Calculates a send rate that is too high– R=RTT & P=Loss Event Rate are ineffective
Link Capacity = 2Mbps max throughput = 0.146
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Outline
• Introduction & Background
• RE-TFRC
• Evaluation
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RE-TFRC
• Rate Estimation TFRC• Goals
1. minimize round trip time2. maintain, or slightly improve, throughput3. solve the “mis-interaction” between TFRC and 802.11 MAC
Layer (w/o changing the MAC layer)
• Optimum sending rate based on:– The number of hops in the flow path– The current loss event rate– Respects TFRC ceiling
• Effectively avoids RTS/CTS Jamming compared to TFRC
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Loss event rate for various RTT
Bandwidth 1.3 MTU
RTT Loss
Floyd97
TCP Westwoodwindow = B x rmin
Simple TFRC
ropt =
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RE-TFRC Rate Estimation
X: TCP Friendly rate
p: TFRC loss event rate
R: TFRC estimated receiving rate
p’: Adjusted TFRC loss event rate
R’: Estimated optimum sending rate
),( prttfX TFRC Simple
),( Xrttfp Inverse
TFRC
),(' Rrttfp optUse R to estimate p’
)',(' prttfR curUse p’ to estimate R’
ReceiverSender
(1) p & R(2) r (RTT)
(4) R’ is used to adjust transmit rate
(3) Sender passes loss event rate & RTT into TFRC Equation
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Optimum RTT = ropt
• Represents the min. RTT during MAC layer saturation
• Helps account for queuing at individual nodes
= avg. MAC layer back off time
= time required to successfully transmit a packet
(r(N)) assumes saturation of the MAC layer and can therefore be used for ropt for an N hop ad hoc wireless network
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RE-TFRC Algorithm
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Outline
• Introduction & Background
• RE-TFRC
• Evaluation
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Evaluations
Core Experiments– detailed analysis of a 7 hop NS-2 simulation– # of hops is varied from 4 to 15 (& multiple flows)– typical Bit Error Rate network environment
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7 hop simulationRE-TFRC has lower probability of retransmissions
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Varied hop count simulationMAC layer drop ratio is reduced 13% to 66%
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Varied hop count simulationRTT for RT TFRC is 5% to 40% lower
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Varied hop count simulationRE-TFRC loss event rate is 8% to 55% less
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BER Evaluation
• 7-hop wireless network with single flow simulation
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References & Acknowledgments
RFC 3488
Floyd97Memo - “TCP-Friendly Unicast Rate-Based Flow
Control”
Floyd00“Equation-Based Congestion Control for Unicast
Applications”
Kinicki04Presentation on “Performance Enhancement of TFRC in
Wireless Ad Hoc Networks”
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Backup Slides
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