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Presented by Max Podlesny

Internet research Needs Better Models Sally Floyd, Eddie Kohler ISCI Center for Internet Research, Berkeley, California. Presented by Max Podlesny. Outline. Motivation Network Model Principles Several problems Phase Effects Active Queue Management: Oscillations TCP Variants Proposal

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Presented by Max Podlesny

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  1. Internet research Needs Better ModelsSally Floyd, Eddie KohlerISCI Center for Internet Research, Berkeley, California Presented by Max Podlesny

  2. Outline • Motivation • Network Model Principles • Several problems • Phase Effects • Active Queue Management: Oscillations • TCP Variants • Proposal • Conclusion

  3. Motivation • Little relationship to Internet reality • Unknown relationship to Internet reality • What divergences are acceptable? • Are used models valid? • Measurements and methodologies have never been synthesized into a convenient , coherent hole

  4. Network Model Principles • The full range of parameters that might affect a simulation or experiment, i.e.: • network topology • traffic generation • end-node protocol behavior • queue drop policies • congestion levels • etc.

  5. Several typical models

  6. Requirements to the model • Model should be specific to the research questions being investigated • Model must go hand-in-hand with measurement • Model should be applicable both to the Internet of the future and to the Internet of present • How do model’s parameter settings affect experimental results?

  7. Example:Phase Effects • Sensitive dependence on precise parameter settings • It is not relevant to the modern Internet • Concrete example: S.Floyd, V.Jacobson. On Traffic Phase Effects in Packet-Switched Gateways. Internetworking: Reseacrh and Experience, 3(3), Sept.1992 • Two TCP flows sharing a Drop-Tail queue • Simulation topology is a simple dumbbell • Long-lived flows • No reverse-path traffic

  8. Results of simulations

  9. Real network • Traffic includes short-lived flows • Traffic consists of small control packets as well as large data packets • More than two competing flows

  10. Example: Active Queue Management:Oscillations • Implicit disagreement about which simulation scenarios are the most important to address • Queue oscillations are considered a serious potential problem with RED AQM • Changes in the traffic mix can affect oscillation dynamics

  11. Model • A dumbbell topology with a 15 Mbps • 10ms congested link with Adaptive RED queue management • Similar, small amounts of reverse-path traffic • All run for 100 seconds • Difference: • Traffic mixes • Flow RTTs

  12. Results of simulations

  13. Actually used models

  14. Example:TCP Variants • TCP Reno • TCP Vegas

  15. TCP Reno • Based on acknowledgements • Two types of congestion event: • Duplicate acknowledgement • Timeout • Works well when only one packet is dropped • Losses often come in bursts • The problem is of how to avoid retransmit timeouts

  16. TCP Vegas • Based on packet delays • Optimized only for environments: • having a few active TCP connections • Sending rate of a TCP connection affects the queue size at the router • Problems arise with higher level of statistical multiplexing

  17. Proposal • Questions around congestion-related mechanisms at router queues • Analysis of the questions is supposed to lead to description of experimental parameters relevant for constructing models • Simulations are supposed to show how parameter settings affect the observed behavior of existing techniques • For settings affecting behavior, new measurement studies and analysis of the measurement literature are supposed to describe how the settings look on real networks

  18. Conclusion • Network research has a great need for better models • Each specific research problem requires its own model • Base of a model must be network measurement if it is necessary • Model should be applicable to the Internet of present, and to the Internet of future • A better understanding of which aspects of models are critical for a particular research issue is required

  19. Questions?

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