Internet research Needs Better Models Sally Floyd, Eddie Kohler ISCI Center for Internet Research, Berkeley, California Presented by Max Podlesny

2 - Max Podlesny – 3/31/05 Outline n Motivation n Network Model Principles n Several problems »Phase Effects »Active Queue Management: Oscillations »TCP Variants n Proposal n Conclusion

3 - Max Podlesny – 3/31/05 Motivation n Little relationship to Internet reality n Unknown relationship to Internet reality n What divergences are acceptable? n Are used models valid? n Measurements and methodologies have never been synthesized into a convenient, coherent hole

4 - Max Podlesny – 3/31/05 Network Model Principles n 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 - Max Podlesny – 3/31/05 Several typical models

6 - Max Podlesny – 3/31/05 Requirements to the model n Model should be specific to the research questions being investigated n Model must go hand-in-hand with measurement n Model should be applicable both to the Internet of the future and to the Internet of present n How do model’s parameter settings affect experimental results?

7 - Max Podlesny – 3/31/05 Example:Phase Effects n Sensitive dependence on precise parameter settings n It is not relevant to the modern Internet n 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 - Max Podlesny – 3/31/05 Results of simulations

9 - Max Podlesny – 3/31/05 Real network n Traffic includes short-lived flows n Traffic consists of small control packets as well as large data packets n More than two competing flows

10 - Max Podlesny – 3/31/05 Example: Active Queue Management:Oscillations n Implicit disagreement about which simulation scenarios are the most important to address n Queue oscillations are considered a serious potential problem with RED AQM n Changes in the traffic mix can affect oscillation dynamics

11 - Max Podlesny – 3/31/05 Model n A dumbbell topology with a 15 Mbps n 10ms congested link with Adaptive RED queue management n Similar, small amounts of reverse-path traffic n All run for 100 seconds n Difference: »Traffic mixes »Flow RTTs

12 - Max Podlesny – 3/31/05 Results of simulations

13 - Max Podlesny – 3/31/05 Actually used models

14 - Max Podlesny – 3/31/05 Example:TCP Variants n TCP Reno n TCP Vegas

15 - Max Podlesny – 3/31/05 TCP Reno n Based on acknowledgements n Two types of congestion event: »Duplicate acknowledgement »Timeout n Works well when only one packet is dropped n Losses often come in bursts n The problem is of how to avoid retransmit timeouts

16 - Max Podlesny – 3/31/05 TCP Vegas n Based on packet delays n Optimized only for environments: »having a few active TCP connections »Sending rate of a TCP connection affects the queue size at the router n Problems arise with higher level of statistical multiplexing

17 - Max Podlesny – 3/31/05 Proposal n Questions around congestion-related mechanisms at router queues n Analysis of the questions is supposed to lead to description of experimental parameters relevant for constructing models n Simulations are supposed to show how parameter settings affect the observed behavior of existing techniques n 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 - Max Podlesny – 3/31/05 Conclusion n Network research has a great need for better models n Each specific research problem requires its own model n Base of a model must be network measurement if it is necessary n Model should be applicable to the Internet of present, and to the Internet of future n A better understanding of which aspects of models are critical for a particular research issue is required

19 - Max Podlesny – 3/31/05 Questions?