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

3. 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.

4. 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.

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

6. DMS 2004 September 9, 2004 6  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). TCP Friendly Rate Control (TFRC) X: Transmission rates: packet size r: round trip timep: lost event rate t rto: : Retransmission time out b: num of packets in each ack

7. DMS 2004 September 9, 2004 7 Hidden Terminal Problem  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. 123

8. DMS 2004 September 9, 2004 8 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!! 123

9. 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.

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

11. 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.

12. 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.

13. 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.

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

15. 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.

16. 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-rate est * rtt min – rtt min 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: rtt opt –Control the sending rate on congestion.

17. DMS 2004 September 9, 2004 17  TCP Friendly equation:  Inverse TCP function: X: TCP Friendly rate p: TFRC loss event rate RE-TFRC Rate Estimation  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

18. DMS 2004 September 9, 2004 18  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. Round Trip Time Modeling

19. DMS 2004 September 9, 2004 19 Round Trip Time Modeling 12345 : estimate of rtt opt for N-hop chain topology : Single hop delay of Ack packet : Single hop delay of Data packet

20. DMS 2004 September 9, 2004 20 Rate Estimation TFRC Algorithm On receiving an ack: 1.Compute R (the original TCP Friendly rate). 2.Estimate rtt opt. using the r(N) approximation. [Assume N can be obtained from the routing protocol.] 3.Compute the adjusted loss event rate p’ using rtt opt and R. 4.Compute the estimated optimum send rate R’. 5.Use the original rate, R, if the new rate, R’, is larger. 6.If there is a rate change, make the change incrementally as TFRC does.

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

22. 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 (n 0 to n N ). –All simulated TRFC flows go from n 0 to n N. –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.

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

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

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

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

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

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

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

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

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

32. 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.

33. 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.

34. Thanks! rek@cs.wpi.edu