The Impact of Multihop Wireless Channel on TCP Throughput and Loss

1. The Impact of Multihop Wireless Channel on TCP Throughput and Loss Zhenghua Fu, Petros Zerfos, Haiyun Luo, Songwu Lu, Lixia Zhang, Mario Gerla INFOCOM2003, San Francisco, April 2003 Presented by Philip Hardebeck

2. Outline • Introduction • Background • TCP Throughput • Several Topologies: Chain, Cross, Grid, Random • Simulations, Experiments, & Analysis • Proposed Solutions • Conclusions Advanced Computer & Comm. Networks: Multihop Wireless Channel

3. Introduction • Do TCP mechanisms work well for Wireless Multihop Networks (WMN)? • WMNs differ from wired networks. • There is an optimal TCP window size for a given topology and flow pattern. Advanced Computer & Comm. Networks: Multihop Wireless Channel

4. More Introduction • Packet losses increase as window size exceeds optimal, up to a threshold. • Link-RED and Adaptive Pacing are proposed to increase throughput. Advanced Computer & Comm. Networks: Multihop Wireless Channel

5. Background: MAC Basics RTS A B C D E CTS CTS DATA A B C D E ACK ACK RTS RTS RTS 8 x A B C D E … random exponential backoff ... RTS A B C D E CTS Advanced Computer & Comm. Networks: Multihop Wireless Channel

6. Spatial Reuse and Contention Interfering Range Communication Range A B C D E F G H I Interfering/Carrier Range of Node B RTS A B C D E CTS Interfering/Carrier Range of Node D DATA RTS A B C D E Advanced Computer & Comm. Networks: Multihop Wireless Channel

7. TCP Throughput • Look at TCP throughput to show how well or poorly it performs spatial reuse. • Typical TCP operation doesn’t do a good job and the throughput is reduced. • Identify window size for highest throughput, and verify with hardware experiments. Advanced Computer & Comm. Networks: Multihop Wireless Channel

8. Chain Topology • Packets of a single flow interfere with one another. • Optimal window size is ~1/4 * number of hops in the chain. Advanced Computer & Comm. Networks: Multihop Wireless Channel

9. Optimal Window Size vs. Chain Length Advanced Computer & Comm. Networks: Multihop Wireless Channel

10. Throughput for 3 Packet Sizes Advanced Computer & Comm. Networks: Multihop Wireless Channel

11. Actual vs. Simulated Throughput Advanced Computer & Comm. Networks: Multihop Wireless Channel

12. Cross and Grid Topologies Advanced Computer & Comm. Networks: Multihop Wireless Channel

13. Aggregate Throughput and Window Size Table III Advanced Computer & Comm. Networks: Multihop Wireless Channel

14. Throughput Summary • Optimal window size exists for all topologies and flow patterns. • Optimal window size derivable only for simple configuration (chain). • Average TCP window size is much larger than optimal • Causes more packet drops and reduced throughput Advanced Computer & Comm. Networks: Multihop Wireless Channel

15. Loss Behavior • Buffer drop probability is not significant in WMN, but contention drop is. • “Network overload is no longer a bottleneck link property, but a shared feature of multiple links.” • Drop probability increases “gracefully” as load increases. Advanced Computer & Comm. Networks: Multihop Wireless Channel

16. TCP Packet Drop Probability Advanced Computer & Comm. Networks: Multihop Wireless Channel

17. UDP Packet Drop Probability at MAC layer Advanced Computer & Comm. Networks: Multihop Wireless Channel

18. Contrasting Drop Characteristics Advanced Computer & Comm. Networks: Multihop Wireless Channel

19. Analysis of Link Drop Probability • Modeling a random topology, drop probability is • Three regions of behavior • Pl ~0: m, number of backlogged nodes, is < B*, maximum number of concurrent DATA transmitting nodes, and m~b~c Advanced Computer & Comm. Networks: Multihop Wireless Channel

20. Analysis of Link Drop Probability Continued • Other two regions: • Pl increases linearly: m>B* and m<C*, maximum number of nodes with a clear channel • Pl stable: m>C* - the amount of contention cannot increase Advanced Computer & Comm. Networks: Multihop Wireless Channel

21. Link-RED Algorithm Advanced Computer & Comm. Networks: Multihop Wireless Channel

22. Adaptive Pacing Algorithm Advanced Computer & Comm. Networks: Multihop Wireless Channel

23. TCP Throughput Comparison Advanced Computer & Comm. Networks: Multihop Wireless Channel

24. Multiflow TCP Throughput Comparison Advanced Computer & Comm. Networks: Multihop Wireless Channel

25. Average TCP Window Size Comparison Advanced Computer & Comm. Networks: Multihop Wireless Channel

26. Discussions • TCP Vegas doesn’t work as well as New Reno. • Optimal window sizes exist for flows with variable packet size, but more complicated. • LRED and Adaptive Pacing improve drop behavior. Advanced Computer & Comm. Networks: Multihop Wireless Channel

27. Related Work • Link-layer retransmission hides channel errors from upper layers • Dynamic ad hoc networks and link failure are studied (routing issues) • Studies of TCP ACK traffic using other MAC protocols • Capacity of ad hoc networks using UDP/CBR flows Advanced Computer & Comm. Networks: Multihop Wireless Channel

28. Conclusions • TCP throughput improves if the window size operates at optimal, maximizing channel spatial reuse. • TCP typically operates with a much larger window, reducing throughput. • Wireless nodes exhibit a graceful drop feature. • LRED and Adaptive Pacing improve throughput by up to 30% Advanced Computer & Comm. Networks: Multihop Wireless Channel

29. Problems/Weaknesses • No explanation for the 10% difference between simulation and experimental results. • Use of aggregate rate and window size makes it difficult to compare results to other papers’. Advanced Computer & Comm. Networks: Multihop Wireless Channel

30. Acknowledgements • Thanks to Professor Kinicki for the opportunity to make this presentation. • Thanks to Shugong Xu and Tarek Saadawi of CUNY for the MAC Basics and Spatial Reuse and Contention graphics. Advanced Computer & Comm. Networks: Multihop Wireless Channel

31. Questions/Comments? Advanced Computer & Comm. Networks: Multihop Wireless Channel