A data streaming algorithm for estimating subpopulation flow size distribution

A Data Streaming Algorithm Estimating Subpopulation FlowSize Distribution

Abhishek KumarMinho SungJun (Jim) Xu

Ellen W. ZeguraNetworking and Telecommunications Group

College of ComputingGeorgia Institute of Technology

{akumar,mhsung,jx,ewz}@cc.gatech.edu

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Problem Statement

Problem: To estimate the empirical distribution of flow sizes inthe network traffic, for arbitrary subpopulations specified after theact of data collection.

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Problem Statement

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Flow Distribution

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Problem Statement

Problem: To estimate the empirical distribution of flow sizes inthe network traffic, for arbitrary subpopulations specified after theact of data collection.

Definition of Flow: All packets with the same flow-label.The flow-label can be defined as any combination of fields fromthe IP header, e.g <Source IP, source Port, Dest. IP, Dest. Port,Protocol>.

Subpopulation: The subpopulation of interest can be specifiedthrough a set of flow classification rules, or asubpopulation filter.

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Overview

Motivation

Related work: Estimation from sampled data

Solution Architecture

Data Collection

Preprocessing

A joint estimation algorithm

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Motivation

• Knowledge of flow-distribution allows us to infer the usagepattern of the network, in terms of:

– The access bandwidth of the user population.

– Application types.

• It also helps in detecting anomalous events such as:

– Incipient worm infections.

– DDoS attacks.

– Route flapping.

• The ability to specify the subpopulation of interesta posterioriprovides additional flexibility and deeper visibility.

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Related work: Inverting sampled packet traces.

• Current measurement boxes collect traces via packet sampling.

• The approach is to invert the sampled distribution to obtain theactual distribution [Duffield et al., SIGCOMM’03].

• High estimation errors due to low sampling rates.

• Practical limitations to inverting sampled traffic [Hohn & Veitch,IMC’03].

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Solution Architecture

Packet stream

Sampling

Module

Streaming

Module

DataPreprocessingSubmodule

Estimation Module

Subpopulation filter

EstimatesSubmoduleEstimation

Header Header Header

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Solution Architecture — Data Collection

• Two parallel data collection modules.

• Measurement proceeds in epochs (e.g. 100 seconds).

• Streaming module maintains an array of counters in fast mem-ory (SRAM).

• For each packet, a counter is chosen via hashing, and incre-mented.

• No attempt to detect or resolve collisions.

• Sampling module performs uniform packet sampling, and ag-gregates samples into NetFlow-like flow records.

• At the end of the epoch, the counter array and flow records arepaged to disk.

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Array of counters

Array of Counters

Processor

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Array of counters

Array of Counters

Processor

Packet arrival

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Array of counters

Array of Counters

Processor

Choose location by hashing flow label

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Array of counters

Array of Counters

Processor

1

Increment counter

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Array of counters

Array of Counters

Processor

1

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Array of counters

Array of Counters

Processor

1

1

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Array of counters

Array of Counters

Processor

1

1

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Array of counters

Array of Counters

Processor1

2

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Array of counters

Array of Counters

Processor1

2

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Array of counters

Array of Counters

Processor1

Collision !!3

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Array of counters — Implementation

• Efficient Implementation of a Statistics Counter Architecture.[Ramabhadran & Varghese, SIGMETRICS’03]

• Small (7-bit) counter in fast memory.

• Large (32 or 64 bit) counter in slow memory.

• Perfectly fits our requirements.

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Solution Architecture — Preprocessing submodule

Packet stream

Sampling

Module

Streaming

Module

DataPreprocessingSubmodule

Estimation Module

Subpopulation filter

EstimatesSubmoduleEstimation

Header Header Header

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Solution Architecture — Preprocessing submodule

< Z,1 >< X,2 > < Y,3 >

Counter Array

Samples200

1

2

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Preprocessing submodule

< Z,1 >< X,2 > < Y,3 >

Counter Array

Samples

Preprocessor

200

1

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< Z,1 >< X,2 > < Y,3 >

Counter Array

Samples

Preprocessor

200

1

2

SubpopulationFilter

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200

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200

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< Z,1 >< X,2 > < Y,3 >

Counter Array

Samples

PreprocessorSubpopulation

Filter

< X,2 > < Y,3 >

< Z,1 >

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200

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200

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< Z,1 >< X,2 > < Y,3 >

Counter Array

Samples

PreprocessorSubpopulation

Filter

< ,2 > < ,3 >

< ,1 >

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• Separate losses in collected data due to collisionsandsampling

• Need to compute the MLE of flow size distributions that wouldcause the observations after these losses

• Joint estimation over the combined data, using the EM algo-rithm

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Termination of the EM Algorithm

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Absolute WMRDRelative WMRD

(a) Proposed algorithm.

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(b) Sampling based estimation.

Figure 1:Absolute and relative WMRD.

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Trace driven Evaluation

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Actual distributionSampling based estimate

Proposed algorithm

Figure 2:Actual, and estimated FSD for flows with source port 80 inthe trace NLANR.

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Evaluation

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Proposed algorithm

(a) Estimate of FSD ofall DNS flows.

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Proposed algorithm

(b) Estimate of FSD ofall HTTP flows.

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Proposed algorithm

(c) Estimate of FSD ofall HTTPS flows.

Figure 3: Actual, and estimated distributions for various subpopula-tions in the trace UNC.

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Evaluation

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Proposed algorithm

(a) Estimate of FSD ofall DNS flows.

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(b) Estimate of FSD ofall HTTP flows.

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(c) Estimate of FSD ofall HTTPS flows.

Figure 4: Actual, and estimated distributions for various subpopula-tions in the trace from USC.

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Monitoring subpopulation flow size distribution – Evaluation

Trace Sub- Actual Sampling Proposedpopulation value based Algorithm

estimates

DNS 31932 38997.87 31389.68HTTP 277583 641874.11 330064.71

UNCHTTPS 27337 72174.95 37928.81Prefix 1.0 388326 673519.43 377266.40DNS 132396.00 161440.75 130686.73HTTP 244773.00 574610.23 283664.58

USCHTTPS 24731.00 64523.73 32660.56Prefix 1.0 637949.00 1017053.20 599140.25

Table 1: Estimates of total number of flows for various subpopulations in tracesUNC and USC

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Conclusions

• Data-Streaming based solution for estimating flow-distributionof arbitrary subpopulations.

• Lossy data structure + Bayesian statistics = Accurate stream-ing

• Data-collection in parallel modules.

• Joint Estimation using EM algorithm.

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Thank You !

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A data streaming algorithm for estimating subpopulation flow size distribution

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