Enhancing Network Models for Internet Research

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?