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TCP Performance and Fairness over Mobile Ad Hoc Networks

TCP Performance and Fairness over Mobile Ad Hoc Networks. Seok-Hoon Yoon. Index. Introduction Issues of TCP over MANETs TCP Performance over MANETs Cross Layer (TCP and network) Approaches TCP-ELFN TCP Fairness over MANETs Network Layer Approaches Neighborhood RED Conclusion.

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TCP Performance and Fairness over Mobile Ad Hoc Networks

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  1. TCP Performance and Fairness over Mobile Ad Hoc Networks Seok-Hoon Yoon

  2. Index • Introduction • Issues of TCP over MANETs • TCP Performance over MANETs • Cross Layer (TCP and network) Approaches • TCP-ELFN • TCP Fairness over MANETs • Network Layer Approaches • Neighborhood RED • Conclusion 2/41

  3. Research Trend • TCP performance over MANETs • Most TCP performance studies are based on simulations and experiments rather than an analytical study • Many approaches • Single layer • TCP • Link, Mac • Cross layer • TCP and network • TCP and physical • Network and physical • TCP fairness over MANETs • Many investigation papers • A few suggestions • Neighborhood RED 3/41

  4. Issues of TCP over MANETs • Lossy channels • High bit error rate • Path asymmetry • Bandwidth asymmetry • Loss rate asymmetry • The backward path is much more lossy than the forward path • It may produce bandwidth asymmetry • Route asymmetry • Due to lack of transmission power • Distinct paths for TCP data and TCP ACKs 4/41

  5. Issues of TCP over MANETs • Network partition • Due to node mobility and energy constrained operation • If disconnectivity > RTO • The TCP sender will trigger exponential backoff • Doubling the RTO • After the network is connected again, TCP is still in the backoff state 5/41

  6. Issues of TCP over MANETs • Routing failures • Very frequent events in MANETs • Due to node mobility and repeated transmission failure from link layer contention • After route re-establishment TCP will face a brutal fluctuation in RTT • Power constraints • Power saving – reducing the power consumption • Power control – adjusting the transmission power of mobile nodes 6/41

  7. Issues of TCP over MANETs • TCP Congestion Control • TCP uses the occurrence of losses to detect congestion • In MANETs, random wireless errors and mobility serves as primary contributor to losses as well as congestion • More than 80% of the losses in the network are due to link failures • Essentially, most losses in ad-hoc networks occur as a result of route failures • If TCP enters congestion control state because of packet losses caused by random wireless errors and mobility, then the throughput of TCP can be degraded significantly 7/41

  8. Why TCP? • Many drawbacks of TCP • New Transport Protocol for MANETs? • ATP • Layer Coordination • Rate Based Transmissions • TCP for MANETs? • A large number of application • Seamless integration with the Internet 8/41

  9. Index • Introduction • Issues of TCP over MANETs • TCP Performance over MANETs • Cross Layer (TCP and network) Approaches • TCP-ELFN • TCP Fairness over MANETs • Network Layer Approaches • Neighborhood RED • Conclusion 9/41

  10. TCP ELFN (Explicit Link Failure Notification ) • Analysis of TCP performance in static, linear, multi hop wireless network • Analysis of TCP in MANETs using expected throughput and measured throughput • Suggestion of TCP ELFN • Simulation results 10/41

  11. TCP performance in simple, static, linear multi-hop network • A simple multi-hop network • TCP-Reno throughput over an 802.11 fixed, linear, multi-hop network of varying length 11/41

  12. ∞ ∞ Σi=0 Σi=0 Performance metric • Performance metric • Expected throughput = t i *Ti t i i: # of hops ti: the duration for which the shortest path contains i hops Ti: the throughput obtained “over a linear chain” using i hops • Expected throughput does not take into account the performance overhead of determining new routes after route failures • It serves as a upper bound of throughput in mobile network 12/41

  13. Performance metric :Expected throughput • Example Δt=t2 Δt=to Δt=t1 S R S R S R Throughput = TH1 Throughput = TH3 Throughput = TH1 Throughput in linear network when # hops is n Expected throughput = t 0*TH1 + t1*TH2 + t2*TH1 to + t1 + t2 13/41

  14. Expected throughput and Measured Throughput • Simulation environment • ns network simulator • TCP-Reno over 802.11 • DSR, BSD’s ARP • 30 nodes, 1500X300 m2 , the random waypoint • The average throughput of 50 scenarios From 2m/s to 10m/s the throughput drops sharply 14/41

  15. Comparison of measured and expected throughput for the 50 different Mobility patterns( 2m/s, 10m/s, 20m/s, 30m/s) 15/41

