When TCP Friendliness Becomes Harmful

1. Amit Mondal Aleksandar Kuzmanovic Northwestern University When TCP Friendliness Becomes Harmful http://networks.cs.northwestern.edu/

2. Introduction • Internet was designed for throughput • TCP probes for available b/w even if it causes packet losses • Interactive applications suffer! • Telnet, ssh, games, chat all use TCP • Telnet delay from typed character until echo • Includes transmission, propagation and queuing delay • It loss, TCP retransmits

3. Upgrading mice to elephants data packets strict priority TCP-fair rate “dummy” packets Padding misbehavior

4. Outline • Source of the problem • Related work • Implication • Padding-induced response time gain • Sustainable countermeasures • Conclusion

5. Outline • Source of the problem • Related work • Implication • Padding-induced response time gain • Sustainable countermeasures • Conclusion

6. Source of the problem How TCP detects packet loss? Retransmission Timeout (RTO) (1 second) Triple duplicate acknowledgment/Fast retransmit (~ 10-100 ms) How does it affect interactive applications? Burst size (~ 1-2 packets) Inter-burst time (~ seconds) Packet loss is detected by timeout, which degrades the response time in orders of magnitude

7. improvement C D A B A deeper look RTO Incentive for misbehavior!

8. Outline • Source of the problem • Related work • Implication • Padding-induced response time gain • Sustainable countermeasures • Conclusion

9. Related Work • Packet marking & differential dropping [Guo and Matta ‘01] • Service differentiation [Noureddine and Tobagi ‘02] • Differential Congestion Notification [ Le et al. ’04] • Explicit congestion notification [Floyd ‘94] • TCP smart framing [Mellia et al. ‘05]

10. Outline • Source of the problem • Related work • Implication • Padding-induced response time gain • Sustainable countermeasures • Conclusion

11. Implication Packet switched => Circuit switched

12. Outline • Source of the problem • Related work • Implication • Padding-induced response time gain • Sustainable countermeasures • Conclusion

13. p 1-p 1-Q Q RTT RTO 1-p 1-p p p RTO 1-p 1-p 2RTO p p 4RTO 2RTO Quantifying padding induced-gain P = packet loss ratio Q = prob. packet loss is detected timeout Correlated packet loss (FIFO) Random packet loss (RED)

14. RED FIFO Modeling Upgrading interactive TCP flows to fully backlogged flows always pays-off

15. Simulations 1) Queuing delay plays a role 2) Consecutive packet losses can affect overall gain achieved

16. Outline • Source of the problem • Related work • Implication • Padding-induced response time gain • Sustainable countermeasures • Conclusion

17. Sustainable countermeasures (1) Differentiated minRTO Application-limited flows use reduced value for minRTO parameter Short-term padding with dummy packets Enable that a packet loss is detected via fast retransmit mechanism Actual packet followed by three tiny dummy packets. A diversity approach TCP sends k (k>1, k is a small integer) copies of the packet without violating congestion control mechanism In reality k=2 is sufficient

18. Sustainable countermeasures (2) Differentiated minRTO does not work Short-term padding and diversity approaches outperform fully-backlogged approach

19. Sustainable countermeasures (3) RED FIFO Short-term padding and diversity approaches outperform fully-backlogged approach

20. RED FIFO Overhead and Sustainability Issue: What happens when everyoneadopts our approach? Short-term padding and diversityapproaches provide significantly friendlier environment

21. Conclusion TCP’s loss recovery mechanism degrades response time of interactive applications relative to long flows during congestion. Incentive for padding misbehaviour and seriously degrades overall networks performance. Explored simple, sustainable and easily deployable solutions.

22. Questions?