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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. Outline. Introduction Background

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performance enhancement of tfrc in wireless ad hoc networks

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

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

DMS 2004 September 9, 2004

introduction
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-Send (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.

DMS 2004 September 9, 2004

introduction1
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.

DMS 2004 September 9, 2004

outline1
Outline
  • Introduction
  • Background
  • TFRC Performance over Wireless Networks
  • RE-TFRC Algorithm
  • Performance Evaluation
  • Conclusions and Future Work

DMS 2004 September 9, 2004

tcp friendly rate control tfrc
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

DMS 2004 September 9, 2004

hidden terminal problem
Hidden Terminal Problem

1

2

3

  • Node 1 is hidden from Node 3:
    • Node 1 and node 3 cannot sense each other’s 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.

DMS 2004 September 9, 2004

hidden terminal problem1
Hidden Terminal Problem

1

2

3

  • 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!!

DMS 2004 September 9, 2004

mac layer saturation
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.

DMS 2004 September 9, 2004

outline2
Outline
  • Introduction
  • Background
  • TFRC Performance over Wireless Networks
  • RE-TFRC Algorithm
  • Performance Evaluation
  • Conclusions and Future Work

DMS 2004 September 9, 2004

tfrc performance investigation
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.

DMS 2004 September 9, 2004

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

DMS 2004 September 9, 2004

rate constrained tfrc1
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.

DMS 2004 September 9, 2004

outline3
Outline
  • Introduction
  • Background
  • TFRC Performance over Wireless Network
  • RE-TFRC Algorithm
  • Performance Evaluation
  • Conclusions and Future Work

DMS 2004 September 9, 2004

rate estimation tfrc re trfc
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.

DMS 2004 September 9, 2004

rate estimation
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.

DMS 2004 September 9, 2004

re tfrc rate estimation
RE-TFRC Rate Estimation
  • Use R to estimate p’
  • Use p’ to estimate R’
  • TCP Friendly equation:
  • Inverse TCP function:

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

DMS 2004 September 9, 2004

round trip time modeling
Round Trip Time Modeling
  • Single hop delay model: [Carvalho03]
  • Multi-hop chain delay model:
    • Divide the N-hop chain into N-2 4-nodenetworksand two 3-node networks.
    • Sum the data/ack packet delay over the N hops.

DMS 2004 September 9, 2004

round trip time modeling1
Round Trip Time Modeling

1

2

3

4

5

: estimate of rttopt for N-hop chain topology

: Single hop delay of Ack packet

: Single hop delay of Data packet

DMS 2004 September 9, 2004

rate estimation tfrc algorithm
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.

DMS 2004 September 9, 2004

outline4
Outline
  • Introduction
  • Background
  • TFRC Performance over Wireless Network
  • RE-TFRC Algorithm
  • Performance Evaluation
  • Conclusions and Future Work

DMS 2004 September 9, 2004

simulation details
Simulation Details
  • NS-2 was used to simulate and evaluate RE-TFRC performance.
  • Wireless Multi-hop Chain Network
    • N-hop network implies N+1 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.

DMS 2004 September 9, 2004

seven hop chain topology
Seven-Hop Chain Topology

CDF of MAC layer retransmissions

DMS 2004 September 9, 2004

loss event rate for multi hop chains
Loss Event Rate for Multi-Hop Chains

Average loss event rate versus number of hops

DMS 2004 September 9, 2004

round trip times for multi hop chains
Round Trip Times for Multi-Hop Chains

Average round trip time versus number of hops

DMS 2004 September 9, 2004

throughput for multi hop chains
Throughput for Multi-Hop Chains

Average throughput versus number of hops

DMS 2004 September 9, 2004

loss event rate for multi flow tests
Loss Event Rate for Multi-Flow Tests

Average loss event rate for various flow scenarios

DMS 2004 September 9, 2004

round trip time for multi flow tests
Round Trip Time for Multi-Flow Tests

Average round trip time for various flow scenarios

DMS 2004 September 9, 2004

throughput for multi flow tests
Throughput for Multi-Flow Tests

Average throughput for various flow scenarios

DMS 2004 September 9, 2004

bit error rate test of re tfrc
Bit Error Rate Test of RE-TFRC
  • Single flow, seven-hop chain topology

DMS 2004 September 9, 2004

outline5
Outline
  • Introduction
  • Background
  • TFRC Performance over Wireless Networks
  • RE-TFRC Algorithm
  • Performance Evaluation
  • Conclusions and Future Work

DMS 2004 September 9, 2004

conclusions
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.

DMS 2004 September 9, 2004

future work
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.

DMS 2004 September 9, 2004