Internet research Needs
Better Models
Sally Floyd, Eddie Kohler
ISCI Center for Internet Research,
Berkeley, California
Presented by
Max Podlesny
Outline
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Motivation
Network Model Principles
Several problems
» Phase Effects
» Active Queue Management: Oscillations
» TCP Variants
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Proposal
Conclusion
‹#› - Max Podlesny – 3/31/05
Motivation
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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
‹#› - Max Podlesny – 3/31/05
Network Model Principles
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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.
‹#› - Max Podlesny – 3/31/05
Several typical models
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Requirements to the model
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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?
‹#› - Max Podlesny – 3/31/05
Example:Phase Effects
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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
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Results of simulations
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Real network
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Traffic includes short-lived flows
Traffic consists of small control packets as well as
large data packets
More than two competing flows
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Example: Active Queue Management:Oscillations
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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
‹#› - Max Podlesny – 3/31/05
Model
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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
‹#› - Max Podlesny – 3/31/05
‹#› - Max Podlesny – 3/31/05
Results of simulations
Actually used models
‹#› - Max Podlesny – 3/31/05
Example:TCP Variants
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TCP Reno
TCP Vegas
‹#› - Max Podlesny – 3/31/05
TCP Reno
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Based on acknowledgements
Two types of congestion event:
» Duplicate acknowledgement
» Timeout
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Works well when only one packet is dropped
Losses often come in bursts
The problem is of how to avoid retransmit timeouts
‹#› - Max Podlesny – 3/31/05
TCP Vegas
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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
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Problems arise with higher level of statistical
multiplexing
‹#› - Max Podlesny – 3/31/05
Proposal
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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
‹#› - Max Podlesny – 3/31/05
Conclusion
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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
‹#› - Max Podlesny – 3/31/05
‹#› - Max Podlesny – 3/31/05
Questions?