  16. Zero Throughput • T = 0s, route fail, packet dropped S A B C R • T = 6s, data packet retransmitted S A B C R • T = 6.1xxs, ACK dropped, due to stale cached route S A B C R • T = 18.1xxs, the second retransmission of data packet, dropped again due to stale cached route S A B C R 16/41 • T=42,90,120s no ACK from the TCP receiver

  17. Some facts • In previous example, only for 6 s of 120 s the network is partitioned • DSR’s stale cached route can degrade TCP throughput significantly • DSR does not retransmit dropped packet when it receives Route Error Msg, and the TCP sender or receiver does not know about the packet loss • The TCP sender waits for occurring time out • Unnecessary RTO back-off of the TCP sender makes problems even worse 17/41

  18. TCP ELFN • Explicit Link Failure Notification (ELFN) • The objective : • To provide the TCP sender with information about link and route failures • TCP sender can avoid responding to the failures as if congestion occurred • DSR’s route failure message is modified • A payload similar to the “host unreachable” ICMP message • The sender and receiver’s addresses and ports and seq number TCP data R S A B C D 18/41 DSR ROUTE ERROR + ELFN Probing message

  19. TCP ELFN • Sender reaction • When a TCP sender receives an ELFN, • It disables its retransmission timers and enters a “stanby” mode • While on standby, • A packet is sent at periodic intervals to probe the network to see if a route has been established • If an acknowledgment is received, • Then it leaves stanby mode 19/41

  20. Simulation for the 50 different Mobility patterns( 2m/s, 10m/s, 20m/s, 30m/s) 20/41

  21. Simulation for the different probing intervals and different window and RTO modification • Different probing interval • If the interval is too large, it delays the discovery of new routes • If the interval is too small, the rapid injection of probes into the network will cause congestion and lower throughput 21/41

  22. Index • Introduction • Issues of TCP over MANETs • TCP Performance over MANETs • Cross Layer (TCP and network) Approaches • TCP-ELFN • TCP Fairness over MANETs • Network Layer Approaches • Neighborhood RED • Conclusion 22/41

  23. Unfairness of TCP in MANETs • Significant TCP unfairness in ad hoc wireless networks • Channel capture • Hidden terminal conditions • The binary exponential backoff of IEEE 802.11 • The RED scheme for wired networks • Keeps the queue size relatively small and drops or marks packets proportional to the bandwidth share • avg = (1-wq)*avg + wq*q • q: current queue size, wq: queue weight • It does not work in wireless ad hoc networks 23/41

  24. RED in MANETs • Simple simulation • 3 FTP connections • FTP2 is always starved 24/41

  25. RED in MANETs • Why does not RED work well in MANETs? • A TCP connection penalized in channel contention drop more packets • It may actually increase the unfairness • Congestion does not happen in a single node • Instead happens in an entire area involving multiple nodes • Multiple nodes should coordinate their “packet drops”, rather than drop independently 25/41

  26. Neighborhood RED • Overview of NRED • NRED extends the original RED scheme • Each node keeps estimating the size of its neighborhood queue (distributed queue) • Once the queue size exceeds a certain threshold, an overall drop probability is computed by the algorithm of RED • This overall drop probability is then propagated to neighboring nodes for cooperative packet drops • However, there is no real distributed queue in ad hoc network, so how to implement distributed queue? 26/41

  27. Neighborhood and Its Distributed Queue • Neighborhood • A node’s neighborhood consists of the node itself and the nodes which can interfere with this node’s signals • Distributed Queue of a Node • The outgoing queue of the node itself • 1-hop neighbors' outgoing queues • 2-hop neighbors’ packets which are directed to a 1-hop neighbor of node A A node’s Neighborhood and it’s distributed queue 27/41

  28. A Simplified Neighborhood Queue Model • Simplified Model • 2-hop neighborhood distributed queue model is not easy to implement and evaluate • A lot of control packet overhead • The packets in the 2-hop neighbors directed to a 1-hop neighbor are moved to the 1-hop neighbor • Outgoing queue – the original queue at a node • Incoming queue – the packets from 2-hop neighbors 28/41

  29. Neighborhood Random Early Detection (NRED) • 3 problems to solve • How to detect the early congestion of a neighborhood? • Neighborhood Congestion Detection (NCD) • When and how does a node inform its neighbors about the congestions? • Neighborhood Congestion Notification (NCN) • How do the neighbor nodes calculate their local drop probabilities? • Distributed Neighborhood Packet Drop (DNPD) 29/41

