The Impact of Multihop Wireless Channel on TCP Throughput and Loss

1. The Impact of Multihop Wireless Channel on TCP Throughput and Loss Presented by Scott McLaren Zhenghua Fu, Petros Zerfos, Haiyun Luo, Songwu Lu, Lixia Zhang, Mario Gerla (UCLA),INFOCOM 2003, San Francisco, Mar. 2003.

2. Overview • Introduction • Background • Throughput in Multihop Wireless Networks • Loss Behavior • Improving Performance • Conclusions

3. Introduction • Improve channel utilization by spatial channel reuse • A TCP window size W* exists at which throughput is maximized by achieving best spatial reuse • Increasing the window size past W* will reduce throughput • Standard TCP typically grows its average window much larger than W*

4. Techniques to improve efficiency • Link-RED • Tune the wireless link’s drop probability • Adaptive link-layer pacing scheme • Increase the spatial reuse of the channel • Allow TCP to operate in the contention avoidance region

5. 802.11 • RTS/CTS messages • Nodes hearing this handshake defer transmission until current transmission is finished • Data is dropped if no CTS is received after 7 RTS retries • Data is also dropped if 4 transmissions are sent without an receiving an ACK

6. Hidden Terminals • A hidden terminal is a node in the receiver’s neighborhood, that can’t detect sender and may disrupt transmissions

7. Nodes are 200m apart • Transmission range is 250m • Carrier sensing and interference range is 550m • D is a hidden terminal of A  B • Cannot hear CTS ( > 250m ) • Cannot hear data from A, A is outside of D’s carrier sensing range • D can transmit to E • Causes collision at B, since D is within 550m interference range for B • Contention loss at B

8. Chain Topology • Best throughput when window size is h/4 • Assuming ideal MAC protocol and equal packet sizes • Max concurrent senders is h/4, where max spatial reuse is achieved • TCP window size < h/4  under utilization • TCP window size > h/4  reduced throughput

10. Cross Topology • 2 TCP flows • Best window W* = 2, measured window = 12 • 20% throughput reduction

11. Grid Topology • 4, 8, and 12 TCP flows • ½ of flows in each direction • Measured TCP windows are larger than max achievable throughput

12. Results

13. TCP Loss Behavior • Using 8-hop chain, all 165 TCP drops out of 12349 transmissions were due to link drops

14. TCP Loss Behavior

15. Corollaries • m – number of backlogged nodes • B* – the max number of nodes that can transmit their DATA packets concurrently without collision • C* – denotes the max number of nodes that can initiate RTS messages • Corollary 4.1 • m < B* • Pl≈ 0 • Corollary 4.2 • m > B* • Plincreases as m increases • Corollary 4.3 • m > C* • Pl remains constant • Throughput reduction due to Wavg >> W*, Pl > 0, Link contention > 0 reducing spatial reuse

16. Improving TCP Performance • Distributed Link RED (LRED) • Adaptive Pacing

17. LRED • Easy way is to improve performance by reducing buffer size, but problems with bursty traffic • LRED exploits dropping in 802.11 MAC • RED provides a linearly increasing drop curve as queue exceeds a min size • LRED provides a linearly increasing drop curve as link drop probability exceeds a min size

18. LRED • Link layer maintains average number of retries • Next packet is dropped/marked with probability based on average number • If average number of retries is small, packets are not dropped/marked • When retries increase, the dropped/marked probability is calculated

19. Adaptive Pacing • Improve spatial channel reuse by balancing traffic among nodes • Exposed receiver problem • Let a node backoff an additional packet transmission time when necessary

20. Adaptive Pacing • Enabled from LRED • If average retries < min_th calculate backoff time as usual • If pacing, backoff time increases by a time equal to the transmission time of the previous packet

21. Performance • Chain Topology • In all cases LRED & Pacing increased TCP throughput by up to 30% • TCP stabilizes at a window size close to the optimal value • The longer the chain, the better the improvement, due to pacing optimizing spatial channel reuse

22. Chain Topology

23. Performance • Cross Topology • Increased throughput and improves fairness (Jain’s) for both flows • TCP NewReno has large unfairness, due to 802.11 capture characteristic (collision of 2 packets, one weaker than the other. The stronger packet is received)

24. Performance • Grid Topology • Also increases throughput and fairness

25. Conclusions • Only when buffer is small do buffer overflow drops dominate • As buffer increases, link-layer drops dominate • Link drop acts as a RED gateway, but is insufficient

26. Questions