Performance Enhancement of TFRC in Wireless Ad Hoc Networks

Travis Grant – grant_travis@emc.com

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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