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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. Introduction Background

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Performance Enhancement of TFRC in Wireless Ad Hoc Networks

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  1. 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

  2. Outline • Introduction • Background • TFRC Performance over Wireless Networks • RE-TFRC Algorithm • Performance Evaluation • Conclusions and Future Work DMS 2004 September 9, 2004

  3. 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

  4. 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

  5. Outline • Introduction • Background • TFRC Performance over Wireless Networks • RE-TFRC Algorithm • Performance Evaluation • Conclusions and Future Work DMS 2004 September 9, 2004

  6. 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

  7. 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

  8. 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

  9. 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

  10. Outline • Introduction • Background • TFRC Performance over Wireless Networks • RE-TFRC Algorithm • Performance Evaluation • Conclusions and Future Work DMS 2004 September 9, 2004

  11. 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

  12. 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

  13. 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

  14. Outline • Introduction • Background • TFRC Performance over Wireless Network • RE-TFRC Algorithm • Performance Evaluation • Conclusions and Future Work DMS 2004 September 9, 2004

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

  21. Outline • Introduction • Background • TFRC Performance over Wireless Network • RE-TFRC Algorithm • Performance Evaluation • Conclusions and Future Work DMS 2004 September 9, 2004

  22. 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

  23. Seven-Hop Chain Topology CDF of MAC layer retransmissions DMS 2004 September 9, 2004

  24. Loss Event Rate for Multi-Hop Chains Average loss event rate versus number of hops DMS 2004 September 9, 2004

  25. Round Trip Times for Multi-Hop Chains Average round trip time versus number of hops DMS 2004 September 9, 2004

  26. Throughput for Multi-Hop Chains Average throughput versus number of hops DMS 2004 September 9, 2004

  27. Loss Event Rate for Multi-Flow Tests Average loss event rate for various flow scenarios DMS 2004 September 9, 2004

  28. Round Trip Time for Multi-Flow Tests Average round trip time for various flow scenarios DMS 2004 September 9, 2004

  29. Throughput for Multi-Flow Tests Average throughput for various flow scenarios DMS 2004 September 9, 2004

  30. Bit Error Rate Test of RE-TFRC • Single flow, seven-hop chain topology DMS 2004 September 9, 2004

  31. Outline • Introduction • Background • TFRC Performance over Wireless Networks • RE-TFRC Algorithm • Performance Evaluation • Conclusions and Future Work DMS 2004 September 9, 2004

  32. 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

  33. 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

  34. Thanks!rek@cs.wpi.edu

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