An End-to-end Approach to Increase TCP Throughput Over Ad-hoc Networks

1. An End-to-end Approach to Increase TCP Throughput Over Ad-hoc Networks Sarah Sharafkandi and Naceur Malouch

2. Introduction • TCP is designed for wired networks • Congestion control : window-based • With IEEE 802.11 PHY & MAC, TCP over Ad-hoc has a low performance: • congestion control and not “collision” control: • TCP react to buffer overflow • "bursty"  traffic • inherent reverse traffic • Objective: Improve TCP throughput without modifying PHY, MAC and NET layers.

3. When collision causes DATA loss? • By hidden nodes: packets sent by D collide with A’s packets at node B preventing B from decoding A’s packets. • By repetitive retries due to “ordinary” collisions: it happens when   C*  rare event • Bybuffer overflow : due to increased waiting times  not considered in this work

4. State of the art • Distributed Link RED and Adaptive pacing [Fu et al. INFOCOM’2003] • If the average number of retransmission retry > min_thresh : • early drop of packets • increase the backoff period •  Improvement: 10%-30% for the chain topology • Increasing retry limit and optimum packet size [Jiang et al. DISCEX’O3] • Increasing the retry limit reduces oscillations in the instantaneous thpt • Increasing the packet size increases the thpt till some thresh • Improving TCP throughput using Delayed ack method [Altman et al. MADNET’03] • delayed ack factor = 2, 3

5. An end-to-end approach to “collision control” ?!

6. Simulation Scenario • NS2 network simulator • Chain topology • The source and destination at both ends of the chain • AODV as a routing protocol • Some modifications to the source code of NS2: • delayed ack > 2 • monitoring without file traces • token bucket: packet version

7. TCP Sends the packets in “burst” • Two experiments to show the effect of “burstiness” • Simulation with TCP using RFC3465 • Simulation with CBR traffic

8. Simulation with TCP using RFC3465 • The “burstiness” of RFC3465 results in throughput reduction despite the gain in the window growth

9. Simulation with CBR traffic: Results • i CBR traffics with rate r/i, i = 1, 2, 3, 4. • Best result is when there is packet spacing  “burstiness” is minimum

10. New approach • Bursty data traffic over Ad-hoc networks results to performance reduction • Shaping : • Controls the rate of releasing packets to the network • No more aggressive traffic • Plus delayed ack  approaches the optimal channel reuse

11. Throughput of TCP with shaper and delayed ack • Shaper increases the TCP throughput by 53%-120%

12. Shaper and Delayed ack • Shaper allow delayed ack mechanism to bypass the limit of d=3 

13. Optimum rate • There is always an optimum rate for the shaper in which TCP has the best performance

14. TCP throughput as a function of Number of hops • Optimum rate decreases when number of hops increases

15. Impact of bucket size • A data can pass through the shaper only if it can get a token from token buffer. • We can use it to test again the effect of burstiness

16. Tokens • Again allowing “burstiness” results to throughput reduction

17. Effectiveness of Shaping in presence of CBR Traffic • Network scenario : • same source/destination for UDP traffic  UDP share all the ad-hoc routers with TCP • Compute the gain while increasing the rate of UDP:

18. Conclusion • TCP throughput drops significantly because of: • link contention caused by hidden terminal problem • An "aggressive“ TCP sender causes an increased contention at the MAC layer • Implementing a shaper at the sender improves TCP throughput by controlling the aggression of TCP data traffic • Delayed ack mechanism plus the shaper → increase spatial channel reuse

19. Future work • An adaptive algorithm for finding the optimum rate • difficulties: convergence and stability • Related work: [ElRakabawy et al. MobiHoc’2005] • same idea: end-to-end solution • BUT : • change TCP protocol for the multihop wireless ad-hoc • based on the esimation of the 4-hop transmission delay • Our approach :