presented by max podlesny

Internet research Needs Better Models

Sally Floyd, Eddie KohlerISCI Center for Internet Research,

Berkeley, California

Presented byMax Podlesny

2 - Max Podlesny – 3/31/05

Outline

Motivation Network Model Principles Several problems

» Phase Effects» Active Queue Management: Oscillations» TCP Variants

Proposal Conclusion


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


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.


Several typical models


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?


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


Results of simulations


Real network

Traffic includes short-lived flows Traffic consists of small control packets as well as

large data packets More than two competing flows


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


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


Results of simulations


Actually used models


Example:TCP Variants

TCP Reno TCP Vegas


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


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


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


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


Questions?

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