  30. Neighborhood Congestion Detection (NCD) • A direct way to monitor the neighborhood queue size • Every node broadcast a control packet to announce its queue size • A lot of control overhead will be caused • A passive measurement technique • An alternate measure related to queue size • Channel utilization • A relationship between channel utilization and the size of both outgoing and incoming queues • When these queues are busy, channel utilization around the node is more likely to increase • How to know the channel utilization of neighborhood? 30/41

  31. Neighborhood Congestion Detection (NCD) • A passive measurement technique data CTS A A A packet is received to any incoming queue A packet in outgoing queue is transmitted 31/41

  32. Tinterval - Tidle Tinterval Ttx Trx Tinterval Tinterval Neighborhood Congestion Detection (NCD) • A node monitors five different radio state • Transmitting (Ttx) • Receiving (Trx) • Carrier sensing busy (Tcs) • Virtual carrier sending busy (Tvcs) • Idle (Tidle) • By monitoring the five radio states, a node can now estimate 3 channel utilization ratio • Total channel utilization Ubusy = • Transmitting ratio Utx = • Receiving ratio Urx = • Tinterval = Ttx + Trx + Tcs + Tvcs + Tidle • Ubusy reflects the size of the neighborhood queue • Utx and Urx reflect the channel bandwidth usage of the outgoing queue and incoming queue at current node 32/41

  33. Ubusy * W C Neighborhood Congestiond Detection (NCD) • To facilitate the implementation of the RED algorithm, the channel utilization is translated into an index of the queue size • The queue size index q = W:channel bandwidth, C: the average packet size • Now the original RED scheme can be applied • The average queue size , • avg = (1-wq)*avg + wq*q • If the queue size exceeds a certain threshold, the neighborhood is in congestion 33/41

  34. Maxp* (Avg – Minth) Maxth - Minth Neighborhood Congestiond Notification (NCN) • Drop probability • Pb = • Normalized Pb = Pb/avg • Current node A broadcasts Drop probability to 1-hop neighbors • The broadcast message  drop probability + life time • Neighborhood nodes choose the largest drop probability, if they receive multiple NCN 34/41

  35. Pb*(avgtx + avgrx) Pb * avgrx Pb * avgtx avg avg avg Distributed Neighborhood Packet Drop • Each node calculate its “share” of this overall drop probability according to its channel bandwidth usage • Pb_local = • Incoming queue drop probability • Pb_lncoming = • Outgoing queue drop probability • Pb_Outgoing = 35/41

  36. Verification of queue size estimation Estimated Queue Size Real Queue Size <<Queue size of Node 5>> <<Scenario>> 36/41

  37. Simulations – Previous Scenario maxp = 0.14 37/41

  38. Simulations – Multiple congested neighborhood • Dropped packets already used the channel bandwidth • NRED tends to keep the wireless channel underutilized 38/41

  39. Simulations – Mobility <<Scenario>> 39/41

  40. Conclusion • The standard TCP is optimized in context of wired networks • Several issues of TCP over MANETs and characteristics of TCP in MANETs has been introduced • In MANETs, the standard TCP shows poor performance • In MANETs, packet losses is usually caused by high bit error rate, route failures as well as congestion • To avoid to enter the TCP congestion control on route change, several improvements have been proposed • The very poor fairness is shown by the standard TCP in MANETs • For better TCP fairness, NRED has been proposed 40/41

  41. References • K. Xu, M. Gerla, L. Qi, and Y. Shu, “Enhancing TCP fairness in ad hoc wireless networks using neighborhood red,” in Proc. of ACM MOBICOM, San Diego, CA, USA, Sep. 2003, pp. 16–28. • K. Sundaresan, V. Anantharaman, H.-Y. Hsieh, and R. Sivakumar. ATP: A reliable transport protocol for ad-hoc networks. In Proceedings of 4th ACM MobiHoc, pp. 64–75, 2003. • G. Holland and N. Vaidya, “Analysis of TCP performance over mobile ad hoc networks,”ACM Wireless Networks, vol. 8, no. 2, pp. 275–288, Mar. 2002. • Z. Fu, X. Meng, and S. Lu. How bad TCP can perform in mobile ad hoc networks. In Proceedings of 7th IEEE ISCC, 2002. • V. Anantharaman and R. Sivakumar. A microscopic analysis of TCP performance over wireless ad-hoc networks.Presented in 2nd ACM SIGMETRICS (Poster Paper), 2002. • F. Wang and Y. Zhang. Improving TCP performance over mobile ad-hoc networks with out-of-order detection and response. In Proceedings of 3rd ACM MobiHoc, pp. 217–225, 2002. 41/41

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